I have a 91z with a 305tpi that starts and runs great, but after a while it starts to miss and then shuts off. It will not start again until it cools for at least 10 hours. Normally over night. I have tested compression and computer, both are good. I replaced plugs,wires,distributor,cap,rotor,tps,iac,map and tested fuel pump and regulator. Computer not sending any codes. Seems to run hot, but the fan is kicking on and it does not over heat.
Any ideas?????

 

ignition control module?

 

Replaced it with the distributor.

 

Also replaced the injectors.

 

Yep , I got an Idea ......

You mention replacing the distributor , and the cap . Hopefully , this includes both the Ignition module and ignition coil as well ?

If so , another possibility could be the ignition switch . If the contacts of the switch that feed the Ign. coil were dirty , burnt , and making a marginal connection , this could very well cause this type of problem . When cold , the switch passes enough current for somewhat normal running . As the poorly connecting switch heats up from the current passing through it , the resistance goes up till there's finally not enough power passing through it to properly fire the coil . once it quits , it cools enough to allow current to flow again , and it repeats ........

The way I'd check for this would be to make a temporary connection to the HEI's big pink wire for a voltmeter , and then run it till it quits . No or low volts on the meter when the engine finally quits = bad switch .

Course , if not , It could be a failure in the ECM that only shows up when hot . No codes isn't a guarentee of a perfectly functioning ECM , , In fact , one of the failure modes IS to be failed with no codes displayed .....

 

I had the same thought, so I replaced the entire steering column. Maybe I need to give more details. When it dies, it seems like it is starving for fuel or spark. I can sometimes get it to startup again, but it runs really rough and then dies and will not start again for a while normally overnight. I did replace the computer and chip, twice. I have another 91 with a 305 and I took the computer from it and put it in with no change. I put the computer from the problem car in the good running car with no issues.

 

Dude , are you DAMN sure it aint fuel delivery ?????

It seems like youve replaced every electrical component that could cause this , and , unless you've got a wiring harness connector somewhere with a high resistance connection , what else could it possibly be ?

You need to run it till it won't run , then do both the voltmeter check at the ign. coil , and the noid light check of the injectors . Any bad connections will show up as a failure of either of those two tests . If the coil never looses 12v , , and the noid light keeps indicating a good injector pulse , maybe it ain't electrical .... Have you pulled a plug wire , put a test sparkplug on it and held it against the block while cranking to see if it's in fact still got a nice fat spark even after it quits ?

Did you do a fuel pressure test AFTER it quit ? If the heat sheild between the fuel tank and muffler wern't put back after an in tank pump replacement , the fuel could literally be boiling in the tank , which as a well documented problem of our cars that you can search out and read about here at Third Gen ... Have you tried removing the gas cap after it quits , to see if a clogged tank vent is building up a massave vacuum in the tank not allowing the pump to push fuel ?

Your problem has a cause . And to keep the engine from running , it's gotta be a pretty blatant cause , ya gotta narrow it down with testing to either the fuel or spark BEFORE ya $pend any more $$$ on parts !

( PS , a mostly , but not completely , clogged Catalytic converter can do this also , so if your really , Really , REALLY sure of your spark and fuel , this could be yet another thing to look into )

Truly best of luck with it !

 

Yes,I have tested the fuel pump. I have 40 lbs when it is running correctly and it does not change when the problems start. I also crimped the return line to see if the needle would peg and it does.
Heat has to be the cause, I am just not sure if it is electrical or mechanical.

All injectors ohm out correctly both when running right and when the issue starts. Same with spark.
I think you are correct about the converter! I think it is closing up when it gets hot and causing too much back pressure. Does this sound possible???

 

This is a duel cat setup.

 

My Friend , It's more than possible , , in fact , darned likely !

I seen partially (mostly) clogged cats do this exact same thing on a couple of different cars !

 

The only thing that gives me pause, there does not seem to be a change in the volume coming out of the pipes when this occures. You would think that there would be a decrease in vapor.

 

I would check the cats.