Murraycroexp
Veteran Expediter
Some of you know that I've been chasing a random misfire (P0300) for several months now. I'm pleased to say I THINK it's been resolved. And in the most obscure place.
Skip to the bottom if you only want the "punch line".
Items checked and/or replaced.
I have a code scanner to help identify pending codes and current codes.
Spark plugs
Plug wires
All four O2 sensors
Took it to shop #1 for them to diagnose. They checked....
Fuel pressure
Fuel flow
Vacuum quality & quantity
Coils for quality of fire
Moved even side coils to odd side to see if coils were issue. Were not.
"Relearned" crank sensor
Checked entire system for vacuum leaks under all RPMs.
Then they ended up saying "Maybe weak valve springs".
Shop #2
Checked all the same items above and only found worst misfire count on Cyl #6.
Replace #6 plug and result was the same. P0300.
Swapped MAF sensor to "test" with no change. Then swapped it back.
Gave it back to me so they could "do more research".
Never hear back from them.
I took it to Smedley's Chevrolet and walked the service writer through everything that's been checked and done.
"I see it periodically. The valve train is just worn out."
I could actually see this being the case as it had, at the time, 403K miles on it.
I ordered valve springs, as a last result, from gmpartsnow.com and replaced them on the 4,6 & 8 cylinders since those were easily accessible by the spring compressor I had.
No change. P0300.
Still had a "random misfire" at 69-75 MPH.
Through ALL of this time I had been reading various blogs about other Chevy owners and their P0300 woes.
I was NOT alone.
But until Wednesday almost everything that I had read had already been checked or replaced on mine.
But on Wednesday I read one blog about a guy with a 6.0L that was stumped and staring at his engine while his kid revved it up to the 2000-2400 RPM range, where he had his misfire. He noticed what I later learned was called "belt slap". It's when your serpentine belt "slaps" back and forth during certain RPMs.
This causes a vibration that shutters through the engine and supposedly CAN disrupt the balance of the crank shaft. The crank sensor picks of the disruption, identifies it as a misfire on the cylinder the crank position is in at that split second, and moves on. Enough of those falsely identified misfires and the computer stops allowing the transmission to shift into lock up. Then kicks out the pending P0300 code. Bummer. That's running 200-300 RPMs more to do the same speeds on the highway. Fuel economy drops.
Well, my tensioner pulley blew up a few months ago right before a pick I had in ATL. I replaced it and the belt in the lot at Advance Auto Parts & made my pick. I was already staring at my CEL for a bad O2 sensor but that code started being paired with the P0300.
So this Wednesday night I replaced my serpentine belt and now, 150 miles later, NO CODES!!!!!!
Check your belts for "belt slap". Occasionally belts are defective & unbalanced. A balanced belt does not slap without the a/c on unless there is some other issue.
Skip to the bottom if you only want the "punch line".
Items checked and/or replaced.
I have a code scanner to help identify pending codes and current codes.
Spark plugs
Plug wires
All four O2 sensors
Took it to shop #1 for them to diagnose. They checked....
Fuel pressure
Fuel flow
Vacuum quality & quantity
Coils for quality of fire
Moved even side coils to odd side to see if coils were issue. Were not.
"Relearned" crank sensor
Checked entire system for vacuum leaks under all RPMs.
Then they ended up saying "Maybe weak valve springs".
Shop #2
Checked all the same items above and only found worst misfire count on Cyl #6.
Replace #6 plug and result was the same. P0300.
Swapped MAF sensor to "test" with no change. Then swapped it back.
Gave it back to me so they could "do more research".
Never hear back from them.
I took it to Smedley's Chevrolet and walked the service writer through everything that's been checked and done.
"I see it periodically. The valve train is just worn out."
I could actually see this being the case as it had, at the time, 403K miles on it.
I ordered valve springs, as a last result, from gmpartsnow.com and replaced them on the 4,6 & 8 cylinders since those were easily accessible by the spring compressor I had.
No change. P0300.
Still had a "random misfire" at 69-75 MPH.
Through ALL of this time I had been reading various blogs about other Chevy owners and their P0300 woes.
I was NOT alone.
But until Wednesday almost everything that I had read had already been checked or replaced on mine.
But on Wednesday I read one blog about a guy with a 6.0L that was stumped and staring at his engine while his kid revved it up to the 2000-2400 RPM range, where he had his misfire. He noticed what I later learned was called "belt slap". It's when your serpentine belt "slaps" back and forth during certain RPMs.
This causes a vibration that shutters through the engine and supposedly CAN disrupt the balance of the crank shaft. The crank sensor picks of the disruption, identifies it as a misfire on the cylinder the crank position is in at that split second, and moves on. Enough of those falsely identified misfires and the computer stops allowing the transmission to shift into lock up. Then kicks out the pending P0300 code. Bummer. That's running 200-300 RPMs more to do the same speeds on the highway. Fuel economy drops.
Well, my tensioner pulley blew up a few months ago right before a pick I had in ATL. I replaced it and the belt in the lot at Advance Auto Parts & made my pick. I was already staring at my CEL for a bad O2 sensor but that code started being paired with the P0300.
So this Wednesday night I replaced my serpentine belt and now, 150 miles later, NO CODES!!!!!!
Check your belts for "belt slap". Occasionally belts are defective & unbalanced. A balanced belt does not slap without the a/c on unless there is some other issue.
