I've got a very strange problem that's been going on for a while: sometimes, when the engine's warmed up to operating temperature, and especially when I've been running the A/C, it cuts out with no warning. When I try to restart it either does nothing or starts and fires what sounds like one or two cylinders out of eight with a nasty knocking sound. Once it's sat for a few minutes, it restarts and runs normally. Engine temperature is normal (90 degC, since I have the factory 195 degF thermostat, indicated both by the PCM and my dash temperature gauge), there are no trouble codes set, and a datalog I took with FreeScan during one of its episodes showed no erroneous sensor readings; the only thing I noticed was that the right bank O2 sensor wasn't crossing stoichiometric as often as the left, and seemed to be hanging around the rich end of the spectrum more. I have an MSD OptiSpark, just over a year old, no fluid leaks onto the opti, and it looked normal when I took it off about 3 months ago. I've also tried swapping PCMs and it doesn't seem to make any difference. I'm about ready to try voodoo--any ideas?

 

ive actually been having the same exact problem, and it wasn't throwing codes until 2 weeks ago. It was throwing 4 codes about my right O2 sensor. I cleared the codes and it ran fine for the night then started to do it the next morning. I'm going to get under the car sometime this week when i have time im goign to troubleshoot the rest of it and replace any o2 stuff that needs replacing. ill keep you posted. I actually have the same exact year and MSD opti i just put in the middle of this summer with new plugs and wires.

 

Have you considered the possibility of the IC Module overheating?

Courtesy of Shoebox:

http://shbox.com/1/4th_gen_tech2.html#ICM_cooling

Factory t'stat in an LT1 is 180*F.

 

Here is my post last week. Does anyone know if there fan is running when it shouldn't?
I had a problem 6 months ago where the car would shut off after running for 3 minutes when i started it after sitting all day. I was able to restart soon after and never had any issues after that. It did it 4-5 times in a three week period and has been fine for a 1/2 year. Today i had the hood open to see if i could hear my exhaust leak from the engine compartment. It ran for 2 minutes, i then noticed the fan come on and run for a minute. Thats weird since the car was sitting for 24 hours and its 40 degrees outside and the car was just started. The fan ran about a minute. I heard a click from under the hood and the fan turned off. One to two seconds later the car died like the key was turned off. I waited a few seconds and turned the key and it started on the second try which isn't unusual. It runs just like normal so whats going on? I have Freescan if there is something i should be looking for. I was thinking the computer believes the car is running hot even though its still just warming up. I didn't pay much attention so the fan could have been off and on. I kept hearing the clicking noise. The SC and motor and headers makes it hard to hear well.

 

Why are you highjacking his thread?

 

i havent noticed that the icm could overheat like that and shut off. But in my case when the car has randomly shut off, it has mostly happened when car was at idle or close to idle while doing some normal driving on a normal not so hot day. Since it doesn't prefer to do it on a hotter day while im sitting in traffic, when the engine would be a lot hotter i dont really think that could be the cause, but hey for a free mod why not.

 

Possible--I'll try doing what Shoebox outlined. It seemed to me that it would be more likely to overheat due to heat soak after the motor stops (no airflow from the radiator fan) than overheat while the motor is running and cause it to die. Given the temperature measurement results he posted I guess heat conduction from the cylinder head is having a much larger effect than convection to the air.

Odd about the thermostat...either I remembered wrong, or I do have the wrong thermostat. In that case, serves me right for trusting a cross-reference manual at Pep Boys. I'll pop the water neck off and take a look tomorrow.

 

I guess I remembered wrong, the thermostat is indeed a 180 degF unit. One thing, when I tested it in a pot of boiling water the upper plate opened normally, but I didn't see any movement of the bottom plate (the one that a regular SBC thermostat doesn't have). Is it supposed to move in response to heat or only react to pressure opposing its spring?

EDIT: Never mind, looks like the wax pellet mechanism isn't set up to cause any movement of the bottom plate. What does it do, anyway? I'm not very clear on how the LT1 water pump's passages are set up.

 

http://shbox.com/1/tstat.jpg