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

 

[iamdub]

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?

All sounds good. I tore mine down to delete AFM right at 200K. The cylinders looked great and all the compression numbers were very good. The VLOM is the control center for the AFM and the first design had flaws which GM greatly improved on. They also improved the design of the AFM lifters.

I don't have the part number for the updated VLOM. But, I'd definitely get a GM Genuine and not aftermarket. The extra cost far outweighs the risk. The VLOM failing and having the lifters pumped or collapsed when they should or shouldn't be is how these engines experience minor damage such as a bent push rod or get completely destroyed by bent valves and cracked pistons.

 

[iamdub]

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

I'd definitely swap the VLOM. Again, be sure you get the updated one. This may even improve your oil pressures. Although, they're fine as-is.

 

[Marbor]

Hey all

Just wanted to update install new vlom and now no change but now I get more random misfires across all cylinders but cy7 still has the most. And oil pressure is hanging at about 10psi at O.T I did just run it on the highway for a good 40 miles and she drove great no misses but once I stop I see the misses on the counter and oil pressure drops. No CEL or track control. But she’s running louder now may be time for new engine.
Hears a short I created

 

[iamdub]

Hey all

Just wanted to update install new vlom and now no change but now I get more random misfires across all cylinders but cy7 still has the most. And oil pressure is hanging at about 10psi at O.T I did just run it on the highway for a good 40 miles and she drove great no misses but once I stop I see the misses on the counter and oil pressure drops. No CEL or track control. But she’s running louder now may be time for new engine.
Hears a short I created

Dammit. I was really hoping it was just the classic VLOM failure. That really sounds like a lifter or something in the valve train. Now I'm wondering if the old VLOM failed and took out a lifter and just did the same thing to the new lifter(s). Or, one of your new components is defective. Popped the rocker covers off yet to check the springs? Get a breaker bar and large socket and turn the engine while watching all the movements of the rockers. They should all cycle up and down the same amounts. Did you look at the cam lobes while you had the VLOM off? Any glitter in the oil on the dipstick?

 

[Marbor]

Dammit. I was really hoping it was just the classic VLOM failure. That really sounds like a lifter or something in the valve train. Now I'm wondering if the old VLOM failed and took out a lifter and just did the same thing to the new lifter(s). Or, one of your new components is defective. Popped the rocker covers off yet to check the springs? Get a breaker bar and large socket and turn the engine while watching all the movements of the rockers. They should all cycle up and down the same amounts. Did you look at the cam lobes while you had the VLOM off? Any glitter in the oil on the dipstick?

Yes I did check the cam lobes while vlom was off with my boroscoped all look good doesn’t look like it’s getting eaten up, also pulled VC prior to vlom replace with engine running and verified all lifters are functional no dead ones and oil is clean no glitter.
I’m guessing it’s probably something with the cylinder possibly a hairline crack something I did wrong with the install of the pickup tube Oring or just a bad oil pump from the manufacturer.
Here’s a cold start video doesn’t sound as bad with OP just when it drops to below 15psi she starts chattering, with rpm over 1k the chatter is minimal but still has somewhat of a tick. Idk I’m at a lost now.
I may do a last ditch effort and pull the pan and oil pump just to see and possibly get the part warranty and if that doesn’t fix I’ll just pull the engine and find something used which I’m trying to avoid.
Again it only misfires when OP is below 15psi and operating temps once moving rides fine.

 

[iamdub]

Yes I did check the cam lobes while vlom was off with my boroscoped all look good doesn’t look like it’s getting eaten up, also pulled VC prior to vlom replace with engine running and verified all lifters are functional no dead ones and oil is clean no glitter.
I’m guessing it’s probably something with the cylinder possibly a hairline crack something I did wrong with the install of the pickup tube Oring or just a bad oil pump from the manufacturer.
Here’s a cold start video doesn’t sound as bad with OP just when it drops to below 15psi she starts chattering, with rpm over 1k the chatter is minimal but still has somewhat of a tick. Idk I’m at a lost now.
I may do a last ditch effort and pull the pan and oil pump just to see and possibly get the part warranty and if that doesn’t fix I’ll just pull the engine and find something used which I’m trying to avoid.
Again it only misfires when OP is below 15psi and operating temps once moving rides fine.

Overfill the oil by 1.5-2 quarts and put your Suburban in a nosedive. See if the idle oil pressure and ticking improves. The lifters are hydraulic so low oil pressure can basically be like they're collapsed. For a regular lifter, this is usually just noisy. An AFM lifter can collapse enough to reduce or stop the valve from opening. Maybe the low pressure and continued operation has damaged an AFM lifter. If a botched O-ring replacement or faulty pump is the cause of the low pressure, that can be fixed easily (but aggravatingly) enough. I'm just thinking you might have subsequent damage. From where did you source and what brand are the lifters you used?

 

[Marbor]

Overfill the oil by 1.5-2 quarts and put your Suburban in a nosedive. See if the idle oil pressure and ticking improves. The lifters are hydraulic so low oil pressure can basically be like they're collapsed. For a regular lifter, this is usually just noisy. An AFM lifter can collapse enough to reduce or stop the valve from opening. Maybe the low pressure and continued operation has damaged an AFM lifter. If a botched O-ring replacement or faulty pump is the cause of the low pressure, that can be fixed easily (but aggravatingly) enough. I'm just thinking you might have subsequent damage. From where did you source and what brand are the lifters you used?

Thanks, I just did 2qt and my driveway is at a 15 deg slope so left at a nose dive and no change in pressure. Parts were kit I found online don’t know the brand 1st mistake form what I’ve read after the fact.
FYI I just reinstalled the old vlom and oil sending unit without the mini screen and seems the misfire went away so I left it running in the drive way for an hour periodically checking and she’s idling smooth just the oil pressure is sitting low see pic but it hasn’t misfired in over an hour idk what i did but she still sounds like a typewriter.

 

[iamdub]

Thanks, I just did 2qt and my driveway is at a 15 deg slope so left at a nose dive and no change in pressure. Parts were kit I found online don’t know the brand 1st mistake form what I’ve read after the fact.
FYI I just reinstalled the old vlom and oil sending unit without the mini screen and seems the misfire went away so I left it running in the drive way for an hour periodically checking and she’s idling smooth just the oil pressure is sitting low see pic but it hasn’t misfired in over an hour idk what i did but she still sounds like a typewriter.View attachment 416705

Oof. Yeah, it's well known that using aftermarket major internal components isn't worth the risk. It's definitely a do it right or do it twice thing. The videos really do sound like a lifter(s), but we still gotta rule out all the other things. Have you verified the spark plugs and wires were tight? Air being pushed out of a cylinder around the spark plug threads can make a clicking sound. The spark arcing from a loose wire can click, too. Both can cause a weak (but not dead) cylinder that might smooth out enough at higher RPM to not be as detectable.

 

[Marbor]

Thanks haven’t done the spark arc test yet may look unplug everything tomorrow and re tighten/verify all ignition components are sound. See pic now it has multi Misfires at 1st it was just 3-7 after restart it seems to be across the board.

 

[iamdub]

Thanks haven’t done the spark arc test yet may look unplug everything tomorrow and re tighten/verify all ignition components are sound. See pic now it has multi Misfires at 1st it was just 3-7 after restart it seems to be across the board.View attachment 416742

And this steers me back to an AFM-related issue. AFM cylinders are 1, 4, 6 and 7. Yes, there's a #8, but its count is much lower and could be a false detection from the adjoining cylinders. Or, it's the low oil pressure affecting all the lifters but affecting the AFM ones more.