Sudden Misfire Problem 2004 LeSabre - P0300

[pcmos]

Unfortunately I was driving my 2004 LeSabre tonight and encountered a terrible meltdown. The car was running perfectly normally when all of a sudden the SES light illuminated and started flashing. I suddenly lost engine power and began to feel a terrible stumble and hesitation as if it was going to stall out. I was able to baby the car home and get it back in my driveway only to determine that it was idling as if it was missing a cylinder.

After connecting my scantool system to the car I was able to scan all of the onboard systems. The powertrain control module had a code P0300 stored indicating multiple misfire events. No other modules reported any trouble codes. After investigating further I can see that the engine is misfiring on cylinders 1 and 5 consistently. Misfire counts occur at almost every stroke of the cylinder. Initially I suspected that cylinder 1 and 5 were ganged to a single coil pack and that I had simply lost that coil pack. Unfortunately cylinders 1 and 5 use two different coil packs.

I immediately pulled out my nice digital compression test kit because I wanted to rule out any serious compression issues like a stuck valve, blown head gasket or bottom end trouble. Cylinder 1 is on the front bank on the passenger side, and Cylinder 5 is also on the front bank on the driver side. Cylinder 3 is on the front bank in the middle and is reporting no misfire condition on the computer scantool. All three cylinders quickly reached compression values of 135 psi after a few strokes. I performed the compression test with the engine actually running by systematically disabling spark and fuel injectors and inserting the tester, I wanted to catch any major problems in the act. I'd pull an injector plug, pull the spark plug, insert the tester setup and start the engine and let it run without that cylinder. All three front bank cylinders reached consistent pressures of 135 psi in the same amount of time. I'm assuming during a normal more appropriate wet compression test procedure those values would be significantly higher. Generally speaking I don't think compression is the problem. I don't see any sign of significant coolant loss and I don't see any sign of unusual oil consumption. Cylinders 1, 3 and 5 behaved identically the same way given the same compression test procedure. The failure was also extremely sudden with no unusual problematic behavior leading up to the event.

The weather is extremely cold here in NJ and is probably the coldest weather the car has experienced since I had it in Arizona. Our temperature was about 24F and the car was operating at a very gentle pace when the failure occured. The engine was also running at its normal operating temperature around 195F. I wasn't accelerating aggressively or driving under any unusual circumstances, it was just humming along normally at about 50mph and just immediately started acting up.

I'm really at a complete loss as to what could be wrong. There are so many possibilities and the problem seems to be very very serious. I'm going to start by checking fuel, air, and spark. I need to make sure that the injector coil resistances are ok and that the cylinder is getting fuel. I'm also going to swap out the spark plugs and make sure they are ok. I'll check the PCV system including the PCV ball valve and make sure it is functional. Any other suggestions would be greatly appreciated. I'm hoping someone with dealer experience might be able to chime in and give me some examples of common problems that can trip P0300 and result in a very sudden consistent misfire condition.

I want to mention that I allowed the scantool to perform the crankshaft position sensor learn test which it reported having completed successfully as per shop manual guidelines. After a cycle of the ignition and engine restart there was no difference in behavior. The car basically idles like it only has 4 cylinders. I'm not sure if a CKP or CMP sensor failure would trip a different code or not?

[maxwedge]

I would check for injector pulse on both cylinders to confirm fuel first, then on to spark. Also using a stethoscope or long thin screw driver see if you can hear/feel the injector clicking, if not look at the harnesses for the injectors, check the resistance of both of them, should be 11.0-12.5 ohms.

[pcmos]

So far I've determined that I have no spark on cylinder 5. I'm using an inline spark tester with a neon lamp to signal the spark pulse. Placing the inline tester between the wire end and the spark plug reveals no spark on cylinder 5. Moving the inline tester back to the coil pack between the cylinder 5 post and the plug wire also shows no spark. I need to do some more experimenting to narrow down the problem. Clearly I have some sort of an ignition problem... could be as simple as a bad wire or coil pack. I'll post a followup when I do some more diagnostics. I purchased a fuel pressure test kit, vacuum gauge, and harmonic balancer puller at harbor freight to explore as many failure points as possible.

[pcmos]

I was able to solve the problem yesterday. After discovering a no spark condition on cylinder 5 I went ahead and used the inline spark tester to check spark on all six cylinders. Cylinders 2 and 5 indicated no spark. Unfortunately my computer diagnostic software was erroneously indicating severe misfires on cylinder 1 instead of cylinder 2. Even with all the modern technology it pays to go back to some basic diagnostics sometimes to confirm computer readings.

Obviously seeing no spark on cylinders 2 and 5 I immediately assumed that the problem was with the #2 coil pack. To be entirely certain I went ahead and swapped the #1 and #2 coil packs only to discover that the problem did not migrate with the suspect coil pack. Once I acquired this result I knew that the problem had to be with the ignition control module or crank position sensor. I went ahead and purchased a new ignition control module at Autozone for about 140 bucks and replaced the unit. After cleaning and resoldering the ground connector, as well as cleaning and polishing the aluminum mounting surface, I installed the new control module with the original coil packs. To my great relief the engine started right away and ran optimally with no misfires. After completing the mechanical repair process I returned to my computer diagnostic software and cleared all of the trouble codes. Additionally I commanded the Crankshaft Position Learn Test in the software diagnostics and followed the procedure for the test, completing it successfully after a few attempts.

In short my diagnostic process began with the understanding that I could be having a problem with compression, fuel, air, or spark. I ruled out exhaust obstructions pretty quickly because I could hear and sense reasonable flow from the tail pipe. I ruled out compression by performing a compression test on the suspect cylinders. After ruling out compression problems and hence major mechanical issues, I went to the diag software and examined the oxygen sensor data and fuel trims. Noting values consistent with a super rich exhaust stream I was fairly certain that the bad cylinders were getting fuel from the injectors. To confirm I went outside to the engine and unplugged all six injector plugs one at a time and found only two that didn't change the engine behavior. I then attached my fuel pressure gauge and found adequate fuel pressure at the rail. All of these things suggested that the injectors were dumping fuel as usual into a cylinder with no spark or no air. In order to rule out air flow problems I went back to the diagnostic software and observed manifold absolute pressure readings. Significant intake obstructions or a stuck throttle body mechanism should momentarily produce unusually strong vacuum at idle. I observed nominal vacuum readings which I then confirmed with a manual vacuum check using a gauge. Finally I knew that I must have a problem with spark ignition. I used the inline spark tester to confirm loss of spark on two cylinders and then proceeded to diagnose the problem by eliminating other variables as described previously.

The car seems to be up and running just fine. Unfortunately my computer work probably eliminated the transmission adapt settings and after resetting everything I had some unusually soft shifts for a few drive cycles. Hopefully everything will remain stable and the car will continue to run well with it's new ignition control module. Thank you for your feedback. Please feel free to ask any questions.

[maxwedge]

Nicely done, if even some of these basic steps were done, much money would be saved on unneccessary parts replacements. BTW some scanners have the ability to perform power balance tests.

[RAMZO358]

My car did the same thing. Replaced the same coil pack, throttle positioning sencor and ignition control module. Just a little over four months the same thing happened minus the throttle positioning sensor. There is also a hesitation in my ignition every 10 or so cranks. What could be causing the reoccurrence of this problem. Any help would be a blessing thank you and God bless.