I haven't started the diagnostic process on the 96 yet, but figured I'd get some opinions before I get started on Saturday.

The car randomly dies and won't start for about 5-10 minutes then fires back up like nothing happened. No signs it's going to die and it runs VERY strong...when it's running. The only thing that's consistent is that it only dies under deceleration and always when the car is warm. Have never had it die under throttle. I've searched a ton of threads, with only two having how they fixed the issue. One was the IAC, the other their OPTI. The opti is a MSD unit, about 4 years old, and the coil is a MSD unit less than two years old. I had this issue twice over the past 6 weeks, now it's happening every time I drive it (well all three times I've driven it the past week). Just changed the alternator before the last three times I've driven it, if that matters. Thoughts?

 

Classic symptoms of an ICM failure.

Unless there are codes present, I doubt it's the MSD opti. For a hard shutdown, it would mean an issue with the optical sensor or the wiring to the sensor. In both cases you would have a code or codes stored since the PCM would perceive it as a loss of the low res signal (the engine will still run....albeit rough...with only the low res signal. It will not run with only the high res signal).

Slight possibility it's the coil due to heat...but the MSD LTx coils are pretty good.

I doubt it's the IAC. The car would start right back up, even if it took a little gas pedal finesse, if it was just an issue of the IAC not being positioned properly. Still wouldn't hurt to pull the tb and clean the tb and the IAC valve.

 

Well, you're making me feel a little better about the whole ordeal. And I found a couple of places that have them in stock. Thanks!

That's the dude that needs the heat sink lube isn't it?

 

Yes. The new ICM will come with the dielectric grease that goes on the back.

 

With a lifetime warranty at autozone and it being around the corner from my house, I'm going to start with the ICM and see how she goes...then I'll bust out the FSM that I can never seem to find anything in if need be. Glad I'm not a mechanic for a living, diagnosing electrical problems is the worst.

 

They do stink, and don't pay well. However, I really enjoy finding them, I get a LOT of satisfaction out of finding the issue and fixing it. Then the car runs right. That is fun, too bad it doesn't pay worth a crap.

Suggestion:
My 93 did something very similar last winter. I have a scan tool, it would start to miss, and then I would shut it down and it would not restart until it would cool down. Using the scan tool, I observed the low and high resolution signals, while I had a crank but no start. I had the signals telling me the Opti-Spark was ok. I had spark but it was not was even as it should be. Turned out my ICM was overheating. I pulled it and discovered all the heat sink grease was gone. I got new stuff and put a ton on it and it works perfect now.

The scan tool might help you narrow this down if you have access to one.

 

I guess I better pick up some hink sink grease at radio shack tomorrow. Hope I have good weather on Saturday! Oh and no scan tool. Flying blind like normal. Hopefully not as blind now. Thanks guys.

 

Ended up being the IAC. My wife even noticed how much better she sounded at idle. Not that it sounded bad before, but she just has a better purr.

 

Surprising. How did you troubleshoot it vs the ICM? Yours is the first IAC I've seen that exhibited symptoms like you described.

 

An IAC does not change with heat, it always acts the same when it's bad. Good luck and hope it doesn't leave you stranded. Need to start working on mine tomorrow.

 

Yeah, I might not be out of the wood work yet. Ran great the last two days. Today it was hard to start at lunch. $&%*

Looks like I might be dropping her off at the shop wednesday, as I don't have any more time screwing with it.

 

"Hard to start" when hot is typically a fuel pressure regulator (FPR) or fuel pump check valve.

Takes 2 seconds to check the FPR...pull the vacuum line off and verify it's dry. If there's fuel in it, the FPR is bad

Takes another minute to check the check valve...put a fuel pressure gauge on the rail. Turn the key to "run" and then to off. Fuel pressure should be ~40-43psi and hold. If it bleeds down and the FPR was good, then it's the check valve or the injectors.

Suddenly dying and then restarting after it cools off is typically the ICM.

