2007 GMC Sierra Denali 6L v8 AWD
I recently got a new engine installed on my 2007 Sierra Denali, along with new spark plugs, spark plug wiring, intake manifold, water pump, and other parts. My mechanic installed everything, tested out the engine/vehicle, and ran into a P0300 code. He warrantied out the plugs/wires, replaced, and tested again. AGAIN, the truck was throwing the code. After thoroughly checking everything, he got in touch with GM about the issue. He claims that he spoke with someone who told him that there have been issues with the crate engines for my truck in which the computer will pick up the code, even without any actual misfires, and this could last for up to about the first 5000 miles. Well, here I am at roughly 8000 miles on the new engine and I still see the code.

While driving at 70+, there are times that the check engine light will flash, then go away. After the flash for one or two cycles, the check engine light will stay on. However, after driving on shorter, slower trips, usually about 2 or 3, the light will turn off. All of this, I'm told, is related to this "known issue." Basically, the check engine light doesn't stay on, and I don't notice any misfires - I'm not feeling anything happen when the light does start to flash. But, the light comes and goes.

Would anyone on this forum be able to confirm that a crate engine for a 2007 GMC Sierra Denali, 6L v8, AWD truck could potentially be throwing a P0300 code for the first few thousand miles of driving? My worry, of course, is that either everything hasn't been installed properly, or that I got a faulty engine straight from GM. The engine is at least under warranty..!
Thanks!

This is thick....You need a more reliable mechanic.

Thanks for the quick response! And, that'* what I'm leading toward...

I'm surprised the tech didn't verify via the Tech2 scanner if there were any misfires.

At the same time, was the fuel trims/learned data reset when the new engine was installed?

I know the mechanic drove around for quite a while with a scanner hooked up and taking readings - but I don't know if he used a Tech2 specifically. I assume that would likely be the best option. And, if I recall correctly, his scanner did not pick up anything.
Not sure if the fuel trim was learned/data reset. I recall them doing a crank learn for sure. I'll have to ask about this one and will certainly post on that response.

Thanks for your input!

Have them do a running compression test.

Thank you for the suggestion!

I agree with the others. This is odd and I suspect the mechanic isn't telling the full story or doesn't have a clear picture of what is going on.

A one-for-one stock replacement crate engine from GM for this car shouldn't have an issue like this, let alone for 5,000 miles as in. Computers don't just randomly come up with this stuff, and the age of the actual crate engine wouldn't change this unless it is poorly built. I suspect GM is not distributing a bunch of poorly-built LS motors.

That your mechanic swapped out plugs and wires without a clear diagnosis of the actual problem says a lot.

What is the fuel pressure while running? Low pressure = lean fuel mixture = misfires.

What was the problem with the original engine?

I'll have to get the fuel pressure checked, likely goes with the other comments on getting the fuel trim reset/learned. If fuel pressure is the issue, can this potentially be fixed by a scan tool, like the Tech2? Maybe a better question is - can the tool correct any of these issues, whatever they are determined to be, or is the tool purely for diagnostic purposes?

The original engine basically had some bearing failures overtime. Ultimately resulted in getting LOW oil pressure when idle/stopped and quite a bit of knocking. There was quite a bit of metallic deposits accumulated in the oil pan. Basically, it was either wait for the engine to completely fail, or go ahead and put a new one in, so I had a new one put in.

Thanks for your time and comments!

Fuel pressure is fuel pressure. Fuel trims etc. are affected as reactions to fuel pressure, but they do not affect fuel pressure being delivered from the fuel pump.

A Tech2 can find issues and make some adjustments, but not fuel pressure.

Bad bearings and metal in pan are good reasons to replace an engine. I wonder how it got that way in the first place, but that'* not important for our purpose here.

While we're talking about it, I'd get a compression test done on this engine.