90 suburban, 5.7 tbi; hesitation with backfire

Hi, I'm an ASE tech and I used to consider myself a bit of a specialist at solving drivability issues, but I'm stumped on this one. Hopin' someone out there can help me.

The history: This 90 suburban, 5.7 TBI, has been setting for nearly 5 years and was towed to me in this condition. I have drained out the stale gas and chemically cleaned the fuel supply from the tank to through the injectors. The injectors were initially frozen but appear to be in good shape now. I replace the inline fuel filter after cleaning. Old oil was drained, refreshed and the crankcase chemically cleaned and flushed again with new oil and filter. New plug, wires, cap & rotor. Timing set to 0 degrees. After extensive driving the scanner shows no codes and all data parameters are within tolerance (TPS, MAP, EGR, etc). There is a puff of blue smoke on initial startup after sitting overnight showing valve seal leakage. Vacuum circuits have been inspected and repaired as needed. Compression for all cyls are 170-180 psi. Has smooth idle and no engine or valve noise. Upper distributor assembly (magnetic pick-up) shows rust, which I have doused with wd40.

The symptom: The suburban will feel doggy on acceleration. If I accelerate from a stop to about 25% throttle it will act weak or lean and then, after a few seconds of lag, as if the system finally realized that the throttle is indeed open, etc., she will come to life and take off like a new truck. If I punch the throttle, from 0% to a sudden 75%, she will backfire through the intake almost every time. This symptom seems less obvious when the system is in open-loop and is in warm-up mode.

Any ideas on this matter would be greatly appreciated. I've got mechanics block and I need a nudge.

My thanks in advance,

Wes

Reply to
Hog Whisperer
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Backfire has to be a timing issue. 5 and 7 crossed perhaps? or the timing chain is in real bad shape, or the pick up you doused with wd40 has had it would be my guess.

Whitelightning

Reply to
Whitelightning

Would a clogged cat cause this? Sorry if this sounds dumb but I'm not a mechanic. Just throwing an idea out to you since you're stuck.

Bob

Reply to
Bob M

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