gtuma

Hi,

I have been after some kind of electrical gremlin for some time now and am looking for some help

I have a 1988 944 Turbo S that will not idle when it’s cold. I pretty much have to keep the throttle cracked for the first minute or it will stall. As it warms up there is a hitch off of idle. Once it’s warmed up the idle is fine, maybe a little bit high (1050 rpm) and it runs well. I can open the screw in top of the throttle body enough to keep it at idle when I start it, but it will end up at 1200+ RPM when it’s warmed up.

Here are a couple of things I have tried so far.

Check Altitude Sensor - Currently closed (I'm in Denver at 6000ft so this is expected)

Check Throttle position sensor - I've heard that if the TPS does not record the throttle as closed the idle stabilizer will not function. There is an audible 'click' as I crack the throttle and the pins on the DME show the switch portion of the TPS opening and closing correctly.

Extra Idle stabilizer valve - Given the difficulty in getting the actual valve out from under the Intake I purchased another, pulled the wiring out and hooked it up to the new valve. I started the car cold and looked inside the valve. It was slightly open even as the car was struggling to idle. It doesn't seem to move much.

Any suggestions would be greatly appreciated.

George

samluke

Could be a bad temp sensor. The temp sensor could be showing a hot engine even when its cold. Once it's warmed up it would run correctly.

gtuma

Just making sure,

The DME temp sensor is the forward most sensor with the full connector, rather than the two wires that press over pins?

Musche

That is correct. The other one is for the gauge.

Musche

From Clark's Garage:

Rough Idle

If the car starts when cold but, idles rough or does not want to stay running, this can indicate a problem with the engine temperature sensor. When the engine is started cold, the engine temperature sensor sends a signal to the DME control unit to provide a richer mixture. As the engine temperature sensor starts to fail, it sends a higher than actual temperature signal to the DME control unit. The DME control unit leans the fuel mixture thinking the engine is at normal operating temperature. This causes the engine to run rough. As the engine warms, it not longer needs the richer cold idle mixture and the engine runs smoother.

If the car idles rough this can also be indicative of bad plug wires, distributor, or rotor. One way to check for bad plug wires is to inspect the plug wire runs with the car running in a very dark area. If the plug wires are bad you should see some arching to the cylinder head or fuel rail.

If the car idles rough or cuts off at low RPM or idle, the cause could be a failing O2 sensor. The O2 sensor is out of the loop at full throttle and will not affect the way the car runs.

A rough idle can also be caused by a problem with the air flow box. The damper for the air flow box can fail mechanically or the contacts inside the box can get dirty and send a errant signal to the DME control unit. This can sometimes be corrected by cleaning the electrical contacts inside the air box. This requires removing the cover on the air box which is held in place by caulk. You will need to cut through the caulk and gently pry the cover off. Once the cover is removed, clean the circuit board inside with a good contact cleaner. When you have finished cleaning the circuit board, reattach the cover using a non-corrosive caulk.

If the car experiences a rough idle or idle fluctuates several hundred RPM, this can indicate a sticking idle stabilizer valve. Many people have removed and cleaned them but, this is normally a temporary fix and the valve should be replaced. A faulty O2 sensor can also cause RPM to fluctuate at idle. Unplug the O2 sensor and see if the idle still fluctuates.

DME Temperature Sensor Testing

Tools


Multimeter
Test Leads
Procedure


Turn the ignition switch OFF.
Disconnect the DME computer electrical connector.
Connect an ohmmeter between terminal 13 on the disconnected DME plug and ground.
Check for the following resistances:

DME Temperature Sensor Resistances
15-30 °C (59-86 °F) At 59 °F approximately 3.3 k-ohms
At 86 °F approximately 1.46 k-ohms



NOTE
The next part of the DME temperature sensor testing assumes that the temperature gauge on the dash is working properly.


Connect the DME plug connector.
Start the car and run until the temperature gauge on the dash indicates approximately 80 °C (see dash temperature gauge section). If you suspect that the dash gauge is not working properly, you can check the surface temperature on the block near the DME temperature sensor.
Turn the engine OFF.
Disconnect the DME computer plug and connect ohmmeter as described in Step 3.
Turn the ignition switch back ON (Do Not Attempt to Start Car) and check dash indication is still reading 80 °C. Alternatively, check the surface temperature on the block near the DME temperature sensor. Compare resistance to the value in the table below.

DME Temperature Sensor Resistances
80 °C (176 °F) 280-360 ohms


If the resistances don't meet the tolerances listed, the DME Temperature sensor should be replaced. If the resistances indicate higher than the specs, it will cause a richer mixture. Lower resistances than the specs will result in a lean mixture.

plumbum

I was fighting the same thing all summer. Check my temp sensor and it was ok, tried adjusting the idle still didn't help when cold. I ended up buying a ICV and venturi deleat kit. Changed those and it is much better (still a bit rough but I dont need to give it gas to keep it running)

I think the Turbo S has the venturi deleat already but it may not hurt to check the vacuum lines see if one is cut or damaged causing your issues.

Good luck...I feel your pain

schip43

Carb spray everything rubber! See if the idle increases.

gtuma

Thank you all very much for your input. I checked the DME temperature sensor. It was at 3000 Ohms with the car sitting in the garage. It dropped down to about 500 Ohms after a trip around the block.

I think I'll take a look at the AFM next, I didn't know that could cause Idle problems. Thats one of the few things that was not changed when I rebuilt the engine (25k miles ago). Assuming the PO never changed it that would put it at 205k miles and 23 years..

Plumbum,
Yes the S does have the venturi system removed. I'll give my vacuum lines a once-over anyway. When you bought the Delete kit, did you remove the ICV completly?

plumbum

I replaced my ICV when I did the deleat. It is still used just less plumbing going to it

gtuma

Update

No luck testing the AFM either. I followed the process on Clarks Garage and everything checked out. On to looking for a vacuum leak.

Luis de Prat

If it idles normally once it reaches operating temperature, you have a bad ISV (leaking).

Or at least that was the case with my car.

friendlyguy64

My problem still continues. Seems like everyone has this problem. I just replaced my head, replaced all new vacuum lines, temp sensor, deleted the ICV and yet my idle still continues to rise to 1500 to 2000 rpms. Any other suggestions?

Crazy Eddie

Any update on this ??
I am getting a up and down idle ( if I don't give it gas ) it will almost stall out
The idle is going up and down
I already replaced the ISV and the TPS
After replacing the TPS, it ran for like 5 seconds like it should, and then reverted back to the up and down idle
The car was running perfectly .. left it for 3 weeks ( vacation ) came back, and started it and this issue developed.
Once warm it seems to run fine .. That would lead me to think it's the ISV but borrowed one from my friend Tom, and it still is not idling smoothly until is warm
This freakin' thing is driving me carzy and that is a very short drive !!
Regards
Ed
88 TS with SMT6 Vitesse Turbo

reno808

Crazy Eddie

AAAhhh..... Reno
Always the rectum !!
I knew I could count on you