00 Denali - Random Misfires under load

[SunlitComet]

Never mind then. Thought maybe you could post a log file or something.

 

[TurboGSR96]

Nah it logs but there is def not way to post a file, I just wish P0306 would leave my life lol

 

[BattelWagun]

Haha i had P0305 in my life for a solid week, a tuneup fixed it though. I got lucky!

 

[SunlitComet]

At least *** is just one cylinder and not more.

 

[TurboGSR96]

Sooo.... This bastard of an issue still exists. Its gotten so bad that the truck is basically undriveable.

Cliff Notes on my particular issue:

New Parts:
MFI Spider Upgrade
Fuel Filter
Plugs, Wires, Cap, Coil & Rotor

Symptoms: horrible fuel economy, P0306, heavy misfiring, rougt idle, truck kicks out of OD after misfiring in high gear, the misfire will clear up if I coast for 10-15sec and if I just creep around with practically no load just trying to maintain 50mph on flat ground but as soon as I increase load or hit a hill its misfire time again. Sometimes it will misfire real bad if I don't get out of the throttle fast enough and when I come to a stop it feels like I am running on 4cyl the motor is shaking and rocking extremely bad, audible misfiring coming from the exhaust at idle at the same time when sitting inside the truck it sounds cammed lol


Whats really left to change out?
Fuel Pump...
Distributor Assy...
Crank Sensor?
What else am I missing?

If it was ignition related wouldn't I experience a constant misfire from idle to wherever? I am going to try and limp it over to a friends house and use his fuel pressure tool to check it out. Does anyone know for sure that 60psi is the pressure I should be seeing with the key on engine off?

Thanks fella's!

Oh, I am starting to feel a slight hesitation in 2nd now which tells me the issue is gradually getting worse. I want to just order a pump and throw it in there but money is tight to non-existent right now, so I am hesitant to just start replacing random parts.

 

[BattelWagun]

did you check your intake vacuum and maybe check your compression?

 

[SunlitComet]

This is what mine is producing right now and it is the factory unit from 236k miles ago.

priming 60psi

Priming stops engine not running 53 psi

engine running 52psi Jumps to 60psi when open wide throttle.

engine off 46psi drop to 45 psi in 5 minutes and holds that for at least 40 minutes.

If you get that then you are good.

If your are running like okay at idle and practically no power while opening peddle you are probably getting starved of gas. Just ensure you don't see anything else wrong before doing your pump and check it power connection too.

 

[TurboGSR96]

did you check your intake vacuum and maybe check your compression?

I did not check either of these, I know I put the plenum back on right so there are no leaks. I can't imagine the truck being down on compression I know a weak motor and this motor isnt weak... never abused and always maintained, been in the family since new. It goes through about 3/4 a qt ever 2500mi so thats not too bad IMO.

This is what mine is producing right now and it is the factory unit from 236k miles ago.

priming 60psi

Priming stops engine not running 53 psi

engine running 52psi Jumps to 60psi when open wide throttle.

engine off 46psi drop to 45 psi in 5 minutes and holds that for at least 40 minutes.

If you get that then you are good.

If your are running like okay at idle and practically no power while opening peddle you are probably getting starved of gas. Just ensure you don't see anything else wrong before doing your pump and check it power connection too.

Thanks I will definately check my results against yours except the WOT test really afraid to do it with this truck she rock and rolls ******* a WOT pull lol. The truck idles ok but I can feel a little shake and a miss every now and then. It doesn't take a long time to start but doesn't just fire like it did when it was new takes about an extra second and it kind of rolls into starting instead of just firing if that makes any sense.

A concern I have is pump pressure vs flow, I am not much on fluid dynamics but isn't it possible for a pump to show the same pressure but have reduced flow as it starts to take a shit?

 

[SunlitComet]

I did not check either of these, I know I put the plenum back on right so there are no leaks. I can't imagine the truck being down on compression I know a weak motor and this motor isnt weak... never abused and always maintained, been in the family since new. It goes through about 3/4 a qt ever 2500mi so thats not too bad IMO.



Thanks I will definately check my results against yours except the WOT test really afraid to do it with this truck she rock and rolls ******* a WOT pull lol. The truck idles ok but I can feel a little shake and a miss every now and then. It doesn't take a long time to start but doesn't just fire like it did when it was new takes about an extra second and it kind of rolls into starting instead of just firing if that makes any sense.

A concern I have is pump pressure vs flow, I am not much on fluid dynamics but isn't it possible for a pump to show the same pressure but have reduced flow as it starts to take a shit?

The WOT was while parked.

If the flow was slowed your pressure would drop off more when the injectors were firing because the flow could not happen fast enough to fill the pressure void. Remember you are testing pressure point between pump and regulator with the injector in between and a filter between the test port and pump. A change in the pumps design could reduce flow but the same pressure and reduce flow suddenly with the same pump is unlikely with say a clogging filter. AS soon as the flow increases in the injector the test port pressure would not increase or actually go down. When you do your pressure test altitude and maybe temp will affect your general highest or lowest reading but look for the the same amount of psi difference between each test phase.

---------- Post added at 11:17 AM ---------- Previous post was at 11:15 AM ----------

really afraid to do it with this truck she rock and rolls ******* a WOT pull lol.

Can we get video confirmation of that?

 

[TurboGSR96]

The WOT was while parked.

If the flow was slowed your pressure would drop off more when the injectors were firing because the flow could not happen fast enough to fill the pressure void. Remember you are testing pressure point between pump and regulator with the injector in between and a filter between the test port and pump. A change in the pumps design could reduce flow but the same pressure and reduce flow suddenly with the same pump is unlikely with say a clogging filter. AS soon as the flow increases in the injector the test port pressure would not increase or actually go down. When you do your pressure test altitude and maybe temp will affect your general highest or lowest reading but look for the the same amount of psi difference between each test phase.

---------- Post added at 11:17 AM ---------- Previous post was at 11:15 AM ----------



Can we get video confirmation of that?

Not like its fast lol but shakes and rattles haha I am afraid of pushing a headgasket out or beating up the bearings its so bad.