I got a 1999 gmc suburban and the number 6 spark plug keeps fouling out replaced the injector what could be the problem

my thinking is when the engine backfired maybe one of the valves got bent and is now hitting a piston and keeping the engine from turning over or a rod may of gotten bent/broken. you could try taking all the spark plugs out than try rotating the engine backwards if it moves backwards try moving it forwards and see if it stops in the same spot if it dose than it is likely either a valve or the rod if you have something to look inside the engine throw the spark plug hole try to see if a valve is bent and try to see if all the pistons are all moving up and down the same amount. you may be able to tell if the pistons are moving by putting a rod throw the spark plug hole against the piston and trying to rotate the engine in either direction and see if the rod moves the same for all the pistons.

if you've changed the spark plug and no difference, then it's time to swap the coil with another cyl. coil and see if the miss goes to the other cylinder, if it does'nt follow to other cylinder, then I would replace that injector !

Hi, I see you have changed a bunch of items, but do we know if the engine is getting fuel pressure? (55 psi at test port), or getting good spark? Could the spark plugs be fouled?I think before you replace too many items, you should start with basics. Good luck, Thanks...

If one cylinder is missing at idle, it would be useful to know which one. Pulling the plug wires one at a time with insulated pliers would tell us if one does not cause a change.
It could be a fouled plug.
You could switch plugs with another, to verify.

If not spark, could be fuel.
If throttle body, then not likely.
If individual injectors, then pull whole rail off and see if all injectors fire, spray pattern like cone, and none dribbling.

If none of above, then consider intake leak, collapsed lifter, valve adj, etc.

It is either a fouled spark plug or the coil for number six cylinder

There is a possibility that your temperature sending unit for the computer is not working correctly. ( not the sending unit for the gage, there are two). If the computer never knows that the engine warms up, it will keep "choking" and over fueling. Thus, the black spark plugs.

P0301 is a DTC code that indicates a misfire to cylinder one. It could be a bad spark plug, spark plug wire, rotor and cap, a dirty fuel injector, or the worst a bad head gasket. Cylinder 1 is the first cylinder on the drivers side, towards the front. Start with the simple and check the spark plug wire and add fuel injector cleaner to the gas tank. Good luck and hope this helps you get you van running 100% soon. If that did not help, then your going to have to do a tune up by replacing the spark plugs, wires and rotor and cap. You can get all the parts you need at Auto Zone and they can give you the step by step to get the job done with common tools in under 2 hours of your time. Keep me posted, be glad to know you got your van running 100% again soon.

You need a scanner to look at fuel and spark parameters. Bad coil or injector Take it for a drive while someone scans it

Fouling occurs when the spark plugs' firing tip becomes coated with excessive fuel, oil, or combustion deposits so that it is unable to produce a spark. A plug can become fouled from continuous low speed driving, improper spark plug heat range (too cold), improper timing (over-retarded), too rich an air/fuel ratio or an oil leak into the combustion chamber. A variety of self-cleaning features are designed into most plugs to reduce fouling.

Per your description, it sounds like you are missing an injector pulse from the ECM.
Check the under-hood fuse box and verify the "ECM1" fuse is good (12 volt supply to injectors).
With the key in the run position, verify you have 12 volts at an injector.
Barrow or purchase a "noid" light to verify you have an injector ground pulse from the ECM.
Let me know what you find and we can proceed from there.
Regards,