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Hey guys, I'm new to this forum despite owning my 1993 pontiac bonneville for almost 10 years. I've fixed all sorts of things, but this one has had me stumped for 2 weeks, so if anyone has any ideas, please let me know! Here's the rundown.

Car ran perfectly fine getting to work one day. When I got in it to leave, it turned over fine but wouldn't start. It acted as if the timing was off: I could hear it try to fire but it usually ended in a backfiring rush of air coming out of my intake.

I tried to connect an OBD 1 GM scan tool to my dash harness, but it doesn't blink any codes. All it does when I turn the ignition switch on is turn on my cooling fans.

I started out with no spark. Switched out the ignition control module and still nothing. Coils are new as are the plugs and wires. I then moved on to the crank position sensor, and after I replaced it I had spark and the car ran great again... For 20 minutes. Then it was right back to the same issue.

Car still has spark and the fuel pump runs, I can clearly hear it. Fuses in both the maxifuse panel in the engine bay as well as both sides of the dash are all good.

I do have a very small vacuum leak at the intake manifold, but it was there before this problem ever came about.

The craziest thing about this whole problem is that when all fuel injector pigtails are plugged in, the car just chokes and backfires when I turn it over and won't run. When I unplug one or two random injectors, the engine will run for at least 15-30 seconds, although it obviously runs poorly! I honestly have no idea how this could happen, but those are my findings thus far.

I am going to replace the injectors anyway considering they're original with 308k miles of abuse on them. However, I want to get the car running again first. Any suggestions or help would be greatly appreciated, I love this car and despite my wife's heckling I want to continue driving it!

All the best to you guys,
Plemmons

My hunch tells me the PCM is flaky.

Does the service engine soon light even work? I have to agree with Wikoff. You may want to check/cleanup your ground busses. Pretty sure they are on both sides, under the carpts, right near the front of the front doors. Bad grounds cause bad behavior.

That's a good part to troubleshoot as well, thanks for your reply!

That was the one part I failed to mention in my first post. I did try a brand new ECU from the local advance auto parts, switched out the PROM and it still made no difference. I had the same hunch when I plugged the GM OBD 1 scan tool in and I got no blinking codes at all, but my cooking fans turned on. The cooling fans didn't work for about a month before this problem arose, so I suspected a faulty ECU. I've read that GM didn't use a PCM until a few years after 1993, from my understanding, the ECU or ECM serves as the PCM in my car.

@RJolly87 thank you for pointing that out too. The service engine soon light hasn't come on for years and doesn't when I plug in the scan tool, either. I'll check the grounds next time I'm at the car and post my findings there!

X2 on the pcm, i found a spare at the junk yard i keep it for back up just incase. These pcms you can do the knock test on, just pull it out and knock it while cranking the engine and see what it does.

If it hasnt worked for years, you may also have an issue with the cluster. The SES light should alwats come on at key on, even when all is well.

If it hasnt worked for years, you may also have an issue with the cluster. The SES light should alwats come on at key on, even when all is well.

Get a scanner on it that will display the codes instead of making the SES light blink.

Alright, so I pulled the trim and carpet to locate the grounds for the ECM. They looked to be in good shape, but I removed them and cleaned them with a wire brush, then reinstalled. No change there. I then tried knocking on the ECM while turning the engine over, and it backfired through the intake again, but this time followed by a loud bang that shook the car, haha. It was almost as if I had rear-ended someone at 5mph or so. Nothing seems to be blown up or destroyed, so I suppose that's a plus, but it hadn't done that until I knocked on the ECM. That's just strange... Anyways, I'm heading over to advance auto parts to see about the OBD 1 scanner and see what that yields.

And in regards to the gauge cluster, the only light that ever comes on at startup is the fasten belts light. The security light used to come on years ago and I couldn't start the car, but after some research I looped the PASSkey 2 module circuit and it always started fine despite the security light staying on permanently. Now I'm down to nothing but the fasten belts, check gages and check oil level at startup. That's all that ever comes on.

Wiggle the wires on the back of the PCM around, it's possible that the connector terminals aren't making good contact. Otherwise I'd be blaming a bad pcm still.

