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tristancboyd@gmail.com

Lopey Idle and intake pops on a cold start

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Its not simple to a shop that dose not how to do it. Or have the correct tooling to work on these cars!

I went to the largest Porsche dealership in the area, and they seemed familiar with the AOS when I asked them to specifically check it as well.

 

 

Just remember one thing about dealerships:  Good techs typically leave dealerships and go to work for independent shops, both because they can make more money, and don't have to toe the corporate line.  A lot of dealerships out there will still tell you that it is impossible to replace the IMS in one of these cars without tearing the engine apart, even though it has been done everyday of the week for over eight years now in the field.

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Its not simple to a shop that dose not how to do it. Or have the correct tooling to work on these cars!

I went to the largest Porsche dealership in the area, and they seemed familiar with the AOS when I asked them to specifically check it as well.

 

 

Just remember one thing about dealerships:  Good techs typically leave dealerships and go to work for independent shops, both because they can make more money, and don't have to toe the corporate line.  A lot of dealerships out there will still tell you that it is impossible to replace the IMS in one of these cars without tearing the engine apart, even though it has been done everyday of the week for over eight years now in the field.

 

This is something I didn't know.  I have really never messed with dealerships much (always worked on my own stuff and could just go to a retail parts place for stuff) and I guess that part of the reason is I never expected much from them other than the corporate bottom line.  This car has been my first venture into something that wasn't your couple-thousand car beater.  I guess with the name and the way the staff has treated me when I went there to order a few things, I expected more from the tech side.  I guess I was little naive there.

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Going all the way back to some of your earliest posts on this, I noted what appeared to be an overly high flow volume in your MAF data, which is exactly what a low level AOS leak would give you.

 

John, this is interesting. Why will AOS leak make MAF read higher?

Unfortunately, it is that simple.  Everyone in the Porsche repair trade knows of the fragility and problem prone nature of the AOS, and should be ready to properly test them.  This is what you should find at any Porsche shop worth its salt:

 

attachicon.gifManometer.JPG

 

It consists of a digital manometer (accurate +/- 0.1 inches of water), some rubber tubing and a modified oil fill cap.  Screw the cap on the car, hook up the manometer, and start the car; you immediately know the health of the AOS.

 

I also hope you ordered an OEM AOS replacement, we have had very poor luck with aftermarket units.

Even I have a setup like that at home :)

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Going all the way back to some of your earliest posts on this, I noted what appeared to be an overly high flow volume in your MAF data, which is exactly what a low level AOS leak would give you.

 

John, this is interesting. Why will AOS leak make MAF read higher?

Unfortunately, it is that simple.  Everyone in the Porsche repair trade knows of the fragility and problem prone nature of the AOS, and should be ready to properly test them.  This is what you should find at any Porsche shop worth its salt:

 

attachicon.gifManometer.JPG

 

It consists of a digital manometer (accurate +/- 0.1 inches of water), some rubber tubing and a modified oil fill cap.  Screw the cap on the car, hook up the manometer, and start the car; you immediately know the health of the AOS.

 

I also hope you ordered an OEM AOS replacement, we have had very poor luck with aftermarket units.

Even I have a setup like that at home :)

 

 

You have hit on the $64 dollar question.  We know from direct experience that MAF's on their way to "aging out" often read higher than expected air volume flows; we have seen this on Porsche's as well as other brands, so it is not an unexpected event.  So when we saw higher than normal MAF air volume numbers on cars with obviously dying AOS units, we kind of wrote it off to an aging sensor.  But then someone brought it to my attention that they had seen similar high air flow volumes on cars with bad AOS units, that subsequently (but not always) dropped back into range when the AOS was replaced, so it was not always a dying sensor. 

 

Always intrigued by a new mystery on these cars, and not wanting to needlessly be replacing expensive MAF units, we started looking at cars we got in that failed the AOS vacuum test (vacuum signal at the oil fill caps greater than 5 inches of water).  And what we saw mimicked what the other observer noted, the air flow volume often changed, which set us scratching out heads.  On the 986 in question for example, the AOS line to the intake system enters behind the throttle body and ahead of the intake plenum, and the MAF is located a couple of feet "upstream" of the throttle body, near the air filter box on the driver's side rear fender; so intuitively it would seem rather too remote to see a change in the vacuum signal level where the AOS line enters the intake.  But it apparently does under some conditions.  Why this happens remains an item of speculation, and the amount of air volume change is not always large, but always seems to be in the same direction when it happens.

 

An interesting experiment would be to set up a car to mimic a failing AOS (which would require being able to "throttle" the AOS crankcase vacuum level up or down, while monitoring both the intake plenum vacuum levels and the MAF air flow volumes) and collect some additional data points in an attempt to see what is going on.  Unfortunately, in the shop we are nearly always in a time crunch to figure out what is wrong and get the car back into the owner's hands, so we have not attempted this.  It would be an interesting R&D experiment however.

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Going all the way back to some of your earliest posts on this, I noted what appeared to be an overly high flow volume in your MAF data, which is exactly what a low level AOS leak would give you.

 

John, this is interesting. Why will AOS leak make MAF read higher?

Unfortunately, it is that simple.  Everyone in the Porsche repair trade knows of the fragility and problem prone nature of the AOS, and should be ready to properly test them.  This is what you should find at any Porsche shop worth its salt:

 

attachicon.gifManometer.JPG

 

It consists of a digital manometer (accurate +/- 0.1 inches of water), some rubber tubing and a modified oil fill cap.  Screw the cap on the car, hook up the manometer, and start the car; you immediately know the health of the AOS.

 

I also hope you ordered an OEM AOS replacement, we have had very poor luck with aftermarket units.

Even I have a setup like that at home :)

 

 

You have hit on the $64 dollar question.  We know from direct experience that MAF's on their way to "aging out" often read higher than expected air volume flows; we have seen this on Porsche's as well as other brands, so it is not an unexpected event.  So when we saw higher than normal MAF air volume numbers on cars with obviously dying AOS units, we kind of wrote it off to an aging sensor.  But then someone brought it to my attention that they had seen similar high air flow volumes on cars with bad AOS units, that subsequently (but not always) dropped back into range when the AOS was replaced, so it was not always a dying sensor. 

 

Always intrigued by a new mystery on these cars, and not wanting to needlessly be replacing expensive MAF units, we started looking at cars we got in that failed the AOS vacuum test (vacuum signal at the oil fill caps greater than 5 inches of water).  And what we saw mimicked what the other observer noted, the air flow volume often changed, which set us scratching out heads.  On the 986 in question for example, the AOS line to the intake system enters behind the throttle body and ahead of the intake plenum, and the MAF is located a couple of feet "upstream" of the throttle body, near the air filter box on the driver's side rear fender; so intuitively it would seem rather too remote to see a change in the vacuum signal level where the AOS line enters the intake.  But it apparently does under some conditions.  Why this happens remains an item of speculation, and the amount of air volume change is not always large, but always seems to be in the same direction when it happens.

 

An interesting experiment would be to set up a car to mimic a failing AOS (which would require being able to "throttle" the AOS crankcase vacuum level up or down, while monitoring both the intake plenum vacuum levels and the MAF air flow volumes) and collect some additional data points in an attempt to see what is going on.  Unfortunately, in the shop we are nearly always in a time crunch to figure out what is wrong and get the car back into the owner's hands, so we have not attempted this.  It would be an interesting R&D experiment however.

 

 

Thanks John for the detailed reply. My thinking is as follows:

 

Suppose everything is working correctly on a warmed up idling engine. Now all in a sudden the AOS diaphragm cracks. That allows MORE crankcase gas (oxygen deprived and even fuel-enriched) into the throttle and LESS fresh air from intake, Both effect will create an instantaneous rich condition. Engine will start to stumble due to wrong a/f mixture. To maintain a minimum rpm, the DME compensates by opening up the throttle a bit more to let more fresh air (oxygen) into the engine to achieve the correct a/f ratio. At the end the MAF will see more flow (in mass per second) and the rpm may be raised a little too (not sure about this though).

 

When you observed the MAF read a bit higher than normal, I assume the rpm was at a normal value and not raised? I mean if the rpm was also raised, it's just natural the MAF read higher.

 

I think it will be quite difficult to simulate the crack inside the AOS precisely. One will need to build an adjustable bypass valve between the AOS vent tube that's connected to the crankcase and the AOS vent tube that's connected to the throttle body. Will be an interesting experiment nonetheless :)

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Going all the way back to some of your earliest posts on this, I noted what appeared to be an overly high flow volume in your MAF data, which is exactly what a low level AOS leak would give you.

 

John, this is interesting. Why will AOS leak make MAF read higher?

Unfortunately, it is that simple.  Everyone in the Porsche repair trade knows of the fragility and problem prone nature of the AOS, and should be ready to properly test them.  This is what you should find at any Porsche shop worth its salt:

 

attachicon.gifManometer.JPG

 

It consists of a digital manometer (accurate +/- 0.1 inches of water), some rubber tubing and a modified oil fill cap.  Screw the cap on the car, hook up the manometer, and start the car; you immediately know the health of the AOS.

 

I also hope you ordered an OEM AOS replacement, we have had very poor luck with aftermarket units.

Even I have a setup like that at home :)

 

 

You have hit on the $64 dollar question.  We know from direct experience that MAF's on their way to "aging out" often read higher than expected air volume flows; we have seen this on Porsche's as well as other brands, so it is not an unexpected event.  So when we saw higher than normal MAF air volume numbers on cars with obviously dying AOS units, we kind of wrote it off to an aging sensor.  But then someone brought it to my attention that they had seen similar high air flow volumes on cars with bad AOS units, that subsequently (but not always) dropped back into range when the AOS was replaced, so it was not always a dying sensor. 

 

Always intrigued by a new mystery on these cars, and not wanting to needlessly be replacing expensive MAF units, we started looking at cars we got in that failed the AOS vacuum test (vacuum signal at the oil fill caps greater than 5 inches of water).  And what we saw mimicked what the other observer noted, the air flow volume often changed, which set us scratching out heads.  On the 986 in question for example, the AOS line to the intake system enters behind the throttle body and ahead of the intake plenum, and the MAF is located a couple of feet "upstream" of the throttle body, near the air filter box on the driver's side rear fender; so intuitively it would seem rather too remote to see a change in the vacuum signal level where the AOS line enters the intake.  But it apparently does under some conditions.  Why this happens remains an item of speculation, and the amount of air volume change is not always large, but always seems to be in the same direction when it happens.

 

An interesting experiment would be to set up a car to mimic a failing AOS (which would require being able to "throttle" the AOS crankcase vacuum level up or down, while monitoring both the intake plenum vacuum levels and the MAF air flow volumes) and collect some additional data points in an attempt to see what is going on.  Unfortunately, in the shop we are nearly always in a time crunch to figure out what is wrong and get the car back into the owner's hands, so we have not attempted this.  It would be an interesting R&D experiment however.

 

 

Thanks John for the detailed reply. My thinking is as follows:

 

Suppose everything is working correctly on a warmed up idling engine. Now all in a sudden the AOS diaphragm cracks. That allows MORE crankcase gas (oxygen deprived and even fuel-enriched) into the throttle and LESS fresh air from intake, Both effect will create an instantaneous rich condition. Engine will start to stumble due to wrong a/f mixture. To maintain a minimum rpm, the DME compensates by opening up the throttle a bit more to let more fresh air (oxygen) into the engine to achieve the correct a/f ratio. At the end the MAF will see more flow (in mass per second) and the rpm may be raised a little too (not sure about this though).

 

When you observed the MAF read a bit higher than normal, I assume the rpm was at a normal value and not raised? I mean if the rpm was also raised, it's just natural the MAF read higher.

 

I think it will be quite difficult to simulate the crack inside the AOS precisely. One will need to build an adjustable bypass valve between the AOS vent tube that's connected to the crankcase and the AOS vent tube that's connected to the throttle body. Will be an interesting experiment nonetheless :)

 

 

I would expect that if the your scenario were correct, the idle should go up and down, and with the repeated A/F issues, the car would also code because the mixture would eventually have to swing out of range one way or the other more than once.

