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P0302 Misfire Code - 2004 Pontiac Grand Prix N/A 3800

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Pooterosa

New member
I am sorry if this thread is in the wrong spot. I was looking for another one regarding misfires but I did not see one.

I have a 2004 Grand Prix N/A 3800 with 140,100 miles on it. It is giving the P0302 Cylinder 2 Misfire code while at idle. It does have a rough idle and shifts into reverse and drive with sort of a clunk too (more so into drive than reverse). You can feel the misfire at idle and you can see it on the tach. Other than at idle, the car runs, drives, and shifts perfectly fine. The CEL light will start off by flashing and if you take off driving, such as from a stop sign/light, the CEL will go off. If you leave the car sit there and warm up, the CEL will go to a steady light and I have to clear it with my scan tool. (I do have an Actron CP9185 and a Scanguage II if anyone needs me to run some tests for a particular reading for troubleshooting this issue).

Before we go through exchanging a bunch of messages about what has or has not been checked, I'll provide a listing below of what I have either verified, or have replaced.

Spark Plugs - New AC Delco Double Platinum - Gap 0.60 (CHANGED TO AC DELCO IRIDIUMS)
Spark Plug Wires - New
EGR Valve - New
MAP Sensor - New
PCV Valve - New
MAF Sensor - New
Camshaft Position Sensor - New
Crankshaft Position Sensor - New
Ignition Coils - All 3 - New
ICM - Used. Tested as good.
Fuel Injection Pressure (at fuel rail) - Tested good @ 56
Fuel Injectors - Noid Light Tested. Number 2 is new. I even went one by one and unplugged them and verified that the idle changed when unplugged and idle restored after plugging them back in. I moved the original #2 injector to #4 and the issue stays on #2, even with a new injector.
Compression Test - May need to be redone but the last time I checked it a couple of months ago it was at 137 on the #2 cylinder. Without having my notes in front of me, the best I can remember, all of them were above 130.
Air Filter - New
Head Gaskets - 6 months old
Upper and Lower Intake Gaskets - 6 months old - No leaks that I have been able to locate. No leaking antifreeze through tailpipes or on the ground, no oil in water or visa versa. Everything checks out fine in that aspect.
Coolant Temp Sensor (and plug) - New
Thermostat - New
Throttle Body - Cleaned

Are there some vacuum lines somewhere that I have not located that could be causing this?
 
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No, but those platinum plugs could be. These ignition systems don't work well with platinums. Throw some stock irridiums in it or some copper plugs. The clunk is likely a worn out engine or trans mount. The flashing CEL indicates an active misfire with possible catalyst damaging.

Why did the head gaskets get done? That almost never needs to be done on a GM 3800.
 
Yea Iridiums are the best overall but I think that if the plugs were the issue, you would have misfires on the other plugs. Swap the #2 plug with another plug and see if the misfire moves to another cylinder.
 
The engine was originally spec'd with the double platinum (AC 41-921) and that is what is still shown in the manuals for 96 and later on the 3800. I had the Iridiums in there before and it was still misfiring on #2.

I bought the car from a used car lot. It was out in the back and had not been into their shop yet. It was a repo car they picked up and just didn't have the time to mess with it then. I could not remove the radiator cap to inspect the coolant so I had expected a blown head gasket from the start. I test drove it around the block and it got hot fast. Bought it anyhow and did a compression test on it when I got it home. Headgasket was blown between #3 and #5 cylinders. After I replaced all the gaskets and got it back up and running, it had a misfire on #3. It eventually went away all on it's own after about a month of running it.

I also forgot to add the lower motor mounts are new, the dogbones are new and the upper motor mounts have the bushings flipped. Flipping the upper motor mount bushings has helped with the "clunk". It really isn't that bad but I just feel that it is more than it should be.
 
