Low compression 5.3L gen IV Cylinder 7

[FamilyForce6]

Hi all,
My 2010 Tahoe started developing a random misfire related to cylinder 7. I went through the usual causes but couldn't track it down so I took into a shop. They stated that the cylinder had low compression, which came when they added a little oil to the top of the piston. Bad rings was there diagnosis. I was really surprised because I keep on my oil changes and only use Wix filters and full synthetic. After looking at the cost of replacement engines I'm shocked and saddened ($8-$9K with labor!)

In my research I came across a TSB that actually deals with this type of issue. It states that cylinder 1 and/or 7 can get overwhelmed with oil due to valve cover design (I replaced with the updated design about 3 months ago) or due to excess oil spray from the AFM pressure relief valve in the oil pan. Interestingly enough, I disabled AFM when I purchased the vehicle.

The TSB recommends trying to put a solvent/cleaner in the top of the cylinder and soak it for 2-3 hours in addition to installing a shield over the AFM valve (though it says the problem is likely only to happen w/ aluminum blocks and not my iron block).

Any recommendations on how to proceed? Should I try to install a shield over the AFM valve (or plug it completely as others have done)? I am going to try cleaning from the top of the cylinder, but any recommendations on the product they'd recommend using would be appreciated since GM doesn't manufacture their recommended product anymore.

Additional helpful info:
* 165,000 miles on the clock (purchased 5 years ago with 125,000 miles)
* AFM disable done almost immediately after purchasing
* Regular oil changes and maintenance. (full synthetic and wix filters)

Thanks community!

 

[Fless]

@donjetman has great info about this -- I think he has a thread documenting what he did (other than the one below).

Mystery Oil Loss - Where is the oil going? 2008 Yukon Denali

Kind of sort of full bore.... But actually through a metered orifice on the models without the PCV valve. Not sure what year they drop the PCV valve, around 2004 or 2005? That orifice is built into the driver side valve cover. if you have a lot of blow by it could potentially get plugged...

www.tahoeyukonforum.com

 

[rdezs]

Before pulling the oil pan to deal with the shield over the oil pressure relief valve, I would 100% focus on getting those rings freed up. I would start with GM's recommendation, they've had some success with it. Then if necessary move on to try other products. Hopefully you get the compression back.

If successful, I would say move forward with an AFM delete. And plug that relief valve at that point.

On an AFM delete I did recently for my neighbor on a 2010 6.2, here's a photo of cylinder number 7.... Had a lot more carbon buildup than the rest, but no compression issue yet

His is a 2010, just under 200,000 miles, and at some point the TSBs were addressed with the updated valve cover and shield over the relief valve in the pan.

Total cost in parts was under $2,000, which included a lot of things to do while in there. New water pump, radiator, all hoses, serpentine belt kit, all new OEM sensors in sending units, machine shop work on the head, etc

 

[j91z28d1]

They stated that the cylinder had low compression, which came when they added a little oil to the top of the piston. Bad rings was there diagnosis.


I have questions. what exactly did they do?

 

[rdezs]

I assume from his post they did the standard diagnosis by adding oil to the cylinder and rechecking compression... And he's inquiring what the best course of action is to address it.
Bad rings doesn't necessarily mean they're broken and or missing, in an LS engine it often means the ring is stuck compressed into its groove often from carbon buildup. If his compression had not gone up, then it points to the head gasket or the head/valves.

A lot of people are fully capable of digging into the engine themselves, but lack the diagnostic techniques to figure out exactly what's wrong, and $200 for a shop to diagnose is often very convenient. Sometimes they get it wrong, but low compression of one cylinder isn't that difficult to get him pointed in the right direction.

 

[Marky Dissod]

@FamilyForce6
If it's worth the time / energy / money to install the shield over the AFM valve,
seems like it can't go wrong, even if you do plug it, as the concern is controlling directional oil spray.

More importantly, think you are looking for cleaners that contain Poly Ether Amines.
At the time of this writing, this list should work, but may need updating as time passes:
Red Line SI-1 Complete Fuel System Cleaner
Chevron Techron Concentrate Plus 01154100-5179P
Gumout Regane High Mileage Fuel System Cleaner
Gumout Regane Complete Fuel System Cleaner
Gumout All-In-One Complete Fuel System Cleaner
Lucas Oil 10512 Deep Clean Fuel System Cleaner
LubeGard Fuel System Booster

You did not mention if your 2010 Tahoe is 'FlexFuel compatible' or not.
If it is, and E85 is at least 20% cheaper than the fuel you typically use, then use E85; it burns cleaner.
Whether or not you can use E85, feel free to use any or all of the cleaners above.

You mentioned 'Regular oil changes and maintenance (full synthetic & Wix filters)',
without mentioning which full synthetic (is it a Group3 or Group4 synthetic?)
or how often you actually change the oil. Valvoline Restore & Protect 5W30 comes very highly recommended.

