91 z28.

seems to start just fine when cold. randomly does not start when hot.

have replaced ignition module and coil with no effect.

timing light shows spark to all 8 cylinders when it won't fire.

once it fires, it runs and drives perfectly for as long as I want to drive.

I can shut the engine down for two seconds and try to re-start and it won't fire.

when being cranked and it's not starting, there is no backfiring or popping. every once in a while it will stumble slightly, sometimes the stumble will lead to the engine starting and sometimes not. But for the most part, it sounds like an engine being cranked without spark.

crank speed is good.
VATS is working correctly
tried spare ECM
no RPM is indicated on a scantool when cranking but not starting, even when spark is present (based on timing light)

Questions:

Is it possible to have enough spark to trigger a timing light, but NOT enough to fire the cylinder?

I'm aware that pickup coils tend to fail when hot, however would I still have accurate timing as indicated by the timing light? with the est lead disconnected from the distributor, cranking shows perfect spark at 6* BTDC (but no RPM indicated on the computer).

As soon as it fires, RPM is indicated correctly on scantool/ALDL and everything is perfect until you shut it down again.


any ideas??

 

My car exhibits the exact same characteristics you have described. One temporary fix to try that worked for me, and I'd be curious to see if it works for you, is that in the no-start condition, I can ALWAYS get it to start with a RUNNING car jumped to the battery. It won't work on a non-running car. I can turn the engine off an on as much as I want until my hand hurts from turning the key. I replaced the coil, module, even distributor (new pick-up coil) to no avail. When I replaced the cap/rotor and plugs/wires, it has so far only not started just one time in the last couple of moths, has become noticably better. I am going to follow this thread as I always have to have a friend in another car come with me when I get gas. (So far not been needed)

The advice I have received and checked:
Grounds, especially the main one between the block and firewall (which I hope is good, cause I don't know how to even get at it w/o yanking the motor, it's in a finite space, LOL).
The battery cables. Connections, "green goo", etc

I'm also assuming the fuel pressure checked out, in the no-start condition as mine did. 47 psi while cranking. If it didn't, that's a whole new can of worms. Also one of my injectors is ohming under 10 ohms, which I'll replace this winter when I get new SLP runners). The guys here say that one shouldn't be enough to cause a no-start, but any more than one could cause that, especially when warm. So there are lots of things to check out.

I have the Service Manual for my L98 and have conducted all the tests for crank, no-start that is published. Everything has checked out so far. I'd like to see where you are compared to me.

I also had the battery checked out, and was good. So maybe we can defeat this deamon together. The only caveat to all this is that I think your car runs a MAP sensor. My 89 is MAF, so that could be a problem too, but the symptoms are just too similar to mine to ignore. Try the jumping it to a running car, if that works, I know we're on the same page!

Good luck!

 

i got an 82 z28 and i'm in the same boat. the car starts with to hesitation when cold, soon as it warms up and i leave it off for ten or so minutes then go to start it, it seems i get one stubling chance when it fires to keep it running with a quick rev up, then runs fine, but if it stumbles to a stop then it wont start, i checked the plugs and it has those dual fire plugs, during the no start time it gets no spark, i've been searching for a solution but cant find one, any one have any progress on this yet?? i got to get this figured out soon, thanks guys

Joel