Misfiring again, PO300

[DenaliHD66]

Ran the truck, was running fine, started it back up after about 15 minutes, misfiring like crazy, SES light came on.

Went and got it scanned, confirmed PO300, multiple misfires.

Background:

Consistently diagnosed with cylinder #5 misfiring. Did compression. Did coil pack. Did spark plugs. Did fuel injector cleaner (helps for a few tank fulls then it comes back).

Do I really need to run fuel injector cleaner through this truck every 500 miles or with every tank or something? That seems to be the only anecdote to help the rough idle, and then times like this, where it prompted the SES light to come on.

 

[BlazingTrails]

I think there is another underlying problem. did you check the fuel pressure?

 

[DenaliHD66]

Not to my knowledge. Would that be related to #5's injector? (Assuming its just that one again). I had the FPR replaced a few months back.

 

[DenaliHD66]

Is there any way to clean the exhaust valves without dismantling the engine? Does running fuel injector cleaner clean them at all? I'm just thinking, at 130,000 miles, and having always used a K&N cold air intake... just wondering if its just buildup that needs cleaned.

 

[IllogicTC]

What year is this, by the way? Pre-05, if there's any issues with pressure at the rail, you might consider a fuel filter change.

 

[DenaliHD66]

2004. fuel filter was also replaced this year.

 

[DenaliHD66]

So it only seems to misfire after the truck is run for a while, shut off 5-10 minutes, then restarted. It will idle very rough and the RPM's will surge up and down. It even happened again today after I filled it with fuel injector cleaner and shell 93 fuel (it only takes 87 octane). Thought the detergents would help clean it out a bit better. Doesn't seem to be a gunk problem at this point.

 

[DenaliHD66]

So based on research, these symptoms seem to be that of an intake vacuum leak. Only misfires after being completely warmed up, then turned back on. How can I further diagnose this?

 

[Texan]

I would check the fuel pressure and manifold vacuum when it is missing.

 

[sunliner]

yours might be one of the vehicles known for having loose intake manifold bolts. My 06 Trailblazer did but in my case, I mostly get codes for running lean, with only an occasional P0300.

 

[DenaliHD66]

Never gotten a code for running lean. Just generic misfires.

 

[Sua25]

I'm having the same problem..p0300 code and even got p1400 code as well. I've changed the spark plugs but I don't know what else to do? And I don't want to take it in so they can run tests on it but they really don't know what to do with my car. So if anyone can help me out

 

[IllogicTC]

I'm having the same problem..p0300 code and even got p1400 code as well. I've changed the spark plugs but I don't know what else to do? And I don't want to take it in so they can run tests on it but they really don't know what to do with my car. So if anyone can help me out

When you changed the plugs, were the replacements the AC Delco 41-103? This motor seems to be very temperamental about any other brand of plug.

There may also be any number of other issues, from fuel misdelivery to ignition coil failure to excessive carbon buildup. Is it just an 0300 code or has it given a specific number, like 0302 or 0303?

 

[DenaliHD66]

Mine only misfires when it is started after already having been warmed up. I'm thinking minor vacuum leak somewhere.

 

[DenaliHD66]

Anyone have any suggestions for diagnosing a vacuum leak? Truck ran fine all winter. Now in the warmer weather it idles more rough, especially after starting up again after already being warm. Engine was surging at idle today and would then sound like it was going to stall out and SES light came on.

 

[IllogicTC]

I think smoke testing might be an avenue to explore, shops can do it, I'm not sure on the feasibility of doing it yourself but I'd imagine it can be done without a ridiculous amount of money sunk into it.

 

[Texan]

Have you checked the intake manifold bolts for being loose? Is this a I6 4.2l?
Is it SWB or LWB? Check vacuum connection on intake manifold for cover(plug)
if SWB. If LWB check vacuum hose to heater hot water shut off valve.
In the old days you could spray carb cleaner on the manifold to head
gasket to see if that effected the running when looking for a leak.