Hello again all,

Once again I bow to your collective wisdom!

Vehicle in question here is a 1994 LT1, no mods.

My C4 has been giving me some problems lately - it has been cutting out randomly while driving, and has (also randomly) been turning over and over but not firing. Eventually I can get it started, usually, but there have been occasions where no matter what I did it would not start up.

I know that replacing parts at random isn't going to solve the issue, but just for info it has had the following replaced within the last 12 months:

Optispark
Water Pump
Spark Plugs
Fuel Pump
Fuel Pump Relay

I have metered the Ignition Coil and it is fine. Ive also replaced the thermal compound behind the Ignition Control Module.

Ive had the car looked at by an American specialist whilst it had the no-start condition and they said that the king lead between the ICM and the Opti had frayed and was discharging into the water pump?
I am going to have a look tomorrow and see if their repair (binding king lead up with tape) has come off/loosened somewhat.

The roadside recovery I have used tested my car and found that injectors were fine, the plugs were giving a good strong spark and fuel was being delivered.

The battery has plenty of charge too.

I know its shooting in the dark as the issue is random, and the ideal circumstance would be to test components at the time of the fault - any ideas where to start looking? Its been a problem for well over 3 months now, on and off and I cant rely on the car anymore.

On a side note, the battery was flattened recently as I got caught in a mega snowstorm and had to abandon the car overnight - when i returned in the morning to get it recovered, the interior light had been left on (by me ) and had killed the battery.
I understand they cant be fast charged to start the car, having read up on it, but it WAS done that way to get it going. Could this have damaged the ECM? Can I test this?

Thanks in advance for any advice - Im at my wits end now.

 

Let me just lend this as possible Solution, My 93 much like your car would Just cut out and soon after restart or Not. So like you between the Opti's and Loosing Mind was looking for a Cause.

After installing a New Opti I was letting the car run waiting for Coolant Temperature to get the Bubbles out, feeling Pleased that the car was Running I was neatening up some of the Wires & Loom's, when I touched the Harness that contained the ICM & Coil wires the Car cut Out Car Would Not Start. Had someone Crank and I wiggled the Wires car started. What I found was the wires at both the ICM & the Coil were not in Great Shape or the the case of the Pink Wire on my Coil Broken just making contact because the Isolation was holding it there.

 

Thanks for the response S Carter, weirdly enough thats exactly the place I was going to start looking, a lot of other forum posts Ive trawled through seem to suggest the ICM or the Coil as giving up at high load when they are hot, so these parts of their associated wiring could be the point of failure - I will do some digging.

Another other suggestions/procedures are welcome

 

Did a little exploring and took the car out (typically started first time) and have been unable to replicate this fault - I took a few shots in the engine bay that might/might not assist in diagnosis:

1: Heat shot of Ignition Module immediately after driving at operating temperature:

https://drive.google.com/file/d/1RIc...ew?usp=sharing


2: And the engine block behind the Ignition Module, showing difference in heat reading (to the right of the picture is the aluminium heatsink with the ICM attached, ie: towards front of the engine, on the left is the engine block):

https://drive.google.com/file/d/1JFU...ew?usp=sharing

Do these temp readings seem reasonable? I have recently replaced the thermal compound behind the aluminium heatsink as the paste was non-existent before (part of the fault finding process) but this cutting out issue was also present before I did this - could the ICM have been damaged by the heat? Can they fail at higher temperatures or higher loads?


3: Additionally, after poking around the electrical connections near the ICM/Coil, I found this terminal in this condition (the plastic guides/insulators have all broken off), can someone identify this connector for me?:

https://drive.google.com/file/d/1yun...ew?usp=sharing


4. Socket for the connector in the previous photo:

https://drive.google.com/file/d/1CWE...ew?usp=sharing

This socket is mounted on the throttle body/front of the engine block on passengers side above the ICM and Coil.
Could the condition of this have any bearing on the operation of the car at high temperature?

If it helps by the way, I'm in the UK, so access to common parts is more limited here - Id have to import. Thanks in advance.

 

Bump - car died randomly again today while out, I was ready for it with my trusty paperclip, but shorting A-G produced absolutely no codes, neither did A-B. Just C12 (begin code) and - - - indicating nothing on any module.

