schimek3624

I had the same problem with my '95 and did nearly the same things you did. Found out that you need to " reset" the IAC valve. In the FSM there is a procedure which if I remember correctly you do the following. Press the accelerator pedal half way, start the car, take foot off accelerator, turn off car. The IAC should be reset and the idle will return to normal.

primalurges

Hey I have a 96 and my idle is about 900 and when I posted the question as to what the idle should be everyone was telling me 900 is normal. So 900 idle is not normal?

rodj

So you have confirmed you have a engine problem ; not a control problem

I have opposite; disconnect IAC ,flat idle @ 400.
Reconnect IAC ,revs jump to 800+
Changed IAC , no difference

Ape0r

IAC is set up correctly. No change after replacing pcv hose.

Here is where I am:
Intake gaskets new
EGR gasket new
Throttle body gaskets new
Disconnected and capped all hoses except pcv and fpr, no change, fpr replaced in last few months
Replaced pcv hose (pcv new a few months ago)
Checked map seal, looks fine
No response from carb spray anywhere
IAC new and operating correctly, has been reset per FSM as required

The only other stuff I can even think of is a warped connection between intake and heads (unlikely) or a large crack in the heads (unlikely). Intake looked fine when I had it off. That, or throttle body shaft leak. Shaft seems tight.

Any other places to look before I try to get the TB shaft out?? I swear I can hear a sucking noise from the d/s of the motor, but cannot pinpoint. I took a coolant hose and put it next to tb and it seemed like air was being sucked through the hose.

Guess I'm going to try the propane trick and then blow apart TB, unless anyone has other ideas?

Paul - Thx for the data. Did get it, wish mine worked like that!

RVY

I had this same problem. I did what you did. My problem turned out to be a slight bind where the throttle cable was fastened to the throttle body. Not much of a bind but just enough to cause the high idle problem.

C409

..... One thing I did notice on your data is that there is no engine coolant temp category ... on Corvettes the ECM and gauge get their input from separate senders ... ECM sensor on LT1 is on the water pump ... gauge sender is in a cylinder head DS I believe ... perhaps the ECM sender is giving erroneous readings to the computer ......

sailorsteve

The "desired idle" parameter in the ECM is 650 for closed loop, or warmed-up, operation. (I believe this is, in neutral or Park, manual or automatic trans)

The reset procedure, p.6E3-C2-5, is stated as "depress accelerator pedal slightly, start and run engine for 5 seconds, then turn ignition "OFF" for ten seconds." Simple enough, worth a try.

surfer93

Have you made sure the IAC connector is on correctly, when my car had idle issues, I had the connector on wrong.

Have you OHM'd the IAC wires?

Have you checked voltage at the IAC?

Ape0r

It is not the IAC. Running IAC shut and then running motor proves this "beyond the shadow of a doubt." Confirmed proper IAC closing when I disassembled TB. Also checked IAC coil resistance on IAC contacts as well as at ECU connection, all good. ECU connection looks perfect.

I am thinking TB shaft leak. Need more time to try and get the cam off the shaft so I can see wth is going on.

pcolt94

Thank You. I could have looked for those sections for ever.....

surfer93

When intake was off, did you clean the EGR opening on the intake, i.e. inside?

Instead of spraying carb spray, which is BAD. Hope you meant TB spray, but spray soapy water and look for bubbles.

Did you check the OHMS and voltage at the IAC connecter, like I mentioned earlier?

surfer93

Here is out of the FSM -

1) check for vacuum leaks
2) check ECM grounds for being clean and proper routing.
3) Injector balance test
4) EVAP(emissions) control system - solenoid with hose going to TB
5) O2's for quick response to throttle changes
6) plugs and wires
7) cylinder compression test
8) weak valve springs

surfer93

Forgot to mention, exhaust leak will cause a high idle, mine idles between 875 and 1200, depending on engine temp.

Ape0r

I tried capping vacuum lines again, this time everything but fpr. No improvement.

I also tried the idiot's solution, which is plugging the base idle hole at the back of the tb (roughly 1/8" hole at back of iac passage, can be seen in pics on google image search). Worked - got the idle down, brought iac counts up to 10 on relatively warm idle, and raised map pressure from 33 to 37 kPa.

