P0304- Drawing Straws Now

[gbsharon]

As per previous posts, am still chasing a cylinder 4 misfire. This happens when I am pulling up long hills and is way worse under load i.e. with passengers or a trailer. I wanted to go into a bit more detail and see if anyone could help. Started noticing going uphill- would get a random misfire code. Took the car to mechanic and was told dirty injectors- car was seafoamed (or equivalent) on machine. Ran well for about 2 months after this (towed and drove up hills no worries). After another 2 months, mis came back. Took to another mechanic who told me it was the injector spider. Had the updated MPFI spider put in along with new upper intake gasket at the same time. Although this got rid of the random misfire, I was now dealing with a P0304. I can feel when the miss is getting bad- front of car starts to vibrate with increasing force. Does not trigger a code until I push it further. Backing off the throttle usually calms her down a bit and the vibration eventually goes away. Took to original mechanic who said they would figure it out- performed leakdown test and compression test, both with positive results. I have since replaced the following myself all with AC Delco or OEM equivalent parts:

Plugs
Wires
Distributor Cap
Coil
Fuel Filter
Crank Position Sensor
T-stat (unrelated but done nonetheless)

Also New:
MPFI Injector Conversion
Upper Intake Gaskets
Fuel Pressure Regulator

What steps can I take to check from here? I really am tired of guess work.

From research, I have narrowed it down to the following:
- Bad ECU
- Bad valve springs
- Bad valve

I was told the fuel pump was good by a mechanic, but I really wanna double check this. Is it possible that it is supplying adequate pressure but not adequate flow?

Car runs really well on flat ground and starts right up every single time.

Thanks,
gb

 

[Black_cirrus]

This is a tough diagnose for any mechanic. If its consistent that's a plus to diagnose. ECU's very rarely go bad, I suppose a weak valve spring could cause this.
Do you have and sort of scan tool? If you can watch the o2's during your driving or record and watch them you should see the o2 stay fluctuating or go to high if it has enough load to go to Power Enrichment. If it goes low you could have a pump issue.
I would check for a vacuum leak on the lower intake or you could have something else happening. Does it use any coolant?

 

[gbsharon]

I have an OBD Bluetooth scanner that I use to pull codes... I'll see if there's an app I can use to log data. Truck doesn't use enough coolant for me to notice, nor does it use oil. I should mention it is at about 202k miles.

 

[Black_cirrus]

If you can look at the data stream check the fuel trims total long term and short term should be under 8% as a general rule. But that is something to look at right before and during the misfire. Something like a borderline pump would drive trims high.

 

[east302]

Was the relearn procedure done when the crankshaft position sensor was replaced?

If you have a Bluetooth OBD adapter, you can download an app to read the oxygen sensor voltages, among other things. DashCommand and CarGaugePro do that.

I'm not finding a diagnostic table specifically for P0304, but GM has issued a PIP (preliminary information publication or something like that) for it...

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Diagnostic table for P0300 which leads to misfires on specific cylinders...

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And general GM diagnostics for misfires...

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