Alright, I've tried 2 total brand new PCMs, wiggled the wires on all three connectors while turning the engine and still nothing. I tried a new camshaft position sensor too just to eliminate that as a problem and still no change. This car ran fine and produced a spark for about 20 minutes right after I replaced the crank position sensor, I'm going to re-test for spark and ensure that nothing went wrong there. What are the chances that the connector could be defective? That thing is stretched rather tightly down the front of the block, I'll test for power there if I get no spark again. I'm certainly open to any other info and suggestions as well!

Thanks for everyone's help thus far!

It shouldn't be stretched too tightly, but it shouldn't be able to flop around into the path of the belt, if you have a DMM you can do a continuity test of the wires as they go directly to the ICM. Also check each terminal in the connector, there is a tab inside that makes contact with the pin on the sensor side of the connector, if the tab isn't touching the roof of the terminal it may be a bad connector.

You indicated something that threw up a flag in my head. You said the dash lights aren't working right anymore.

I would check power and ground, to make sure you are getting power where you should. Test light will go a long way here.

That's what I believe I'm gonna have to do. The dash lights didn't go out all at once, it was a slow, gradual burnout over the years, I'm pretty sure. I have a digital multimeter, and I'm going to get a test light tomorrow and start checking all the circuits. That connector for the crank sensor is stretched as tight as a drum, so it could have pulled apart somewhere behind the engine mount brackets for all I know. Thanks for the input and I'll post my findings tomorrow evening.

Ok, I've got a friend that's gonna round up a code scanner sometime in the next couple days. I don't have the $$ to fork out for one of those and no parts stores have one that they connect to customer's cars, so I've been flying blind with all this. I'm definitely tired of throwing parts at the problem and getting nowhere, haha. It may not be tonight, but I'll let you guys know what codes it throws at me and hopefully I'll get somewhere. Thanks for all your help!

Alright, I finally had a chance to put a snap-on OBD 1 scanner on it and it produced a ton of information. However, the top of that reading was the most confusing. It read "no codes present, ok to drive." Just seeing this made me want to throw bricks, haha. Granted, my battery was getting pretty weak before all this troubleshooting and cranking I've done, so it may have cleared the codes when it didn't have power for a while, I'm not sure. Anyways, this post may get a bit long from here, but here are all the readings I got today:

Open/closed loop= open
Exhaust oxygen_rich
TPS(V)=.058
Throttle %=0
LT trim 128
Trim cell 0
IAC position 143
O2 crosscounts =0
Airflow g/s=0
Desired idle =1313
CCP duty cycle =0
CCP solenoid =off
Weak cylinder =0
Load/LV8 =0
Spark advance =0 degrees
Time =0:00
Knock =no
Knock retard =0
Coolant temp =-6C
MAT =-40C
Cooling fan 1=off
Cooling fan 2=off
EGR solenoid 1,2 and 3 = off
PROM ID =1284
Quad driver A = lo
Quad driver B = lo
A/C request = no
A/C clutch = off
PRNDL switch = P
Command gear =1
P/N switch = P-N
2nd, 3rd and 4th gear = P1
Battery (V) =10.7
Vehicle speed =0
TCC slip RPM = 0
TCC duty % = 0
TCC brake switch =open
TCC mode =off
CC speed =0
CC servo =45
ACT CC servo % =0
DES CC servo % =0
Cruise mode = disengaged
CC on/off switch =on
CC res/acc switch =off
CC set/coast sw =off
028mV =443
Fuel trim =128

This is everything from the engine scan. Is it possible that my timing chain has broken? I don't know how I would be getting spark if that were the case, considering I did install new crank and cam sensors. The TPS sensor did give a slightly lower than tolerance reading, but my battery is also down to 10.7 volts at the time of the reading. Let me know what you guys think!

If you had disconnected the battery, you cleared the codes, and it won't have any till it runs again.

My two thoughts right now:
Check the fuel pressure.
Is it possible you've swapped some plug wires? The two posts on one coil aren't specific two one cylinder, but each coil is specific to two cylinders.

I know It sounds as if I've mixed up the spark plug wires, for sure. But I've checked my wiring and made sure it wasn't that. I'll get ahold of a fuel pressure gauge and see what I've got there. Thanks!