First 4th gen HELP

The first two places folks tend to start with a "no start" condition is the basics: Fuel and Spark.

When the no-start condition is present, is there spark? Yes? Check fuel system / injectors. No? Diagnose Ignition System. You could even spray some starter fluid into the intake while the engine is cranking to see if it lights off. If so, you have a fuel delivery issue. If not, check spark.

When I had a strong "raw" fuel smell from my T/A, it took a while to track down, but I eventually found it. ...under the driver side door the steel fuel lines connect to the Nylon (or whatever they are) fuel lines that run into the engine compartment. 2 are a proper mechanically coupled connection, the other one just uses a rubber fuel line as the coupler. The rubber perished on mine and allowed the fuel vapors to escape to atmosphere. I used 2" of scrap fuel line from my lawn mower in it's place. Problem solved.

The fuel pressure test and observations are important. Engine-off/key-on, it may point to a weak pump, leaking injector(s), etc. Things could also be fine on startup, but at the 20 minute mark you notice fuel pressure fall to ZERO as the engine stalls, and then the pump will not prime again until time passes; and it cools down. ...indicating a "thermal" failure with the pump itself or maybe even the relay.

That said, you might have TWO unrelated problems.

Maybe it's not the fuel system at all, and the fuel smell isn't even related. Maybe it is the fuel system and the smell isn't related...

The ICM has a hand in spark control and is prone to Thermal Failure (after a reasonable service life) due to it's location. It's like a CPU and has a heatsink. Being mounted on the head, it gets really hot, and electronics can only take so much thermal cycling and/or heat before they let go. The thermal paste dries up with age and loses effectiveness...

A failing ICM tends to make the engine cough and spit a bit before it dies in my experience, and once it start this, it doesn't take long before it's just dead 24/7. Two or three ignition cycles at most after the first failure.

Call your local AutoZones and ask if they can bench-test an ICM, one should have the test equipment on hand. They will probably have to heat it to induce failure if it's thermal related.

Or, you can use the Factory Service Manual (or the directions at SHBOX) to test the ICM and OptiSpark in place as soon as the stall/no start condition appears.

The OptiSpark is the Alpha and the Omega of the ignition system; and it can fail from thermals too.

Actually, it seems like only AFTERMARKET OptiSparks fail from thermals. I dealt with this over 10 years and 7 aftermarket Opti's. When my engine got to 160F, it cut off. Dead. No spark until it cooled way down.

There are supposed to be 2 codes for a failed/failing OptiSpark. One code says the high-resolution pulses are missing, this is no big deal, the engine runs fine but can not be timed "as precisely". ...you'd never notice there was an issue if the Check Engine light wasn't lit. The other code, for the low resolution pulse, is major. Without this pulse, the PCM has no idea where any of the pistons are in their stroke. As such, it can't time a spark properly, and could be so far out of time it could damage the engine. ...so the PCM shuts everything down.

Thing is, the aftermarket OptiSparks tend to fail without ever setting a code. They drop their ground apparently, and when that happens, the OptiSpark just "disappears" from the system. No pulses, No codes, No Spark, NOTHING!.

Here is SHBOX's procedure for testing the OptiSpark, and for grins, the coil too:




My first ignition failure was the ICM, but a mechanic said "it's always the OptiSpark when that happens". Opti's were still over $1,200 at GM plus labor so I bought a $500 unit from AutoZone and installed it myself. While the RTV I used was curing, I tore apart the OEM OptiSpark destroying it in the process. Next morning, went to start the engine. NO START! ...this is when I learned of the ICM. Replaced it for $90 and a helluva lot less labor and the engine fired right up. 6 months later, the aftermarket OptiSpark failed. ...and each subsequent replacement failed within 10,000 miles. Always different brands, never a code. Not even once.

Bought a remanufactured OEM OptiSpark almost 10 years ago, never had an issue since. I have replaced the ICM again though. I'd like to think that if I had never destroyed the original OptiSpark, I'd still have it, and would have just replaced the cap and rotor 3 times now.

