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Clayso, Master Automobile Technician
Category: Chevy
Satisfied Customers: 880
Experience:  ASE Master Certified Automobile Technician. 30 Years Experience.
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1998 Tahoe 5.7, misses/backfires horrible over 1/2 throttle.

Customer Question

1998 Tahoe 5.7, misses/backfires horrible over 1/2 throttle. Have replaced; distributor, coil, ignition module, plugs, wires, crank sensor, spider injection assembly, fuel filter. Fuel psi is 55 koer, no codes.
Submitted: 1 year ago.
Category: Chevy
Expert:  Clayso replied 1 year ago.

Hi There, I'm Clayso a certified master automobile technician with 30 years of automotive repair and diagnostic experience.

From your description most likely the distributor cap is at fault. Anytime I see a flat cap with driveability problems the cap and rotor are the first things I replace. If you're not using AC Delco parts for the cap and rotor I'd address this first. These engines also have a problem with the crank sensor being to close to the reluctor and causing driveability problems. Loosen the Crankshaft Position (CKP) sensor hold down and place a small paper clip in under the CKP's flange and snug the hold down. If this solves the problems there is a shim kit from GM #12565102 to address this problem.

GM Shim Kit #12565102

Kindly rate my assistance in a positive manner so I may have credit for helping you.I hope this information is helpful. If you have any questions let me know. You can reply back to me through this thread at any time.

Positive ratings are greatly appreciated.

Customer: replied 1 year ago.
Ok. I'll try the paper clip, the distributor is a complete AC Delco, and I even warrantied it out and replaced again because I was so sure that was the issue. It has Delco cap and rotor.
Expert:  Clayso replied 1 year ago.

That's cool. If shimming the CKP doesn't change anything check the engine vacuum and let me know how many inches of vacuum at idle.

Let me know.


Customer: replied 1 year ago.
Shimming didn't work. When we got to vehicle this morning it took several attempts to start up, fuel pressure was at 50 koeo and went down to around 40 while cranking. Finally it started and idled around 50 psi and accelerated it would go to around 55psi but still cut out very bad. The customer supplied a ac Delco fuel pump and wanted it changed to fix the sending unit, so we are going to do that now. And if it's the same I'll check vacuum again and let you know the readings.
Expert:  Clayso replied 1 year ago.

Shimming the CKP has only helped me out a couple of times, but it's easy and good to know. Fuel pressures a bit low and may account for the hard starting, misfire and bucking. It should be 60-66 psi and 66 psi at open throttle. Like you say if the AC delco pump doesn't change anything let me know the engines vacuum.

I'm sure you set the cam retard at 0° when the strib was replaced.

Customer: replied 1 year ago.
Yea, I had to elongate the mounting fork hole to get to 0 for some odd reason. The fuel pump fixed it, the pressures are pretty close to the same, but the flow is a ton better. The fuel psi never dropped under a load on road test with old pump which was also strange.
Expert:  Clayso replied 1 year ago.

Looks like AC Delco wins again. I have old specifications 50 psi minimum, revised editions are at 60. I've run into to problems with most under 60 psi. 59.9 psi just isn't good enough especially under a load. No one shows a flow specification, I guess that's the most important part.

I had a problem with the 5.7 in an Express van, it was setting a cam retard code and bucking very bad, wound up the cam was walking. With the cap off rotate the engine slightly back and forth and watch the rotor. The difference in rotation will give an indication of any slack in the timing chain, this may account for cutting the strib hold down.

Kindly rate my assistance in a positive manner so I may have credit for helping you.I hope this information is helpful.

Positive ratings are greatly appreciated.