Weird Engine/Electrical Problem

I hate these weird problems. I've had this issue before a few years ago and thought I fixed it, but it's back.

What happens is this (It's an OBD1 1993 Firebird LT1):

Driving along on freeway, always at least 10min for the first occurrence, and the engine will suddenly act like I've lifted off the throttle, except pushing the throttle does nothing. The revs will steadily drop until the engine stalls out. I get a check engine light, but it never stores a code. Once I stop and turn the car off, I can restart it just fine. However this sudden loss of power will happen again in 5 min or so of driving. Only once the car has cooled off will I get another solid 10min of freeway driving out of it.

In the past, I thought I fixed it by replacing the optispark (now with a GM Perf Parts body and MSD vented cap), replaced the wiring from opti to the computer, replaced the TPS, and had a computer tune to turn the fans on much sooner. The car has run fine for 2 years now in even the hottest weather with no problems until today. :(

It's not like the engine suddenly dies, it is literally like I've lifted off the throttle and let it coast. Since I am already going 70, I can coast for quite some time and the engine will run until the revs are so low it can't sustain anymore.


Help!

have you checked the MAF and TPS sensors yet?

I have previously replaced the TPS sensor (back when it first happened two years ago). I have not replaced the MAF sensor, but two years ago, when I disconnected the MAF with the engine running, it doesn't do what happens to me on the freeway. Also, the engine still runs fine after restart. I shall check them again this weekend just to be sure though. I should add though, that I have tried letting the car sit in the drive way idling for 30min and nothing ever happened. This issue has only happened on the freeway at 70mph or higher.

Check all your grounds. I know it seems like a ground problem would be sustained but they can be fickle sometimes. I've had a ton of "come and go" problems like this that are caused by faulty grounds. it's just always a good place to start.

I would want to know why it's dying......is it losing fuel or spark? The first thing I would think is that you're opti is bad again if it fixed it the first time for 2 years. I would want to verify though that it was spark that's causing your problem and not a fueling issue.

My LT1 friend has "Always check the OPTI 1st" on his tool box........:lol:

I am going to say the ICM.

It sits on a bracket with the coil on the head and they heat soak causing it to fail.

Get some longer bolts and a few nuts and washers and space the bracket away from the head.

It's too late for this ICM but it will save the next one.

Got an update: It has gotten much worse since the first post. Stalled out on me 6 times on the way home from work :( However, it's almost a good thing because it's now consistent. The engine just shuts off when it reaches 3k rpms. Even in park. When it stalls out, it kinda stumbles and I smell gas. I'm thinking bad fuel regulator. I'll be checking it this weekend and will post back with my findings.

Transamtom: when this first happened 2 years ago, I did replace the ICM and did mount it with extra washers.

SStrokeME00: I'll check the grounds too. I know my engine ground strap is looking pretty weak lately.....

Also check your MAF connector,I have seen bent tabs cause intermittent problems.

Check opti and fuel. Has to be something in that area. If both are ok, then only other thing I would guess is maybe a ECM going out... I hate that opti, what causes me to get rid of my 93 and get an 02 with an LS1. Good luck chasing the issue.

OK, another update: TransamTom was correct (thank you), my ICM did fail. I was mistaken about replacing it, I still had the original GM unit. Two years ago, I must have removed it to put some fresh grease on it and did the washer mod, but never bought a new one. I tested just about everything according to my service manual and it pointed me to an "open ground circuit" inside the ICM. I bought a lifetime warranty Duralast Performance ICM and car started right up and never stalled at any RPM :)

However, I am now having some other, possibly, minor issues. I'm going to ask one here, but I'll make a new post about all three in a minute.

Since I replaced my ICM, my scan tool reports a different RPM then my tach does. For example, my tach reports 2000rpms but my scan tool says 1825. It didn't use to do this before....do you think it's related to the ICM? The service manual doesn't cover this problem....