I've had a weird idle issue for some time now. 95lt1 will idle just fine about 98% of the time.

Issue arises when I take it out of gear or push clutch in. It will drop low to about 500rpm. Then regulate back up to its normal 700-800 range.

It's can be noticeable on start up too. It will kinda struggle to get up to normal idle, about few seconds worth.

So IAC? TPS? PCM ISSUE?
Not sure but want to check with forum knowledge before randomly purchasing things

 

Your IAC and/or throttle body could be dirty. Pull the TB and give it a good soak in carb cleaner after you remove the IAC. Blow out the orifices with air. Then clean the pintle of the IAC and check for any galling on the taper. Do a forum search on how to properly re-install the IAC.

 

........ After you clean up the IAC and its passages , make sure you lubricate the throttle shaft where it passes through the throttle body ... a good silicone or Teflon based spray lube works well ... Then Set the Minimum Idle (per the FSM) ... THEN reset the TPS ... If your tune is OK (plugs , wires , O2S) , it should run/idle like new .........

 

There is no minimum idle adjustment of LT1 or LT4 engines. You can CHECK the TPS -- you can't "reset" or "adjust" it.

 

OP - are you talking about the car in your profile (1987) or a different car?

 

....... Thanks for "schooling" me ... do you have anything for the OP ??? ............

 

I think Cliff gave the appropriate advice, given the OP's post. Let's look a little closer........I don't know what you guys saw, but *I* read "95 LT1".

 

You are correct. The OP's post does say that. I just wanted to make sure we weren't talking about the OP's profile car.

 

It is the car in the picture. It's an 87 but 95 lt1 in it. I did clean everything and same issues. I did not try to lubricate it though. I'll check everything again. But idk if issue continues may just try replacing things

 

Sounds to me like your throttle blades are closed to far. One question, has it always done this? If it has always done this, then I would suspect that as you coast the iac tries to close to slow down the idle but it cannot because the car is in gear keeping the rpm high. When you push the clutch in the car that was driving the engine is removed and the closed throttle and closed iac cause the idle to drop and then the iac has to recover and raise the rpm up. If the throttle is opened slightly farther this could not happen. Is engine all stock? The slow start is another clue.

 

Yea. Has done it since I put this engine in the car.
Engine is stock though
I didn't look but I'm guessing you can adjust the throttle plate's resting position

 

Yes, you can adjust the throttle blade resting position.

 

Is it normal for the plate to be open more like that. I always thought IAC did this job

 

Does this describe your issue? It is from the FSM.

"Rough, Unstable, or Incorrect Idle, stalling"

Definition: Engine runs unevenly at idle. If severe, the engine or vehicle may shake. Engine idle speed may vary in RPM. Either condition may be severe enough to stall the engine.

 

IAW the FSM troubleshooting procedure.

1. Refer to symptoms, important preliminary checks before starting. - Check for vacuum leaks.
2. Check PCM grounds
3. Check air filter
4. Perform injector balance test
5. Check for low fuel pressure
6. Check for restricted fuel filter
7. Check for incorrect minimum idle speed. Using scan tool, monitor IAC postion. IAC should be between 5 and 5 counts.
8. Check each injector harness is connected to the correct injector according to the firing order.
9. Check for injectors that are shorted internally.
10. Check for fuel in pressure regulator vacuum hose.
11. Check for foreign material accumulation in the throttle bore, coking on the throttle valve or on the throttle shaft. Also check for throttle body tampering.
12. Check Crankcase ventilation valve for proper operation.
13. Using scan tool, monitor TP sensor voltage. Should be less than .9 volts.
14. Using scan tool, monitor KS system for excessive spark retard activity.
15. Check for proper ignition voltage output with spark tester.
16. Check spark plugs for Wet, Cracks, Wear, Improper gap, Burned electrode, Heavy deposits.
17. Check engine mechanical: Compression, sticking/leaking valves, worn camshaft, valve timing, bent push rods, worn rocker arms, broken valve springs, excessive oil in combustion chamber.
18. Check Traction Control Adjuster assembly for out of adjustment.
19. Check PNP switch

 

The idle is not severely bad. I will say it stalled once when it was colder and I just started driving out of the driveway. Did that taking it out of gear.

Condition only arises coming out of gear or starting the engine. And it's inconsistent too. Sometimes it goes to normal idle just fine.

TB is clean also and I did plugs and wires already.

When I did take IAC and put back, I thought it was fixed for a bit but symptoms came back after a little driving

 

I re cleaned the Throttle Body. It looked little black. I also cleaned the MAF.

It seems to have helped a little but the issue still occurs about 40% of the time when taking out of gear. Starting issue is still there too.

Could this be any kind of air restriction? I have the LT1 from a 95 but I have the stock 87 air filter that leads to the MAF. Could this be starving the engine of the correct air flow?

 

have you checked for vacuum leaks?

 

I have thought it could be that but I do not know where to look. I know some of the lines but not all. Anyway to easily check beside eyeing it?

This is very custom application because I have 87 lines integrated into the LT1. I think it may just be the line from the canister I ran to purge valve.

The AIR lines from the exhaust manifold are on, but there is a line that points down on the driver side. I did not know what this went to and I just put a plug on it. Could this affect the idle?

 

Sounds like that is the line that goes to the AIR pump in the driver side fender well. Don't know if that would cause an idle issue but I don't think so.

I have seen people use starting fluid to track down vacuum leaks. They would spray around a suspect vacuum line and if the idle RPM went up then the line or connection was bad. This is because the starting fluid was being sucked in through the vacuum lines and being burned in the combustion chamber as unmetered fuel.

Have you gone down the check list I provided from the FSM?

Without seeing or hearing the car in person, I believe you have a vacuum leak caused by a bad hose/connection or an open vacuum port or you have a bad FPR.