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JeTexas

Idle not dropping when I clutch

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I finally had a nice weekend with no rain, so I could pull the cab top forward and get to the DME and relays. I cleaned the connecter to the DME just in case, but it was spotless. However, when I pulled the big green relay 96461521500, there was oxidation on the prongs and running between the prongs on the plastic. I cleaned it all off, but I'm ordering a replacement from Sunset in the morning.

The other relays all looked very clean, but I sprayed a brushed them all anyway, and I rotated all the duplicate relays into different slots. When I put it back together and restarted the car, it would barely idle. It's acting like my old Mustang used to when it had a bad spark plug wire. It keeps missing and is really chugging until it warms up. However, the only code it threw was P1602, which just means the battery was disconnected.

Anyway, I'm going to get those replacement relays ordered. Maybe that will finally solve my issues.

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Nobody in town has the relays in stock, so they're over a week away coming from Sunset. (I do miss the days of domestic car ownership when I could get any part I wanted any day of the week at Autozone. However, Sunset's prices were half what my local dealer wanted. Thank you, Jeff.)

So while I wait, I'd like to test the relays I have to see if I can determine which one is bad.

I have the 996 workshop manual, and I'm guessing there's a procedure in there for testing them with a multimeter, but I can't seem to find it.

Anyone know?

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JeTexas,

Interesting thread.

Most relays use a common pattern. If you put a multimeter in the ohm's setting between terminals 87 and 30, it should read 1 or infinity. Then apply 12v to 86 and ground 85, the relay should close and resistance should drop to almost zero.

Edited by rb101

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I can't claim victory yet, but I had a major breakthrough this morning. I finally found a pattern to the intermittent behavior!

I hadn't been paying attention to my voltage readings because they were plenty high, but the problem seems to lie in the fact that they're too high.

When I start the car, I get as much as 14.2 volts. Then, as I drive, the voltage ever so slowly drops until it finally settles at 13.6 - 13.5. Once the voltage finally drops to 13.6, that's when the idle problems go away, and the car runs normal again.

I think it may actually be the voltage regulator in my alternator causing the problems!

Of course, that's if the voltage is the cause, not just one of the symptoms. Any other ideas?

Addendum: I just read that alternators should run 13.5 - 14.5 volts, so my 14.2 really shouldn't be causing the problem. In fact, it seems that the fact that my voltage is dropping as the battery is charged shows the regulator is working correctly. However, I've verified on many data logs that 13.6 is the magic voltage when all my problems stop. I guess that means some other component in the electrical system is at fault.

Edited by JeTexas

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I did suggest a few pages back that you check the diode ripple generated by the alternator and connections. I had something similar happen on a jaguar.

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I took the car to Autozone at lunch as their testers check ripple -- although the guys there had never even heard the term before. The diodes checked out good. Everything in the charging system seems to be working perfectly.

Back to the drawing board.

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So moving the relays did or didn't fix the high hanging idle at speed?

You still have the original problem, and moving the relays added rough idle at cold start?

I suspect the drop to 13.6 is a relay going on or off.

If you connect some wires to the relays one at a time and look at the voltage while driving, you could

at least identify which one was tripping at the time the problem starts/stops.

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So moving the relays did or didn't fix the high hanging idle at speed?

You still have the original problem, and moving the relays added rough idle at cold start?

I suspect the drop to 13.6 is a relay going on or off.

If you connect some wires to the relays one at a time and look at the voltage while driving, you could

at least identify which one was tripping at the time the problem starts/stops.

Moving the relays didn't fix the hanging idle problem but did add a rough start problem. However, after a few days of driving the car, the rough start has pretty much gone away. I'm wondering if the DME just had to recalibrate -- although it seems like it should have done it much faster. I'm still waiting on all the replacement relays to show up. I don't think they've even showed up to Sunset yet, much less shipped.

