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?

 

[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?

I also wanted to add that it's absolutely fascinating that a cam position sensor that was in a motor that had 268,000 miles on it, and the sensor itself being quite dirty, still works better than brand new O'Reilly's special

 

[iamdub]

I also wanted to add that it's absolutely fascinating that a cam position sensor that was in a motor that had 268,000 miles on it, and the sensor itself being quite dirty, still works better than brand new O'Reilly's special

Can't leave any stone unturned. Many times people have gone in circles and ignored the culprit because of a new part.

 

[iamdub]

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

I don't see why not. Start fresh, ya know?

 

[Fless]

I don't see why not. Start fresh, ya know?

Agreed. Can't see that it would harm anything.

 

[Fless]

And I'd give it some running time to get any excess fuel out. Might need an oil change if there's any fuel in it.

 

[MrBishop]

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?

Based on what you had going on, yes, although it will learn itself eventually. There are several cells that contain the learn fuel trims for various driving conditions. Idle, cruise, accel, decel, etc, and all those with AC on and off and with purge and no purge. Each time you enter that condition, or cell, it uses the last know value and begins to learn again but each learn takes a bit. The reset takes you back to the factory numbers with no compensation for age, fuel type, etc.

 

[NardDog]

theres a lot of post here so forgive me if you already mentioned this. I had a similar issue and decided since my truck was close to twenty yrs old i would replace many of the EVAP system components like Charcoal canister, purge and vent solenoids all at the same time. Seems like you’ve already replaced allot parts already .

 

[Nicolai8775]

Well here is an update. I gave it a week before clearing ltft in EFI live. I was experiencing some surging on acceleration but only for a a couple minutes until truck fully warmed up. I'm seeing way improved mpg, but my ltfts jump to like 15% or higher again when I give it gas, even after fuel trim reset. It hangs out around 5% at idle. My vacuum tester I got from harbor freight, hooked up on top of intake where PCV goes, told me it was on the low side of normal operating range(still in the green). Torque is reporting 5.4-5.5psi of vacuum in the intake which I believe is in the acceptable range.

Truly the only thing I can think of is a slight pinhole leak in that stupid EGR hose, or maybe the IAC isn't functioning right. I've never been able to get torque to show me IAC steps on these GMT-800s. I removed the IAC and inspected visually. Absolutely no carbon gunk build up on the plunger or in the TB hole that would restrict it's movement.

Or compression. Or dirty intake valves, I noticed a good amount of oil in the intake. Clogged cats? Or even the EVAP system not functioning right.

Still no pending or current fault codes for lean condition or misfires or EVAP codes. Have had my scanner plugged in for weeks and it still hasn't been able to tell me if the EVAP system test ran or passed or anything.

 

[Nicolai8775]

Well, still chasing slightly high LTFT in the 15% range only when I accelerate. I put mufflers on it cause I was making the neighbors angry and I don't hear a tick like an exhaust leak.

The LTFT behavior is odd. At idle sometimes it's -.75% ltft, and sometimes both banks are between positive 4-6% at idle. When I accelerate it jumps to positive 12-15%. If I let off the gas and it settles back down to around 5%.

I have the tiniest of kinks in the fuel line right before the filter by the frame. Could that be restricting flow when the engine is under load?
I did the fuel pressure test as mentioned before but what should fuel pressure be if I rev it to 2000rpm?

Otherwise I'm still suspecting EGR is leaking somewhere. Or perhaps the o-rings on the IAC and TPS are bad. Idk.

Also wanted to add, that based on these videos I'm sharing, changing the injectors was unnecessary lol.

These two videos really helped me understand fuel trims, in case anyone else finds this thread in the future.

part 1

part 2

 

[gpracer1]

Run it warmed up at idle with a scanner/app that shows O2 sensors live going up and down……get a propane torch and NOT LIT spray propane around the injector orings and see if the O2 hangs high. My vette stumped my for years with issues till I did this and all injector orings were hard as glass.