Having problems with your 1998 GMC Suburban ?

Rough idle and engine stumbles at highway speed

\015 The vehicle is a 1998 GMC Suburban with a 350 cid engine and auto trans, two barrel throttel body and 180000 miles. Check eingine light works but has not iluminated for these problems.\015\012 Engine dies at idle or after driving and taking my foot of the throttle,( slowing for a stop or turn). The engine seems to have great power when taking off at full throttle. It does not miss a beat and will chirp the tires when hitting 2nd, but at cruzing speeds, whether at 35 or 75 the engine will misfire occasionaly and backfire. Most of the time it just stumbles without backfiring. At idle the engine goes back and forth from 500 to 1200 rpm and will die at some point. \015\012 The tach is bouncing also. I know it is reacting to the engine rpm but it has it's own 2 to 4 count beat per second at idle and at highyway speed the tach will stay steady for a period, but say at 2500 rpms it will bounce to 3000 rpm. The greater percentage of time the engine will stumble when this happens but not always. These problems will happen through warm-up and operating tempeatures.\015\012 I do not have a code reader and cannot get the engine light to flash when I short the terminals on the plug to get any codes.\015\012 I found problems with the fuel pump and ditributor and replaced them. I've replace the egr valve, egr selonoid and checked function and voltage, all seems good. I checked function and voltage on the MAP, Oxygen sensor and temp sensor for the ECM, again all seems well.\015\012 I removed the throttle body, dismantled and clean it. I replaced all soft parts in the unit. I also replaced and set the throttle positioner and it seems to be functioning properly. I replaced the idle air control valve and checked voltage and function and it checked good, however while the engine was running I disconnected it and it did not affect the running of the engine. I did recheck the IAC operation and it checked OK.\015\012 This has been going on for about 6 months. It seems to run a bit better after replacing the fuel pump but still has problems. I have had it to a shop with no success.\015\012 Any help would be greatly appreciated.\015\012Thanks, Jack \015 Posted / edited by AnonymousUser on : 17-09-2018

Answers :

I have run across this problem and discovered that it was the mass air flow sensor.
Repair Help & Product Troubleshooting for 1998 GMC Suburban

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