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I'm just asking because if you put new wires on when you installed the iridiums and kept them when you put the platinums in and still have a misfire, theres a chance the #2 plug wire is defective
 


It had the Iridiums in there when I bought it. The wires were all good back then too. I had tried flip floping the old wires and the new wires and the results are the same. The old wires still ohm out fine, resistance wise, but I changed them anyways just to make sure. I've found out in the past that even though a wire may still ohm good, that doesn't mean that it can carry and deliver the full amperage to the plug.
 
I also tried swapping the #4 and #6 wires with #2 and the misfire stays with #2.

However, I have not tried removing the metal shield(s) to see if that makes any difference. Even if it would clear the misfire, there would still be an underlying problem that is causing this.
 
I also tried swapping the #4 and #6 wires with #2 and the misfire stays with #2.

However, I have not tried removing the metal shield(s) to see if that makes any difference. Even if it would clear the misfire, there would still be an underlying problem that is causing this.
 


You should get some new manual's.

Stock plugs at AC Delco 41-101 Iridiums, call GM or any Chevy dealer and they will give you same part number. To my knowledge all waste spark ignition system used Iridiums because they can handle the "Abuse" of being fired on every engine stroke.

I'd even try lowering the gap to .050, it will hurt nothing if you do but could stop the issue.

I'd even pull the valve cover and look from a broken valve spring.
 
Yes, it was changed to the Iridium's. I'm not arguing that point. All I am saying is that the double platinum's are what were spec'd originally. I got the double platinum plugs this time because they were available. The parts stores I visited that particular evening were 0 stock, except for one store that only had 4. I needed to replace them that evening so my daughter could use her car the next morning. I know I could have ordered the Iridium's and changed them all out once they were received, or I could have just put back in the old ones for the time being, but I wanted to verify whether plugs or wires could have been the culprit here. I can put the old Iridium's back in when these double platinum's wear out, or whenever. I kept them because it turns out there is nothing wrong with them. None were fouled or burned up. #2 was a little dirty compared to the others but nothing that would require a replacement. I could have just wire brushed it and put it back in but that is not what I was after at that particular time. I wanted a whole complete set of new plugs to put in and test with.

I'm going to try adjusting the gap as you suggested and if the misfire remains, I will pull the valve cover to see what is going on in there. I did notice a little chatter coming from that side but I did not go into visually inspecting it yet because I didn't think the noise warranted an inspection at the time. Being that you have brought it up though, I am going to proceed with doing that if the plug gapping change don't correct the problem. Would I re-gap only the #2, or is it recommended to do all of them? Should I really put the Iridium's back in now as well? I could go purchase a new set of Iridium's if you really think it would make a difference at this point. The store does have them in stock now.

If by chance the valve spring is broken, can it be changed with the head still in place?
 


The manual does not say is whether that was spec for the Series II or Series III specifically. All it says is 3800 - 1996 and later. I don't believe they are going to revise a manual just because of a spark plug spec change. I do agree that it had the Iridium's in it. It had Iridium's in it when I purchased it as well. It only has the double platinum's right now because of the reasons I explained above. Basically, it was an availability issue and for a troubleshooting verification only.
 
@HighOctaneRacing - I thought I remembered reading something about the air needing to be applied to hold the valve in place. I have that adapter and this is something I can do, if needed. I'm going to try the re-gap suggestion first though. Should I re-gap all of them or only the #2 and see what happens?
 
you''ll also need a spring compressor and a good strong magnet for the valve locks.

put the air in the cylinder, tap the top of the valves to seat them, then take the springs off. theres a few vids on you tube.
 
Thanks for the reminder about the spring compressor. I have to go pick mine up from a friend I loaned it to, if this is the route I have to go. I have a small magnet with a 16lb hold capacity but I do not know if that is adequate. I have a larger one with a 25lb capacity, if needed. Or is this a special type of magnet that would be required such as something like an extendable parts pick up magnet that is small in diameter? I have one of those that can hold a 3/4" open end wrench, if that is a strong enough magnet for the locks. Never checked the weight rating of it per say, but I know won't hold a wrench any bigger than that. If I have to go this route, I'll search for a few videos on doing this before attempting it.
 
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