 

We observed the higher MAF volume numbers at a normal (and steady) idle speeds.

 

The basic problem with the AOS design is that it is way too complicated, too fragile, and has too many ways of going south on you.  We thought about how you would have to modify one to test our observations and it quickly turned into a plumbing and electrical nightmare.  Sometimes the view just ain't worth the climb.............

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This discussion is amazing.  Just reading over this, I bet an experiment like this could yield useful information.  I just got back from a meet a little bit ago.  I experienced a few times so popping and lack of power at low RPM's or some take offs from a slow coast or stop.  I'm probably going to go out in a few an look over the sensor readings with it idling for a bit and also look at the temp readouts as well.  The AOS will be replaced when it comes in a few days from now.

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This discussion is amazing.  Just reading over this, I bet an experiment like this could yield useful information.  I just got back from a meet a little bit ago.  I experienced a few times so popping and lack of power at low RPM's or some take offs from a slow coast or stop.  I'm probably going to go out in a few an look over the sensor readings with it idling for a bit and also look at the temp readouts as well.  The AOS will be replaced when it comes in a few days from now.

 

More than the data this could yield, I'd still like to know what the crankcase vacuum level is on your car; one min. of diagnostic time would immediately confirm or deny the AOS is going bad.  I'd be willing to bet that we would see a vacuum signal somewhere between 5 and 10 inches of water; enough to be annoying and causing running issues, but not yet shot enough to disable the car and turn it into a mosquito fogger.  Oh the joys of long distance diagnostics...........

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Going all the way back to some of your earliest posts on this, I noted what appeared to be an overly high flow volume in your MAF data, which is exactly what a low level AOS leak would give you.

John, this is interesting. Why will AOS leak make MAF read higher?

Unfortunately, it is that simple. Everyone in the Porsche repair trade knows of the fragility and problem prone nature of the AOS, and should be ready to properly test them. This is what you should find at any Porsche shop worth its salt:

attachicon.gifManometer.JPG

It consists of a digital manometer (accurate +/- 0.1 inches of water), some rubber tubing and a modified oil fill cap. Screw the cap on the car, hook up the manometer, and start the car; you immediately know the health of the AOS.

I also hope you ordered an OEM AOS replacement, we have had very poor luck with aftermarket units.

Even I have a setup like that at home :)

You have hit on the $64 dollar question. We know from direct experience that MAF's on their way to "aging out" often read higher than expected air volume flows; we have seen this on Porsche's as well as other brands, so it is not an unexpected event. So when we saw higher than normal MAF air volume numbers on cars with obviously dying AOS units, we kind of wrote it off to an aging sensor. But then someone brought it to my attention that they had seen similar high air flow volumes on cars with bad AOS units, that subsequently (but not always) dropped back into range when the AOS was replaced, so it was not always a dying sensor.

Always intrigued by a new mystery on these cars, and not wanting to needlessly be replacing expensive MAF units, we started looking at cars we got in that failed the AOS vacuum test (vacuum signal at the oil fill caps greater than 5 inches of water). And what we saw mimicked what the other observer noted, the air flow volume often changed, which set us scratching out heads. On the 986 in question for example, the AOS line to the intake system enters behind the throttle body and ahead of the intake plenum, and the MAF is located a couple of feet "upstream" of the throttle body, near the air filter box on the driver's side rear fender; so intuitively it would seem rather too remote to see a change in the vacuum signal level where the AOS line enters the intake. But it apparently does under some conditions. Why this happens remains an item of speculation, and the amount of air volume change is not always large, but always seems to be in the same direction when it happens.

An interesting experiment would be to set up a car to mimic a failing AOS (which would require being able to "throttle" the AOS crankcase vacuum level up or down, while monitoring both the intake plenum vacuum levels and the MAF air flow volumes) and collect some additional data points in an attempt to see what is going on. Unfortunately, in the shop we are nearly always in a time crunch to figure out what is wrong and get the car back into the owner's hands, so we have not attempted this. It would be an interesting R&D experiment however.

