Hi guys,

I have an 89 C4 coupe that idles a bit high at 800-1000. When going through the base idle adjustment one has to open the blades with the adjustment screw 2-3 turns to get the engine to not stall. After that the rpm will be 1400-1500 with the iac and dist cable reconnected. I don´t seem to be able to reset the iac by the five sec start. At start after adjustment the blades are about 1/8 inch open at the bottom and the ECM opens the iac as if the blades were closed. If I disconnect the iac the rpm will return to 500. Dist cable out makes no change. After I ultrasound cleaned the throttle house disassembled i set the blades to as closed it could be without sticking so that means the idle is set by the iac also meaning that the vacuum hole connected to the area under the throttle plate will be outside the throttle house whatever that might affect. At start the idle will be a little rough until temp is about 70 C. So should the blades be a little open or as closed as possible at idle? Car runs great with no codes but I can´t get anything through the ALDL cable that I wasted money on and no shop carries the old obd1 reader.

New iac, tps, O2, plugs and air filter. Vacuum 19, ign 6 deg before, smoke tested. So can the IAC be reset? Should it be done in closed lope or with engine cold? Can it be repeated?
Hope for help!
Peter

 

Hi idle could be due to air leak; check manifold vacuum at idle.

See pp. 6E-C2-20 in your FSM for IAC diagnostic procedure.

 

As I said I smoke tested the engine and no smoke came out from engine or inside the car and 19 is a good vacuum I think. I went through the section 6E3-C2-20 and Chart C-2C and no problem. What I meant with my question was that there are two way to supply air for idle. Either by open up the throttle blades a bit or by letting the ECM open the iac valve. The later means you have to do the base idle adjustment (see 6E3-C2-16) and in most cases it means you must turn the idle adjustment screw a couple of turns clockwise so that the blades are slightly open so you can maintain 425 rpms. So blades open and iac closed. Adjust TPS and reconnect IAC and DST. After restart 5 sec and wait 10 sec ECM should know better than to open IAC to much but for me idle was 1300-1500. It says drive 20 miles varied conditions and ECM should learn. Not for me. Took the TB apart and cleaned it with ultrasound for an hour. Connected the IAVC to IAC cable to see how it behaved and it closed perfectly in one second with no leak. Put it together with blades closed and that means I made the first choice in supplying air through iac for idle of 800-1000. I would do the base idle adjust if I could only get the ECM to close the iac after reconnecting it. I am a bit in the blind when I can´t get that ALDL cable to work with my laptop or find someone that has a scanner. One thing is that the car was sitting since 2015 cause it failed emission test. I got that fixed and if the car has been sitting outside on a lawn there might be ground problems or oxidized connections. During the long winter I will take it all apart and clean. Trans is leaking too. A little drip from cover. Can I take out the converter and change the seal without removing transmission?

 

I don't think you can adjust the idle like that. You say you smoked the engine so it SHOULD be good. Have you bought yourself a scanner so you can set it by IAC counts and see what the ECM is thinking before you try to outsmart the ECM and make changes? If you have, I can try walk you through a way to see if it is able to be set or not.

 

I´m not smart enough to outsmart any computer but I am looking for a way to drive my car in a relaxed way and that is not the case with 1300-1500 rpm having to break most of the time not t speak of being in neutral at red light. The OPEL shop has a tech1 but no cartridge for Chevrolet, I have tried to find a scanner for sale that has the OBD1 connector without success. As I said I can´t get the cable to work with Tunerpro 5 or WinALDL. Cleaning the TB made a great difference and lowered the RPM by 200. There is a car show taking place tonight and I will go there to enjoy and like detective try to trace anybody that might have a scanner. On this small island named Gotland there should be close to 100 corvettes and even more Chevy trucks so I am optimistic about it. I actually just have to borrow it to do one scan.

 

Or you can buy one from 1320 electronics and use your android phone to scan data.

 

The linkage gets worn and the throttle plates can stick open slightly

 

Thanks Aklim, that is what I´ve been looking for. I will order one today and maybe I will get one before next week ends.

 

Deepa, that is what I did after the cleaning. It was a big difference from when I just used carb cleaning that I sprayed on the TB. Linkage and plates were solid and the sticking was gone. I still gave the adjusting screw just a touch to clear it from the TB bottom trying to keep the vacuum hole above the top of the passenger side plate in the plenum. When I did the base idle adjustment plates had to be more open to sustain 450 rpm (according to FSM) and then the hole ended up in front of the plates. I don´t know what that hole is good for except feeding TB top and the connecting hose with vacuum.