My 1999 gmc sierra misses, backfires and stumbles under load.
While driving the tach will indicate a sudden drop of 500 - 1500 rpms. The truck will buck and shudder while driving ... mostly when accelerating. The truck has a rebuilt engine with less than 30,000 k but no sensors were replaced. This all seemed to start right after I changed the fuel pump. At that time I also changed the Crank Position Sensor because I was getting a 1435 CPS to Cam position sensor alignment code. That code has not returned. The only code I am getting now is P0102 MAF input low. I have changed the MAF a couple of times with used MAFs and cleared the code but it returns. I removed the EGR valve. The mechanical operation seemed fine but I noticed the OEM gasket was broken. I replaced the gasket and reinstalled the EGR. When I started the truck it ran great. I drove home happy, but when I started it the next morning the problems had returned. Can anyone suggest a troubleshooting procedure from here? The EGR is around $400 so I don't want to replace it to find out the truck still runs crappy afterward. Thanks for any help.
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Backfiring is usually associated with a lean condition. Is fuel pressure correct? Is this vehicle TBI and are both injectors working properly? If timing related it should spark rattle under load and the timing would be way out and not run/start well at idle
Is this problem only on decel does it have a stable idle An egr that is not closing on decel could account for missing even stalling but the backfire indicates excessive fuel in the exhaust stream and egr gas is inert and is only used to dilute the fuel mixture to reduce combustion chamber temperature and control the formation of NOX gases
could be a number of things. timing may be off. timing chain could have jumped but then you would have lack of power and backfire. could have a vacuum leak. throttle body have carbon buildup around the butterfly valves? Valve problems. compression problems. many things. Hope its nothing major I know I am not being much help but those are some things to look at.
Try this, remove (MAF) Mas Air Flow sensor,(be sure key is OFF) so you don't set a trouble code when disconecting wire conector, with some throttle body/air intake cleaner from the outlet side of MAF, spray the sensor clean, let dry, reassemble.Remove snorkle from throttle body, and with a old tooth brush, clean throttle body& plate with cleaner, may take a couple of cans if throttle is badly 'coked' up. Hope this helps.
Sounds like the fuel supply is inadequate. Check for a plugged fuel filter, or possibly the fuel pump is losing pressure. I had a 93 Chevy V6 that acted the same way, and after checking everything else I replaced the fuel pump, which solved the problem.