Lil'John

I took my LQ4 converted Land Cruiser to the shop for some non-drivetrain related work.

It now has a low RPM, high load misfire.

Engine is ~40k mile stock from 2004 Silverado 2500HD. The only three changes on the engine are hooker center dump headers, custom air filter, and no diverter valve at front of engine.

Stock, the engine came with 4l80e. I used NV4500 and had tune created for it.

Basically, if I am at about 2k rpm and push heavy on the throttle, it misfires VERY bad.

Today, while trying to diagnose it, when I shut the engine off it sounds like a rattle

I pulled all the spark plug wires to check if they burned through but found no issue.

While the engine was running, I jiggled each plug wire to see if it made a difference. It made no difference, but two of the wires had a physical pulse to them It was third back from the front on the driver side and the very front on the passenger side.

I put maybe a total of 100 miles on it with this misfire... while minimizing any high load gassing.

I've checked the oil and it didn't smell funny nor was it low.

I used a generic OBD2 code reader and got P0507 (IAC system RPM higher than expected) Unfortunately generic reader doesn't allow me to see any GM specific real time data such as coils.

I checked the throttle body to see if it was loose. It was not.

I've driven the engine ~1500 miles since the conversion and this is the first time for this issue.

Where should I be looking to diagnose this?

badtomatoes

Make sure the spark plugs are torqued correctly. I've seen a 5.3 in a tahoe with "moving" plug wires. Half the plugs were finger tight. Idled fine, misfired under load.

Good luck.

Pop N Wood

iTrader: (1)

Sounds like detonation to me. You might have gotten some bad gas. Try burning this tank up and fill it back up with higher octane.