 

Yeah I changed the ICM and IAC. Changing the ICM didn't do anything, but after changing the IAC, she ran great. Disconnected te battery to reset the ECM and ran just fine on the way home. Let it idol in the driveway and she surely would have died before Saturdays parts swap. Hoping it just needed a fresh reboot but will check the regulator. I checked fuel pressure and it's good (I think it was 46 though). Haven't checked for bleed down though.

 

No fuel in the regulator vacuum line. Went to check fuel pressure and the needle broke on the gauge! Par for the course. Will drive it the rest of the week and she how she goes. Will let y'all know. She fired right up when I went to test the fuel pressure. Once she got hot before, she wouldn't do that. Seems better, but like she's still got a slight cough. Maybe she needs a ride on the highway. She rarely sees it any more.

Have an autocross on Sunday, I really hope I get her straightened out by then!

 

Well, after three days of driving around town and one blazing hot autocross yesterday, she looks like she's back to normal. Again, the IAC was the culprit.

 

And it's back... Stalled on the way home from work Friday, but fired right back up and was fine getting home. Turned it off and fired it right back up. Then on Sunday it wouldn't start at all (still cranks, no fire). Will have more time this weekend to dig deeper. May get flat bedded to my mechanic next Monday.

If it turns out to be the opti...yet again (this would be the third replacement), I'll be adding the delteq to the new opti!

 

Check the wiring harness that runs from the passenger side of the intake to the opti. On that harness, check for corrosion on the pins or the locking tab being broken off.

When it cranks but will not start, is there spark?

Typically if it's an opti issue, there will be codes when it dies without warning (and eventually restarts) since that means the low res signal was lost (the engine can run without the high resolution signal...albeit rough). If there was oil or debris in the opti, a worn cap or rotor, or a grenaded rotor it wouldn't restart and run well or likely at all.

Did you use a GM ICM or an autoparts store variety? If the latter, those can be hit or miss. Did you put thermal grease on the backside of it?

FWIW, I believe Delteq is out of business which means installing a used unit at your own risk.

What does it do when it dies? Just quit or briefly run rough and then die. If the latter, check your fuel pressure. Could be a fuel supply issue.

 

The car has had wiring issues before (Rats made home in the engine bay before me), but I thought it had an entire new engine harness put in. But knowing the stealership that did the install, they probably didn't. So it's an area I've got to spend time looking through.

Spark is the issue. She's got plenty of fuel, at the fuel rail anyway. And yep, no codes. All the seals on the front of the engine are less than a year old, so there's not even a drop of oil. I did use an autoparts store ICM. I'm going to replace the new one tonight with the old GM and take the old one in to have it tested (I've only found one shop in the area to test these)...and see if it's fires up with the old GM ICM. And yes, I used thermal grease on the backside. Hmm, I guess I better get some more before putting the old one back on just in case it works.

Damn, didn't know Delteq went belly up. I still don't think the opti is the issue as I'm not getting any codes, but I'm certainly not ruling it out yet either. When it dies there is not a hint of it coming. No roughness, no missing, just quits.

The strangest part to me, is the fact it'll run great for a couple of weeks, then I won't be able to start it for days. And without doing a thing, will just start up one day for me and run again fine for a few days. I've even beat the hell out of it at the autocross a few weeks ago and she performed flawlessly on a 98 degree day some 25 miles away from home. Even drove back in a pouring rain and it didn't mind it in the least.

 

I went through this whole ordeal. Changed the opti and would not run, changed the icm and coil, would run till it got hot and die. Took out the autozone crap [ coil & icm] put in GM parts and have not had a problem. Put in the autozone coil back in for a test and no start, put in GM coil and started right up. No more important parts from autozone anymore. Also put in a GM waterpump, you can see the difference just by looking at the quality of the impeller. Bty found it much easier to put the opti on with the impeller cover off the pump. You just gently move the impeller back and forth to get the shaft in.

 

Yeah, I've been pondering that exact issue.