Thanks John for the detailed reply. My thinking is as follows:

Suppose everything is working correctly on a warmed up idling engine. Now all in a sudden the AOS diaphragm cracks. That allows MORE crankcase gas (oxygen deprived and even fuel-enriched) into the throttle and LESS fresh air from intake, Both effect will create an instantaneous rich condition. Engine will start to stumble due to wrong a/f mixture. To maintain a minimum rpm, the DME compensates by opening up the throttle a bit more to let more fresh air (oxygen) into the engine to achieve the correct a/f ratio. At the end the MAF will see more flow (in mass per second) and the rpm may be raised a little too (not sure about this though).

When you observed the MAF read a bit higher than normal, I assume the rpm was at a normal value and not raised? I mean if the rpm was also raised, it's just natural the MAF read higher.

I think it will be quite difficult to simulate the crack inside the AOS precisely. One will need to build an adjustable bypass valve between the AOS vent tube that's connected to the crankcase and the AOS vent tube that's connected to the throttle body. Will be an interesting experiment nonetheless :)

I would expect that if the your scenario were correct, the idle should go up and down, and with the repeated A/F issues, the car would also code because the mixture would eventually have to swing out of range one way or the other more than once.

We observed the higher MAF volume numbers at a normal (and steady) idle speeds.

The basic problem with the AOS design is that it is way too complicated, too fragile, and has too many ways of going south on you. We thought about how you would have to modify one to test our observations and it quickly turned into a plumbing and electrical nightmare. Sometimes the view just ain't worth the climb.............

Actually I expect it to stabilize and not oscillate because once the throttle plate cracks a little bit more, more O2 will be sucked in to neutralize the rich condition. If my thinking is right, we should observe the throttle plate angle is larger than normal. To have the (working) MAF read higher, either the vacuum suction from the engine increases or the throttle plate opens more. I expect the leaked crankcase gas will decrease the vacuum so if the rpm remains the same, I would expect the throttle plate opens more than notmal.

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Agree with John if you switch out the AOS before measuring the crankcase vacuum, you lose a valuable and critical data point/clue.

Also keep in mind a new AOScan fail out of the box so a manaometer measurement is needed anyway to verify the new AOS is indeed working. Blindly swap out components may introduce more variables and lead you chasing your tail.

This discussion is amazing. Just reading over this, I bet an experiment like this could yield useful information. I just got back from a meet a little bit ago. I experienced a few times so popping and lack of power at low RPM's or some take offs from a slow coast or stop. I'm probably going to go out in a few an look over the sensor readings with it idling for a bit and also look at the temp readouts as well. The AOS will be replaced when it comes in a few days from now.

More than the data this could yield, I'd still like to know what the crankcase vacuum level is on your car; one min. of diagnostic time would immediately confirm or deny the AOS is going bad. I'd be willing to bet that we would see a vacuum signal somewhere between 5 and 10 inches of water; enough to be annoying and causing running issues, but not yet shot enough to disable the car and turn it into a mosquito fogger. Oh the joys of long distance diagnostics...........

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That and to verify the new AOS is working properly. If you don't measure, you may also change out a good AOS unnecessarily.

These meters are getting cheaper.

http://www.amazon.com/gp/aw/d/B00N3PPZZY/ref=mp_s_a_1_2?qid=1456019143&sr=8-2π=AC_SX236_SY340_FMwebp_QL65&keywords=digital+manometer

Alright, I had one day left of amazon prime trial with free two day shipping so I ordered it.  Beats paying someone else to do it.  Now I just need to rig a cap right?

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Great. Yes, I just modified my old cap, bought and installed a new one on my engine.

  • Upvote 1

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I decided to do some data logging this morning on the way to grab some breakfast and as I was going, it dawned on me to add throttle position and vacuum to the list, of course one thought came after the other.  When I got back home, and looked it up, I saw that I have been logging for days without realizing it because I had not turned the app off.  The most recent log has vacuum and throttle position.  There is a longer trip from yesterday where some of the pops and hesitations occurred too, I'm just not a data genius.  I'll review it and circle it though since I remember where in my trip one instance specifically happened.

