2003 Pontiac Grand Prix GTP

Hi all,

I have a Grand Prix that I have owned since it was new. It now has roughly 180,000 miles. About 8 months ago my son was driving it (he is in college and went to see friends). As he was driving it home it overheated, we ended up towing it home and it has sat in my driveway since. I dont have a lot of time to work on it, but, have spent a few weekends looking at it and going over what might be the problem, I have not been able to really diagnose the issue so, am looking for some help here.

In a nutshell, the vehicle cranks but does not start. When it cranks it will sputter with a backfire every once in a while, but, only once about every 5 turns of the ending.
There are no codes from the engine, the battery went dead and the scanner does not report any codes at all.

I have checked the basics, Fuel pressure is in spec, compression is not perfect, 5 cyl. are close and good, #1 is lower but still high enough, I suspect I have a valve in there not seating correctly. I dont want to work on that until I can get the vehicle running again, but, I do believe there is more than enough compression for it to run.

I believe the problem is with the spark... when we checked it, we noticed it did not look right, we would get 2 or 3 sparks within 1 turn, then, the engine would turn 3 times before sparking again... I marked the crankshaft with a chalk mark and counted, in 10 turns, it sparked over 7 times, but, of those 7 there was at least 2 times we counted the 3 sparks at the same basic time....

So, my first question is, without having a scope, Is there a better way to test out the spark? I'm not sure what to even test first? Is there specific tests I can do for the ICM?

Should I be looking at something other than the ICM?

thanks,
chris

For our info, which engine is in this? 3.8L supercharged? What was the issue with overheating, specifically?

If spark is your primary suspect, then I'd begin by looking into the 3X crank signal circuit. Visually check that the crank sensor wiring isn't rubbing on the serpentine belt, and that there isn't coolant in the crank sensor connector.

First, Thank you very much for the reply... I have a bit of a dump of info below, hopefully not too much....

Yes, this is a 3.8L supercharged.

The previous overheating was a couple of things,
One we did not put the 'Orange' coolant in it, we used the old fashioned 'Green', that actually ate the plastic tubes that this engine uses to move the coolant around, once I figured that out, I was able to drain all of the coolant, replace the two pipes by the water pump and it stopped leaking coolant causing the overheating. Car had about 90,000 miles when this happened (roughly)

Two, and this might be related to the above issue, just took longer, the intake manifold gaskets deteriorated and caused both overheating and a vacuum leak. We pulled the SC and the intake manifold changed out the gaskets and the problem was fixed. Car probably had 110,000 to 120,000 miles when this happened.

I believe when it overheated this time it was cause the engine was leaking coolant (again), but, my son in college was driving it, and, I'm not sure he was paying much attention to any leaks (this engine leaks a LOT, both oil and coolant).

I have wondered about the wiring from the crank sensor, I have had that apart, and the wires looked ok, but, I did not give it the attention I should have. I checked both of the wires for continuity between the crankshaft sensor, and the harness into the ICM, had good connection, and did not see where either of the wires seemed to be grounded out... I will for sure go take a look at them again and make sure there isn't something rubbing while it runs.

I did pull the crankshaft sensor, it was quite a mess, hoped that was the problem, and I replaced it, but, that didn't seem to change anything, I convinced myself this summer that the timing belt jumped a tooth (or 5), took it all apart to check it, but, all of the markings were right on, chain tensioner was tight. When I moved #1 to TDC, the timing gears were lined up correctly. I'm fairly certain when/if I get it running, I will have a leak around the timing cover, but, I can fix that if I can get it running again. Since the new gaskets are so expensive, I decided to put the old gaskets back in and replace after I get it running.

I am fairly old school, grew up in the 70's, worked on Hotrods as a teenager, then, after high school I was a Diesel mechannic in the 80's, and have always been a bit of a gearhead... Over the years I have re-built lots of engines, from Honda/Toyota/Ford/Chevy... most notably about 5 years ago, I built out a chevy 350 to a 377 stroker (we went with cyl bore .010 over instead of .030 to get a 383), and i really like not having to deal with the sensors and computer, I am very out of sorts trying to figure out 'why' this engine wont start, and I for sure do not understand how to even properly diagnose the issue.....