So, its not intake gaskets, its not egr gasket, its not tb gaskets, its not fuel injector seals, its not any of my vacuum devices (although I imagine they aren't helping).

WHAT ELSE IS LEFT THAT IT COULD BE SUCKING AIR THROUGH!?????

surfer93

It does NOT have to be a vacuum leak!!!!!!!!!!!!!!!

And I have given you 10+ options, that you have not replied to.

AVL94LT1383

I looked at the data. I will try not to write a book.
Things that look right.
ECM calls for 650 idle and seeing 900 starts pulling timing, injector PW, and IAC counts.
MAP @ idle looks ok for stock.
What doesm't look right...
IAC at 0, PW at 1-2, timing down to low teens. but O2s showing slightly rich condition.
So: Have the injectors been changed to larger ones from stock?
Is the Fuel pressure at 43 psi.?
Might want to rule out injector issues at this point based on information.
Question... What is the minimum PW in the constants section?
Basically the car is calling for reduction in idle speed and using controls to lower it but it is running out of adjustment...
Oh, less dangerous than propane... can of Quick start?... be very light in use I hear... but I think that the carb cleaner would have done the trick...
The temp thing that was mentioned might be skipped since the data is showing the ECM is telling the controls to go to 650 RPM....

AVL94LT1383

Oh the TPS looked to be working also.... which sends us back to... is it getting too much fuel at idle...

Andy P is well aware that Ben Barnes (Barnes & Reese Racing Engines) of Asheville, NC is preparing the Chevy engines for the Davis operation. Ben Barnes was the engine builder for Jack Ingram during his championship years and has built the power plants that have propelled Kertus to top ten runs at Talladega.

Ape0r

Injectors are indeed not stock - Bosch III's from FIC. Swapped them in four or five months ago as something like five of the stock injectors were leaking. I checked fuel pressure before I changed injectors - it was 39 psi with key on engine off (and dropping, so maybe a touch higher before I got around the car to check the gauge - fuel pressure holds fine now that I've changed out injectors and fpr, and tightened hose clamps in the hose that the PO used to replace the pulsator!).

surfer:

(1) Hard to believe it is a vacuum leak at this point, you are indeed correct. I do have a hard time seeing how it can have a higher idle with no extra air, but I guess it must?

(2) Don't see any reason to suspect ECM issues based on data I am getting. IAC is definitely working correctly.

(3) How does one conduct an injector balance test? See above re FIC Bosch III injectors.

(4) Probably not evap since IAC counts drop to zero before engine reaches operating temp? Engine does throw evap code when I have the valve disconnected with vacuum connections capped

(5) Can't say I'm qualified here, but hard to imagine O2 sensors would be a factor when IAC is already rammed shut and injector PW is cranked down.

(6) Plugs were replaced six months ago. I also swapped the coil to opti wire and the wires on the p/s of motor. As you probably know, wires on d/s are a bear, been waiting for an excuse to have all that other crap off to make swapping those four easy. Thoughts on this?

(7) Have not checked compression

(8) Guessing you wouldn't get to this unless you didn't have good compression

Car runs like a bat out of hell off idle, everything is great when on the throttle. Really idles fine too, although much rougher than my truck.

Where do I go to from here? As I stated above, plugging the base idle hole in the TB got the IAC counts to 10 and dropped the idle to ~the desire idle, maybe +50 rpm.

Ape0r

Coolant temp is good, just didn't remember to click to include it in the output.

screamin_conure

Hey Ape0r,

I had the same problem on my '92 as well, and did most all of the things you've done in troubleshooting. The thing that finally solved the high idle situation for me was replacing the EGR valve itself. I guess, and I really don't know how this could have been, the valve must have had some sort of internal leak. Maybe the rubber diaphragm was broken or something. I also removed and cleaned my TB thoroughly and also replaced my IAC valve too. This helped some, but it wasn't until I replaced the EGR valve that the idle situation improved.

Man, it seems like C4s really suffer from idle issues. And it appears that once a car starts to have idling problems, it seems like they just NEVER go away no matter what you do. Even now, I still have some weird idle issues, though it has gone from a high idle situation to an idle that is now somewhat low and a bit rough. And every once in a blue moon, the idle will start to creep up to where it's at 600 - 650 in gear when fully warmed and I'll see my IAC counts go from 30 - 50 at idle right down to zero.

Frustrating, I know.