I am getting a fault code now. It's P0446 Fault Code 95 - Shutoff valve activated charcoal filter (function). I got that code a couple months ago, cleared it, and it just came back this week. I don't know how it could be related to the hanging idle problem though.

Edited by JeTexas

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Could this fault be allowing extra fuel vapors or air into the intake keeping the idle up?

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Could this fault be allowing extra fuel vapors or air into the intake keeping the idle up?

I don't think so. There would have to be a vacuum leak for the intake to pull that much air through the EVAP system, and it would take a lot of evaporated fuel vapor to raise the idle like that. Plus, I don't think it would just shut off at 10 mph and not cause any effects when I am at a stop.

I have new relays and a PST-2 on the way. Maybe I'll have a breakthrough this weekend.

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Changed out all three little gray 141951253B DME/fuel pump relays, the green AC 96461521500 relay (which updates to a smaller green relay 99965701890), and the Secondary Air Pump relay 99361511501.

I didn't bother changing out the two spoiler relays or the starter relay.

My rough start and chugging problems were instantly gone, so I'm happy that the relay replacement wasn't just another exercise in futility. However, it did absolutely nothing for the hanging idle situation.

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Thanks to a lot of help from tholyoak, I was able to switch out my DME this weekend with a known working unit. I got absolutely no change in my symptoms.

So at this point, I can say it is definitely NOT:

- An intermittent vacuum leak (at least one that myself or three different mechanics have been able to find ... )

- Sticking throttle, cable or pedal

- Idle Control Sensor

- Throttle Position Sensor

- Mass Airflow Sensor

- Coolant Temperature Sensor

- Alternator Diodes

- Bad Ground (that anyone can find ... )

- DME or fuel pump relays

- DME

All my percieved patterns of when the problems starts or stops have gone out the window. Whenever I seem to find something temperature or voltage related, within a few days of tracking the problem, the percieved pattern will change. The only thing that can for sure make it happen, if sometimes only briefly, is turning the car off and turning it back on.

I'm not sure where to check next. I think maybe the fuel pump regulator or one of my friends suggested the cruise control system although it happens whether the cruise control is turned on or off, and the cruise still works normally when it's turned on.

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Well you seem to have covered all the bases.... I really have no idea, replace the ignition switch, it causes odd things to happen...lol.

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How about checking the lower AOS bellows for tears? That is common and the mechanics may not have checked there for leaks.

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If I were you, at this point I would invest in the Durametric cable and use the actual values logging function while you drive the car. You can see if things like throttle position change in an unusual way. The Durametric is well worth the money if you already have a suitable laptop to use with it.

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If I were you, at this point I would invest in the Durametric cable and use the actual values logging function while you drive the car. You can see if things like throttle position change in an unusual way. The Durametric is well worth the money if you already have a suitable laptop to use with it.

I've been through the Durametric. Now I'm on the PST2. Still not finding the fault.

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If I were you, at this point I would invest in the Durametric cable and use the actual values logging function while you drive the car. You can see if things like throttle position change in an unusual way. The Durametric is well worth the money if you already have a suitable laptop to use with it.

I've been through the Durametric. Now I'm on the PST2. Still not finding the fault.

You're not looking for a fault, but a sensor which indicates something is outside parameters.

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Are you certain this isn't a problem with the mechanical throttle linkage? Have you monitored the actual throttle position when the hanging idle occurs?

-Todd

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Are you certain this isn't a problem with the mechanical throttle linkage? Have you monitored the actual throttle position when the hanging idle occurs?

-Todd

According to the Durametric the throttle position is 0 degrees when the problem is occurring. The only non-normal reading is that the Air Mass is around 60 instead of 20 (not sure of the units on these measurements. I think kg/sec, but I can't find anything to verify that), which corresponds to the 2000 rpm idle. Then I hit 10 mph and the air mass drops to 20 and the idle drops to 800 rpm - back to normal.

