2003 Tahoe 5.3 VIN-Z - P0300 With a Twist

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mridgewayii

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SWAG I wonder if the O2 heater circuits are working. If not, it could prevent the sensors from coming alive. But I think there's a code for that. (?)
I agree and I thought that in the beginning but after driving it 10-15 miles … I would think the O2 sensors would have been hot enough
 

Tonyv__

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Well, your post made up my mind. I’m going to pull the valve covers this week to see. I’m pretty competent at troubleshooting and can’t remember a time something had me this stumped. Time to go back to the basics and see if the mechanical parts are working.
Keep us updated and good luck
 

Fless

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I agree and I thought that in the beginning but after driving it 10-15 miles … I would think the O2 sensors would have been hot enough

Think, but know. I'd be testing the heater circuits to make sure they can carry the load. Several YouTubes out there showing how using a test light that draws some current.
 

Mudsport96

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If you can get a helper looking at moving rockers is easier.
Remove coil packs and valve covers and pull the fuel pump relay. Have helper crank the engine while you watch rockers. Look for any that aren't moving a whole lot, or seem to have slop. Otherwise, use a socket and breaker bar to rotate and check for slop in the rockers.
 

V327839

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I had an intermittent limp mode issue for the longest time. I could even fix it just by slamming the drivers door on occasion. Since you’ve done so much work to your engine, I would check the place I found my problem. The underside of the main harness as it crosses the intake manifold. In my case, the engine cover has squashed some wires to the point I had a bunch of shiners. After some wire repair, the issue was fixed. It’s worth a shot as I found that the main harness as it’s pulled across the manifold is very tight and rubs in several places, especially if you’ve moved it yourself. You’ll never get it back perfect. Thats my experience.
 
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mridgewayii

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Hi Everyone
Thought I would provide an update. Pulled the valve covers and lifters were fine. Ran a compression check and all were even except #5. #5 was higher and the more I cranked the higher it got. We all know that’s not how it is supposed to work.

Pulled the engine this weekend. Has to be a cracked head as the #5 exhaust valve was severely rusted. The Tahoe has been losing coolant over the years but very, very minimally. Maybe a half quart a year? I chalked that up to being in a very dry climate but guess I was wrong on that one.

As info - these were 862 head castings and NOT the Castec version (or at least it didn’t have the typical castec head markings.

5.3 is staying out and picked up a 150K mile LQ4 6.0 core today. I’ll be rebuilding it and breathing new life into the old girl.

Thanks for everyone’s input and suggestions.
 

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