Point is, do the diagnostics. Even if the fuel smell is actual flooding, you need to know why. Excessive fuel pressure, leaking injector, etc.

Guessing is expensive, and in today's times, the replacement parts likely aren't as good as the near 30 year old parts you'll replace needlessly as you play parts-roulette. As the old saying goes, "If it ain't broke...".

Let us know where you're starting from. Lack of fuel or lack of spark. When you know that, you're on your way to resolution.


If you don't have a multi-meter, now is the time to buy one. Anything around $10 is just fine.

...and if it hasn't been offered yet, member GaryDoug has scanned most of the Factory Service Manuals and provided them to us. It's a great asset. You will need Adobe Reader to search through the document as it's several thousand pages long; and worth it's printed weight in Gold.

The '96 Manual is there. Grab it!

P0372 is for the high-resolution pulse. In post #10 I explained all of this, including the fact that a missing high-resolution pulse will not shutdown the engine or prevent it from starting. The high-resolution pulse allows for precise timing accuracy. Drop the high-resolution pulse and worst case the engine runs a little rough, maybe even an occasional little backfire, but most people don't even notice a difference. P0372 while a "problem" is nothing serious, or anything that will leave you stranded. Basically, your timing is now as accurate (or inaccurate) as an old-fashioned distributor. Certainly good enough to get the job done, but lacking the precision timing it is capable of.

Failure of the LOW-RESOLUTION pulse will result in the PCM shutting everything down right there on the spot the millisecond the signal drops out. Again, post #10.

Download the Factory Service manual I linked an follow the Diagnostic Flow Chart for your P0372. Section 6E, page 654

If you look the code up online, you will see reference to Crank Position Sensors, Hall Effect Sensors, etc... You don't have any of that. The OptiSpark does the job of these "later" modern sensors. Basically, the code you have is more applicable to and intended for the LS engines, but the diagnostics in the Factory (year specific) Service Manual are correct for you and your situation. ...the things the Internet says to do for this code are NOT intended for you, they are for LS owners.

Factory Service Manual, DOWNLOAD IT and USE IT!
Multi-Meter, MANDATORY equipment!

I suspect you have an aftermarket OptiSpark and it is done; but I hesitate to say that because it sounds like you will just replace the OptiSpark without doing any diagnostics. I had suggested the ICM could be an issue, but also stated how it tends to fail, which is not the failure mode you experience; but you replaced the ICM without even having the free test from AutoZone performed on it. You could have simply Ohm'd out the coil during a failure and seen it was fine/within specification too; but you replaced it anyway.

All wasted money that could have been applied to source of the actual problem.

IF you decide to continue just throwing money at this until it's resolved, get an OptiSpark from Petris Enterprises. NOBODY ELSE. It might solve your problem, it might not; but at least you'll know you have a good OptiSpark you can rely on.

Consider yourself warned... ONLY PETRIS ENTERPRISES. Every OptiSpark on the market now with the exception of Pertis Enterprises comes from China. $500 OptiSparks are no more reliable than $20 OptiSparks these days. Petris builds theirs to a higher specification than even the Factory unit was built to.

Buy anything other than Petris and you'll be here again within 12,000 miles. I promise. There was a time when remanufactured-OEM OptiSparks were the best choice, but lately it appears that some counterfeits and aftermarket units have entered the reman-oem supply chain, so even if you can locate one, avoid it; it's a huge gamble. Petris Enterprises OptiSpark or an LS ignition system conversion are our/your only options at this point. Petris isn't cheap compared to the rest of the market, but they are reliable. Converting to LS style ignition costs about $2k all in, but, that completely eliminates the OptiSpark and also gives you the benefit of on-the-fly tuning with a laptop just like modern cars.

Of course, feel free to disregard everything I have said and buy an OptiSpark from Advance, AutoZone, NAPA, or O'reilly. Just make sure it has a lifetime warranty. You'll need it. You can also look to eBay and Amazon. They have the same Chinese junk at even lower prices. Not sure about the warranty, but I doubt they are "lifetime".

All the major retailers I mention do honor the lifetime warranty, no questions asked, so there's that; and that's worth a little premium.

Good Luck!