Of course, that's only with the A/C on. If I turn the A/C off, the idle hangs at 1100 rpm and tends to fall very slowly when I brake, no matter the speed of the car, but it then corrects itself to around 700 RPM when I get under 10 mph.

I picked up a power inverter this weekend, so I can monitor the live values with the PST-2 this week and also the idle position switch while I'm at speed since everything checked out normal in the driveway.

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I'm having the same issue with a '99 986 which is the same basic setup(mecanical throttle body with IAC). Drive the car over 2000-3000 rpm it will intermitently hang. The client replaced the iac himself, so I took his word on it that he replaced it and went on from there. Also the DME wont fix it either I programmed a used control unit and same problem, so dont bother with that attempt. My next step with mine is I'm going to back probe pin 2 of the iac (supply voltage) and see if the supply voltage is going away when the iac hangs open. If I find anything out with that I will let you know.

Tom

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Well I have good news. You need the correct size matched tires. Your car has Traction control built into the abs control unit. When the abs control unit sees a difference in speeds front to rear at speeds the abs control unit sends a signal to the dme to hold the idle high incase the clutch is reengaged. The abs/tc gets confused because it is seeing a slower wheel speed in the rear than the front so it thinks the rear end ir locking up when you back off the gas. the cheap fix is to just turn the TC off. and you will be ok other wise you will need to get correct matching tires for that car.

Unfortunately I have spent way to much time on this problem....I believe I have about 22 hours clocked onto tracing this stupid problem out that I will probably get paid 3 for but at least I have the glory of saying I figured it out :)

please repost if this works.

Thanks Tom

Porsche of North Scottsdale

  • Thanks 1

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Well I have good news. You need the correct size matched tires. Your car has Traction control built into the abs control unit. When the abs control unit sees a difference in speeds front to rear at speeds the abs control unit sends a signal to the dme to hold the idle high incase the clutch is reengaged. The abs/tc gets confused because it is seeing a slower wheel speed in the rear than the front so it thinks the rear end ir locking up when you back off the gas. the cheap fix is to just turn the TC off. and you will be ok other wise you will need to get correct matching tires for that car.

Unfortunately I have spent way to much time on this problem....I believe I have about 22 hours clocked onto tracing this stupid problem out that I will probably get paid 3 for but at least I have the glory of saying I figured it out :)

please repost if this works.

Thanks Tom

Porsche of North Scottsdale

The 99 C2 TC has a cable controlled throttle and the TC does not work in the way you described. I don't see how this would effect the idle.

Edited by 1999Porsche911

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Well I have good news. You need the correct size matched tires. Your car has Traction control built into the abs control unit. When the abs control unit sees a difference in speeds front to rear at speeds the abs control unit sends a signal to the dme to hold the idle high incase the clutch is reengaged. The abs/tc gets confused because it is seeing a slower wheel speed in the rear than the front so it thinks the rear end ir locking up when you back off the gas. the cheap fix is to just turn the TC off. and you will be ok other wise you will need to get correct matching tires for that car.

Unfortunately I have spent way to much time on this problem....I believe I have about 22 hours clocked onto tracing this stupid problem out that I will probably get paid 3 for but at least I have the glory of saying I figured it out :)

please repost if this works.

Thanks Tom

Porsche of North Scottsdale

The 99 C2 TC has a cable controlled throttle and the TC does not work in the way you described. I don't see how this would effect the idle.

correct it also has an iac, and in this situation the tc is holding the iac wide open because the rear wheels are showing a lower speed signal than the fronts or it is possible that it is a programming flaw in the abs/tc programming. On the 986 I was working on it had 40 series tires in the rear instead of 35 series, with a piwis tester I watched the wheel speeds up to 35kph they were 4-5 kph off from front to rear. I do find it very strange that the abs would have anything to do with changing the iac setting, but it is.

Edited by flatsix36

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I'm headed out to my car to turn the traction control off and see what happens.

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