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 ?

 

[Quasar C4]

An update on a few things.
On the third run (after it misfired, backfired and stalled on hard acceleration on the previous two occasions), it stalled whilst coming up to a junction. Re-started straight away then stalled again within 1/2 mile. It wouldn’t re-start for 30 mins after which I managed to drive the couple of miles home.

Testing yesterday….

Fuel rail pressure holds at 45psi on ignition and whilst running.

I replaced the coil and ICM which I bought from RockAuto, but I also bought a Moates ALDL USB lead and managed to hook up the car to TunerPro.

I also fitted an inline spark tester between the coil and Optispark.

Fitted the coil and ICM and ran the car on fast idle. When warm it stalled. It wouldn’t start on the new ICM or old one until it cooled down after which it would start using either ICM.

When failing to start, there’s:
- No glow from the spark tester, ie. No spark
- No pulsed 5V EST output from the ECM to the ICM
- No rpm data to the ECM when cranking (which is derived from the Optispark LOW RES output - TunerPro just shows RPM flatlining)
- No rpm shown at the tachometer during cranking (input derived from the coil so without a spark this is to be expected)

Throughout troubleshooting the car it has thrown up no fault codes to indicate Optispark failure. It flagged a code 41 which is an EST open circuit which happened whilst we cranked the car with the ICM disconnected.

During other testing yesterday, the coil resistance on two injectors is down at 8.3 and 10.8 ohm whilst all others are at 12.1 - 12.2 ohm.
This could indicate the injectors are on the way out. Idling was a bit uneven.

Also the TunerPro showed a periodic flicker of ERROR on injector driver 1 and injector driver 2.

The misfire and cutting out only happened when the engine was hot but now it won’t start when cold. The missing EST input at the ICM seems to be the cause of no spark.

Reluctant to change the Optispark unless I can prove it’s the source of the problem.

Would welcome ideas from anybody who’s experienced similar issues with a C4 LT1.

 

[teamzr1]

Sounds like two issues
Injectors that old surprised they worked that long even with Ethanol in gas
Can buy a full set for under $200 US

Common for coolant to seep out of water pump and gets inside the OptiSpark so
would suspect that as the main problem ?

Due to its location, the optispark is subject to moisture from several sources. First, driving through high water can submerge the opti completely. Automatic car washes with undercarriage wash can also soak the opti. Secondly, if the water pump suffers bearing failure and coolant comes out the weep hole, the opti can be soaked with coolant.
Lastly, NEVER WASH AN LT SERIES MOTOR WITH A HOSE OR HIGH PRESSURE SPRAYER. To wash your motor, use a damp soapy rag and take your time.

Moisture buildup inside the opti can cause the precise parts of the optical side to rust up. The bearings are also weak and prone to failure, particularly from misalignment or a too-long cam dowel pin distorting the drive mechanism. Last, high voltage causes ozone buildup inside the cap which is to be considered a wear item just like any traditional cap/rotor distributor.
After market ignition amplifiers make the ozone buildup worse and can significantly shorten the life of your opti.

What are my replacement/upgrade options?

There are four basic options.
(1) replace with a new GM unit. (rebuilt parts store units are of questionable quality). MSD now makes an upgraded cap/rotor of supposedly higher quality than stock if your optical section is in good working order.

(2) The Dynaspark is a totally re-engineered unit with a billet body and improved moisture sealing.

(3) the LTCC is an LS1 coil-pack conversion adapter to which you supply your own coils. It uses the signal from the optical section of your opti to work, so if you suffer optical section failure you'll still have to replace the opti. But, by removing the high voltage section from operation, you can significantly prolong opti life.

(4) the Delteq opti eliminator converts the LT1 to a Northstar coilpack setup using either the Opti optical section or a stage 2 which eliminates the Opti all together.

Replacing Optispark requires removing the water pump, and crank pulley :-(

How to

Replacing the Optispark Distributor on Your LT1 Corvette (1992–1996)

When GM introduced the LT1 engine in 1992, Corvette owners got a completely new ignition system. The Optispark distributor was a major departure from the

www.corvettemagazine.com

 

[Quasar C4]

This morning, we tested the high and low resolution signals to and from the Optispark. Both pulsed at 5V and the car ran OK. We disconnected the injectors and cranked the engine to see if the ECM was seeing the engine speed during cranking which it did (RPM derived from Optispark low resolution signal).

When the engine warmed up it cut-out and would not re-start. No spark.
Checked both Optispark signals and both were fixed at 8.5V. Removed the 12V supply to the Optispark and the signal voltage returned to 5V. Re-applied 12V and signal voltage returned to 8.5V.
When cranking with no spark there was no engine speed recorded at the ECM. We were able to see this on my laptop using the TunerPro RT software.

Let the engine cool for 1 hour. Switched on ignition. High and Low resolution signals back at 5V. Car started and signals pulsed for a few seconds then cut-out. Signal voltage now fixed at 8.5V again.
Disconnected the Optispark harness at the Optispark to see if the problem was due to a breakdown in the harness but signal voltage from the ECM was now back at 5V.

In conclusion, the Optispark high and low resolution signals are failing at temperature probably due to a breakdown in the sensor insulation causing the 12V supply to short across to the return signals.

Also a couple of the injector coils are on the way out!

Only been able to diagnose with TunerPro RT software, Moates ALDL to USB cable and an online spark tester between the coil and the distributor.