Help! Weird Issue L99/6L80E
 

We have a 2010 Camaro L99 (ECM and harness by Speartech) in my father's '59 Impala.

The issue usually happens when the car has been driven, then allowed to cool off. We get a no start condition where I have spark, but no injector pulse.

Although everything tested good, I replaced the crank and cam sensors and ignition switch "just because."

I don't see anything on datastream that looks out of place.

Thoughts?

Not a clue?

I would think it's an issue with the ecm not getting an rpm signal. Not sure why it only does it after a cool down. Have you looked at the battery voltage when cranking in this condition? If it has any kind of heat soak issue or similar and the volts are dropping to 10 volts or so it will cause what you're seeing. Had this on a friends car and a new battery with more cranking amps fixed it. Thing is the engine seemed to spin over fine, but looking at the voltage when cranking told us what was going on.

But wouldn't I lose spark, too?

If the battery voltage drops under 11 volts or so during cranking all sorts of weird things happen. I'm pretty sure that's what my buddies car was doing, spark but no fuel, but I'm going off memory. It's a simple thing to check though. As a test next time it happens plug in a battery charger and see if it starts. Like I said before his seemed to turn over normally, not slow at all, but it wouldn't fire.

It's something to think about... though the fact that it will then start on the third or fourth try (and start immediately, regardless of how long you had been cranking it previously), seems to demonstrate the battery has more than enough juice.

I do really appreciate the thoughts, though! Like I said, it's a weird one!

This is a real thing as Jody says. When I was a mech we'd have several models that wouldn't start, but engine would turn over like a new battery. We noticed various electrical things wouldn't work right, tilt column or auto folding mirrors would stop mid travel while cranking, all while the engine was turning over just fine. We chased those for a while until deciding to try a new battery and that fixed it. It then became a quick battery diagnostic tool once we were aware.

But would they start up just fine after a combined total of, say, a minute or so of cranking? In fact, it starts instantly after such (on the third or fourth try).

I'm not trying to be a pain, but I don't know of any batteries which gain voltage whilst cranking for such a period.

This might be helpful for others!
 

Spoke with a friend/GM tech.

Apparently GM has trouble with the connection at the ECM. They call the issue "micro abrasions" and they are brought on by vibration.

The fix, according to GM, is to unplug the harness from the ECM and plug it back in, then repeat for a total of 4 or 5 times, then unplug the ECM and apply a thin smear of dielectric compound to the female terminals and plug it all back in.

According to my friend, this little issue causes all manner of odd, intermittent issues.

In our defense, you hadn't given us that info to start with. Hopefully the connector fixes it. Loose connectors, pins not fully locked in the connector, etc. also cause countless random issues.