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Poor MPG - Rough Idle?


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I have a 2002 Boxster S with replacement Porsche engine with about 30K on it. The car itself has about 65K - I have noticed since I have owned it (2 months) that I get extremely poor MPG. It averages about ~14, with my average less than 200 miles per tank. I do not drive wreckless nor overly aggressive. I have all records from Porsche Austin where the work was performed originally including the 15K, 30K, 60K service including replacement engine, RMS job, new clutch etc. Car was extremely well maintained according the records. Since purchasing the car, I have taken the car to 3 different Porsche shops to have various misc repairs and maintenance done (everything basically to make the car as brand new as possible - mostly all cosmetic) - and to research the poor MPG issue --- no resolution, but I don't trust the effectiveness of the shops in my honest opinion.

SYMPTOMS:

Upon starting cold, it will idle at 1000RPM then drop down to about 800RPM, sounds slightly rough with a strange idle flucuation or vacuum that will drop it briefly then kick back in at 800 or so....To me, it sounds like a leak, but I am just guessing. I get no CEL codes, but throttle response and idle and definitely the gas mileage issue does concerns me greatly. Here is my printout from my Durametric vehicle log.

Can someone please help me go through this and figure out what it all means? (specifically # of ignitions range 1 and 2, hours, etc) - What tests should I choose to run on my Durametric to check for this issue further?

Motronic 7.2

Vehicle Data

Porsche part number 98661866006

Supplier part number 0261204790

Software version number: 004157029210

Number of ignitions range 1 12289 / 2128h

Number of ignitions range 2 1 / 1180h

Operating hours counter 2132

VIN: WP0CB29862S660238

Type ID DME control unit _986SUS02

Order type 986320

Country code C02

Engine type 9621

Engine number 672

Transmission type G8620

Transmission number 0

Radio code 0

Body color/convertible top color 041A10

Interior equipment SA

Thanks,

Andy

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I don't no what concerning Durametric, but with the PIWIS you can look in the Actual Values menu.

There is an "Actual Values" tab, but it has about 100 selectable items to monitor. Didn't know what items would be the preferred ones considering my specific situation. Would guess throttle, MAF, fuel delivery etc.

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Please do a search here - it's been covered many times.

Ok, got it all done. Idle after running it seems to show up somewhere in the 650-700 range. Seems a whole lot smoother and more responsive with the cleaning. Was a little bear to get that rigid air intake pipe off, but I managed. It was gummed up moderately, but probably enough to affect the proper closing/opening. Figured that after bringing it to 3 Porsche repair centers, they could have at least cleaned the throttle body and the engine compartment. I just don't know where the heck customer service has gone --- I doubt any of them even opened the engine compartment. Feel like taping a $100 bill to the engine and see if any of them actually find it.....people are far too reliant on their fancy diag testers and don't perform proper maintenance and customer care, IMHO.

Thanks for the info --- hope it will help my gas mileage out, but I cannot imagine how much more fuel I was using at idle at 1000rpms vs. 650-700 ----

Going to pickup a 16oz of Techron and run it through likewise.

:D

:renntech:

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Please do a search here - it's been covered many times.

Ok, got it all done. Idle after running it seems to show up somewhere in the 650-700 range. Seems a whole lot smoother and more responsive with the cleaning. Was a little bear to get that rigid air intake pipe off, but I managed. It was gummed up moderately, but probably enough to affect the proper closing/opening. Figured that after bringing it to 3 Porsche repair centers, they could have at least cleaned the throttle body and the engine compartment. I just don't know where the heck customer service has gone --- I doubt any of them even opened the engine compartment. Feel like taping a $100 bill to the engine and see if any of them actually find it.....people are far too reliant on their fancy diag testers and don't perform proper maintenance and customer care, IMHO.

Thanks for the info --- hope it will help my gas mileage out, but I cannot imagine how much more fuel I was using at idle at 1000rpms vs. 650-700 ----

Going to pickup a 16oz of Techron and run it through likewise.

:D

:renntech:

I had the same problem several months ago and cleaned the intake side of the throttle body. I didn't have the proper tools to actualy take the body off and couldn't get to the other side of the throttle body. The idle smoothed out but started to come back a couple of weeks ago. By then I had the tools to remove the throttle body and cleaned the back side of the throttle body. It was very dirty. That completely solved the problem. I have a 2002 S and your idle is the same as mine. Sometimes it's the simple things.
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Please do a search here - it's been covered many times.

