DI Ignition Problem

Bluesman123 · January 28, 2004

I have a problem with a 1997 L31 I'm running, and I'm about out of ideas. I would appreciate any new leads you folks might be able to give me.

A few weeks ago I had the VCM reprogrammed and it runs great...except for every once in a while when it just dies. What it sounds like is that the timing suddenly get WAY off. If I catch it quick enough I can goose it and it sometimes goes away...other times it sounds like it is dieseling or running back through the intake manifold. I don't think it actually COULD be running back through because the 1997 MFI is a dry manifold system...no gas until it gets all the way down to the runners, but that's how it sounds.

If it stops it won't start. I mean it is stone cold dead with no spark (I checked). So I hop out of the cab...grin stupidly at the cars stuck behind me...pop the hood...unplug and replug the connectors going to (1) the coil and (2) the ignition module (they are both on the same bracket with the DI ignition)...drop the hood...wave at the guys still stuck behind me (using all my fingers, not just one like they are)...it starts right up and runs. It seems to mostly do this at idle.

There are no error codes and both the coil and the ignition module check fine. I've cleaned the contacts in both those connectors.

This is not the older HEI system. With this system spark and advance are both controlled by the computer supposedly with inputs from the crank and cam shaft position indicators. I am suspecting problems in the camshaft position indicator even though it doesn't throw a code, but it is terrible to get to. The engine is jammed into a 1991 Sonoma and I may have to get out the cherry picker to reach down behind the distributor and get to it.

So...sounds like the timing sometimes gets off (at least that is the sound...like when you've put a distributor back wrong)...wiggling wires helps, but the wires themselves check out OK...no error codes.

Thanks for your thoughts, folks!!!

Dave

Jeremy WI · January 30, 2004

So does this thing have the 4l60e behind it? What kind of fuel pump setup? 59-66psi fuel pressure? Stock EGR? oxygen sensors? Have you pulled the crankshaft sensor to check for obstructions? Timing is based on the CKT, the injector timing is on CMP. Do you have it set up for OBD2 scanners(diagnostic port)?

Bluesman123 · January 31, 2004

OK...let me see if I can answer.

1. 4L60E behind it

2. Fuel ps good at 63-65

3. Stone stock though-out, including EGR

4. HO2 sensors check OK

5. I have not pulled the Crank Sensor to check for obstructions. What should I look for? Just clean on the end??

Thanks for the info on timing based on the CKT. I thought it was on the CMP.

My OBDII scanner shows no codes, which is confusing to me (of course it doesn't take much ;)

Someone told me the ignition module can cause this problem and throw no codes. Does this make sense to you?

Thanks for your help,

Dave

Jeremy WI · January 31, 2004

Does your scanner give you real time data, particularly camshaft offset or cam retard? Watch this value for a while and see if it doesn't change when the vehicle struggles, I have heard about CMP's coming loose or breaking loose. A lot of codes won't set off the SES unless they happen so long or for so many drive cycles or so on

Bluesman123 · January 31, 2004

Good suggestion. Yeah, I can save data and it does give advance/retard figures. I'll try that this afternoon.

Thanks,

Dave

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Later...

That's what it is! I got the engine from a junkyard and it looks like in moving it around I (or they) bumped the crank-end just enough to damage the CKP and the CKP connector. I've replaced the CKP (easy but expensive) and am now trying to rewire the connector (cheap but hard).

Thanks a million!!