1999 GMC Sierra truck misfire and backfire

  • 1999 GMC SIERRA
Engine Performance problem
1999 GMC Sierra V8 Four Wheel Drive Automatic

my truck had a large vacum leak causing it to miss and I fixed that and it ran fine for a couple days now its missing worse than before and it is backfining I think it might be the ignition coil that is going bad and I took it to the auto parts store and they put the computer on it and it said that but it wont say what cylinder its on it just lists letters and in the repair manual it only gives numbers for the cylinders can yall help me out cuz the auto store wants me to replace all 8 and thats really expensive when its just one or two thats bad
Do you
have the same problem?
Saturday, June 13th, 2009 AT 9:32 PM

1 Reply

Misfires can be caused by worn or fouled spark plugs, a weak spark (weak coil, bad spark plug wire), loss of compression, vacuum leaks, anything that causes an unusually lean fuel mixture (lean misfire), an EGR valve that is stuck open, dirty fuel injectors, low fuel pressure, or even bad fuel.

Dirty fuel injectors (cleaning the injectors often fixes this).
Bad MAP (manifold absolute pressure) sensor
Bad TPS (throttle position) sensor
Bad or dirty MAF (mass airflow) sensor
Low fuel pressure (leaky fuel pressure regulator or weak fuel pump)
Vacuum leaks (intake manifold, vacuum hoses, throttle body, EGR valve)
Bad gasoline (fuel contaminated with water or too much alcohol)

Sometimes, what feels like a hesitation is actually ignition misfire rather than lean misfire. The causes of ignition misfire may include:

Dirty or worn spark plugs
Bad plug wires
Weak ignition coil
Wet plug wires
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Monday, June 15th, 2009 AT 6:16 PM

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