C4ProjectCar

Thanks, I'll take it off and clean it today.

Now that I understand how the IAC works, I realize that my IAC must be working. When I go through the procedure to set the idle (jump ALDL A and B, wait 30 seconds, disconnect IAC plug, turn off ignition, remove connection between ALDL pins A and B, disconnect EST timing connector, start engine, turn screw on throttle body to adjust base idle, turn off engine, reconnect IAC and EST) the idle drops to the base idle like it's supposed to.
Nonetheless, I still have an idle that's too high. The base idle wasn't exactly 450 RPM, but I got it as close as I could. Once the idle got below about 500 the engine wouldn't stay running without me using the gas. After I finished the procedure, the idle was back where I started at 1000 RPM.

cudamax

If u got the butterfly air gap cleaned and the IAC is moving freely and the pintle is seating. Start looking at the coolant temp sensor and coolant temp sensor harness pigtail and vacuum leaks and the motor just running like crap

Cliff Harris

I always try to explain WHY you should check something so people understand what they are looking at and what they should see.

In cadamax's advice, the reason is that if there is a problem with the CTS (Coolant Temperature Sensor) and the ECM thinks the coolant is cold it will advance the idle speed and richen the mixture.

If the connection to the CTS is open the ECM will think the coolant temperature is -40°. This could be caused by a bad connection or a broken wire. The CTS itself could also be bad. You can measure the CTS resistance and check it against a resistance/temperature chart to see if it is reading a reasonable temperature. This chart is found in several places in the FSM:

cudamax

Found the answer to ur question for the exact procedure on checking resistance on the IAC coils that pull on the small worm gear
http://shbox.com/1/iac2.jpg

cudamax

Check out all of These
http://forums.corvetteforum.com/tags/iac.html

cudamax

Here is a post that ended up fixing a high Idle with a TPS Adjustment
http://forums.corvetteforum.com/c4-t...ange-help.html

C4ProjectCar

Thanks, I appreciate explanation in conjunction with advice.
I think you told me about measuring CTS resistance and gave me the chart a while back, but I forgot to get around to it. I plan to check it tonight.

C4ProjectCar

Thanks Cuda, I'll check those out after I measure the CTS resistance.

C4ProjectCar

Okay, I was having trouble with the CTS so I moved on to the IAC valve. I believe the CTS resistance was 2600 ohms - about what it should be, 2900 ohms - but due to difficulties getting the test leads on the sensor pins and problems with the digital multimeter I'm not sure. I might end up removing the CTS so I can be sure I'm getting a correct reading.
But anyways, on to the IAC valve. Resistance across pins A and B was about 54 ohms and it was the same for pins C and D. Resistance was infinite between all the other possible pin combinations. Long story short, everything was as it should be. So that means my IAC is not in fact the problem?

cudamax

No, it is and at the same time, may not. Be the problem

cudamax

The IAC is what is raising your Idle speed at a time that it Shouldn't.

C4ProjectCar

I'm afraid I don't entirely understand your somewhat cryptic answer.

It is controlled by the ECM, so the problem could be in the ECM thinking the idle needed to be higher, right?

cudamax

If it isn't physically binding, { The IAC } and has smooth motion in and out. Then something, somewhere is calling/commanding it to Step and open to allow more air to pass for the higher Idle or your throttle linkage/butterfly is not returning fully to its rest stop position. Have you grabbed the linkage lever physically to see if you can force a lower Idle speed? Did you stab your CTS wires past the harness connector about 6 inches or more to test for readings at that point to see if the CTS harness connector is good? The sensor it self may be good and check out but the wires may be damaged and not sending the proper readings back. Are you positive you cleaned out all carbon buildup on the throttle body so u can see light past it because it needs to have a real small about .002 to .003 space all around the butterfly to allow a small amount of air to pass around it for the right idle speed and if its not getting that the IAC will give it and be too much causing a higher idle speed than needed. Also sometimes people screw with the TPS Sensor and that can move the throttle causing a high Idle. The ECM could be calling it also when its not supposed to. Is it getting/going into closed loop and maintaining 14.3 A/F? I suspect though, because of ur older problem with real low oil pressure and the dash oil light coming that someone might have got in there to force a higher than normal Idle speed so the light wouldn't come on.

cudamax

Bingo

C4ProjectCar

Thanks for the comprehensive answer.

The throttle body is closed all the way snug up against the set screw, which is set at a base idle of 450 RPM.
I have not pierced the insulation on the CTS yet, but that sounds like a good idea. I'll do that next.
I cleaned out the throttle body pretty thoroughly. I didn't soak it (a trip to the local GM dealership is pretty far for a single item, and other than what you suggested I wasn't sure what to use) but I was very thorough with spray TB cleaner and a toothbrush.
I believe the TPS is self-adjusting on 1990 onward, right?
I'm not sure if it's going into closed loop or what the A/F ratio is. You have to have a scanner to find those out, right?
That's what I was thinking too. The cap was off the TB set screw so I know the mechanic messed with it.

C4ProjectCar

Okay, I pierced the insulation of the wires on the CTS about a foot and a half from the connector and measured the resistance. I got 1540 ohms, which corresponds to a little over 100 F; it was about 75 in the garage at the time. However, I did measure a value that was correct (2600 ohms) when I just measured the sensor. I could see why a break in the wire would cause a greater resistance, but why would the resistance be lower? I don't think there could be a short in the wires since they are individually insulated. Even if there was, I would think that would have a significantly lower resistance than 1540 ohms.

EDIT: I tested for a short by unplugging the plug and testing resistance between the two wires. I got infinite resistance, so there's no short.

cudamax

O.K. REAL Quick. I'm a single dad and its homework time and cooking a late Dinner. Huge pork chops and Alfredo mosta.
The throttle body doesn't have to always be removed and soaked. You can have someone hold the throttle floored and use spray and a toothbrush to clean off right where the butterfly rests at an Idle. Thats where a carbon ledge builds and must be removed so when its closed a small amount of air passes.

cudamax

Not sure on ur 1990 TPS but if the TPS is slotted that could/may be ur problem

cudamax

If the coolant is reading and sending its readings to the ECM then when it warms it will get into closed loop and if its not the ECM will think its cold and kick the Idle speed up. Alway when checking any sensor you have to check the sensor itself then make sure by its pigtail its sending those readings back to the ECM and then u have to make sure by the cause that its responding.
So u check sensor
then check sensor pigtail harness a little down line

C4ProjectCar

Mmm, that sounds good. Now I'm hungry Chef, mechanic... you do it all!
I ended up taking it off just for the heck of it. It didn't really have a noticeable "ledge", just a sooty looking area.

I'm not sure what the difference is between slotted or unslotted, but I'll figure it out. To Google I go!
EDIT: I'm thinking you mean slotted as in the mounting holes are slotted? Mine are not; it is not adjustable.

Ahh, I see.
Is there an easy way to see if it's in open or closed loop?
I'm checking the sensor itself again with the help of my dad to get a more certain reading. My suspicion is that coolant that dripped is shorting out the plug a tiny bit, causing resistance to drop. We'll see.

EDIT: Got 2340 ohms, and since the garage was about 85 F this time that's about right. I'll dry off the plug, plug it back in, and verify that the reading is correct.