Hi,

I have a small problem with my idle. When I start my Corvette, the idle is OK, running at 7 RPM and very well but after 2 minutes (2:06:00) the idle starts fluctuating; it speeds up and slows down repeatedly. I can’t identify the cause? So far, I suspect a bad sensor or a bad signal. I know it has something to do with the mixture of air and fuel. For this reason, I eliminated a vacuum leak as a possible cause. When I drive, the engine runs well, the RPM is regular and works great. The problem is only at idle. What else can I check? Which sensor could affect the ratio of the mixture (air and fuel)? I have 2 theories, first it has something to do with the cold start. Second it could be related to the intake tempera sensor in the plenum. I suspect the issue is related with temperature because 2 minutes after the engine is started, it starts running badly. If I wait a few minutes and I restart the engine, the idle will start fluctuating immediately. If I wait a few hours to restart the engine, then the idle is ok for 2 minutes and the issue reappears

 

Please start with the year so we don't have to guess what engine you have miles is also a big help. A hunting idle is most times a bad or dirty IAC valve

 

What is an IAC valve and where is it located?? I still have the same problem..Nerve wreaking and I'm getting Pi##ed off...I'm getting rid of this piece of craps and going back to a C3..way to much crap in this motor

 

Yes Floridamale,

It a 1986 C4, with 5.7 L98, 240 000 KM or around 120 000 milles

Thanks , I appreciate.

For the IAC valve, how I can check it, if it bad or dirty ?

Marc

 

A bad CTS (Coolant Temperature Sensor) will cause this problem. It's located on the front of the intake manifold under the throttle body. A bad connection or broken wire could also be the cause of this.

If the ECM thinks the engine is cold it will richen the mixture to compensate. When the ECM goes into closed loop mode the O2 sensor will show rich and the ECM will increase the air with the IAC (Intake Air Control) to compensate. Then the idle goes up and the ECM will reduce the IAC to fix that... ...and it just goes 'round and 'round.

A vacuum leak will also cause a wandering idle. There are lots of hidden places you can get a vacuum leak, so the classic "spray test" could pinpoint the location. Spray something flammable around the throttle body and plenum. Flammable means something like carburetor cleaner. Many people use an unlit propane torch.

The cold start injector is controlled by the cold start thermo switch (also located on the front of the intake manifold under the throttle body -- it has a connector that looks like a fuel injector connector). It only works while cranking (it gets power from the starter solenoid). It has a thermal element inside that times out after 8 seconds. It will not work if the coolant temperature is over 95°F.

The MAT (Manifold Air Temperature -- sometimes called an IAT for Intake Air Temperature) sensor is used to enable EGR. It has no other function.

See above about closed loop mode. The ECM goes into closed loop mode under several conditions:

Coolant temperature > 40.7°C = 104.4°F
Engine run time > 300 seconds (cold) if coolant temperature < 58.5°F
Engine run time > 206 seconds (warm) if coolant temperature > 58.5°F and < 158.5°F
Engine run time > 50 seconds (hot) if coolant temperature > 158.5°F
Oxygen sensor reading between 0.195 and 0.686 volts for > 10 seconds

It's the O2 sensor that is the last to meet these conditions. It must get hot to work. That is why it's located as close as possible to the exhaust manifold. In some cases (long tube headers, for example) the ECM will never go into closed loop mode because the O2 sensor never gets hot enough to start working.

For the IAC valve, you need to remove the throttle body to get access to it. Then unscrew it. In many cases the passages have a lot of black stuff in them. When you go to put the IAC back into the throttle body, push or "screw" the pintle into the IAC first. If you don't do that it's possible to damage the IAC (been there, done that). It is NOT necessary to measure the extension of the pintle before installing the IAC, as many people recommend. The IAC is calibrated by the ECM every time the ignition is turned on, so there is no "tuning" needed.

 

Hi Cliff,

Thanks for all your answers.

For the CTS, I previously changed it but saw no improvement. It’s not the sensor but could be related to the connection or the ground. How would I test a connection? How can I validate that the sensor cables actually reach the EMC.

How can I tell if it’s a vacuum leak? The reason I discarded this cause was because when I drive the car, the RPM is stable. There is no up and down. I had a vacuum leak in the past with constantly unstable RPM or idle or on the highway. Currently, the problem is only when the engine is running idle.

I also changed the O2 sensor with no improvement. This seems to suggest that I need to check the cables and ground but I’m not sure how to proceed.

I didn’t understand your suggestion concerning the IAC. I previously removed the IAC but didn’t need to remove the Throttle body. I simply removed it with a wrench and nut. I cleaned the tip and measured it to be 1 1/4. How do I know if it is broken or if it’s sending wrong information to the EMC?

What I understand is when the engine runs in open loop mode, everything is OK. The issues arise when it switches to a closed loop and the EMC receives incorrect information.

The only thing I failed to mention before is that I have a code 36. I do not know if there is a correlation with the fuel air mixture at idle.

 

Hi,
I found a leak and now the engine runs better. There’s still some up and down idle, but now stable idle stretches are longer. I probably have to reset or adjust my minimum idle. The idle runs around 5 and 6. It’s a little bit low and sometime goes up to 7 for short periods. I suspect it’s due to a very small vacuum leak.

 

so what was it ? a vacuum leak?

 

Yes, it was a vacuum leak!!! I never see this leak, it was the line between plenum and HVAC, the line touch the manifold and make a hole in the line

 

The IAC is a motor that is controlled by the ECM. The motor turns and the pintle moves in and out. It adjusts a "controlled air leak" around the throttle plates.

When the ECM is in open loop mode it uses various sensors to determine what is happening with the engine, mostly TPS, CTS and MAF. In closed loop mode the O2 sensor is used to determine what the AFR (Air/Fuel Ratio) is. The ECM adjusts the fuel injector pulse width to maintain the correct AFR. It also uses the IAC to control the idle speed.

Error code 36 is a MAF burnoff fault. When the ignition is turned off the ECM turns on the MAF burnoff relay for several seconds. That causes the sensor wire in the MAF to get very hot (you can see it glow red if you look into the MAF while this is happening). The ECM monitors the output voltage of the MAF while burnoff is occurring to make sure it is really happening. You could have a bad MAF burnoff relay (very common). There are two relays mounted on the firewall behind the battery. One is the MAF power relay and the other is the MAF burnoff relay. They are relatively cheap so it's easiest to just replace both of them. On the '86 cars they are NOT the same so make sure you get the correct relays and plug them into the correct connectors. The connectors are the same so it's easy to get them wrong. If you do that the MAF burnoff will not work (that could be your problem -- or wrong burnoff relay). Here are some part numbers ('86 ONLY):

10094701 ... AC Delco 212-305 ... MAF burnoff relay
10067925 ... AC Delco 212-300 ... MAF power relay

In '87 and later years the two relays have the same part number.