CFeria

Recently bought 1991 C4. Looks great, runs great, BUT... the Idle is off! I have looked for vac leak and found nothing, I have done the propane and carb cleaner spray and got nuthin. Have done the idle reset several times, but still nothing. Have replaced IAC, but nothing!

about the car, its a 1991 base C4 with vortex ram air, hypertech stage 2 chip, 160 or 180 thermostat, and high flow cat. I was told all this prior to purchasing the vette. I didn't buy because of this, but because it was in the best shape of all the others I looked at. I was wondering if the hypertech chip would raise the idle?

Also, My Vette has the electronic package where it allows me to see the rpm on the temp gauge, I take what ever 2 digit number shows up and divide by 25 to get actual RPM. So I checked that against the tach and noticed the factory tach is off by about 300 rpm. OK, I thought cool but my Idle is still off! On temp gauge in car I get 26 when at idle so 26/25 is roughly 1000 rpm, that's actual. Tach shows between 1200 and 1300.

When I have tried to do the idle reset, by unplugging the Iac then backing the idle set screw out a bit the motor stalls when I back the Idle screw out like 1/2 turn. What this tells me is that I am choking the engine, the throttle blade is not allowing any/enough air in, so this is another reason I think there is not vacuum leaks?

Other than this problem the Vette runs great. But it bothers me that I cant get this car running correctly! only has 98k miles! Trans rebuilt in 2009 and Vette was painted in 2010. Previous owner was planning to restore the Vette, but instead decided to get married! (bad choice I think!) So he had to sel the Vette. Anyways.... can some one please help?

Also TB has been cleaned thoroughly, IAC replaced, O2 replaced, TPS set to 0.54. Would the fuel filter have anything to do with this? I don't think so, but thought I would ask?

THANK YOU!!!

Cliff Harris

What do you consider "off"? How do you know?

The ECM controls the idle, so if it's off then that is the way it's programmed.

I suggest a minimum idle adjustment (it sounds like you didn't do it correctly):

http://forums.corvetteforum.com/1570563689-post1.html

eatmydst1234

Look in the stickies and do the complete min idle reset. You shouldn't have to mess with the screw on the throttle body, but since you have you will also need to get that back correct.

When you are doing the minimum idle reset inspect the wiring for the IAC. I was chasing a high idle on my car for a long time and it turned out one of the wires broke loose from the terminal in the plug, had to replace the pigtail for the iac.

65Z01

Define "idle is off"...??
Did you use the full procedure to set base idle?
-jumper ALDL pins A&B
-turn ign to On; wait 30sec
-unplug IAC cable
-remove ALDL jumper & start the engine
-back off the idle screw to base idle of 450rpm
-set TPS to 0.54 Vdc
-turn engine Off and plug in IAC cable

Ozzstar

That is not a full proof way to test for a vacuum leak. Have you performed a 'smoke' test?

65Z01

Possible even a vac gauge might be useful to see if manifold vac is ok.

xrav22

It is supposed to die when you back out the idle screw, You need to stick a screwdriver in between the screw and get it started then use the screwdriver to back it down to 450 rpms, then adjust the screw until it touches the throttle. It will barely run at 450 at this point then continue with turn off, plug in iac, ect. Good luck
It should eventually idle at 600rpm.

jv9999

Mine idles at 450 fine. Check for air leaks past the throttle plate. Make sure the throttle body bolts are tight. Check vacuum at idle should be 20" or so. Also check timing with advance disconnected (6 deg). If none of these pan out, you need a scan tool that can read the data steam in real time to see why the ECM is making the idle decisions that it is.

CFeria

'JV9999' thanks! Can you or someone recommend a scan tool? I currently have a actron 9175 but its only for obd II, if possible would like something cheap since I prob will not keep the vette much longer, hope to buy a newer 2000+ model in possible 1 yr?
Just annoys me I can not fix this idle issue!

CFeria

Thing is Idle in always around 1100 when warm, it doesn't stumble or search always goes from 1300 (cold) to 1100 when warm. This is always consistent. Idle set screw makes no difference. Only work done to vette by previous owner was hypertech stage 2 chip, vortex ram air, and hiflo cat. There is no way the hypertech would raise the idle?? Stupid question, and I know its not difficult but how would I remove hypertech so I can run motor with out see what difference it makes. What is an easy way to open up the ECM and remove the hypertech piggyback chip? I would imagine battery must be disconnected, but do I need to completely remove the ecm from the vehicle or can chip be removed by just opening the ecm cover?

ANTI VENOM

Unbolt the ecm from the plastic holder and flip it over. there is a small cover on the back that needs removed. Then you will see the memcal. If hypertech used an adapter and your memcal, then you can just unplug it and remove the adapter and chip. Reinstall your memcal.

eatmydst1234

When the car is idling high, what happens when you turn the AC on? Does the idle drop or does it stay the same?

CFeria

If I unbolt ecm from holder do I need to unplug the ecm? Also I need to disconnect - bat cable right??


turning ac on makes no difference or it will drop as soon as ac is turned on but then goes back to normal idle or normal for this car which is 1100.

C4vettrn

If you don't step back and reset your idle adj. per the manual you will be chasing your tail. Once this is done correctly then check your TPS voltage. Doest it have an adjustable TPS on it?

ANTI VENOM

I didn't need to unplug mine in my 92. I didn't even disconnect the battery and I did this at LEAST 50 times while tuning over the course of a year. I'm sure there are many on here that don't disconnect their battery but it is probably the safest way. Your call.