DanielRicany

Hey guys, I have a 1985 Corvette and I am tracing an idle surge, looked in a lot of places for vacuum leaks, haven't found any, checked all sensors and everything. Because of the combination of problems I've been having, I am thinking that it might be a vacuum leak.

Can someone or a few people give me their IAC counts at idle and the MAF reading and specify the idle RPM so I can compare? This must be a car with a smooth running engine with no problems.

Thank you very much, I want to use this information and compare to see if my IAC counts were lower than what should be, it would indicate a vacuum leak. Thanks!

Cliff Harris

IAC counts depend on the physical setup of the throttle body, so they wouldn't mean anything for your specific engine. The O2 sensor also gets involved. The ECM uses the IAC to dial in the RPM that it wants.

BLMs are a much better indication of what the ECM is seeing. The ECM uses them as the basis for calculating the injector pulse width. Cell 0 is the idle cell, but it can also slip into the neighboring cells depending on conditions, especially if the engine is cold.

c4cruiser

My '87 also has a idle surge problem with as much as a 200-300 RPM swing when cold. This occurs until coolant temp reaches about 120-130 degrees. After that, idle is fairly constant to within <50 RPM.

No vacuum leaks, EGR is connected, new O2 sensor (it currently located at the header reducer, but had been right under the exhaust manifold and had the surge issue then).

IAC is new along with a 52mm TB, TPS voltage is right at .54.

Cliff Harris

99.99% of the time an idle surge is a vacuum leak. The extra air causes the idle speed to go up and the ECM adds fuel to keep the AFR constant. Then the ECM sees that the idle speed is too high so it backs off the IAC. When the idle gets too low, the ECM adds more air and it goes back into this vicious cycle, up and down.

Vacuum leaks can be very subtle and not from places you would think. I had one where the PCV valve vacuum source attaches to the intake manifold. The hose got loose when the engine got hot and made the rubber more flexible. I found that one because the leak made a noise. The EGR valve can cause a vacuum leak if it doesn't close completely (usually due to carbon buildup in the EGR passages). Sometimes the brake booster diaphragm splits and leaks. The canister purge solenoid defaults to OPEN, and will leak vacuum if the ECM can't switch it to closed. it's only supposed to go open when the RPMs are higher and the engine can take the extra air.

MikeT 90 C4sixer

When engine is cold, you are in open loop, so it might not be able to control idle very well. i would guess at 120-130 deg you hit closed loop engine control, and the ECM can hold the idle more tight, by adjusting PW.
Just saying.......you may need to tune open loop idle. (Since you have all of the other mechanicals replaced.)

DanielRicany

I know you were talking to the other guy but I also have some extra information to share.

I haven't really noticed the idle surge during open loop, only when I first try to start the car, it will search for an idle and then eventually seems to straighten out. Also takes two tries to start it, the first time it stalls. I think, if I remember correctly, it tried to idle a little lower in open loop than closed loop. It runs really lean in open loop though, like 19:1 AFR. No knocks or pings.

Would an exhaust leak before the oxygen sensor cause an idle fluctuation? I have hedman headers and they leak at the collector.

Any help is appreciated. Thank you.

DanielRicany

Okay, well I started the car up tonight, this time it did not stall on the first start. But it did idle surge about 2 - 3 times until it found an idle at around 800 rpms. The AFR was a little rich, high 12s and low 13s. As time went on, still cold and in open loop, it gradually worked its way down, 800, 750, 700, 650, 600 and stopped around 600. At this point I think it was around 14 - 15 AFR. The engine kind of sounded like it was struggling and it was very bouncy inside the car. Time goes on a little further and the AFR gradually went up to around 15-17. The car then decided it wanted a little higher and went to 750-800 RPMs. Then it started surging, still in open loop, between 700-800, not very aggressively but still surging. As it warmed up to operating temperature, in the 190s, it was idle at a good RPM, I think it tried to stay around 750ish, and the AFRs seemed pretty good, it tried to stay around 14.7 with a little up and down.

I shut it off before it surged in closed loop, I think it will only do this if I drive it and come to a stop light.

I'm not really sure where to go from here. I'm stuck. Unless there is a problem with the manifold not lining up with the head ports properly, there's nothing else that I can think of.