Ok, got it all done. Idle after running it seems to show up somewhere in the 650-700 range. Seems a whole lot smoother and more responsive with the cleaning. Was a little bear to get that rigid air intake pipe off, but I managed. It was gummed up moderately, but probably enough to affect the proper closing/opening. Figured that after bringing it to 3 Porsche repair centers, they could have at least cleaned the throttle body and the engine compartment. I just don't know where the heck customer service has gone --- I doubt any of them even opened the engine compartment. Feel like taping a $100 bill to the engine and see if any of them actually find it.....people are far too reliant on their fancy diag testers and don't perform proper maintenance and customer care, IMHO.

Thanks for the info --- hope it will help my gas mileage out, but I cannot imagine how much more fuel I was using at idle at 1000rpms vs. 650-700 ----

Going to pickup a 16oz of Techron and run it through likewise.

:D

:renntech:

I had the same problem several months ago and cleaned the intake side of the throttle body. I didn't have the proper tools to actualy take the body off and couldn't get to the other side of the throttle body. The idle smoothed out but started to come back a couple of weeks ago. By then I had the tools to remove the throttle body and cleaned the back side of the throttle body. It was very dirty. That completely solved the problem. I have a 2002 S and your idle is the same as mine. Sometimes it's the simple things.

Thanks, Tom.

What is the proper tools to remove the bolts? It appears to be a Torx --- I really wanted to remove it completely and clean both sides, but even the intake side did seem to help. Thanks for taking the time to reply.

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Once you put the throttlebody back together, grab some carb cleaner and start spraying around the intake tract and intake manifold. There are times where you'll get a intake manifold leak thats minute enough not to flip the CEL but enough to give you the problems that your seeing.

Get the car to normal operating temp. Then go to your garage with the door open and have someone watch your idle. Start spraying carb cleaner past the MAF all the way down to the intake manifold, esp around those connections. If your buddy see's a sudden jump in rpm you might want to investigate. If you can get ahold of an OBDII reader, you can also monitor your o2 readings. What your going to be looking for is a lazy o2 sensor thats giving you somewhat linear readings instead of the up and down voltage their supposed to give you. Every once in awhile you'll get a dying o2 sensor thats just not ready to throw a CEL just yet but enough to throw off your mpg.

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Once you put the throttlebody back together, grab some carb cleaner and start spraying around the intake tract and intake manifold. There are times where you'll get a intake manifold leak thats minute enough not to flip the CEL but enough to give you the problems that your seeing.

Get the car to normal operating temp. Then go to your garage with the door open and have someone watch your idle. Start spraying carb cleaner past the MAF all the way down to the intake manifold, esp around those connections. If your buddy see's a sudden jump in rpm you might want to investigate. If you can get ahold of an OBDII reader, you can also monitor your o2 readings. What your going to be looking for is a lazy o2 sensor thats giving you somewhat linear readings instead of the up and down voltage their supposed to give you. Every once in awhile you'll get a dying o2 sensor thats just not ready to throw a CEL just yet but enough to throw off your mpg.

Thanks alot for the knowledgeable reply. The idle is pretty solid at about ~650 now when at normal operational temp. It flucuates just a tiny bit which would lead me to believe that there is still some better cleaning to do on the manifold side of the throttle body or it may have a leak in the air intake as you suggested. Will my cheap Duramtric Pro unit provide any details on the o2 readings or do I need to have a PIWIS or other type OBDII unit?

Also, Can you please shed any light on my initial info pulled?

Motronic 7.2

Vehicle Data

Porsche part number 98661866006

Supplier part number 0261204790

Software version number: 004157029210

Number of ignitions range 1 12289 / 2128h

Number of ignitions range 2 1 / 1180h

Operating hours counter 2132

VIN: WP0CB29862S660238

Type ID DME control unit _986SUS02

Order type 986320

Country code C02

Engine type 9621

Engine number 672

Transmission type G8620

Transmission number 0

Radio code 0

Body color/convertible top color 041A10

Interior equipment SA

Thanks,

Andy

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From the Duarmetric website...

