P303 Code On 99 2.4l Alero

Service Engine light is on solid (P303 -- #3cylinder missfire). Engine runs rough at idel -- especially rough when you hold the break down while in drive, but it smooths out when you accelerate.

I replaced the plugs, inspected plug caps for cracks, replaced fuel filter, and even replaced a cracked emission hose by t.b. (real short one). The light is on solid... had it erased several times at AutoZone. I do have 145000 miles. I tried several tanks of fuel injector cleaners.

I think its the injector but i'm not sure.
1. Does anyone know what this might be?

2. Should i take it to the dealer and pay $80.00 just to let me know whats wrong or should i spend $100.00 at the dealer and have the injectors cleaned?

[Oldsman]

i would check your coils. if getting misfire and plugs are new. then it can either be the coils or the ignition module.

I thought about replacing the coil but since each coil energizes two cylinders, i thought i would have multiple missfire codes.

you dont have to go to the dealer to get them cleaned, just go find some place and it should cost less than a 100..........it wont hurt and it will ensure your injectors are fine and even if thats not the problem your injectors are going to work perfect......i would check the coil first just because it powers two there could be a short or a wire thats not connected on one coil only.......then check the module.......good luck......

[Oldsman]

you could take your coils off and see if there is any crap on the coil contacts that go to the plugs.

say if you live in the chicago area i have a extra set of coils you could try.

i was going to buy new coils but since AutoZone was already closed, i removed the coils and checked the resistance. I read ~ .4 ohms on the primary and ~6.5 Kohms on the secondary on both coils which is just right according to the repair manual. The coil contacts looked pretty shinny, (supriseingly clean).

Can the coils change resistance as they warm up? Maybe an intermittent open circuit?? Should I still try installing new ones or does this point straight to the ignition module.

The car does sound like timming is off while acceleraing from dead stop. (Kind of like running on three cylinders) But seems fairly smooth at higher speeds.

[Oldsman]

i would say try replacing the module first. but coils can change with heat. Heat is a big enemy of coils. I would say take it one step at a time.

I checked prices for the ignition module...
Dealer $270, gmpartsdirect.com $162, AutoZone $145 with lifetime warranty.
I'll replace it tonight and post the results... I will select the AutoZone part unless someone knows of a High-performance ignition module i should go with -- if there is such a thing.

[Oldsman]

there is no such thing.

well he could do msd couldnt he or something like it.......but yea let us know how it goes....

[Oldsman]

MSD on the 2.4 goes inline from module to coils. so if module is bad this won't help same with the coils.

After replacing ignition module, the car definitly is acting different. It is ideling much lower ~650rpms vs 900-950 on the old ignition module. The acceleration doesn't sound like bad timming anymore... its alot smoother. But this didn't fix everything.
After installing the module, i drove the car for about 15 miles hoping the engine light would clear. Then I drove to AutoZone and had the code cleared. As soon as i started the car, the light was back on -- i figured they didn't erase all the errors ( they were very busy so i just went home and unplugged the battery for 15min. Drove the car some more and the engine light came back on. I drove to AutoZone and now the code read P0300 --- random cylinder missfire detected. Upon closer inspection at the exhaust, i can still hear the sputtering every 3 to 6 seconds. So, even though the ignition module made it run alot better, the engine is still missfiring. I bought and installed those plug boots and inspected the new plugs while i was back in there -- everything looked normal. It didn't make a difference. I ohmed out the coil cover as well and it looked good. To sumarze, i replaced the plugs, boots, ignition module, fuel filter, one cracked vaccume hose, ran several gas tanks with fuel injector cleaner and i'm still getting cylinder missfirings although the code moved from P0303 to P0300.

I'm ready to replace the coils if that should be the next step, but i did ohm them out and they do read what they are supposed to.

Please Help.... any ideas welcome

i would say maybe an injector......i dont know how hard it is to get into the injectors but taking a look at them might be able to tell you something about it.and also looking at the plugs, specifically the cylinder thats misfiring.....i think there is a chart in the manual that shows different pictures of plugs and the things that have caused them to go bad.....

[Oldsman]

well i would say take it to a shop and have them diagnos it. sounds like the end of the rope. n i would suspect either coil or injectors now.

I took it in this morning to the Dealer for diagnostics. They called me a few minutes ago and said that two injectors are clogged. I told them to go ahead and do the cleaning. Hopefully this will be the end of the mystery. Thanks for all your help. :thumbsup: