Corvette 1992 LT1 - misfire

[Quasar C4]

My C4 badly misfired and then backfired when flooring it a couple of days ago. It happened at about 3500-4000rpm. Having backed off the throttle it recovered itself after a few seconds and then ran fine below 3000rpm. It did the same again today. Runs fine below 3000rpm.

Checked the relevant work done on the car.
New Optispark distributor and ignition control module in Nov 2018 (42,240 miles)
Injectors cleaned, new plugs & HT leads and fuel filter fitted by me in Mar 2022 (43,212 miles) just after buying the car when it was only running on 6 cylinders.
Has run fine since. The car’s only done 50,000 miles and is in great condition.

Has anybody had a similar experience.

 

[Oneball]

When Ive had similar symptoms (not in a C4) it’s usually rotor arm, dissy cap or ignition module. I’d try changing the rotor arm. Relatively cheap and easy to do.

 

[teamzr1]

Check fuel rail pressure, using the test port, should be around 43 PSI at idle
when was the last time the fuel filter was replaced ?

Are those the original fuel injectors ?
When was last time they had a good cleaning with an injector cleaning machine or maybe degraded due to age and also Ethanol in the gas they were not designed for and are carbon'ed up
Check the O-rings at both ends of injectors as leaking and pulling in air

Check for exhaust leaks like at the exhaust manifolds and check sparkplug tips and gaps

Unit I have cleans and tests injector for proper flow and not leaking



.

 

[Quasar C4]

As far as I know they are the original injectors.
We ultrasonically cleaned them and checked the spray pattern in March 2022 at 43,212 miles and fitted a new fuel filter. We did this when we bough the car as it appeared to only be running on 6 cylinders and was underpowered but never badly misfired or backfired.
Two injectors were blocked and fuel filter was almost blocked.
We fitted new injector O-rings.
We also fitted Champion plugs and AC Delco OEM HT leads
We checked the fuel pressure which was OK. The car has run well ever since.

I sold the car in 2023 to buy and C7 but bought it back earlier this year as it is a beauty with a 6 speed manual box.
It was running fine but even then I noticed
- it didn't start instantly like it used to. You needed to crank it for just a few seconds which is OK but not as instant as before
- on start up there is a whiff of petrol that I never previously recalled.
The car has run fine all summer until now. Problem started a day after filling up at Asda with E5 superunleaded but I think that is a coincidence

I was going to
1. test the fuel rail pressure and see if I can observe it whilst running. Looking for 40-47 psi
2. check the fuel pressure regulator to see if the diaphragm is damaged and leaking fuel
3. check the injector spray pattern and coil resistance. Looking for 12-16 ohm
4. Check ignition coil secondary resistance

Trying to consider anything else that wouldn't trigger a fault code
Code 36 would indicate fail optical sensor due to missing high resolution pulse to the ECM. It could still rotor arm / distributor cap
Code 44 would indicate lean running if there was fuel starvation on acceleration (also never had this fault in 2022 when it was only running on 6 cylinders).

 

[teamzr1]

GM OBD DTC 44 is lean for Bank 1 only
I would be looking at the O2 sensors and their wiring, intake and exhaust leaks for B1 and injectors
Best of course is using an OBD 1 scanner and see what the ECM reports as to fuel trims, O2 sensors,
in monitoring and record modes

This 92 C4 have Optispark and if so how old and any signs of water getting into it ?

I do not trust the quality of any parts related to fuel system as what damage Ethanol causes to this older vehicles
You might have cleaned the injectors back in 2022, but that does not mean they are good today
I see sets of C4 injectors new for about $100 on Amazon

Oxygen Sensor Error (Lean Condition Indicated)

The ECM receives a signal from the O2 sensor based on the amount of oxygen in the exhaust.
If there is excess oxygen in the exhaust, it is in lean condition and the O2 sensor voltage will be low.
The ECM uses this information to adjust the amount of fuel delivered to each cylinder.

Fault Code Description

• 44 - Oxygen sensor error (lean condition indicated)
• On V-type engines with two O2 sensors, code 44 would indicate the left bank sensor is lean

Symptoms

• Lack of power

Common Problems

• Faulty O2 sensor
• Engine vacuum leak

 

[teamzr1]

I may be wrong, cannot recall back in my C4 days, but there was 2 DTCs 36
1 was for the C4 ZR-1 and different meaning to the base C4s : DTC 36
Does your C4 have a MAF ?

GM OBD Trouble Code 36 (depending on which C4 model)​

1. Mass Air Flow Sensor Error​

2. 24x Crankshaft Position Sensor Circuit Error 0r
   3. 4T60-E Shifting Error

• Depending on the model, code 36 can refer to three different C4 models.
• Choose from one of the above links for specific information on each system.

Mass Air Flow Sensor Error​

The ECM receives a signal from the Mass Air Flow sensor depending on the amount of air entering the engine. The ECM uses information from this sensor to help calculate fuel delivery and spark timing.
Fault Code Description

• 36 - Mass Air Flow sensor error

Symptoms

• Engine stalling
• Lack of power

Common Problems

• Faulty Mass Air Flow sensor
• Broken wire to Mass Air Flow sensor
• Dirty sensing element in Mass Air Flow sensor

24x Crankshaft Position Sensor Circuit Error​

The ECM receives a signal from the Crankshaft Position sensor depending on the engine speed. The ECM uses information from this sensor to help calculate fuel delivery and spark timing.
Fault Code Description

• 36 - 24x Crankshaft Position sensor circuit error

Symptoms

• Engine may not start
• Engine stalling

Common Problems

• Faulty Crankshaft Position sensor
• Damage wiring to Crankshaft Position sensor

4T60-E Shifting Error​

The ECM monitors the transmission shifting performance. Code 36 will set if the shifting performance does not meet factory specializations.
Fault Code Description

• 36 - 4T60-E Shifting error

Symptoms

• Transmission stuck in "fail safe" mode
• Erratic transmission shifting

Common Problems

• Blown transmission fuse
• Internal transmission fault

 

[teamzr1]

As to cleaning fuel injectors back than you looked at pattern but
did you test at what volume each sprayed ?