How do I fix random misfire

[jimmy125jr]

I have a check engine light on and has been on a long as I've owned the truck. The guy said it was because of bad dash board which does have problems. So I took that answer. Well the code is p0300 random misfire. I started noticing (do not know if because I'm suspicious now or has just started actually within last few months) the idle sounds a little off occasionally when by the exhaust. I have changed plugs, wires, and coil packs. I did fuel pressure test and it maintained 58lbs. I did stethoscope check on each injectors. Cleaned the mass air flow sensor and throttle body. I am out of thoughts as to what to check. I just tried pulling my camper the other day and struggled to get up on ramp and up to speed. Any help would be greatly appreciated. I also noticed that previous owner must have cut out 3 of the 4 catalyst converters it has three sections of straight pipe replaced before muffler, not sure if that effects this code or not?

2005 GM Savana 2500 4.8L

[MountainManJoe]

Hi Jimmy. Welcome to the forum.

Please do not post Savana questions in the Astro section. Thanks.
I'll move it to "Express & Savana" for you.

There are many causes for misfires. While you're in there scanning codes, always check your fuel trims, MAF rate, and other live data. As well as freeze frame data for when the DTC was recorded. Always check your basics first:
- what did the plugs look like? Did you visually confirm engine is getting good spark?
- make sure all electrical connectors are clean , not corroded, and full seated
- Make sure you've got a clean air filter.
- a common cause is vacuum leaks.
- Load test your battery/alternator
- compression test the engine cylinders

I did stethoscope check on each injectors.

This isn't a definitive test. They could still be obstructed.You would have to perform a flow rate test.

I also noticed that previous owner must have cut out 3 of the 4 catalyst converters it has three sections of straight pipe replaced before muffler, not sure if that effects this code or not?

Did the O2 sensors (2 upstream, 1 downstream) get butchered in the process?
Maybe he cut out 3 because they got plugged, and now the 4th is plugged too.

[Bret Schmerker]

On my M11006, I've narrowed the residual miss down to the ignition module (DTC: semi-consistent P0305) after replacing the spark plugs, secondary wires, HVS cap and rotor, even the ignition coil to systematically rule out possible causes - I had the HVS break on the road, traced the root failure to a fractured center secondary contact (thus the miss on #2 regardless of throttle position – since remedied), but could never completely eliminate the miss on the #5 cylinder at idle; suspect I compromised the stock module during the brokey on the road. In my case, the central location of the six fuel injectors at the top of the manifold contraindicates stethoscopy.

I concur with MountainManJoe on ruling out air, fuel, and exhaust as well as ignition and other electrical components. Exhaust catalyst bypass would be illegal in the seven States (USA) that follow California's lead for emissions testing and servicing. I cannot rule out intermittent connections in your engine harness depriving the coil packs of primary current.

[MountainManJoe]

This van uses electronic ignition , so it doesn't have a distributor, but you can try spraying some water on the secondary components (coils and wires) in the dark and watch for arcing.

Removing the catalytic converter is against Federal law, and therefor illegal in all States. Fines can be thousands of dollars.

[jimmy125jr]

2 of the new spark plugs were cracked. Replaced and issue is taken care of. Thank you

[MountainManJoe]

I'm glad it was an easy fix. Thanks for reporting back.