P300 Random Misfire - Troubleshooting Advice?

My 99 Intrigue 3.5 has an occasional random misfire. It sets code P300, but nothing else (no specific cylinders or sensors).

The symptoms are a 'blip' or 'lull' in power while driving, could be at any speed. On cold-weather start-up it also runs rough until warmed. I do not get the acceleration/power this car used to have (I've had it since new).

I have changed the spark plugs, air filter, fuel filter. Had an 'emissions clean' done (chemical clean the intake through vacuum sys).

Reading through the forums I see several possibilities (Fuel pressure regulator, crank position sensor, EGR valve, etc.) but I'm looking for help on where to start or how to isolate. Sooner or later it may throw me a bone with a new code.

Any suggestions on a Service manual are also welcome.

check all your vac lines are hooked up properly, same with pcv, is it missing on the same cylinder during warmup every time? Check all the air-flow boots by the throttle body and air-flow meter, etc. for rips, holes, or falling off. Maybe you can have a tech read the Misfire Bar Graph while the vehicle is warming up to determine a miss on a certain cylinder. Maybe the service dept/tech would know specifically what P0300 means? Maybe you have an intermittently clogged injector - try some injector cleaner. Maybe one of the spark plug connectors has a problem?

And maybe it's just a bad crankshaft sensor. Gmpartdirect.com has them for $53.00

P0300 - Engine Misfire Detected

http://myweb.accessus.net/~090/dtcobd2p.html#pow

Unfortunately that code is so vague additional troubleshooting with the Tech 2 is probably required. Unless the technicians who know this car have had experience with this code and have associated it with a certain condition or fault. That's why it would be a good idea to have a Tech 2 hooked up while the car is cold and running poorly - if a certain cylinder is misfiring you might be able to tell by watching the "Misfire Bar Graph" and find out which one the miss is on.

BTW, I have the Helm manual, the same one the dealer has. They are pricey, though. I had to fork out $135 for the 2002 manual - (2 books)

Thanks for the quick responses...

My limited understanding of the codes is that P0300 (random misfire) indicates that it is not limited to specific cylinders. If it was recurring in the same cylinder all the time, I would get P0301 to P0306, depending. I am leaning towards something common to all cylinders (fuel, voltage, MAF, etc.) rather than leaking rings or head gasket.

I'll snoop around the hoses/intake for anything obvious.

The 'emissions clean' should have cleaned the injectors as well.

I don't think it's related to vacuum lines because that would cause a problem at idle (not while you're driving) and it probably wouldn't cause a misfire either. I've also noticed that GM PCMs are very quick to throw code P0442 [Evaporative Emission (EVAP) System Small Leak Detected] if a vacuum leak is detected.

Have you had your battery or alternator checked lately? Are the battery terminals and are the ground leads clean and tight? The PCM may just be starving for power and intermittently cannot provide the correct spark signal to the coils necessary for ignition.

A bad fuel pressure regulator usually causes a hard start condition after the car has been sitting for a while, say an hour or more.

A bad crank position sensor will cause a no start or stalling condition in almost all cases.

How many miles are on the car?

I have a 200-500rpm blip-down that can cause a stall. This happens when warming up --when the temp guage is less than 1/4. Once the engine is warm I don't need to worry. I drive in 2nd gear around town to keep the revs up (2000-1500rpm)and once warm I can drive normally. The blip is only for 1/8-1/4 a second, but the torque drop off is immediately noticeable. The blip seems to only happen once the engine hits a certain temp-then everything is ok. Even at 1200 rpm the blip can cause a stall--then you need to put it back in park to restart the engine which can be dangerous.

My codes are P0335 Crank-Shaft Position Sensor (CKP) A circuit performance. P1372 CKP Sensor A-B correlation.

So with my car the crankshaft position sensor is probably causing the stall. No other codes have ever been given out-no other engine problems either. 2000 Intrigue 75k miles.

the solution for the P0300 code is rather complicated. And I wouldn't expect any codes to be stored after a P0300 shows. From Alldatadiy:


Any data previously stored in Freeze Frame will be overwritten when this DTC sets. The only exception to this is if a fuel trim DTC was already recorded in Freeze Frame. In this case, the fuel trim data stays in Freeze Frame and the Misfire Data will be stored in Failure Records.

hmmm.... haven't been looking at the forum in a while. To answer my own thread, the solution came from Andy G: MAF sensor.

Car ran fine with the sensor just disconnected (easy test), but shouldn't likely be left that way. Put a new sensor on, and it has been fine (other than the wheel falling off, and a worn out wheel bearing - I think they are unrelated :-)

Glad you got it figured out !!

Andy