Took readings from the dash as soon as it happened, just in case but doesn't seem out of the ordinary:

Coolant 89C/192F
Oil 89C/192F
Volt 12.3

One thing I did notice, the Oil Pressure dial jumped slightly to the first notch, then dropped to zero again, with the car off. I'm not sure if this is of any consequence either.

Any ideas where I can look? This is my daily driver and it's getting annoying.

 

Bump

 

Still have this issue, but it has evolved now - car has been in and our of a repair garage about three times since last post.

The car doesn't conk out anymore, but the randomly the speedo drops to zero and the car will act as if there is no power for a couple of seconds: at this time the SES light will illuminate.

Next, the SES light goes out.

Then the speedo will return to the proper position and everything will be fine again?!

Finally, the SES light comes back on and stays on until I turn the car off.

Codes from car:

MODULE 1: H72
MODULE 4: H36
MODULE 9: H72

Can anyone offer any advice? I had the optispark replaced last year with a vented type as mentioned previously, so I don't think this is the issue, despite the error code, unless it was replaced with another crap opti?

The garage I took it to refuse to accept that the opti could be the problem (they are the ones that replaced it for me last year) and they seem to think that there could be an issue with the wiring harness between the opti and the ECM.

This has been going on in one form or another for months now and Im starting to hear "set it on fire in a field somewhere" in my head!

 

Any ideas anyone?

 

I had a number of these problems on my 94, turned out it was from multiple causes. One was the opti, there was oil on the disk and one of the screws and washers holding the rotor in was flying around. I also had a cracked coil, it would become intermittent when warm...if you are testing this with the hood open it might not get to fail temp. The last one for me was the fuel pressure regulator which had a bad diaphragm. It would cut out under spirited driving, but when the diaphragm reset it would seal and work again. I can also see the wires you already suspect causing the problem, the coil to Opti seems to be one that deteriorates inside the connectors. I would not think ICM. I have read about fuel filters internally coming apart and the paper element blocking the exit port as well as the fuel modulator leaking in the tank bleeding pressure. It could also be the ECM, those are known for bad solder joints and can sometimes be temporary corrected with percussive maintenance (hitting it). Good luck!

 

Thanks for the suggestions, lots to go through there! Hopefully it isn't as complex. I've asked the garage I use for major repairs to try another Optispark, to see if that improves things, not a fan of throwing parts at the problem but I can't fault find this one myself.

 

How did you test your injectors ?


My 92 sat for 15 yrs. before I rescued it......(22,433 miles)
I changed the fluids/ filters and flushed the fuel Sys.
New tires and a battery and I was in business .....


That is until the water pump leaked all over the opti.
I changed that and added a vented MSD cap and rotor kit.( still had the factory cap)
And threw in new plugs also.
Ran great until it got hot.....
Then would bog and stumble and even stall out....
Final straw was when it stalled and wouldn't restart.
Thankfully it was at the foot of the drive way ! lol
Turns out that the injectors are wired together in a way that one bad injector can affect all of them !
I had 2 injectors that were bad when they were hot but checked out fine with an ohm meter when cold/warm........


A set of injectors cured all my problems.

 

Come to think of it, that is a possibility.

The first time that the car wouldn't start it was on my driveway, and because I ended up iust scratching my head I had to call out recovery; the engineer they sent tested (among other things) one of the injectors (but only one) and they were 'cold' because the car hadn't run, so that's not a bad shout actually.

Maybe the next thing to check. I have only limited equipment in my own garage, so I'm having a local specialist check it over again, it's very frustrating because it runs beautifully when it actually works, it's just this completely random shut-off that's ruining it all.

Any other advice is very much appreciated, I'll try to report back when I hear from the service station.

 

It sounds like Opti once again . Water pump probably is leaking but you can not see that unless you place on lift or take a picture .My water pump looked fine but destroyed the opti. The insides were completely rusted

 

Thanks Fake,I've instructed the garage to try a new opti. I'm praying it isn't the distributor, but knowing my luck, I am sure it will be. They did fit a vented one, but whether or not it has survived is another question - I will update when I get the car back (hopefully the end of this week, supplier was out of stock of Optis....)