P0304 won't clear..looking for help before having to pull the head

[jimkg62]

Hi. I've got a 03 1500 Z71 with a Vortex 5.3lt V8 with a P0304 code that I can't clear. Its only got 16000 on the motor and this code just appeared during this virus situation so the truck really hasn't been driven in the last 8 months. Maybe 100 miles this year...
I brought it to my mechanic as I'm disabled now and cannot do it myself. He checked it for vacuum leaks, swapped the ignition coils around...changed the plug (looked good) swapped plug wires, water tested...it was missing at 1000 and it was causing cylinder 3 to misfire as well (about 300 misfires per cycle on the tester he uses) but 3 wasn't throwing a code..
He finally found a leak on the exhaust manifold, changed that, cleaned the throttle body, Its running great but still missing and throwing a code. You can feel it slightly at idle but not driving and now the tester is reading about 300 misses instead of 1000. My mechanic thinks the valve is burnt and it will probably run another 100k and is advising me not to repair the top end. Unfortunately, I have to register this thing in California by February.
I really can't afford much more. I spent 80 to diagnose it and 330 for the testing and the exhaust manifold gasket and I got it back with a code. I was hoping someone here might have an idea of something that might have been missed

 

[2004Slickside]

Its running great but still missing and throwing a code.

I'm sorry but if you can't work on it yourself....... you need to find someone who knows how to work with, and can diagnose/rule out a false misfire code. Just because it is reporting a misfire doesn't necesarily make it a real misfire. Too many possibilities to really discuss in depth unless you have the equipment and are going to work on it yourself. You current "mechanic" must be using the S.W.A.G. system of diagnosis - he never even checked the compression? There is just way too much incompetence out there! Everything from cracked flex plates, bad balancers, anything driven by the serpentine belt, bad engine bearings, etc - all can cause false misfires. I personally had an engine with some bad rod/main bearings that gave false misfire codes.

Misfire detection: The PCM has a waveform from the crank sensor stored in memory that it compares to during routine misfire detection. When an engine runs it produces a series of accelerating rotational pulses as each cylinder fires. If the waveform timing for a particular cylinder is off by more than a programmed amount, it is detected as a misfire.

First step is to verify that the misfire detection system can correctly detect a misfire on a normally OK cylinder. Pull a plug wire and ground it so spark doesn't jump. Now start engine and look at counts for that cylinder. They should be rapidly increasing on just that cylinder.

Second step is to perform a cylinder balance test. This is a test built into the PCM that tests the RPM drop when each injector is removed from the system. If an individual cylinder doesn't drop RPM, then it is misfiring. If they all drop the same, then you don't have a misfire. You have to have a capable scanner to do this test.

Depending on the results for the first two tests, you are directed to focus on the cause of a real misfire or the cause for a false misfire.

If you find the misfire is real, make sure the technician does both static and running compression tests as the running compression tests can be used to diagnose things like flat cam lobes and broken/weak valve springs. https://support.alldata.com/article/tech-tip/running-compression-tests

If the misfire is false, remove the serpentine belt and see if harmonics from any of the driven things like alternator, water pump, P/S, tensioner, A/C compressor is causing the counts? If not, then try to do a crankshaft sensor relearn (again capable scanner needed) and see if that helps. If cause isn't found, start thinking about harmonic balancers, flex plates and bearings.

DO NOT let anyone disturb the crank sensor unless you plan on doing a crank sensor relearn.

Good luck.

 

[ygmn]

Well Slick seemed to cover it pretty well...
Time for a different mechanic... or dealership.

 

[jimkg62]

Thanks guys. Yes. They did check compression and there is a small drop in the compression at idle on cylinder 4 on the outtake. I need more info from him I guess. I am also considering trying another mechanic. Unfortunately I just spent what I could so I have to wait a bit. The only recommendation I really got was to register it in a different state!! Seriously problematic...do you guys know what could also trigger this code. (a false P0304) because I don't think that was a consideration due to the exhaust manifold leak...to his credit, that is fixed....I honestly cant tell if its misfiring...Its seems to run great

 

[2004Slickside]

Seriously problematic...do you guys know what could also trigger this code. (a false P0304)

Can't diagnose a P0304 off zero info through the Internet. Anything that you get people to say fixed theirs will be just that - a guess! Sorry.

From Post #2:

Everything from cracked flex plates, bad balancers, anything driven by the serpentine belt, bad engine bearings, etc - all can cause false misfires. I personally had an engine with some bad rod/main bearings that gave false misfire codes.

If the misfire is false, remove the serpentine belt and see if harmonics from any of the driven things like alternator, water pump, P/S, tensioner, A/C compressor is causing the counts? If not, then try to do a crankshaft sensor relearn (again capable scanner needed) and see if that helps. If cause isn't found, start thinking about harmonic balancers, flex plates and bearings.

 

[jimkg62]

Hey folks...this raised a red flag in my mind
"If the misfire is false, remove the serpentine belt and see if harmonics from any of the driven things like alternator, water pump, P/S, tensioner, A/C compressor is causing the counts? If not, then try to do a crankshaft sensor relearn (again capable scanner needed) and see if that helps. If cause isn't found, start thinking about harmonic balancers, flex plates and bearings."
I took the A/C controller out of the dash earlier this year because it need to be replaced and never put it back because I haven't been driving so...The A/C compressor has been running but without the controller installed... would replacing that controller make a difference at this point?? meanwhile...waiting for a call from my local chevy dealership

 

[2004Slickside]

It really shouldn't make a difference but you could unplug the compressor clutch electrical connector to make sure.