 

http://view.torque-bhp.com/

 

Device ID: 990005434631165

Edited by tristancboyd@gmail.com

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Correction, I remember it happened turning left onto a street or at an intersection yesterday, I don't see which one it could have been though.  

 

I also wish it was logging when I first left the house this morning, it started up pretty decent for a change (but its also somewhere between 60-70 degrees out today) but as soon as I pulled out of the drive, it was a dog and wouldn't hardly go at all.  

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And some epoxy or sealant to seal it too.

 

 

Or a couple of good compression seals to go under the bulkhead fitting nuts.

 

Quoted you guys just for notification.  I'm waiting for the new cap to come in.  I have a log this morning, started logging before I started the car, The car started pretty fair but if you zoom in to around the 6:28 mark, you can see the idle get erratic until I get back in the car to leave.  I also noticed that it claims speed is reported in MPH but that's definitely KMH.  I do not drive that fast with my son in the car!

Edited by tristancboyd@gmail.com

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      MORE NEW CODES;
      P2A04 O2 SENSOR CIRCUT RANGE/ PERFORMANCE BANK 2 SENSOR 2
      P2099 POST CATALYST FUEL TRIM SYSTEM TOO RICH BANK 2
      P0153 O2 SENSOR CIRCUIT SLOW RESPONSE BANK 2 SENSOR 1
      P2198 O2 SENSOR SIGNAL BIASED/ STUCK RICH BANK 2 SENSOR 1
      P0171 SYSTEM TOO LEAN BANK 1
       
    • By hugo kohl
      1999, 996,  117k miles
       
      Due to leaking I recently changed my coolant reservoir tank, which involved disconnecting and re connecting 2 fuel lines..  Upon restarting the car I had a rough idle and CEL.  I purchased a $100b code reader, read a bunch of stuff online and decided to clean the MAF sensor, change the air filter, spark plugs, plug tubes and coils.    
       
       
      upon today's startup I get an erratic/rough/low rpm idle, flashing CEL and the following codes:
       
      P0102  mass air flow circuit low
      P1319 misfire emission related
      P1318  misfire cylinder 6 emission related
      P1315 misfire cylinder 3 emission related
      P1313 misfire cylinder 1 emission related
       
      Somehow I went from a leaky tank to some bad stuff based on a do-it-myself moment.
      I wonder if the coil packs might not be perfectly seated?
      I'm thinking a new mass air flow sensor and all four O2 sensors need replacing??  
      Please advise.
       
      Hugo Kohl
    • By Los996
      Hello all,
      This is a continuing repair to what started as a camshaft deviation out of spec leading to the replacement of the chain tensioner pads / brake pads / chain ramps on my 2000 911 - 996 5-Chain engine. Seen here:
      That odyssey has since been resolved but I now have a new issue.

      Quick background:
      Removed camshafts, readjusted the camshaft chain timing "marker" positions, replaced tensioner pads, reassembled everything per factory specs.
      On first start up got very rough idle and bad misfires. At first I thought it may have been my timing was off but went back and visually inspected all and all looked correct (visually). Upon further inspection realized parts guy had given me Bosch 7403 plugs instead of Bosch 7413 (2000 Porsche 911).

      Current issue:
      Switched out and installed the correct spark plugs and no more continuous misfires / backfires. Did the initial reset (wait 1 min with ignition on / turn ignition off / wait 10 sec) On the first start up it did misfired / back fired once but I assumed that was left over fuel.

      Car now runs but very rough and threw 2 codes:
      - P0300 : Porsche Fault code 62 - Misfire damaging cat. converter
      - P0301 : Porsche fault code Cylinder 1 misfire damaging to cat. converter
      Also, took these readings:
      Actual angle for camshaft bank 1: -12.53
      Actual angle for camshaft bank 2: 3.05

      On positive note the main reason for doing all this work was the original problem / issue of tensioner / brake pad wear which was giving me a -10 / -2.92 reading is now:
      Camshaft position 1 deviation: 0.00
      Camshaft position 2 deviation: 0.00

      So the question is now are the Actual angle for camshaft readings within specs and could incorrect timing be attributed to these new issues?

      And... Do I have to give the DME time to "relearn"?

      Txs all
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