I did my first mountain pass tow with the truck this past weekend (and our 8,000lb trailer). Had some major pinging/detonation when we got to the steep part of the pass (6% grade) and the engine had almost no power. Traveled the pass at 25-30mph adjusting the throttle regularly to avoid the pinging. Fuel was 87 octane (mix of Costco and Chevron). I was pissed...

Refilled at our destination with Shell 92 octane and the tow home went much better. But the CEL did flash a few times when the engine jumped up into the 3,800-4,000 RPM range before I could back off the throttle and slow things down.

The engine felt stronger with the higher octane fuel, but the MPG's were the same both directions...6.6mpg (hand calc'ed).

Got home yesterday and filled up with Chevron 92 octane since we're taking a trip over the mountain pass again pretty soon. Figured I'd just keep the lower octane fuel out until our trips are finished. Then tonight the CEL came on under heavy (but not full throttle) acceleration on a freeway onramp. I was unloaded, but using the tow-haul mode. She's throwing a P0300 code, which I'm told is a "Random Misfire" error.

The engine feels fine, but I'm taking it easy until I can get to the dealership tomorrow.

Any ideas? On a side note (any maybe not related), the exhaust sounds deeper after my pull and there is a rattle that sounds like a loose heat shield.

 

I had a similar experience a few years back, I had filled up at a Pilot station In Central Point Or and started back to Brookings Or. When we started climbing a 2K pass on hwy 199 (without trailer) the truck started missing and the CEL came on. I was hoping it was bad gas and after blending some new gas with the old the truck started running close to normal again. I ran the tank just about empty before it was refilled again and the problem was gone.

Denny

 

I had that same rattle that was cause by the flange that connects to the manifold, both bolts were rotted right off, and the rattling drove me nuts, but at the same time the truck was skipping while traveling up hills. might want to check out the flange. it seems like any exhaust leak on these engines makes a big difference in performance, maybe it's just me.

 

Survey says...straight from the Dealership...

I have a bad ignition coil, causing the #2 cylinder to miss. I also have a crack in the intake snorkel letting in unfiltered, un-metered air. Towing amplified the problems.

The rattle in the exhaust is a loose heat shield. They can't fix it, so I'm getting a whole new muffler.

Thankfully this is all covered under the warranty. Can't wait to haul another mountain pass and see how the fixes work. Would be nice to not drink premium fuel during the summer towing months.

 

New update...

The dealership took apart my exhaust today to replace the muffler and found the rattle was from the cat destroying itself and throwing parts down the pipe. So I've been driving one cylinder down for at least a week, including the 350 mile mountain pass tow.

I tell you what, she pulled pretty good for a V-9.