 

[j91z28d1]

I assume from his post they did the standard diagnosis by adding oil to the cylinder and rechecking compression... And he's inquiring what the best course of action is to address it.
Bad rings doesn't necessarily mean they're broken and or missing, in an LS engine it often means the ring is stuck compressed into its groove often from carbon buildup. If his compression had not gone up, then it points to the head gasket or the head/valves.

A lot of people are fully capable of digging into the engine themselves, but lack the diagnostic techniques to figure out exactly what's wrong, and $200 for a shop to diagnose is often very convenient. Sometimes they get it wrong, but low compression of one cylinder isn't that difficult to get him pointed in the right direction.

I trust no shop anywhere. so I'd would honestly be amazed if they went as far as actually doing a compression test, let alone add oil and redo it.


that's why I asked for details. like did they actually say they did that, or did he happen to see them do it.

 

[donjetman]

@FamilyForce6
The TSB you posted above has been revised or superseded twice with:
https://static.nhtsa.gov/odi/tsbs/2014/MC-10114272-9999.pdf

When I did the piston and combustion chamber soak in 2018 I used Gunk brand Motor Medic and I soaked mine for 2 days.
In 2018 at 130k miles the TSB immediately cured my oil consumption, and today at 200k miles it still doesn't need any makeup oil between 5k mile oil change intervals.

 

[rdezs]

While the TSB's are an indicator that GM recognizes an issue, something tells me that if you change your oil before reaching the 50% of oil life left according to the meter, you're far less likely to have these issues in the long run. I don't have any hard data to back that up, just personal experience with a few different LS engines.... I never let mine go beyond 4500 miles even with synthetic. I think oil is cheap when taken into account the extended lifespan of an engine that receives frequent changes.

 

[donjetman]

GM sold new 2007 thru 2011 LS powered vehicles with defective driver side pcv valve cover that allows oil into the combustion chambers and sticks the ring(s) in their piston groove. How often and what oil is used has little to zero effect on the problem. Doing the TSB will free the ring(s) and installing the new improved driver side valve cover as stated in the TSB is the fix. I have personal experience with stuck rings caused by the stock original DS valve cover with our 07 Denali.

Since buying our 07 Denali in 2018 w/130k miles, I have OLM etc data from putting 70k miles on it and results from 8 used oil analysis reports.

At some point in time? GM changed the OLM % algorithm to 50% of what it was when it left the factory. My 07 hasn't ever had this software algorithm change, so I have learned with hard data to change its oil about every 5,000 miles which is at about 50% on the OLM. I don't tow or hotrod with it.

 

[Marky Dissod]

@donjetman, is it reasonably possible for a Gen3 V8 (L59) to make use of the Gen4 post-TSB valvecovers?

(Always thought that the conceptual logic of the Oil Life Monitor was sound, except that it was simply too lenient.
It's nice to know that, at some point in time, GM agreed with me.)

 

[donjetman]

@donjetman, is it reasonably possible for a Gen3 V8 (L59) to make use of the Gen4 post-TSB valvecovers?

A TSB valve cover should work good.

2.5 yrs ago I bought a stock 2002 C5 Corvette w/gen3 LS1 w/60k miles. When I started driving it the 1,000 miles each way between our TX & CO homes I was bummed to see it was burning 1 qt of oil per 2,500 miles.
I replaced the stock pcv valve on the stock 2002 gen3 LS1 with a "fixed orifice" pcv valve, #12572717. I also installed a catch can and did a piston/combustion chamber soak. These measures have reduced consumption to 1/2 qt per 5,000 miles.

If these changes hadn't work as well as they have I was gonna install one of the improved TSB gen4 valve covers.

 

[solli5pack]

Shouldn't the shop have done a leakdown test to see where the loss of compression is going?

 

[j91z28d1]

A TSB valve cover should work good.

2.5 yrs ago I bought a stock 2002 C5 Corvette w/gen3 LS1 w/60k miles. When I started driving it the 1,000 miles each way between our TX & CO homes I was bummed to see it was burning 1 qt of oil per 2,500 miles.
I replaced the stock pcv valve on the stock 2002 gen3 LS1 with a "fixed orifice" pcv valve, #12572717. I also installed a catch can and did a piston/combustion chamber soak. These measures have reduced consumption to 1/2 qt per 5,000 miles.

If these changes hadn't work as well as they have I was gonna install one of the improved TSB gen4 valve covers.

in theory the ls6 valley cover swap would be better than a newer valve cover. but I just spent a few days in the mountains with my buddy 04 ls6 zo6 and his 30$ Amazon catch can was full at the end of each day. so I'm not sure it's any better in practice.