You can plot any actual readings.

post-1-1195833084_thumb.jpg

Loren,

Thanks! Doesn't the hours reading on the engine (2132) seem unusually high for a new engine less than 25-30K miles ago? (Car has 60k)

Is this hour counter engine specific or will it keep reporting the same hours on a replacement engine like the odometer does with the car as a whole? (One would think that is was totally engine specific based upon the remainder of the engine variables, details and figures reported, but just asking)

I know from my piloting hobby, I start getting nervous as the engines reach their TBO limits...I was hoping that this engine would be good for a while

since it seemed (at least initially) to have good #s SMOH. What is the TBO hours guideline limit for a Boxster 3.2 engine?

Thanks,

Andy

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It's hard to say. Some cars owners idle more than others, heavy traffic areas mean less miles more hours, etc., etc.

The engine hours "should" be reset when changing the engine - but I do not think that always happens.

Porsche doesn't quote TBO for their street cars only their race cars (which is usually 50 operating hours).

We have already seen many early Boxster go well over 150,000 miles without a rebuild.

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It's hard to say. Some cars owners idle more than others, heavy traffic areas mean less miles more hours, etc., etc.

The engine hours "should" be reset when changing the engine - but I do not think that always happens.

Porsche doesn't quote TBO for their street cars only their race cars (which is usually 50 operating hours).

We have already seen many early Boxster go well over 150,000 miles without a rebuild.

Simply amazing..... I reset my OBC for the rough MPG readings and I am observing low 20s for the first time ever! :) Quite amazing that I would go from 13-14 to 19-21 by just cleaning out the throttle body and tightening up most of the fasteners on the intake supply system. Well, at least fuel is not rushing out of my Box now.... just wish that some of the "speciality" shops could have came up with 1/2 of the solutions that people here on the forum were able to offer.....Thanks Tom/Loren and the rest of you! Truly appreciated.

Andy

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  • 2 months later...
Please do a search here - it's been covered many times.

I am going through the same problem with my 01 US Boxster - need throttle body cleaning....did a search, but could not find a doc on how to do it for the 986...

Thanks,

Gustavo

Edited by gandrade1
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Please do a search here - it's been covered many times.

I am going through the same problem with my 01 US Boxster - need throttle body cleaning....did a search, but could not find a doc on how to do it for the 986...

Thanks,

Gustavo

It is easy to do. In fact the job I did also was the first time I had opened up the engine compartment at all. The biggest issue for me was the stiffness of the intake plastic tubes. You really have to yank on them pretty hard and loosen up the end opposite of the throttle body intake (to get a little play in there) --- I found a few online DIY places for instructions on getting access to the engine, then I just figured out the rest.

If you have any questions, let us know.

Best of luck.

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Please do a search here - it's been covered many times.

I am going through the same problem with my 01 US Boxster - need throttle body cleaning....did a search, but could not find a doc on how to do it for the 986...

Thanks,

Gustavo

It is easy to do. In fact the job I did also was the first time I had opened up the engine compartment at all. The biggest issue for me was the stiffness of the intake plastic tubes. You really have to yank on them pretty hard and loosen up the end opposite of the throttle body intake (to get a little play in there) --- I found a few online DIY places for instructions on getting access to the engine, then I just figured out the rest.

If you have any questions, let us know.

Best of luck.

Thank you for the prompt input...

How long did it take you to do the whole thing...BTW it is also going to be my first time opening the engine compartment

Thanks,

Gus

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  • 2 weeks later...
How to with pictures

I couldn't find it here using the search but there is this rather complete description here

Thanks Mike,

I will do the cleaning this weekend...Couple of Questions:

(1) Do I need to disconect the batery before I do it to avoid any CEL codes? Can this process lead to CEL codes at all?

(2) What pressure should I put when puting the screws back together? I just bought a torque wrench and have been looking for an opportunity to use it... :jump: Hahahahah

(3) Should I go ahead and take the opportunity to clean the MAF ? If yes, where do I find instructions...

Thanks,

Gus

Edited by gandrade1
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  • Admin

1. No need to disconnect the battery. You should not cause any faults unless you forget to plug the MAF or throttle electronics back in.

2. These are all very low torque screws - unless you have an inch pounds torque wrench I would just get them tight with a short wrench (not too much leverage that way).

3. Might not hurt since you will be in there anyway.

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