Good Old Random Cylinder Misfire Mystery

[Nicolai8775]

One thing you can do is an injector balance test (injector removal not required), which measures the difference between the initial fuel rail pressure and the pressure change when each injector is fired. Ultimately an injector flow test might be needed to measure the volume of each injector's contribution, but that requires removal of the fuel rail.

Think I saw a video on that. Need that special tool that fires the injector in short bursts?

The MAP sensor doesn't do anything as far as calculating how much fuel to spray does it? I think the trailblazers are like that.

 

[iamdub]

What's more likely to cause a long crank? A vacuum leak, or bad injectors?

If it were injectors, why isn't it misfiring? I don't see any seepage around the injectors indicating bad o-rings. I will check more thoroughly when I get home. Or is it internal failure, like a clog. Cause my fuel system did leak down a little after like 45 minutes or so

Vacuum isn't a factor when cranking.

If it were bad injectors causing extended cranking, they'd likely cause problems when running.

I questioned the brand of the new CKP and CMP sensors since they're vital to operation and play a role in cranking. Even if they were new, and even if they were a new good quality brand, one could be defective.

 

[Nicolai8775]

Vacuum isn't a factor when cranking.

If it were bad injectors causing extended cranking, they'd likely cause problems when running.

I questioned the brand of the new CKP and CMP sensors since they're vital to operation and play a role in cranking. Even if they were new, and even if they were a new good quality brand, one could be defective.

I hear ya. Is it Delco or nothing with these or luck of the draw with the aftermarket parts?

 

[iamdub]

I hear ya. Is it Delco or nothing with these or luck of the draw with the aftermarket parts?

With sensors, OEM brands such as AC Delco and Delphi are the standard go-tos. There are some aftermarket ones that are generally safe, but I'd still kinda avoid them for major drivability-related sensors. Something like a coolant temp or even oil pressure sensor, I'd feel okay with getting a quality aftermarket brand if the OEM brands weren't available.

 

[Fless]

I don't recall if a compression test (or a relative compression test) has been done, but it might show something, or be good enough to rule out that kind of problem.

As commented earlier by another poster, the secondary ignition (coil, spark plug wire) could be swapped one at a time with other cylinders to see if the misfire follows.

Eric O. at South Main Auto shows how to run an injector balance test in this video (the early part of the video included diagnostics to lead him here, so it might be useful to watch that part, too). Obviously you'd need a bidirectional scanner like a Tech 2 to do this test:

 

[Nicolai8775]

I don't recall if a compression test (or a relative compression test) has been done, but it might show something, or be good enough to rule out that kind of problem.

As commented earlier by another poster, the secondary ignition (coil, spark plug wire) could be swapped one at a time with other cylinders to see if the misfire follows.

Eric O. at South Main Auto shows how to run an injector balance test in this video (the early part of the video included diagnostics to lead him here, so it might be useful to watch that part, too). Obviously you'd need a bidirectional scanner like a Tech 2 to do this test:

Ok I think I can do this with efi live

 

[Nicolai8775]

I don't recall if a compression test (or a relative compression test) has been done, but it might show something, or be good enough to rule out that kind of problem.

As commented earlier by another poster, the secondary ignition (coil, spark plug wire) could be swapped one at a time with other cylinders to see if the misfire follows.

Eric O. at South Main Auto shows how to run an injector balance test in this video (the early part of the video included diagnostics to lead him here, so it might be useful to watch that part, too). Obviously you'd need a bidirectional scanner like a Tech 2 to do this test:

As of now the misfire is completely gone. Just have slightly high fuel trims

 

[Nicolai8775]

It's been a long night. Took off the intake, took off the fuel rail, removed all injectors. Replaced all of bank 2 with old injectors from my Tahoe. Was missing 2 and only had 6 so I replaced 2 on bank 1. I put new o-rings on all before putting them back on the fuel rail. Replaced the cam position sensor with one I had bought. I have a code for that CPS now but at idle my ltft is sitting at +2.34 on both banks. Will update as I drive around!

 

[Fless]

Nice! You will probably want to do a crank/cam position correlation; should be done any time either of those sensors are changed.

 

[Nicolai8775]

Ok so. I ended up taking the cam position sensor out of my 2500HD cause Delphi was at AutoZone 45 minutes away. Ordered it but didn't want to wait. No more Cam Position Sensor voltage code. Had the p0300 so I did the crank relearn. It SEEMS my mpg has improved significantly though my LTFT still hang around 10-14% when I press the gas or at cruise. My understanding is that these are long term learned values based on the performance of the engine.

Would it be worth it to reset LTFT in EFI live?