I've got a 1991 6-speed that I've been working on for a while. It's hot where I live and I want to use the A/C, but the car doesn't like it. Everything is fine when I turn the A/C on, the ECM adjusts the idle and it blows cold. But if I'm driving and I push in the clutch, the car will stall when the RPM's drop to idle speed. Also, when I start the engine hot, it'll jump up to 2k rpm or so and fall back down, and will occasionally fall to 400RPM or so and almost stall before it comes back up to idle at about 700.

Some other things that might be related:
The engine has a hard time starting when it's cold, and doesn't rev easy until it warms up. If I try to drive while it's cold, it bogs down bad whenever I let the clutch out no matter how slowly I let it out or what RPM the engine is at. When I start it, it doesn't crank for a super long time but it starts real rough and the RPMs sort of climb up is the best way I can describe it. The idle will fluctuate around 1k rpm for a few minutes, going up and down, before eventually settling to about 700RPM.
The IAC, TPS, CTSensor + CTSender, ECM, fuel pump, FPR, spark plugs, distributor, and all upper intake gaskets and seals are brand new. Fuel injectors are re manufactured Bosch III's. I've replaced all these solving other issues, none were replaced to solve this particular problem but they are new regardless.

Also, the car used to have a bad vacuum leak that caused it to run very badly when it was hot and stall. However, it ran perfect when it was cold. After fixing that issue, now it doesn't want to run cold but works great hot.

Any ideas are appreciated

 

Google minimum idle adjustment for Corvette C4. do that and report back.

 

I did the minimum idle adjustment. The guide I used said to adjust the set screw with the IAC unplugged until the idle is 450 RPM. I did this, but the engine won't stay running that low. The lowest I could get the idle down was 600 before it stalled. Even after plugging the IAC back in I just couldn't keep the engine running at that speed without the gas pedal.

 

can u post link to the guide u followed

 

https://www.digitalcorvettes.com/thr...your-c4.58873/

 

ok, gm updated the guide for the 85 (that guide was weitten for the 85) to be 550 minimum idle.

So try for 550.

do not have ac on when doing the procedure.

try that and report back

 

if you cannot gain control of idle, read my situation and the whole thread:


https://www.corvetteforum.com/forums...post1603469586

 

The car will idle at 550, but it doesn't like it and I have to give it some throttle on startup and sort of ease it down to 550 or it will stall. Turning on the AC (After I finished the adjustment) stalled the engine.

I read your IAC thread. I have this https://www.oreillyauto.com/detail/c...+control&pos=0, I'm not exactly sure what part number it is or if it has the large or small pintle.

I would like to mention again that the car handles AC request flawlessly and doesn't change idle at all when I turn the AC on, but stalls when I push in the clutch while the AC is on. I did have the idle set a little higher though, around 700-800 RPM. Do you think a wrong IAC might cause the car to not want to idle any lower?

I have to go for a while, I'll return to this thread later tonight

 

so one thing about the c4’s is that “scanning” while running is pretty invaluable.

The way i do it is using a laptop, a program called tunerpro rt, and a cable from aldlcable.com

once you learn how to set it up it is quite easy and you can see what is going on.

To me, it seems like your car isnt able to get control of its air (it does the air part by commanding the iac to move, like a fine tuning of the air allowed, and ofcourse whatever you have for the minimum allowed air through the tb which is what you are adjusting during the min idle adjustment. and the fuel part, it does by adjusting the length of time an injector is allowed to spray fuel.

it uses the o2 sensor to gauge whether or not to add or subtract fuel and to move the iac around.

So when u r scanning, u can see the “position” of the iac. You could see for example, if the computer is commanding the iac all the way closed, and yet you still have a high idle. That air is coming from somewhere, ie, a vac leak.

So scanning the car while running allows a person to pinpoint issues. There is no reason to replace a cornucopia of sensors if you can see them via the scanner working properly. It allows a more quick and accurate diagnosis.

 

I have a scanner and tuner pro. I haven’t used it for this particular problem but I will when I get the chance. I didn’t replace all the sensors for this particular issue, I had the engine apart and most of the plastic connectors were broken so I decided to just replace them as I put it back together.
Thanks for the help, I’ll update as soon as I can get my scanner on and check it out

 

Great, let us know what the IAC counts are when it is dying or struggling to idle.

 

Random thoughts...550 rpm sounds a little low. Took a gander in TPRT 91AXCN 6 speed bin and the target rpm at 92*C is 700 rpm. No added rpm for a/c on. Make sure the minimum idle adjustment is done on a fully warmed engine. Agreed to check IAC but I would check IAC counts when it's idling at 550rpm. Just can't imagine that the IAC is pegged all the way that it can't hit target rpms unless the passages are completely clogged.

 

Sorry for the wait, it's been difficult to find time. I collected as much information as I can
Interestingly it was difficult to get it to stall. The car idles much lower and wants to stall when the engine is about halfway to operating temperature, but seems to do much better at operating temperature. Unfortunately I've been driving it all day and by the time it cools down enough to test it it'll be way too late. I can test that later but I'll give you what I have now.
Firstly: target RPM is 700, with and without A/C, and the car does a good job keeping it between 700-750 according to TunerPro.

