Cylinders misfiring


Asked by Jan 31, 2015 at 04:19 PM about the 2003 GMC Sierra 1500 SLT 4WD Extended Cab LB

Question type: Maintenance & Repair

Cylinders randomly misfires when hot. Manifold gaskets, fuel injectors on the odd
number cylinders, M.A.F. sensor, crank shaft sensor, spark plugs, and O2 sensor
were replaced. The funny thing is, after every items were replaced [in these order] on
test runs, truck runs fine, until engine gets hot. The latest diagnoses was cylinders
were slightly misfiring truck could still run good and catalytic converter not performing
up to par which maybe due to it being plugged. Did as recommended fuel treatment
were placed in fuel tank. Truck ran really smooth for a period of time where I thought
problem was solved until it started major misfiring again. Any thoughts or suggestions?
I would appreciate it.

7 Answers


Sounds like an injector problem, try putting in a bottle of Techron injector cleaner, then when fuel gets to a 1/4 tank fill it with Chevron or a fuel that contains Techron, after that you can go back to your regular fuel but keep adding the Techron every other tankful. That should help, it did my Tahoe.




Appreciate your answer. At the suggestion from a mechanic, I did fill my gas tank which was half empty w/chevron techron gas. Also added a packet of X-treme fuel treatment at his advise I guess to mix w/the present fuel? As I mentioned, truck did run smooth after running it a couple of miles. It continued to run smooth for I estimate another 2-3 miles and started misfiring again?

1 of 1 people found this helpful.

Have you checked the ignition coils? Sounds like they may be breaking down when hot. HTH. -Jim


Thanks. That's my next move. The misfiring are randomly happening with all of the cylinders when hot. It started with 1-3-5-7 cylinders, jumped to 5 and 6, now and then 2 and 4 would pop up. The last read out was most of the cylinders including 8. It's just jumping around at random.

1 of 1 people found this helpful.

Did you check the plug boots for hard spots or cracks? Did you put a little dielectric grease around each plug boot when you replaced the plugs?


Yes, the boots seemed to fine. Did not see any cracks or signs of wear. No, I didn't know that the boots needed to be greased?

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