1992 Lumina Z34 idle problem when cold

[TNT1962]

My son drives a 1992 Chevrolet Lumina Z34. When you start the car first thing in the morning it idles very high (often bouncing off of the Rev limiter). Once the car warms up it runs fine. The two codes that set on the car are for the IAC and EGR valve. I took the EGR valve off of the car, cleaned it, put it back together, and tested it. It appeared to work fine (pretty simple, 12V input and the computer supplies the ground to actaute the solenoids). The car ran the same once the valve was re-installed so it doesn't appear to be related to the idle problem.

Has anyone experienced this problem? I read the Idle Set thread (have not tried it yet), but the car only runs rough (much higher RPM than normal) when it is first started in the morning. I appreciate any help/guidance you can provide; I am afraid continued starts like this will damage the engine/transmission (he doesn't always wait until it is warmed up, so he often shifts it into gear at very high RPM).

Thanks!!

[LMP]

Yes...my son's '92 Z-34 runs high on start also..not up to the rev limiter though. but still annoying. We tried a few things last summer...to no avail. My son got the car last summer and I do not know if it was like that before but it behave this way . Since it ends up setting the idle speed to a proper level when temperature gets higher, I intend to trick the temp sensor to make the computer believe the engine is already warmer. I've done than with my Transport 3.8 to relieve excess rich mixture in cold temperatures with great success on fuel consumption. The trick is to switch a 2200 ohms resistor in parrallel with the temp sensor..which amounts to tell the computer the temp is up to 80°F. Other values can be used but the resistor must be switched out before start when it is really cold cause it might not fire up.
Here is the temp vs resistance chart:
www.avigex.ca/xport/thermistor.jpg

[maxwedge]

If the temp sensor and iac are working properly why is this procedure necessary?

[LMP]

IN the case of the Z-34, this would be for investigation purposes. In the case of the 3.8, it is a definite fix for a poorly matched temp/time/mixture curve that would otherwise require a change in the EPROM firmware.

[maxwedge]

So you are saying the temp/fuel curve mapping for 3800's is improperly programmed?

[LMP]

Yes..with nuances. I'm talking of VEry COLD temperature operation.
GM has ensured the engine starts: and it does. But they have oversimplified the code, assuming these 2 elements:
- below a given low temperature limit, give all the fuel possible, so to speak, so it fires up. OK here. THis translates in VERY rich fuel mixture..and this is not worn out components: it was like this from the first day I bought it, end of '93...and it was cold. YOu can smell the unburnt fuel odor.
-GM has then assumed that this abnormal condition would not last for long, because as soon en engine would heat up to a more "acceptable" temperature, it reverts to a more normal ratio. They do not live in Quebec.

...but when it is -25, this takes a LONG time..very LONG time...and during that time, the internals of the engine are warm enough to accept a leaner mixture, but the coolant sensor is still below the mark. THis is when I "kick in" the shunt resistor (simulates coolant temperature around 80F) and the mixture goes to normal right away. At idle, that lowers the RPM just a bit. This is why I have stated that the mixture curve should be temp +time related. So my operation of the switch, in a way, is the time factor in the equation.
For short run - low speed, low temp - city operation, the engine simply does not reach normal temps fast enough and burns enornous amounts of fuel to no avail. The shunt resistor helps lower fuel consumption a lot. In those low coolant temp conditions, the computer cannot switch to close loop operation so it relies on raw coefficient numbers for open loop management. So it never sees the unburnt fuel or inexistant excess O2 in the exhaust. I just help it a little.

THis has no effect in summer operation.
The resistor I have chosen is such that once the engine is really warmed up, the sensor resistance completely overrides the shunt resistor, the ratio being lke 1 to 10...so at that point, being in or out makes no difference and by then, anyway, it runs closed loop.

[TNT1962]

Is there a way to check the operation of the temp sensor and the IAC? We have had this vehicle since 1994 and it has not always done this. It started some time after my son started driving it to school (not exactly sure when). Would it make more sense to take it to a repair shop and have them check the compter?

[maxwedge]

Originally Posted by LMP

Yes..with nuances. I'm talking of VEry COLD temperature operation.
GM has ensured the engine starts: and it does. But they have oversimplified the code, assuming these 2 elements:
- below a given low temperature limit, give all the fuel possible, so to speak, so it fires up. OK here. THis translates in VERY rich fuel mixture..and this is not worn out components: it was like this from the first day I bought it, end of '93...and it was cold. YOu can smell the unburnt fuel odor.
-GM has then assumed that this abnormal condition would not last for long, because as soon en engine would heat up to a more "acceptable" temperature, it reverts to a more normal ratio. They do not live in Quebec.

...but when it is -25, this takes a LONG time..very LONG time...and during that time, the internals of the engine are warm enough to accept a leaner mixture, but the coolant sensor is still below the mark. THis is when I "kick in" the shunt resistor (simulates coolant temperature around 80F) and the mixture goes to normal right away. At idle, that lowers the RPM just a bit. This is why I have stated that the mixture curve should be temp +time related. So my operation of the switch, in a way, is the time factor in the equation.
For short run - low speed, low temp - city operation, the engine simply does not reach normal temps fast enough and burns enornous amounts of fuel to no avail. The shunt resistor helps lower fuel consumption a lot. In those low coolant temp conditions, the computer cannot switch to close loop operation so it relies on raw coefficient numbers for open loop management. So it never sees the unburnt fuel or inexistant excess O2 in the exhaust. I just help it a little.

THis has no effect in summer operation.
The resistor I have chosen is such that once the engine is really warmed up, the sensor resistance completely overrides the shunt resistor, the ratio being lke 1 to 10...so at that point, being in or out makes no difference and by then, anyway, it runs closed loop.

I have to tell you my 98 3800 exibits no such symptoms at -25, but this is an obd11 car with preheated sensors. I applaud you for your fine tuning this cold fuel map issue, but it does not really apply to the OP's issue in my opinion.

[airboatphil]

Have you cleaned the throttle body lately ?
Phillip

[1996LTOwner]

Try pulling the EGR off and plug the ports. See if this changes anything. I believe all three ports for the EGR are suppose to be closed at idle. I'd also check the IAC solenoid adjustment. If it's the same as the '91, I believe the over all extension should be about 1-1/8 inch. But do not turn the key with the IAC out and connected, it will throw the adjustment off.
Good luck.

[guess919]

I have a 93 Z34 with the exact same problem. mine was the intake manifold gasket and heater nipple dealer replaced it fix problem good luck.

[richtazz]

that engine is known for an intake vacuum leak when cold that will go away once the engine warms up. The IAC is responsible for adjusting the amount of air allowed into the engine at cold start, part and closed throttle, so if it's out of whack, it would cause idle issues. Normally it causes a surging or high idle in the 2,000-2,500 range, not redline. I think your issue may be a combination of both the IAC and intake vacuum leak.