The following engine mods consist of:

Mini Ram
Flat tappet version of ZZ409 cam
Hedman Longtube headers
Gutted catalytic converter
AIR delete
EGR delete
MSD 8.5mm low resistance ignition wires
10 Degrees timing
Descreened MAF sensor
Air filter lid cut
58mm Throttle Body
K&N Air filter
Heated o2 Sensor conversion

There may be a few things that I forgot to list but that's the main modifications.

Before I changed out my throttle body, I couldn't get the min air idle setting right because the engine RPM was always jumping around. I had initially gotten IAC counts of like 60 - 80 before I adjusted the min air, then I had gotten like 45, then I got 20, and when I got 20 it would keep dropping down to like 10, and then at my last one the counts were actually 0. Then I installed the new throttle body and left the preset adjustment. I am not touching it because the last time I did that the car was running lean and then rich and all this crazy stuff. I have not checked the IAC counts for the new throttle body.

I have tested my TPS, my MAF sensor I swapped with one from an 86, no change, my o2 sensor is new, my CTS is new but that reports 160 when it is really at 199, so it is still in closed loop, my MAT sensor is new that came with the manifold, my timing is set to 10 degrees, and I checked for vacuum leaks, I used propane, starting fluid, cigar smoke, and found nothing, pulled off all the vacuum lines and plugged them and no change. So I'm at a loss here.

Please help.

Cliff Harris

Is your O2 sensor heated?

DanielRicany

Yes

MikeT 90 C4sixer

Wow, alot going on there. You sure got yourself a nailbiter. You might not like to hear this, but the more mods you make to your engine, the harder it is to tune it. You may have just maxed out things with a MAF setup. I try to give you some direction, but i am no expert.
Gotta get that coolant sensor working right. The ecm uses CTS for alot of settings. Double check for vac leaks, again. Sorry! Is your 9th injector working, for cold start? What mask and def file are you running?
Also, check the MAF sensor again. I know you did it once, but it is a very important sensor. When they go bad, it will really mess things up. And you must have the correct one for your app. I think one of the early years, the TPI used a different sensor, like for just one year. Maybe a frequency based sensor, vs an analog sensor? Check it out. I fought with a replacement sensor on my IROC, and it was the wrong one, had to get a correct Bosch reman, which costs more of course.

As far as things in the bin to look at. Crank fuel tables. Startup fuel tables....IAC tables and constants....dont be afraid to make radical changes. You can always go back if something does not work.
Good luck.....

bjankuski

You may want to try setting your base timing at 6 degrees instead of 10. A lot of times an idle surge is due to too much timing at idle which causes the engine to respond faster then the IAC can respond so it goes into a uncontrolled rolling idle. To solve this you may need to remove idle timing in the effected RPM and load positions in the main timing table to get the car to respond slower so the IAC can contol the idle. You may also want to increase your open loop fuel in the warmer engine operation areas since your AFR hits 17 to 1.

skip myles

i have an 85 corvette have checked everything 2 to 3 times when I first crank it I idle in the 11's then drop to 7.5 when temp comes up but some time it will crawl up to 12 or 13 and will not drop to the 7's until I shut off and restart drove 100 miles and stopped car woudnt drop idle below 9 then next stop it dropped to 7.5 been driving me crazy is it possible to have a bad ground on computer cause idle problems

1 John 2:2.

First, I'm not a mechanic. I've had this 94 for 14 years and it runs great, except at stop, in drive with warm engine or a/c on. It will surge suddenly and requires good brake pressure to hold it back. I've been shifting into neutral at stop and that cures surge. Had Chevy garage test for error codes...none. No problems except at idle. Any suggestions?

skip myles

I found a 1/2 inch hose going from valve cover to throttle body was leaking didn't show up on vacuum test put clamps on it and idle is right now

Thank you very much, I want to use this information and compare to see if my IAC counts were lower than what should be, it would indicate a vacuum leak. Thanks![/QUOTE]

1 John 2:2.

[QUOTE=skip myles;1589664475]I found a 1/2 inch hose going from valve cover to throttle body was leaking didn't show up on vacuum test put clamps on it and idle is right now

Thanks for your reply. I put two clamps on the hose of which you spoke which enters the throttle body at the IAC. It cured about 75% of the surge but it still has some. Did you take off the plastic cover above the valve cover to clamp the end of the hose where it enters the valve cover? I didn't do that yet.
Thanks so much for your good advice.