Intermittent spark on one cylinder

[levsmith]

Ok everyone, I need some expert advice. I started with a p0300 code (random/multiple misfire) and replaced a bunch of parts trying to fix it. I have replaced the fuel filter, spark plugs and wires, cap and rotor, and coil. Finally I got a fuel pressure gauge that worked correctly and found out it was loosing fuel pressure after the pump shut off. I isolated it to the fuel pump, not the injectors. I replaced the pump and sending unit assembly and that fixed the fuel pressure loss.

Now I am getting a p0306 code. I checked compression and it is at 150, just like the number 4 cylinder, so I don't think compression is the problem. The new spark plug has some carbon fouling already, so I'm guessing it's getting fuel. First thing i did was swapped the cylinder 4 and 6 wires to rule out the new wires, no fix. Then I swapped the same spark plugs and no fix. I bought an inline spark tester and found it missing a spark every so often at higher rpms.

What could be the culprit of this? It has to be something in the distributor, correct? Unfortunately I already threw out the old cap and rotor, so I can't swap them back in to see if something is wrong with the new cap and rotor. Is there any other way to test the cap and rotor without buying another set? Am I better off replacing the entire distributor? Anything else I am overlooking? Thanks for any help!

 

[GarrettGmc]

If you just replaced them and they're already fouling up I'd be looking into either the valves or possible crack, couple years ago I kept getting misfire on one cylinder tried everything you did, finally took it to a shop and found my head was cracking. Ended up both head were cracking but one side more then the other.
I'm sure other will chime in with other possible ideas

 

[levsmith]

Thanks for the reply Garrett. Since its skipping a spark on only one cylinder, I'm going to try replacing the cap again. I didn't have a cylinder 6 misfire issue until I started getting the random multiple misfire which I assume was related to the fuel pump not being able to keep up with it leaking inside the tank. At that time, I replaced the cap and rotor. I'm hoping its a bad cap and I can move on. Will keep you updated though

 

[Barry cowan]

Hey man check out some of the threads on the injectors if you have the vortec spider injector setup. Your problem could be a spider leg of your system I know I had a similar problem when I got my 97 Tahoe it had to get a new injector set. I think my pressure regulator was bad because I was getting po300. I do know that the regulator is suppose to hold pressure for a certain amount of time I believe 5 minuets @levsmith

 

[Barry cowan]

Also if anyone knows I’m having an issue with mine. The truck runs ok when its cool outside around 60 outside. But when it gets hot outside I have trouble getting the truck to start I have a very hard time leaving work after the truck has sat for about 8 hours. It’s bad enough it drains my battery. My battery’s about had it anyway I can get it running usually by spraying starting fluid down the throttle body so I’m assuming it’s a fuel related problem. But the problem is I have already replaced the injector assembly with a new fuel regulator. I put that cause I know most people think fpr and I myself do too. So far Ive replaced injectors mpfi set distributor spark plugs wires coolant temp sensor swaped coil and icm with another one. I am getting codes po300 occasionally also po306 that codes not coming up often and another code I’m getting is occasional running rich on the number 1 side of the engine I’m stumped guys the only idea I have left is a bad fuel pressure regulator since it’s not that old and I hate to rip of the upper plenum again. I hate to get rid of the Tahoe it’s not perfect cosmetically but it is dependable when it’s running lol I have drove it about 10k miles since I got it leagal in November. If anyone has any ideas please let me know

 

[94burbk1500]

Also if anyone knows I’m having an issue with mine. The truck runs ok when its cool outside around 60 outside. But when it gets hot outside I have trouble getting the truck to start I have a very hard time leaving work after the truck has sat for about 8 hours. It’s bad enough it drains my battery. My battery’s about had it anyway I can get it running usually by spraying starting fluid down the throttle body so I’m assuming it’s a fuel related problem. But the problem is I have already replaced the injector assembly with a new fuel regulator. I put that cause I know most people think fpr and I myself do too. So far Ive replaced injectors mpfi set distributor spark plugs wires coolant temp sensor swaped coil and icm with another one. I am getting codes po300 occasionally also po306 that codes not coming up often and another code I’m getting is occasional running rich on the number 1 side of the engine I’m stumped guys the only idea I have left is a bad fuel pressure regulator since it’s not that old and I hate to rip of the upper plenum again. I hate to get rid of the Tahoe it’s not perfect cosmetically but it is dependable when it’s running lol I have drove it about 10k miles since I got it leagal in November. If anyone has any ideas please let me know

You'd probably have better luck posting this in a new thread, not sure why you hijacked this one.

 

[east302]

Agreed that you’d probably have better luck with a new thread, but a fuel pressure test would be in order. Spec is 60-66 psi on 96+. That it starts and runs with starter fluid sounds like a dying fuel pump.


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[Barry cowan]

You'd probably have better luck posting this in a new thread, not sure why you hijacked this one.

Not sure why it mattters as long as were all helping each other out that’s the whole purpose of this site i though. By the way I wasn’t sure how to start a new one

 

[east302]

Here’s a screenshot, just hit the “post new thread” and you’re all set.

You must be registered for see images attach

The suggestion for a new thread was merely because your issue seemed different than that of the original poster: a hard start when warm (possibly fuel related) versus a lack of spark on one cylinder (ignition). Someone casually going through the threads may skip it, not even seeing your query.

Hope this helps.


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[east302]

@levsmith

Here’s a good write-up on ignition system testing. Page 3 (test #2) looks to be where you’re at:

http://easyautodiagnostics.com/gm/4.3L-5.0L-5.7L/ignition-misfire-diagnostic-tests-1


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