Its an RS with an L05/T5. The car doesn't throw any codes. I've checked the fuel pressure, and Im obviously getting spark, alternator is charging at 14.5. New plugs,wires,cap,coil. This started suddenly. Worst part is, some days it does it, other days it purrs like a kitten. It does it across the rpm range. I'm thinking spark control or esc module, but I don't want to start throwing parts at it.

 

ESC or spark control module should not do that-check the reluctor and pickup coil in the distributor for heavy rust buildup-that is a common "small cap"HEI problem caused by ozone buildup under the cap.

 

Thanks for the reply! They do have a little bit of surface rust, but nothing I would consider heavy or out of place for a 30 year old car. How much rust is too much? If I was to replace parts I was going to start with the dizzy.

 

Also, its intermittent...which leads me to think its electrical and not mechanical.

 

the small cap distributors are a known failure problem with age.

the dist can be pulled, cleaned and a new pick up coil installed. the shaft should also be checked for run-out (play).

by the time you invest in those parts and time, you're near the cost of a replacement. a replacement should also come with a new control module.

if the one now show's rust under the cap and is of unknown age I'd likely swap it out with new.

 

If the ICM is the original, I'd suspect it first. You can find troubleshooting info here: https://easyautodiagnostics.com/gm/4...ed-icm-tests-1

 

Installed a new dizzy today. Same issue.

Something I've noticed is that when I disconnect the wire for the timing advance, nothing changes. When its connected it times the same, and doesn't pull any advance.

Does it only adjust the timing in open loop? If so it may be that I didn't give it enough run time.

 

pretty "soft" misfire-almost more surging...could be a large variety of causes:wiring,ECM,possibly even fuel pump going bad.Pull out and reinsert all the fuses-the tinned blades of the fuses will develop an oxide where they meet the brass contacts of the fuse panel after they have been in place for 20 or more years...See what the injector spray looks like when car is doing this.

 

I did pull all the fuses and check them. I have good consistent fuel pressure. The spray is good until it bogs and its starts to die off with everything else.

 

A few things I've found through the years with chevy TBI that will NOT throw a code. As stated a relucter ring in the dizzy. I've found them cracked before. You can have good fuel pressure coming to the throttle body but low IN the injectors from a broke FPR spring.....seen it many times. But, one of the first things I suspect with a surging/rough idle on a TBI system is the temp sensor in the front of the intake (black and yellow wires). That little thing will cause all kinds of crazy issues without throwing a code. Last, check out the IAC motor, if they get real carboned up they start sticking. Maybe just cleaning it would help too.

 

A few days ago I remember my 90 had a no codes issue. 20 years ago it was a coolant temp sensor issue. So i picked up a cheap sensor the other day and installed it. I ran the car for about an hour with zero issue. I'm fairly confident that was it.

 

Great! I actually used to work on cars a lot 15-20 years ago. BEFORE OBD2. I still have a local shop owner that comes by to pick my brain when he gets an older vehicle in giving fits. The new guys just don't know where to look if the computer won't tell them. More recently it's been ATV's and Seadoos that dealers can't seem to find the issue.

 

If it was electrical or electronic most likely the ECM would throw a fault code. Have you checked or tested your fuel injectors?

 

the temp sensor can be ruled out by jumping the leads to the sensor. this tricks the ecm into thinking the coolant is full hot. don't see this causing a mis-fire though.

check fuel pressure when it misbehaves? throw a gauge on it and tape it to the windshield?

the timing should advance even with the esc disconnected from the dist. there is a 'step' advance built into the ICM. you should be able to see this with a timing light. at around 1200-1500 rpm the timing will jump up maybe 15 degrees or so above base. either way, incorrect timing adjustment shouldn't cause a mis-fire, maybe a mild surge.

ohm each of your plug wires out? double check the condition of your cap and rotor?