Mechanics

CHEVROLET SILVERADO ENGINE PROBLEM

1999 Chevrolet Silverado • 70,000 miles

I have a 1999 Chevy 3500HD with the 454 and automatic transmission. Mileage on body is approx 70,000 while the mileage on the engine is 16,000 miles less than that. Original engine threw a rod while sitting in the dealership service bay. Yes, I do get lucky now and then. Anyway, heres the problem. For the last 6 years or so, the local dealership has had this truck in its shop for the same complaint and has been unable to fix it. The truck starts fine, idles fine, and for the most part will drive decently. BUT, it suffers from what I'll describe as vapor lock. The engine will suddenly start running rough then eventually die. This has happened in around town, stop and go driving as well as highway cruise speeds where the truck will suddenly lose power and coast to the shoulder of the road. If you pop the hood you can bleed off hot gas vapor at the fuel injection test valve on the front of the fuel rail. If you bleed it off a few times, you finally will get to liquid fuel and the engine will restart and run. Yes, we do it carefully to keep fuel from the hot surfaces under the hood. During this problem cycle no DTCs are thrown. The dealership has replaced the crankshaft sensor, and the rear tank fuel sendor. We have also replaced the cap, rotor, wires and plugs, two sets of intake manifold gaskets due to gaskets getting sucked in over time, the in tank fuel pump, all fuel filters, dropped and cleaned both tanks. We run premium fuel, 92 octane Shell gasoline. Recently using a scan tool, not the primo dealership model but a decent tool with monitoring capabilities, I was able to notice that in park, the timing will advance like normal from about 17BTDC to high 20sBTDC when blipping the throttle. Pull the transmission into anything other than park and the timing retards to about 12BTDC, stepping on throttle results in more timing being pulled until it goes past TDC, sometimes as far as 7ATDC before firing the plugs. Thinking it may be a knock sensor, I tried unplugging both of them so that no signal was sent to the ECU. This resulted in no improvement, nor did it cause any worse issues. There is no obvious sounds of knock or ping I can detect with my ears. The engine coolant temp remains in the 220 range or slightly less once the engine is warmed up, unless I sit an idle for a bit when it will climb to the 230 range. It never overheats to boil over. The underhood temps become grossly hot resulting in the inability to hold your hand on the hood or fenders after driving the truck for great distances. I'm thinking due to the retarded timing and possibly sending unburnt fuel into the exhaust where the manifolds are heating up. The cat is fine and is not plugged up. I'm at a loss as is the dealership who's abilities I've come to question. They are now insisting on a complete tune-up before hooking up their scan tool to monitor the timing. As I stated, the plugs, wires, cap and rotor have less than 2000 miles on them and initial timing checked with a timing light seems to be spot on. What sensors may be responsible for telling the ECU to pull timing as it is doing when pulled from park into any other position in the automatic transmission? Would the ECU function correctly only in park? Timing issue is there with transmission in neutral as well, so I don't see it tied to a load on the engine. Thanks for your time. Rick
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Superdutyrick
December 17, 2012.




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