Hello! I have a problem with my C4 (1991 convertible l98 manual). The idle speed of the car was always around 1000/1100 rpm, I thought the Idle Valve Control was malfunctioning, so I replaced it. I followed the procedure indicated by the official manual to reset correct idle speed (slightly push the throttle, turn on the engine, let it run for 5 seconds. Turn it off for 10 seconds, and then check idle speed), but idle speed lowered only around 900 rpm. Does someone know if there is nother procedure to resed idle speed? Many thanks!

Ps. I guess that the idle speed of the engine should be around 600 rpm, is this right?

 

You need to set the minimum idle. The idea is to temporarily fully extend the IAC to the closed position and take it out of the loop while setting the blades with the adjustment screw. Going on memory:

1. Jump A and B on the diagnostic terminal.
2. Ignition on
3. Unplug IAC
4. Start car
5. Adjust screw until idle is near 600ish. Different specs for auto vs manual but 600 is close for both. May need to remove plug to access screw.
6. Shut down.
7. Connect IAC
8. Adjust TPS

This should be completed with the engine at operating temp. Others chime in if I missed something. Edit, maybe it should be 450 rpm? Can’t remember. It’s been a while.

 

here is a good write up on the subject.

http://zr1.net/forum/showthread.php?...hlight=minimum

 

Thank you for the answers and for the link. The link given by Sleazy Rider is for ZR1s..is the procedure the same? I was also wondering: both the first answer and in the link is mentioned a screw to adjust the idle. Where can I find that screw? Because the screw on the throttle body is fixed there...thank you again for your help!

 

 

the procedure should be the same for both the lt1 and the zr-1, just the locations are different. the zr-1 has the iac on the side of the t-body while the lt1 has the iac on the bottom of the t-body. both iac's look to be the same. the lt1 throttle stop has a t-15 torx screw on the back side that will most likely be adjusted with the t-body off the plunum.

 

If you don't know what the idle speed is, ask. Don't assume. Get yourself a scanner and see what the idle speed should be based on what the ECM wants.

The official way to do it is wonderful IF you want to make assumptions. I would, and do, forget that entire thing and check things before throwing parts at it. Are you sure that the timing is right? Make sure the harmonic balancer is not moving on you. After all these years, IF it is original, it might. If it does, your timing will probably be wrong. If that is wrong, setting the timing is difficult. TEST for good balancer. If the balancer is right and the timing is at 6 degrees, check the fuel pressure. Does it spike up to the right numbers? Does it hold pressure after shut off? If so, time to move on to other things. Plugs and wires, cap and rotor. If those are good get yourself a scanner. You will need it sooner or later. Not a code reader, a scanner where you can see live data.

With a scanner, you can see what the ECM wants the idle to be and not what we say. Drive it for a while and let it get to closed loop and operating temperature. Check what the ECM wants at this point and see what the ECM says the rpm is. Your tach could be inaccurate and IIRC, they often are. Check what the IAC counts are. IF they are higher than 30, the ECM is asking for more air. Turn the adjustment screw in to open the blades. Rev and check again after it settles down. If it is lower, turn it out to close the blades. Now if it is 0 and you can't do anything about it, you have an air leak. Once that is set to 30 counts, give or take a few, you are done. The TPS cannot be set.

 

Check for vac leaks first. By far the most common issue and causes high idle

 

Thank you for your answers and suggestions. The problem is that we don't have the official corvette assistance here in Italy and it is quite hard to find someone who has worked on this kind of cars and that actually knows where to put his hands. There is just one big company specialised in american cars, but it is located in another region and with covid restrictions is not possible to get there at the moment. I'll keep you posted on this issue. Please continue to post your advices and suggestions on this matter, so that when I'll be finally able to get the car to this company I will have clearer ideas!

 

What makes you think your idle speed is high? Are you using a scan tool to determine true idle speed? If you are using the tach in the instrument cluster they are frequently way off, usually reading high. Mine was off about 400 rpm. As I remember it a little resistor on the tach printed circuit board gets wacky and causes an erroneous reading on the tach.

 

Buy one of THESE gizmos and see if the rpm is high or not. The tach is only so you can guess at what it might be. I have checked it with a timing light and a scanner and it was off till I had it recalibrated and it is now right. That said, if I ever want to see what the RPM is, I still prefer to use a scanner. I have one that sits in the storage box.

 

Hi, thanks for your reply. The point here is that we are not talking about few rpm more or less: the idle is distinctively high, around 1000 rpm. (Now just a little better, after replacing the Idle Control Valvew). I'll think about buying the scanner suggested by Aklim (thanks for this). It might indeed be useful to have one, especially considerning the fact that we don't really have an official Corvette service.