thomastl

95 LT1 Automatic. 109,000 miles. New to me.
Has what I would describe as a Dead Miss at idle (only when A/C is off) and a slight pinging under light acceleration (no downshift) with 93 Octane fuel.

Otherwise runs good. No Miss at speed that I can detect.
No codes set (I have checked).

Again, miss only occurs at idle when A/C is not turned on. If A/C is on, idles perfect but still has slight pinging under acceleration.


List of things done recently:
New Air Filter and clean intake system.
New IAC Valve and cleaned throttle body port.
Cleaned Throttle body including under inspection plate although not very dirty at all and no oil.
New EGR Valve and cleaned intake around EGR valve port.
New O2 Sensors
New PCV Valve
New Optispark, Plugs, and Wires (about 3000 miles ago by previous owner) I have also verified that each is firing and no stray firing using Timing light on each plug wire and looking at night for stray arching.
New Ignition control module (with proper thermal grease)
New MSD Coil
New Mass Air Flow Sensor
Antifreeze flush and fill (temps running proper)
Clean Fuel Injectors by removing and running Carb Cleaner thru while cycling power (good stream and operating properly). Also verified operation and 12.5 ohms.
New Fuel filter
Verified fuel pressures are good.
Verified smooth reading on throttle position sensor thru operating range.
Checked for vacuum leaks and replaced any vacuum lines that were suspicious. Cruise works good.
Air pump works properly when first cranking car.

Some parts were bad, some were in need based on mileage, some were too dirty to get clean (MAF).

I am at a loss. Everything tells me that it is EGR valve or vacuum leak based on pinging but I cannot understand why the idle would be perfect with A/C on and have a miss with A/C off.

What am I missing?

Thanks.

antfarmer2

I would blow smoke from a cheap cigar in the vacuum lines to look for the leak. And would make sure all the plug wires click twice with a touch of dielectric grease in both ends. OHM your injectors hot and cold.

thomastl

Plug wires have been checked and verified good. Dielectric was used.
Injectors hot and cold - no difference - 12.5 ohms.
Smoke the intake. I have listened for leaks with stethoscope, I have used propane with no change, I have used brake cleaner (the flammable kind) with no change, I have unplugged each line at the intake and closed off intake port to verify no change. No vacuum leak that I can find. I don't smoke and don't know anyone that does, so hard for me to do the actual smoke test. But, given all I have done, I can't imagine it is a vacuum leak at this point.

But if it was any of these, then it would not clear up with A/C on as far as I know.

antfarmer2

The wires clicked twice? OHM your injectors hot and cold and post the results.

thomastl

Wires clicked twice.
Ohm was 12.5 cold and hot as far as I remember but will recheck tomorrow.

But if it was this, how would it clear up with the A/C turned on?

antfarmer2

All at 12.5 sounds low

thomastl

According to this posting: https://www.corvetteforum.com/forums...-the-same.html Post #3, LT1 injectors are 12 to 13 ohms. Anything above 10 is good.

I have also tested fuel pressure leak down and there is no problem there.

antfarmer2

Are they original might give Jon a call at FIC. Have they been cleaned?

thomastl

They are original.
As stated, I removed and Cleaned and tested myself.
I connected to 12 volts with a switch. Ran 2 Oz of Carb Cleaner thru each one with a syringe connected to provide pressure while cycling power. They all have a very clean, good, spray pattern.

Plus, if they were bad, how would it be ok with A/C on and bad with A/C off.

I am thinking it must be some kind of controller or sensor issue that changes with the A/C being turned on. But not sure what all the A/C on would effect sensor or control wise.

antfarmer2

Missed that but a bit more load on it will change things. If it was mine I would clean the egr. Will sleep on it but stumped too. Did you put new gaskets if you took the TB off and check the bolts to see if tight?

thomastl

EGR is new. Intake port cleaned and new gasket as well. (Old one would not hold vacuum). Tested and verified operational.

antfarmer2

Kick it.

thomastl

I did use all new gaskets with everything that was removed.
All bolts are tightened properly.

I am lost on this one.

pcolt94

The miss might be masked with the AC on because the PCM commands a higher idle changing the IAC setting.

Have you checked out the EGR vacuum relay?

Is the new EGR the correct part?

You need to troubleshoot this for the miss at idle as you would any other car and find out what cylinder may not be firing. Either pull the plug wires off one by one or easier to disconnect one injector at a time and see which one does not make a difference.

thomastl

Idle speed does not seem to change with AC on.
Just smooth and no miss.
Have not checked the EGR relay but I have verified that the EGR valve is closed at idle.
EGR matched up with the old one and was purchased from Advance Auto Parts.
I have unplugged each injector one at a time. Results: With A/C off, miss remains and overall you do notice a small decrease in idle and increased roughness. All 8 have the same result so was no way to verify which cylinder was the problem.
The miss is not always there. It will miss a beat and then be fine for a second or so and then miss a beat again. More like a stumble ever so often than always missing on one cylinder.

Another way to describe would be as if the computer is commanding a complete closing of the IAC for one revolution and then opening back up again. I know that sounds weird but that is best I can explain it.

1993C4LT1

Post a video of the sound. 93Rubie fixed that problem with new injectors. Not saying injectors are your problem, just mentioning it has cured another person's problem. I've read that a few of us with 93's(I know your's is a 95) have this noise at idle. AC on, it goes away.

aklim

I never bother to Ohm the injectors. Way too misleading. All it says is that you don't have a short. Is the filter basket collapsed or clogged? Is the pintle clean? Is the volume right? A proper test will have fuel like liquid flowing through at 43 psi and the spray pattern and volume measured. How you did all that with a syringe is puzzling.

"ok"? What determines "ok"? SOTP is worthless. You need to see if the RPM is going up as seen by the ECM and not your tach. You need to see if the ECM is compensating or not. You need to see if there is an air leak the ECM is trying to compensate for. For all that, you are going to need a scanner.

aklim

At stable operating temp with everything but the motor off, how many IAC counts? What is the command idle speed?

aklim

I would suggest getting it to FIC or RC Engineering or some place with a bench to see what the volume and spray pattern is and not rely on a DVOM, syringe and some brake cleaner.

thomastl

I will try a take a video today.
I have an OBD2 scanner (actually a Bluetooth adapter and the software on my phone). But it won't work on this 95.
I did the jumper wire to pull codes and there are none to include no history codes after resetting from testing setting off codes.

I have read that the Actron CP9185 will read this car.
Is that true or is there a better one that I can use to make this work?