On startup the IAC was 56, then it increased to 65 and went back down to 59. When the idle settled at 700 the IAC was 40. The IAC slowly dropped as the engine ran, the lowest I got was 22. After giving the car some throttle it would usually decrease the IAC when it returned to idle. Turning the A/C on put the IAC at 64, and the car did not stall.
I DID get the car to stall if I turned on the A/C blower before the A/C. Doing that, the IAC was at 26 with the blower on, and turning on the A/C it jumped up but not quick enough and the car stalled.
I need to get more testing done when the car is having a more erratic idle, as it was holding well when I tested it just now. Like I said it seems to want to stall when the engine is about halfway to OT. Since my gauge on the 91 only has a high and a low temp, I can only guess what that is. 140-150 would be my guess. My car stays at about 200 at OT (according to TunerPro)

Other information: The MAP voltage was 1.45 and the temperature was 203 degrees.

One more thing that seems significant: The O2 sensor status said "not ready" and TunerPro indicated the car was in open loop, despite being 200 degrees. There is no check engine light, however it does come on occasionally when I have the car at low throttle for a long time, and reads a code 13. It also does this while climbing a hill close to my house. It's a long and steep hill and the only way to make it up without the light coming on is to accelerate and coast and repeat that going up. The light being on doesn't change how the car drives.

The O2 reading held steady at 450.84 until the car settled at idle, then fluctuated back and forth between that and 455.26. O2 status remained at not ready and still no check engine light. It had a persistent check engine light when I bought it for a code 13, so I replaced the O2 sensor and the light went away except for the circumstances I explained above. Also TunerPro indicates that the O2 is sending a lean status to the ECM.

 

Your O2s shouldn't flatline like that. Perhaps that code 13 is still a persistent issue. When in closed loop, it should look more like the attached pic...

 

Yeah I think the O2 is an issue. The problem is, the O2 is brand new and so is the ECM. The ECM is getting a value in it's expected range, and it's moving enough to prevent a code 13 under most circumstances. It almost seems like it was set up to keep it in open loop without setting a code. I would imagine that if the circuit was shorted somewhere it would be giving the ECM a value out of it's expected range. I'm not really sure how to fix this.

 

Your '91 if I'm not mistaken is essentially the same as a '90 well at least L98 and 1227727 ECM. Do you have a factory service manual and have you tried following the logic tree for a code 13? At this point with everything Never Ever Worked, the only thing left is to check the wiring in that circuit. Are you able to post a datalog and the adx that you're using to acquire the data?

 

Yes it has the 1227727 ECM, and yes I have an FSM and have followed the tree. I replaced the sensor (twice because I originally suspected a defective replacement), and the ECM is also new. The ECM is getting values in its expected range which makes me think there's some sort of emulation going on or something? But I'm not sure because it says the O2 is never ready.

Another thing I should've mentioned sooner but I forgot: I don't have any cats. Could this cause this problem? I don't have a heated O2 either.

I can post a datalog soon but not at this moment. I only have a desktop and will need to borrow a laptop to run the datalog.
I do know that my ADX is called 8D_90-91_Corvette.adx
It's on the TunerPro bin definitions download page, in the second box from the top under General Motors.
https://www.tunerpro.net/downloadBinDefs.htm#GM

 

Do you have long tube headers?

Have you checked your base timing with the esc wire disconnected?

Have you taken the throttle body off and cleaned it?

Have you checked out the TPS readings? What brand did you replace it with? Anything but ACDelco in my car acts up and changed throttle response.

IAC replacement - there are two different pintle designs. Does the new one match the old one?

Just throwing ideas out there.

turning the blower motor on before the AC shouldn’t make a difference.

as far as ac pressures and condition of the system. AC adds a large load to the motor. If it’s overcharged or the compressor / clutch is failing, it could also cause an issue.

I still think getting your car into closed loop is the first step.

 

no I have the stock manifolds. I’ve got a corsa system on it and I cut out the cats because they got clogged up real bad.

My base timing is around 4 degrees before.

Yes I’ve cleaned the TB. The TPS readings are good, .54 when closed and the brand is AC Delco. Unfortunately I didn’t keep the old IAC so I can’t really confirm that.

I don’t know about the condition of the compressor, but it seems ok to me, I can turn the compressor pulley by hand pretty easily, and the clutch works exactly like it should.
The A/C does blow very cold and I’m not getting any unusual noises from the compressor or pulley.

I’ll try to run the A/C again when I get back in the car and I’ll update how that goes. I haven’t actually driven with it on since I started this thread and I’ll let y’all know what happens.

 

Spec timing is 6 degrees before TDC with ESC wire unhooked. Some go up to 8 degrees. I’d bump that up to 6 and see if things improve.

still need to get it in closed loop though.