I have a couple missfire codes driving me crazy,in a 2004 Impala showing PO302 and PO305 for codes.Has the 3400 and this Impala is a customer'* car.I did check compression on those two cylinders,that is good which are over 150 psi.The customer just bought this car last week,the seller installed new spark plugs,plug wires just before putting it up for sale.I noticed the spark plugs were E3s and I believe they are the problem.I wonder if the E3s are the problem.The plugwires are AC Delco plugwires.Been working on it on Friday,the whole day.

kind of funny, my mom had a 03 3400 i just put those same parts into and she just got rid of it. i am guessing its the lim or uim gaskets though. you can switch plugs/wires/coil packs around to see if the code moves. if that doesnt do it its likely the gaskets. they were supposed to be the good redesigned ones but i believe 03-04 kind of faded in. my 03 monte and her 03 imp both 3400'* lasted i think mine was 180 and hers over 150 with no internal engine work

Brand new intake gaskets too.I am going to pull the intake too,I believe the previous person screwed up in the installation of the intake gaskets.I use the upgraded Dorman intake gaskets made out metal a lot,these are the plastic intake gaskets.I noticed weld marks, on the left side intake.may have broken it removing it forgeting to remove the bolt.

i like the felpro ones, just have never had an issue with a felpro gasket.

I had a 3.1 when I worked at the dealer that had a random misfire on cylinder 4. It had been through most of the other mechanics in the shop before me so it had a new injector, coil, plug, plug wire. It also had good compression so we all were scratching our heads. I decided to start tearing into it it and found a broken valve spring. The inner spring made it run at low RPM'* and the way the spring broke it would rotate and catch and act normal at times. What really blew my mind is when I went to the parts department we actually had a valve spring in stock for the thing!

I did pull the intake and had to call the customer to see what to do next.Found a collapsed valve spring on these two cylinders,both intake.Customer is picking up another engine,headgaskets out of it too.

why wouldnt you just get just those springs and the complete new design felpro head gasket set with all the other gaskets is like less than a hundred bucks.

Got it way too hot,washed the rings right out.It was in for an overheating problem too.The customer'* 16 year old daughter needs to learn to shut it off when the temp gauge goes to hot.Drove it hot with the temp gauge pegged out.I smelled antifreeze out of the exhaust pipe.Customer bought a 3400 out of a 2004 Pontiac Grand Prix with 77,000 miles on it and runs great now.Finished it up this morning and had the engine in yesterday afternoon.

got it, lol. must be if its still running its fine. had to rebuild a tractor at work for that reason, if they would have stopped and let it cool i wouldnt have had to rebuild the engine too

Missfire is gone and the customer'* daughter does not get it back yet.I told the customer'* daughter if the temp gauge goes to high,get to off the side of the road and shut it off asap.Would of saved it if she did this.The replacement engine started up right away running like a watch.