1988 - 1998 (GMT400) Section for all discussion related to the 1987-1998 Chevrolet and GMC trucks.

97 gmc 5.7l

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Old Jun 7, 2019 | 4:01 PM
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Default 97 gmc 5.7l

Runs rough and very rich, the check engine light is flashing! It was stumbling at low RPM and I changed the plugs. It got way worse!?
The question is why?
Any guidance / suggestions would be appreciated.
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Old Jun 7, 2019 | 4:32 PM
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maybe a poor connection with the plug and wire. but need to see what the code is and go from there.
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Old Jun 10, 2019 | 4:56 PM
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just a shot in the dark but have you tried unplugging mass air flow sensor yet.
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Old Jun 10, 2019 | 5:11 PM
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Like what Irish said, check wires, etc. Hell, I put in new plugs, wires, one-at-a-time even and cap/rotor and still managed to get #3 and #5 crossed over!
Easy to knock a vacuum line off too, check all the obvious.

This little reader is great for the money. Does the basics plus a little more. Just hook it up to you laptop and see what's going on.
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Old Jun 11, 2019 | 3:33 PM
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Thanks all, I have traced all the plug wires, all on correctly, I replaced both upstream oxygen sensors, no change. I did unplug the mass air flow sensor and 'no change' at idle. I will double check the vacuum lines and head to the pawn shop and buy a nice scanner my buddy Kelli has for $50!

Last edited by JNNelsen; Jun 11, 2019 at 4:07 PM.
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Old Jun 14, 2019 | 2:07 PM
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i want to say if you unplug the MAF sensor and there is no change in anything thats a sign its bad
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Old Jun 14, 2019 | 5:57 PM
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Thanks, changed the MAF, no improvement.
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Old Jun 14, 2019 | 6:14 PM
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ok lets start with the basics. double check your plug wire connections and make sure you didnt cross number 5 and 7 wires. then test your fuel pressure
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Old Jun 14, 2019 | 6:54 PM
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Originally Posted by Irish_alley
ok lets start with the basics. double check your plug wire connections and make sure you didnt cross number 5 and 7 wires. then test your fuel pressure
OK, a little more background. New fuel pump and injectors in Feb, fuel pressure is fine. All the plug wires are on correctly, the plugs are new and the cap and rotor were replaced in Feb. The misfire is now 'random'. I will be looking at the timing and checking the intake for leaks when I return from vacation.
Thanks for the suggestions.
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Old Jun 24, 2019 | 4:28 PM
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Sometimes the ECU won't throw a code, hence you get no CEL. Before you get another headache, run a scan. Again, even if you have no CEL
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