My GMC 1500 Sierra


Asked by Sep 10, 2016 at 04:09 PM about the 1998 GMC Sierra 1500

Question type: Maintenance & Repair

Help !! I have a 1998 GMC Sierra 1500 I have done
everything I can possibly do by changing anything I
can possibly change and it's still misfiring and it is
running so bad is there any suggestions anybody
can get me I would so appreciate it

14 Answers


Vortec "spyder" injection system is not the best, you may finally have to replace the system, Have you tried run some SeaFoam in the fuel? See pic.


Doesn't have a Spyder


96, 97, 98, 99 That is what it is suppose to have. Has it been modified?


It's a 98


It's throttle body injected

1 of 1 people found this helpful.

So that would make it an 88 they were TBI. So then have you got the codes out of yet?


Yes it came up missfire


Random multiple, or did it give you a cylinder number?

1 of 1 people found this helpful.

I just bought a cam shafts positioning sensor I'm going to see if that works if not I don't know what to do LOL


Did you check for a bad intake manifold gasket?


No I will but would that cause misfiring? I mean I'm going to pull my hair out I've done all I can think of


Yes especially if it is leaking internally in the engine valley.


Have you done a compression test? Checked the ignition timing? Checked the ignition coil? If it is an '88 model it's OBD I and won't give misfire codes. Is the check engine or service engine soon light on? If so, what trouble code(s) do you have? Have you done a tune up? Distributor cap, distributor rotor, spark plugs, and spark plug wires? Does it run poorly all the time? Or only when hot? Only when cold? Only when damp/raining/high humidity? HTH. -Jim

The common problem is anti freeze is leak on the inside of the engine onto the spark plug causing a misfire and needs either a head gasket replaced or the gasket and grommets inside of the valve cover under the lifters , either way it needs to be repaired internally.

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