08 suburban LTZ 5.3 misfire at idle

[Marbor]

Hello all been awhile since I been in a Chevy past year been rolling an Audi and BMW, but now back in a suburban.
Got this rig as a basket case for dirt cheap body and interior really good condition PO had a lifter drop and was done with the truck and got a new one.

So long story short I pulled engine cleaned up and did a minor rebuild. New cam lifters oil pump (HP), timing chain and sprockets all new seal and gaskets. New plugs wires no coils.
Now the rig runs solid but has a cylinder 7 miss at idle once idle is brought up over 1k rpm the miss goes away once at idle for longer than 5 minutes the Mis starts up and CE starts flashing.
I’ve cleaned and bench tested injectors swapped coils plug’s everything to cylinder 3 and no change same symptom. Compression tested bank every one was 200+ cylinder 7 hung around 185 for around 10 minutes with no loss. Boroscoped 7 while turning both valves are opening and closing as normal.
What else can I check any help would be great I’m pretty stumped at this point.

 

[Joseph Garcia]

I cannot personally assist you with your issue, but other members of this Forum much more knowledgeable than me in this area will chime in.

 

[iamdub]

Hello all been awhile since I been in a Chevy past year been rolling an Audi and BMW, but now back in a suburban.
Got this rig as a basket case for dirt cheap body and interior really good condition PO had a lifter drop and was done with the truck and got a new one.

So long story short I pulled engine cleaned up and did a minor rebuild. New cam lifters oil pump (HP), timing chain and sprockets all new seal and gaskets. New plugs wires no coils.
Now the rig runs solid but has a cylinder 7 miss at idle once idle is brought up over 1k rpm the miss goes away once at idle for longer than 5 minutes the Mis starts up and CE starts flashing.
I’ve cleaned and bench tested injectors swapped coils plug’s everything to cylinder 3 and no change same symptom. Compression tested bank every one was 200+ cylinder 7 hung around 185 for around 10 minutes with no loss. Boroscoped 7 while turning both valves are opening and closing as normal.
What else can I check any help would be great I’m pretty stumped at this point.

Sounds like a misfire from compression and not ignition or fueling. The valves might be moving when scoping it, but things can be different when its running, especially with lifters controlled by manipulated oil pressure. I'd suspect a lifter or the solenoid in the VLOM that controls that lifter. Since it's easier than pulling the head, try a new VLOM. Be sure to get the updated design that came out around 2010-2011.

Is it high mileage? Did you take note of the carbon buildup when you had the heads off? Cylinder #7 is the one most affected by the oil consumption through the PCV system. It gets the most of the oil dumped into the intake manifold and cokes up the chamber and piston rings. The rings can stick and wear far more than those in the other seven cylinders, causing weak compression in, increased blow-by, higher PCV pressures, more oil in the PCV system, etc. and the downward spiral feeds itself. Again, compression when static is one thing, while running is another.

Ah, the joys of AFM...

 

[Marbor]

Sounds like a misfire from compression and not ignition or fueling. The valves might be moving when scoping it, but things can be different when its running, especially with lifters controlled by manipulated oil pressure. I'd suspect a lifter or the solenoid in the VLOM that controls that lifter. Since it's easier than pulling the head, try a new VLOM. Be sure to get the updated design that came out around 2010-2011.

Is it high mileage? Did you take note of the carbon buildup when you had the heads off? Cylinder #7 is the one most affected by the oil consumption through the PCV system. It gets the most of the oil dumped into the intake manifold and cokes up the chamber and piston rings. The rings can stick and wear far more than those in the other seven cylinders, causing weak compression in, increased blow-by, higher PCV pressures, more oil in the PCV system, etc. and the downward spiral feeds itself. Again, compression when static is one thing, while running is another.

Ah, the joys of AFM...

Thanks for the response when head was off all cylinders on the bank were similar. I did clean them up with a brass wire wheel. I attached some before and afters. Motor just turned 201k cylinder walls were great still had some cross hatchings,PO travel the Midwest so he stated all highway miles from when he purchased at 60k. I will look into VLOM cost any suggestions were to get one a good cost? Also any way to test the vlom?

 

[OR VietVet]

Sounds to me like a vacuum leak. Use a stethoscope with a hollow tube attachment and a rubber cone on the end and listen for a vacuum leak. Use a squirt bottle and squirt water all around there and see if idle changes. Use a smoke machine if have access.

 

[Marbor]

Sounds to me like a vacuum leak. Use a stethoscope with a hollow tube attachment and a rubber cone on the end and listen for a vacuum leak. Use a squirt bottle and squirt water all around there and see if idle changes. Use a smoke machine if have access.

I have all the tools needed, I didn’t think of a vacuum leak due to good fuel trims

 

[OR VietVet]

Fuel trims look good at all speeds? Double check the spark plug seat and if is tight? Those back plugs sometimes require a long reach to tension down and installer is afraid of side load crack on plug porcelain.

 

[Marbor]

So I figured ide mess around a bit since I’m bored and seen a post online a guy having the same issue on cy6. Said to unplug vlom plug so I did rig runs like shit but oil pressure is better and no misfire so far or the traction control either. May be the vlom
I’m going to run it till it gets up to temp then plug back in and see if the misfire comes back.

 

[Marbor]

Sounds to me like a vacuum leak. Use a stethoscope with a hollow tube attachment and a rubber cone on the end and listen for a vacuum leak. Use a squirt bottle and squirt water all around there and see if idle changes. Use a smoke machine if have access.

Just did some spraying around the intake ports and any plugs connecting to it nothing noticeable.

 

[Marbor]

Just wanted to update the group and see if I’m on the right path. So with vlom plugged in I get the cy 7 misfire and can see them counting on my top don scanner. When I set the rpm to 1k no misfires drove around then block a few time no misfire during acceleration when at a stop I get misfire. Only on 7. When I unplug the vlom and just let idle I get no misfire on counter just a rough idle and the codes for vlom. I am able to actuate the dod cylinders on my scanner and they all click and I can feel the difference in idle once active and not.

My guess is the vlom has clogged ports or something wrong so at idle there’s not enough oil pressure, at or over 1k rpm the pressure builds enough to get through the ports which clears the misfire's. BTW I just changed the oil sending unit and screen with a fresh oil change on Friday due to low oil pressure msg coming on.

So would u swap vlom with this info?
Also I do notice my oil pressure goes down to just below 20 psi on the gauge at operating temps, cold start is 50+ psi then drops normal driving stays around 40ish.
I’ve seen a few post saying this is common and my buddy’s truck at work has the same oil pressure as mine and has had no issue