intermittent code 42

[csft66]

New wires and coil, three different modules, two different caps and rotors, new Delphi distributor, tried another ECU.
Still get intermittent code 42.
I do lose power so the timing is going to base of 0 degrees. No other running problems or codes.
Wires from computer to module are good.
Mechanic friend printed a GM trouble shooting guide and it says replace the prom.
Has anyone replaced a prom and it fixed it?
I found a used one (ANLW) on eBay for $30.

 

[csft66]

It looks like my new GM coil was the problem, put the original one back on and so far no code 42.
I had read that a bad coil could kill the module, so I guess the coil was sending stray voltage to the module and causing the issue.
That's what I get for changing everything at once.

 

[csft66]

The problem came back so I'm going to use a regular mechanical/vacuum distributor with just a pickup and wire it to the TBI module remotely (leaving the timing wire disconnected) that way the computer will get a proper reference signal for the fuel injection.

 

[YakkoWarner]

Code 42 is always for me the worst thing to see - it doesn't really have any single discrenable thing that can cause it, troubleshooting it is always a hit and miss thing because so many things can trigger it, and just when you think you might have solved it, boom its back again.

It never occured to me to just juse a normal mech/vac distributer and let the ECU see the trigger for the injector pulse. Only downside I can see to that is that the "normal" system always fires the injectors when the engine is at 0 or 4 degrees relative to TDC (dependant on the engine) whereas using a traditional mech/vac advance will have the ECU triggering the injectors in sync with the advance curve because the pickup is moving relative to the advance mechanism.

 

[gmbellew]

I would spray around the distributor cap and external coil with soapy water mist to see if you hear any cracking or popping from electricity. Also make sure the plug wires are not touching any of the ECM or sensor harnesses. Most code 42s I have had have been traced back to electrical interference or leakage of some sort from plug wires or the wire from the cap to coil or a cracked cap. Especially when it is random. If it happens when you hit a bump, I’d look for an intermittent open or short to ground in the EST wire or the bypass wire from the ICM to the computer.

Code 42 will only hit with extra voltage, short to ground, or an open in the EST wire or bypass circuits

 

[Ricko1966]

If you use salt water and spray at night ignition leaks are easy to see.

 

[gmbellew]

Code 42 is always for me the worst thing to see - it doesn't really have any single discrenable thing that can cause it, troubleshooting it is always a hit and miss thing because so many things can trigger it, and just when you think you might have solved it, boom its back again.

It never occured to me to just juse a normal mech/vac distributer and let the ECU see the trigger for the injector pulse. Only downside I can see to that is that the "normal" system always fires the injectors when the engine is at 0 or 4 degrees relative to TDC (dependant on the engine) whereas using a traditional mech/vac advance will have the ECU triggering the injectors in sync with the advance curve because the pickup is moving relative to the advance mechanism.

Pretty sure the ECM needs to see the RPM data and the high reference signal from the ICM for the fueling to work? Not sure you can mix the mechanical distributor and TBI fueling. And even if you could, it would always have code 42 bc of the lack of information

 

[YakkoWarner]

Pretty sure the ECM needs to see the RPM data and the high reference signal from the ICM for the fueling to work? Not sure you can mix the mechanical distributor and TBI fueling. And even if you could, it would always have code 42 bc of the lack of information

Its not an experiment I'm likely to try anytime soon, because I expect it could have some odd consequences. It is an interesting idea though, and if it does work it could be a useful "limp this thing home instead of calling a tow" sort of thing. I havn't had to chase the 42 in a while, probably because the thing wouldn't stay running long enough for it to happen. I'm sure it will come back...and be expensive when it does.

 

[csft66]

It started up, idles perfectly but stumbles while driving.
My understanding is that the module can alter the timing somewhat on its own for a limp mode and I assume that is what is causing the problem.
I can try a 4 pin module but was afraid to feed a 12v tach signal to the computer for reference.
Some videos on YouTube show it done like that, some say it works fine but others say it causes rich fueling.

 

[bucket]

In my experience, stumbling while driving has never been the fault of an ignition module. Not in the small cap years anyway. Usually it's a fueling issue or basic tune-up issue like cap/rotor, etc.

 

[JamesSam]

Watching this one.

It seems there is always more to learn about the: TBI/ECM/ICM/OBD1.

I keep my paper clip jumper in the bench seat console.