I've searched and searched but I cannot find an exact problem as mine.
Basics: 1986 all stock except performance chip.
My problem is the idle is slow to return after I let off the pedal.
I have the FSM and have set and reset the basic idle many times. On the first drive after setting the base idle the car runs perfect and returns to idle all the time. I can drive it a mile or 30 miles and it runs like it should Then after I shut the car off and restart it the idle stays at 11 to 1200 for 5 or 6 seconds before returning to 700ish.
I've replaced the TPS, three times. IAC twice and all the relays for the MAF, the MAF, CTS and the ECM. Not all of the items mentioned were changed because of the idle problem.
I'm sure there are no vacuum leaks as I've changed most of the lines and besides it works great on the first drive after I set the base idle.
I'm stumped.........the only thing I haven't done is to re-install the stock chip.

 

Your performance chip may have a higher number listed for initial startup position for the IAC stepper motor.

Or, if you did not set/reset the IAC correctly after the reinstallation, that would cause the same thing.

 

......... So , Let me see if I got this right ... You're throwing parts at your car and complaining about 5-6 seconds of an annoying high idle ? You're more high maintenance than my ex ........

 

Your not alone, My 89 does the exact same thing ever since I put the new motor in last May.. If I disconnect the battery it works fine for about a day, but after the car is started a few times it goes back the same way.. When I back out of a parking space and shift from reverse into drive I have to wait about 5 seconds in between gears for the rpms to come back down..Also anytime I put it in neutral the rpms go up for about 5 seconds, then come down to where they belong..I sure would like to get this fixed because its a pain in the butt...WW

 

The ECM "learns" IAC positioning. I wasn't aware it took a whole day but its the best explanation I can think of. More importantly, I'm absolutely certain having your idle screw mis-adjusted will cause this problem.

When you install a new motor (or significant performance enhancement), you need to open the blades more so the IAC doesn't have to do the work. When it does, you get that floating idle.

 

Gregg,When I started my tuning Mick had me open the throttle up to 40-45 idle air flow and set the tps to .60.. When I put my new IAC in I didn't reset the new one, I just swapped it out..Do you think I need to reset the new IAC to fix this problem?..WW

 

This is why I set the idle by IAC counts and NOT using the FSM. The FSM works for perfectly stock vehicles but once you put stuff on it, things change. Use a scanner and see what the IAC count is and set it to about 20 counts, give or take, using the idle screw. Make an adjustment, rev. Repeat till you get the target counts. Timing must be on and everything tight. Set TPS after that. All other systems must be off. No AC, no Stereo, etc, etc.

 

My money is on a small vac leak. You need to spray around a bit with some starting fluid. If the engine gets the slightest wiff, idle speed will increase. If not....you can look elsewhere for your problem.

 

For me, it was a higher IAC setting that caused my idle float to be more pronounced. Factory setting is 15-20.

Hook your logger up and drop IAC count to 15 if the fans aren't running and it's at running temp. (20 is fan is on) Reset the TPS to .54-.55.

See if you like that better. If not, you can always put it back.

(Disconnect battery as before...for ECm to relearn new position(s).)

 

Thanks both Gregg and Aklim,
I'll give your settings a try. Like I said earlier, mseven had me set my "IAC motor position" to 45 when we first started doing my tune. He said this was to make sure the engine was getting enough air at idle..He also had me set my TPS at .60 instead of .54.. Maybe the 45 IAC counts are to high for my engine.....WW

 

I'm not in the habit of throwing parts at cars. I have the FSM and the reason for having to install two or three of the same items is because the crap you get at parts stores these days is just that, crap. The first TPS I replaced lasted just three days before it fell apart on the car. The second one kept giving me a code 33 that was fixed earlier by changing the MAF.
The IAC's are made by the same company that made the TPS.
So my advice to all is to not buy the AutoZone parts.

 

I have the FSM and have set and reset the basic idle many times. On the first drive after setting the base idle the car runs perfect and returns to idle all the time. I can drive it a mile or 30 miles and it runs like it should Then after I shut the car off and restart it the idle stays at 11 to 1200 for 5 or 6 seconds before returning to 700ish.

It seems to me some of you guys aren't reading the full post.
True...I don't work on cars for a living but I am a certified aircraft mechanic and english is my native language so the FSM isn't difficult to understand.

Not to be bashing anyone but it's a little frustrating to read post that offer suggestions that I've already covered in the post.

Thanks to all.

I see I will need some computer software and a cable or two so that I can access the ECM.

 

You can get something like Auto X-ray to see what the ECM is doing. Who wrote that performance chip program? What is the command idle speed that the ECM wants to see?

 

Ahhhh,...parts that break quickly and need replacing often. No wonder AutoZone's stock has performed so well in the past year!

What have you done to set the IAC? Where is it set at idle? I read your OP. Did you read mine?

 

For the lowest cost option, I suggest buying a cable from ALDL.com. USB type is what you want if you have that laptop port.

Download TunerPro. With a couple of setup steps, you'll be ready to monitor, get codes, and datalog.

BTW...Saying you're a certified aircraft mechanic doesn't negate the fact you bought a performance chip -- for an otherwise stock corvette. I'll admit I'm not familiar with the 86 timing curve, but we've recent seen posts showing they're certainly rich enough. From $6E bins that followed 1986, their timing curves are VERY aggressive. So, aggressive, I have to think they're running into the ESC system often enough that it was used as a technique to make sure maximum timing is always present in any Corvette. That's just my opinion.

The overall opinion of the forum is aftermarket chips are a total waste of money (even if you DO have other performance mods)! They aren't specific to your situation, driving, or environment. I've even seen where someone reverse-engineered a HyperTech chip only to find one or two parms changed -- like fan temp and shift-light parms. It was B.S.

The perception of people who do buy these types of chips is they don't research and/or understand what they're doing. They're buying into hype. Expect responses from people with that in mind.

 

Just thought I would update this thread.. Moving my IAC knock counts from 45 to 20 and setting my TPS to .54 didn't change my idle problem, so I set knock counts back to 45. So it looks like the hunt is still on for the problem...WW

 

The chip came in the car when I purchased it and I will have to find the box the guy gave me to see which one it is. I have the stock chip and will try setting up the idle with it instead.
This is the oldest of the threes C4's I've owned, none of the previous cars were modified at all and I was perfectly happy with the performance of those cars. (94 and 96).
I am going out to work tonight and won't be back to work on the car until the 2nd so it will be a while before I can relay any feed back.
Thanks for the info. I will be ordering a usb cable to go with the program I downloaded.

 

Thanks for posting this, my car has always taken forever to idle down after a re-start, especially when it is warmed up. I'm going to check this out.

 

With your car in closed loop, shut everything off and see where the IAC counts are at. If it is higher than target (20-30), it means the IAC is opening more because the throttle blades are too closed so turn the screw in to open it up a bit more. Rev the engine and let it settle. Recheck. Each time you adjust, rev. If it is too low, close the throttle blades by turning the screw out. rev and recheck

 

I went out & tried it, I started out with 58 counts so I started the adjust/rev pattern and just when I thought I had it coming down to where it should be it would take off & start climbing again. Once I got that far I couldn't bring it back so I did the min air idle procedure and now the counts are 9-15, the idle is a tad high at 850, (chip is tuned for 750) but it goes right down to 730-740 when I put it in gear. All in all it is better and I plan on playing some more with it this weekend. It was getting late & the neighbors would have been throwing fits if I had kept revving it.