misfire....help me think thru this...

B

Bt08231

New member
Hey guys,

I've searched and it seems like the p0300 code can be anything. However, I have some clues. The truck is a 99 3.4 168***. New plugs and wires at 151000, new a/f sensor at 151000, cleaned MAF. The other day I thought it was running out of gas (maybe I clogged an injector). I filled it up and it was fine for a few days. After that I drove it all day at work and then on the way home it started to stumble, shake, sound like an old shitty boat, and misfire like crazy. I mentioned this to my brother who drove it earlier in the week and he said he noticed it too.

Scanned it at Advanced auto and got 4 codes: p0300, p0301, p0306, p0420. I'm not worried about 0420 because it was throwing that before and I know a misfire will screw up the exhaust readings. I think that bad af sensor a year ago cooked the cat....

I am going to try to pull plug 1 and 6 today and see if I can find anything odd going on. I find it very unlikely that 2 different coils would fail at the exact same time. And even if 2 coils did fail, wouldn't i have 4 misfires? It also seems odd that 2 injectors would both fail. The misfires are occurring on different banks at opposite ends of the engine.

What are your thoughts?
 
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When does the misfire happen? Only under load, at idle or at all times? That will help narrow it down a bit.

A bad fuel filter usually will affect only under load when it needs more fuel volume and pressure to the engine but could be the cause.

Injectors don't fail all at once like that but it could just be a very random coincidence. Not too likely. Pull the coils and spark plugs and see if there's anything abnormal going on there. If the injectors are bad you should be able to tell.
 
I just had the same issue happen to me.
P0300 would come up for over 2 months even after clearing it over and over. Finally changed the fuel filter, cleaned out the PCV valve, ran a full can of Sea Foam in the gas tank, and also replaced the spark plugs.
Now the rig runs like a champ and getting better gas milage.
 
I don't actually think filling the tank helped, the mis just went away. The truck starts up and runs fine for a while and then once it is shut down and sits it starts to run very rough. It does it at idle as well as under load, I had to floor it on the highway to keep my speed up.
 
That doesn't sound like injectors. If an injector is bad it doesn't wait until you shut it off and start it hot to act up. Doesn't sound like a fuel filter either.

Have you checked the spark plugs yet in cylinders 1 and 6? While you are at it, if you have a multimeter check the resistance (ohms) of your spark plug wires. Maybe when hot they are acting up.
 
Pulled lugs in cylinder 1 and 6 after work. 1 was looser than 3 and 5 and also grayish almost white. 6 was completely white almost pinkish. No question cylinder 6 is running VERY LEAN. Based off of the plug, 1 is also running lean. Wires were in excellent shape. Coils as well. We cleaned the Maf for shits and giggles, no change. Truck started and ran fine. Drove it for 15 minutes to pick up maf cleaner. Restarted truck and it was misfiring like crazy. Drove home and ran terrible, misfired the whole way. Based off the plugs, I'm going with a new fuel filter and new injectors 1 and 6. Thoughts??
 
P0300 is multiple misfire if memory serves me right... (???)
Why you have the rail off replacing the injectors, check the seals where the rail meets the injectors. I just went through this yesterday. Apparently, I had pinched the o-ring seal and tore it a bit which caused fuel to leak around it. This caused a multiple misfire issue with mine. It was on the first one after the fuel pressure regulator, so all the others were affected.
 
Bt08231 said:
Pulled lugs in cylinder 1 and 6 after work. 1 was looser than 3 and 5 and also grayish almost white. 6 was completely white almost pinkish. No question cylinder 6 is running VERY LEAN. Based off of the plug, 1 is also running lean. Wires were in excellent shape. Coils as well. We cleaned the Maf for shits and giggles, no change. Truck started and ran fine. Drove it for 15 minutes to pick up maf cleaner. Restarted truck and it was misfiring like crazy. Drove home and ran terrible, misfired the whole way. Based off the plugs, I'm going with a new fuel filter and new injectors 1 and 6. Thoughts??
Click to expand...

That definitely sounds like injectors, odd that it cuts in and out like that. There's no damage to the wire harness or any sort of corrosion right? White/ash deposits definitely seem like it would be the injectors, I can't think of anything else that would cause them to do that.

I suppose your could always try to run some seafoam (twice the regular amount) or injector cleaning stuff through the gas tank to see if that unclogs them but no guarantees it'll work. I don't know the going rate for injectors, so hope they are not too pricey.
 
Thanks for the help guys! Harness and connections are in great shape. We swapped coil 1 to coil 5. The misfire then continued on cylinder 1. Cylinder 6 no longer sends a code. We then swapped spark plug 1 to 5 and 5 to 1. Misfire continued in cylinder 1. At this point, I think it has to be a fuel issue. Could cylinder 1 cause a mis in cylinder 6?

It misfires really bad right off of idle and then around 3000 rpm it smooths out. When we floor it, the check engine light flashes.

I'm going to try an injector I think.
 
I had the same thing on my 99. I sent my injectors off to Witchhunter performance in Washington. Best thing I ever did.
 
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