My buddy has a 4WD Tahoe, I'm not sure what year but it's most likely a 1998 to 2003. Has about 90K on the clock. It is exhibiting a few of issues:

Check Engine Light About a month back he had codes read at AutoZone which indicated TPS and one other part which I don't remember, probably MAF. He replaced both, but the TPS light has come back twice. I warned him that generic codes are not necessarily remedied by plug-and-play parts replacement. What are the common failure points?

Dieseling Holy cow, this thing diesels like crazy! Happens when applying heavy throttle while moving. Sounds like an SDL for about a second, then levels out. He runs 87 in it, I believe the manual specifies it. I told him to put 93 in it for a couple of tanks to see if it levels out. It has been 105 here after all. However, shouldn't everything automatically detune if preignition is detected? Maybe his piezoelectric (knock) sensor is blown? But wouldn't that throw a code or invoke limp?

Engine knock When the dieseling levels off, a faint knock can briefly be heard while still under load. Valve train has been ruled out as the noise is not present when revving unsprung and is most apparent under moderate to heavy load. I'm thinking mains or rods? How can that be, when it only has 90K? Is that a known issue on these engines?

As a side point, he is running 5W30 in this vehicle - yikes! This is a severe duty vehicle, often being idled for extended periods in the heat with the A/C on. I don't know what his manual says, but I'm almost certain that 5W30 in an engine with 90K on the clock running in 105* heat is not ideal. I told him to run 15W40 DELO or 20W50 in it. I am not attempting to start another oil thread here (flame suit on), but I do think 5W30 is a little light. Any thoughts?

Mechanically speaking, these Tahoes are supposedly fairly reliable. While Consumer Reports gives thumbs down to the 4WD, it is only due to consistent rollovers in avoidance maneuver testing; the 2WD is actually a "Recommended" vehicle as it exhibits average reliability.

I am aware of the famous piston slap issues plaguing 98 and up GM engines and wonder if some of the engine noise is related. I don't see direct reference to the Tahoe at http://www.pistonslap.com, but I don't exactly know which engine is under his hood either. Is this similar to the BMW alusil issues? Will this engine be seizing shortly?

TIA to all the experts...

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