I will do a visual inspection of the wiring tomorrow, if I remember correctly, when we pulled the timing cover off, that wiring harness was easy to remove, so, I think its fairly easy, I think can pull the harness completely out and get as good an inspection as possible.

Any other ideas? Is there any way for me to check the ICM without just replacing it? or will the ICM even cause something like this to happen?

chris99 wrote:

Any other ideas? Is there any way for me to check the ICM without just replacing it? or will the ICM even cause something like this to happen?

I don't mean to be insulting, but I'm trying to think of some ideas for you! :silly: I probably rely too heavily on scopes in my diagnostics. :blush: Gotta remember that people were diagnosing these things without scopes for years.

Not too much of an info dump. I was mostly concerned that coolant made it into the crank sensor connector when the coolant elbows popped. But that was way too long ago to be the case.

The ICM is definitely a suspect, but I'd like some more evidence. ICM's are a bit too expensive to gamble on, IMO. I might suggest backing up a bit. Does every ignition coil behave the same way you noted? Do the injectors pulse in an erratic way?

easyautodiagnostics>gm 3.8l Icm and crank sensor test.check that site out, hopefully some info there for you

I've followed along here and I don't think I'm satisfied this is not an engine base issue I'd like to know what the compression results were ? maybe retest without the core in to see if that makes a difference, I haven't tried it with a conventional gauge as I use a pressure transducer on the oscilloscope but it could probably be observed on a needle as well(others may have tried and maybe able to tell you what to expect ?what is the cranking cadence of the motor is it smooth or does it sound off/ low compression (I here people say it sounds like the motor is cranking slow and wants to start but doesn't and sometimes backfires when it's actually spinning faster your just not hearing the regular compression events . Also if valves aren't opening it can still pass a conventional compression test as the air isn't getting out of the motor. I vaguely remember a 3800 years ago that overheated and wouldn't run seems like it had to do with camshaft breaking or something.... I do believe it passed a compression test but the rockers weren't rocking when observed through the oil cap. 1 last thought how is spark consistency being checked? I tried using g a timing light flash to check for spark and intermittent flashing was exactly what it would do even though the ignition was firing every revolution I imagine you could have this problem with any inductive pickup.also as Tyler suggested is it the same on all 3 coils and does it affect injectors as well

Sent from my SM-G781V using Tapatalk

easyautodiagnostics.com/gm/3.8L/ignition...-crank-sensor-test-1

Have a look at the link, it may help you with how to test the ignition side of your problem child.

Hi Ben, sorry it has been so long since I started this topic, but, I have had a bad 9 months!

So, I did pull the front valve cover off (1, 3, 5), all six valves were operating normally... pulling the rear cover is a bit of a different story.

Spark was very intermittent, would observe 4 or 5 sparks in a single turn, then, multiple turns of the engine without any spark. I found a grounded wire in the harness going to the ICM, so, replaced it with a junk yard part, really do not want to spend a bunch of $ if this is an internals breakdown... now the spark is much better, but, still the same issue.

The starter Cadence is not great, and it is using a lot of power to crank the engine. I'm a bit concerned its a mechanical problem inside also, but, have not found anything that really points to that yet.

Hi, sorry it has been so long since I started this topic, but, I have had a bad year so far, hoping things start getting better soon!

ok, after looking through this article (and re-reading) the article says the ICM cannot be bad if there is spark going to the cylinders. I'm going to check that all 6 are getting spark, after replacing the harness.

lol, not insulting to me.at all, I really wish I had my distributor and carburetor, things IMO were much easier to diagnose.

I did find a grounded wire in the ICM harness and replaced, but, same issue. The engine does crank erratically, it's really tough to 'see' timing, or how quickly/correctly the injectors are firing. With the backfires, it changed the cadence of the cranking, maybe 1 backfire every 3 to 5 turns?

I did also notice, that all of the plugs checked (1, 3, and 5) seem to have a reasonable cadence.

I know the info is not the greatest, I come from the world of no computers and, am still trying to figure things out with all of this being 'easier' now.