2008 Tahoe 5.3L Flex P0300 Help

[JimTahoe]

I could use some help from the group. I’m getting the P0300 misfire code with a rough idle and seems to be dumping fuel in the right bank. Ive done a significant amount of work with no results.
- pulled heads and replaced AFM lifters and trays. Old lifters showed no damage. New valve seals and reseated valves
- new head gaskets and bolts.
- replaced VLOM, MAF sensor, O2 sensors, and gaskets for throttle, intake, exhaust.
- calibrated throttle
- cleaned and tested injectors
- new spark plugs, coils, and wires
- compression tested with a range of 165-180
- seafoamed and blew out after three days with daily hand cranks.

The odd part is that the engine runs smooth when throttled to 3k rpm driving in 1st gear. I took it to a local Indy who ran tests and could not find the issue. Any thoughts or recommendations?

 

[strutaeng]

I could use some help from the group. I’m getting the P0300 misfire code with a rough idle and seems to be dumping fuel in the right bank. Ive done a significant amount of work with no results.
- pulled heads and replaced AFM lifters and trays. Old lifters showed no damage. New valve seals and reseated valves
- new head gaskets and bolts.
- replaced VLOM, MAF sensor, O2 sensors, and gaskets for throttle, intake, exhaust.
- calibrated throttle
- cleaned and tested injectors
- new spark plugs, coils, and wires
- compression tested with a range of 165-180
- seafoamed and blew out after three days with daily hand cranks.

The odd part is that the engine runs smooth when throttled to 3k rpm driving in 1st gear. I took it to a local Indy who ran tests and could not find the issue. Any thoughts or recommendations?

You need a better scanner that will give you specific cylinder misfire location and misfire count to help you chase down the issue.

Also need scanner to show live data with things like fuel trims, O2 waveform, MAF/MAP, ECT, etc.

It could be as simple as a vacuum leak (since you mention it smooths out at higher RPM.) But the data would show that if you know what to look for.

Mechanically, seems like everything is good.

What is the history with the vehicle? You've owned it for a while? Ran good before? Just bought it with this issue? That's all good information to share.

 

[donjetman]

What is the engine and vehicle????????

 

[strutaeng]

What is the engine and vehicle????????

He put in the title: 2008 Tahoe 5.3 Flex fuel, whatever that RPO code is.

 

[Fless]

As @strutaeng noted, the MAF reading at warm idle in g/sec should be roughly the same as the engine displacement. O2 sensor activity should be documented.

Check fuel trims at idle, then at an elevated rpm. Dumping fuel at idle can indicate a vacuum leak.

Check fuel alcohol percentage (running anything over E10 gas?); is it as expected based on the fuel in the tank? IIRC there is an updated calibration for the alcohol percentage calculation for the trucks that don't have the physical sensor.

What's the regulated fuel pressure at idle?

 

[JimTahoe]

I am the original owner. The Tahoe has 100k miles on it. I have kept up on all the maintenance. It started originally with #6 misfire, and I could hear a faint ticking. After reading a bunch of threads on this site, I assumed it was a failed lifter, hence the laundry list of work done. I’ve been running Costco 85 octane in it for years. I also have a KN air intake that I installed 14 years ago. The rubber couplings and hose that feeds the right head still look good. I’ll check fuel trim and MAF readings and get back to the group.

 

[swathdiver]

Second sentence gives a clue. Any fuel soaked spark plugs? You did most of the mechanical and spark possibles, now do the fuel possibles.

 

[Fless]

I am the original owner. The Tahoe has 100k miles on it. I have kept up on all the maintenance. It started originally with #6 misfire, and I could hear a faint ticking. After reading a bunch of threads on this site, I assumed it was a failed lifter, hence the laundry list of work done. I’ve been running Costco 85 octane in it for years. I also have a KN air intake that I installed 14 years ago. The rubber couplings and hose that feeds the right head still look good. I’ll check fuel trim and MAF readings and get back to the group.

Is the KN a wet filter? Those can dirty the MAF and skew what the MAF reports.

Using 85 octane -- not that it matters but are you at a fairly high altitude?

 

[j91z28d1]

As @strutaeng noted, the MAF reading at warm idle in g/sec should be roughly the same as the engine displacement. O2 sensor activity should be documented.

Check fuel trims at idle, then at an elevated rpm. Dumping fuel at idle can indicate a vacuum leak.

Check fuel alcohol percentage (running anything over E10 gas?); is it as expected based on the fuel in the tank? IIRC there is an updated calibration for the alcohol percentage calculation for the trucks that don't have the physical sensor.

What's the regulated fuel pressure at idle?

the alcohol percentage calculation is the one I always forget.

 

[JimTahoe]

I put a new MAF sensor in yesterday and it has been about 6 months since I oiled the filter. Atlanta base