My 1999 Grand Am 3.4L V6 is giving an engine code of P304 defined as cylinder 4 misfire. It has 30,000 miles on it. I've replaced the plug, checked for spark and compared compression to the other cylinders. Spark and compression seem to be OK. I threw a bottle of injection cleaner in the tank just for "grins".

How does the computer know it is misfiring on cylinder 4? My guess of what to look at next is the injector and/or the injector connector.

Any ideas? GMMerlin do you have any?

So you've checked the wire leading to #4? Also could it be the distributer? Too many things to go wrong, lol. GM should have bought out rotary engines, not very many moving parts, less to look after. :biggrin: you'd only have to worry about that whole exploding thing.

My 1999 Grand Am 3.4L V6 is giving an engine code of P304 defined as cylinder 4 misfire. It has 30,000 miles on it. I've replaced the plug, checked for spark and compared compression to the other cylinders. Spark and compression seem to be OK. I threw a bottle of injection cleaner in the tank just for "grins".

How does the computer know it is misfiring on cylinder 4? My guess of what to look at next is the injector and/or the injector connector.

Any ideas? GMMerlin do you have any?


Ok here we go.....the PCM measures the crankshaft speed as the engine is running...if a cylinder misfires the crankshaft will slow down for a millisecond..the PCM sees this momentary loss of speed and registers it as a misfire...when it reads constant misfires over a predetermined time, the PCM will turn on the SES and store a code.

If you feel confident that the plug is firing and the compression is good, check #1 cylinder out...(it shares a coil with #4 and can actually be your problem) if you dont see anything there, most likely you have a injector problem..either in the injector control circuit or the injector itself...an injector balance test will show the condition of the injectors.

So you've checked the wire leading to #4? Also could it be the distributer? Too many things to go wrong, lol. GM should have bought out rotary engines, not very many moving parts, less to look after. :biggrin: you'd only have to worry about that whole exploding thing.

I know that with the plug removed from the head and grounded it seems to fire as well as #2 or #6. That tells me that the rest of the circuit should be fine - as long as it fires under compression. This engine is distributorless.

The compression in #2, 4 and 6 are about the same - I forget exactly but somewhere around 120 lbs.

GMMerlin thanks for the info.

Isn't there some sort of injector circuit tester you can buy at an auto parts store too?

Briefly how is an injector balance test done?

Thanks!

Injector balance is done by charging the fuel rail and firing each injector individually..you record the amount of fuel drop and compare it to the others..if 1 injector drops too much or not enough, that could be your problem.

They do make tools to check the injectors...I usually get mine from a tool distibutor like Snap On


try www.snapon.com