First of thanks for the great Forum.. I have picked up a new to me 1990 with a l98.

I had a issue with it wanting to die cold from the get go and issues with the idle with a mind of its own after it was warm. Sometimes it would be 1300 and other 2200 RPM... Anyhow I did some research on here and cleaned the IAC and port, set the TPS for .54 volts, followed the procedure for putting the computer into diagnostic mode to reset the IAC valve\computer.

The idle now is perfect once I keep the RPMs up for a minute or so to warm it up some. Any ideas on what might make this thing so cold blooded?

I have a few questions.. I do notice 1 of the cooling fans turns on imediately when i start the engine... Is this normal? Typically im used to a motor warming up before it turns any fans on. If it isnt normal what would cause it to turn on imediately (im assuming one of the temp sensors)?

I do have a FSM coming but I'm wanting to deal with some of these issues now.

Thanks in advance,

Mike

 

Your symptoms sound like a problem with the CTS (Coolant Temperature Sensor). If the ECM thinks the temperature is much higher than it really is it will turn on the fans and use the "hot" fuel and spark tables. The problem could be with the CTS itself or the connector or wiring.

I suggest setting the minimum idle and checking for error codes:

http://www.batee.com/corvette/dcrg/r...scuss_sim5.htm
http://corvettephotographs.com/c4vettes/codes.htm
http://forums.corvetteforum.com/1576116036-post35.html

 

I replaced the coolant temperature sensor today and that resolved the fan issue.. I would have thought the idle problem would have been ok also but it isnt.


I did go through the procedure for setting min idle and that still didnt fix it either.. I think I may try IAC valve next as the idle hunts 50 rpm or so even when its hot.

Only code i got was a 12 which means all is ok.

 

Have you checked for a vacuum leak.. Vacuum leaks and a dirty IAC are the 2 most common problems causing hunting rpms. Also a vacuum leak will cause hunting when the engine is cold or hot...WW

 

If I ever think now that the IAC is bad I replace it. In the past many years I have cleaned them and sometimes they worked better but was only temporary. It never really held up and usually came back to bite me.

I just don’t mess around anymore with a $25-$50 item that when defective will really never check bad. If it does not fix it at least I know I have a good one in there.

Just my 2 cents and experience.

 

In regard to the fan running all the time. I just corrected that problem on my 92.

The previous owner had cut the wire from the primary relay to the ECM and directly tied the relay to ground which caused the fan to run as soon as the key was turned on. Spliced the wires back togeather and the fan runs as it is supposed to.

Do you have a set of FSM for your car? They are priceless for locating problems.

 

I'm pretty baffled as to what is going on here... I'm starting to think I have a ECM issue.

It still wont idle cold regardless,I have to keep it at 1500 or so for a minute and it will barely idle without dying until it warms up. The primary fan is now back to running all the time. The hunting of the idle seems alot better with a new IAC.

The very first time I fired it up after replacing the coolant sensor it worked like it was suposed to.

Right after replacing the IAC i could not set base idle below 900 in gear with the screw backed all the way out. I finished up the procedure and verified the TPS voltage was ok. The very next day i start it up and it wont idle now at all even when its warm. So i run through the procedure once again and I have to move the screw in about 1\2" to move the butterfly open more and can get the 650 in gear... Hmm ok,,, Verify TPS @ .54 again

Drove it today and the idle seems ok when warmed up.


I have sprayed starting fluid around the intake and around vacuum lines an also plugged whatever ones i could find.. I cant find a leak

Also my temp guage still does not go over 130 no matter what. The only time i did see it go any higher was when i ran it after I just replaced the temp sensor on the intake. I think a thermostat is in order.


A few questions...

The fitting on the passenger side valve cover for where the PVC valve would normally go is just a plastic fitting. The FSM states they use dual PCV valves but they dont say if both of them are in the valve covers...


Are the PC based OBD1 softwares capable of showing the inputs\outputs on the ECM? I think i can pin down this fan situation if i can see if something is calling for fan demand (AC pressure switch,ac request from the climate control ect ect).

Another wierd thing is the PO had it at the dealer because the AC compressor not coming on. They installed a switch that you have to turn on when you need AC. The reciept didnt state where they tapped the wiring in. I was assuming it was an issue with auto climate control unit being bad and they jumped around around it to send the signal to the ECM calling for the compressor. I pulled the climate control unit out when I was switching the radio expecting to see the wires spliced in back there but they werent. I havent had a chance to chase them down yet... This issue could be related to the fan problem even though the compressor isnt running.

 

First off the TPS on a '90 or '91 is no longer adjustable; the ECM takes care of things in that area.

Yes a OBD1 scan tool would be very useful as it will show inputs to the ECM.

Your L98 does not have MAF, it uses the MAT & MAP sensors to calculate mass air flow into the intake. If either of those sensors is faulty it will present problems, though I'm not sure if that is your problem.

I would also suggest picking up a vacuum gauge to check manifold vacuum, as it is sooo important on your engine.

 

Im really baffled with this thing!
Nice day today and decide to take a cruise.. It started and for once it idled when it is cold... But at 2000 RPM! I notice something is different this time.. The cooling fan it not running now as it has in the past.. I let the car warm up to operating temperature and jump a-b on the diagnostic plug, turn the key on and give it 30 seconds for the IAC to close, disconnect the IAC and turn off the key and remove the jumper... Fire the car back up and it wont idle so i turned the screw to open the butterfly, set the idle at 650 in gear, shut the car off and plug the iac back in. I fire the car back up and its idling fine and the fan shuts off after it cools the motor down a little.. I go for a ride and its fine.

I did notice another odd thing.. I had the temperature set at 74 on the climate control from when i drove it a few days ago in chilly weather.. It was warm today so i feel cold air coming out of the vents like the air is on.. I look at the switch that was installed to turn the compressor on and it was off. When i get home i shut off the climate control unit and the compressor shuts off followed by the fan about a minute later... Everythig is working the way it is suposed to as far as fans, ac and idle..

I'm really starting to think that I really do have a bad ECM unit as I cannot fiqure out anything else that would cause the idle to be completly different from day to day and the fan running when the engine is first started.The compressor is not on at the same time the fans so ive ruled that out. Unfortunatly the reciept from the dealer that the PO had didnt describe what was the issue that they had to install a switch or where the switch was wired in.


It seems i can get a used ECM pretty cheap without the PROM.. I take it isnt a big deal to swap the Prom out.

Any other idea's before i try another ECM?


Thanks again for all your help so far..

Mike

 

Well the final issue was i couldnt get this thing to idle under 2300 no matter what I did.. Bought a used ECM off of Ebay and now everything seems back to normal again.. Ive started it for the past 4 days and it starts right up and idles fine.. Crossing fingers.