I feel like the best LS I've been around for not drinking oil was by 02 ls1 camaro. got it new, left it bone stock, over a 100k of very hard miles. I never changed oil before the light popped up, beat the car to death and don't ever remember adding oil between changes. and literally used any oil that was cheap to free except the one before a trip I knew it would be extremely abused, including towing the bike trailer there.


how or why gm consistently made the ls pvc system worse and worse as the years when by is byond me.


have you considered a catch can on the clean side of your c5? or maybe the clean air separator they call it that replaces the oil cap with a mesh filled housing before running to the air box port? with such low mileage and lack of abuse. I feel like it's gotta be using the half qt thru the pvc system and not the rings?

 

[rdezs]

I think it was simply too much PCV the way they had it set up. I have a 2003 lq4 in a Hummer, that always had the typical little pool in the intake manifold of oil. Updated to the same valve cover as on our 2014 Escalade, at the same time I went through the heads.... Under 113,000 miles, I thought the deposits on the intake valves stems were a little excessive. Started chasing a problem with condensation building in the crankcase, verified by oil sample analysis. Then I realized the new valve covers have a real small orifice.... With engine running I put a vacuum gauge on the fresh air side and it took about 1 minute to even register a couple inches of vacuum. Drilled that orifice out to 3/16ths.... Condensation issue disappeared, and a nice steady vacuum on the fresh air side. The fresh air intake to the engine is downstream from the mass air flow sensor, which apparently adjusted and it runs just fine. Also, I don't get the oil pulling in the intake. Uses zero oil in 3500 miles. My guess.... Just a guess.... Is that excess oil in the intake drips down into cylinder one or seven when the engine shut off, maybe causing the issue with the rings in those cylinders. This is way before AFM existed, no relief valve in the pan to be spraying oil upwards under the piston.

Not 100% sure of the dynamics behind GM's problem, but it does seem to be an issue with the right amount of crankcase ventilation.

On most modern engines, if you pull the dipstick with the engine running you should hear the idle change as you create a vacuum leak. That's why you have that little o-ring around the dipstick. That was my first clue the updated valve cover wasn't quite adequate...

 

[j91z28d1]

I think it was simply too much PCV the way they had it set up. I have a 2003 lq4 in a Hummer, that always had the typical little pool in the intake manifold of oil. Updated to the same valve cover as on our 2014 Escalade, at the same time I went through the heads.... Under 113,000 miles, I thought the deposits on the intake valves stems were a little excessive. Started chasing a problem with condensation building in the crankcase, verified by oil sample analysis. Then I realized the new valve covers have a real small orifice.... With engine running I put a vacuum gauge on the fresh air side and it took about 1 minute to even register a couple inches of vacuum. Drilled that orifice out to 3/16ths.... Condensation issue disappeared, and a nice steady vacuum on the fresh air side. The fresh air intake to the engine is downstream from the mass air flow sensor, which apparently adjusted and it runs just fine. Also, I don't get the oil pulling in the intake. Uses zero oil in 3500 miles. My guess.... Just a guess.... Is that excess oil in the intake drips down into cylinder one or seven when the engine shut off, maybe causing the issue with the rings in those cylinders. This is way before AFM existed, no relief valve in the pan to be spraying oil upwards under the piston.

Not 100% sure of the dynamics behind GM's problem, but it does seem to be an issue with the right amount of crankcase ventilation.

On most modern engines, if you pull the dipstick with the engine running you should hear the idle change as you create a vacuum leak. That's why you have that little o-ring around the dipstick. That was my first clue the updated valve cover wasn't quite adequate...

it's a problem across the board too. my neighbor has a gm 4 cyl ecotech or whatever they are called. the pvc port in the head is plugged up. very common from my research and messes everything up. blows out crank shaft seal and stuff. fix seems to be pull half the engine apart to poke it out with a wire and then install a catch can.

I Just pulled the intake off a v10 Ford at work to replace a leaking intake manifold gasket. poured what felt like a qt of oil out the intake.


I still feel like the old rattle pvc valve was good at blocking oil, or at least helped.

 

[donjetman]

have you considered a catch can on the clean side of your c5? or maybe the clean air separator they call it that replaces the oil cap with a mesh filled housing before running to the air box port? with such low mileage and lack of abuse. I feel like it's gotta be using the half qt thru the pvc system and not the rings?

No
My dirty side catch can doesn't catch much between oil changes.

 

[j91z28d1]

No
My dirty side catch can doesn't catch much between oil changes.
View attachment 455538

ahh. you think the half qt is just getting by the rings?

 

[donjetman]

ahh. you think the half qt is just getting by the rings?

yes
In 2024 I ran a mix of M1 and Val PB "Restore" to cleanup the rings lands, etc. I believe it worked (less consumption).
In 2025 I'm running HPL 5w-30 PCEO, a top shelf oil brand, made with premium ingredients.
Next UOA report for the LS1 will be in Sept 2025.

 

[j91z28d1]

interesting.

be curious to see what you see