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2008 6.2 No Compression in two cylinders

mbfireboat

By mbfireboat
in Ask A GM Technician

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mbfireboat Enthusiast

mbfireboat

Posted
Posted

My 2008 Yukon Denali with 105K miles suddenly started running really bad this week. The traction control/stabilitrac light came on at the same time.

I took it to a local shop which said I need to take it to a dealer. They felt possibly it was a bad fuel pump module (which has physical damage) and a new one would have to be programmed by the dealer.

I drove it a few miles to Williamson Cadillac here in Miami. The service advisor called me the next day and said that diagnostics points to no compression in cylinders in 2 and 4. He said that he has seen this several times before with the 6.2 where it is a broken valve spring. However, it will be a week before they can fully check out the engine.

I have always cared for my Yukon with regular maintenance and do not drive it hard.

Obviously the engine is out of warranty but I was wondering how big of an issue is the broken valve and/or spring with the non-AFM 6.2 engine?

I haven't seen much on the internet as everything seems to be related to the AFM engines.

I am hoping it is just a simple valve spring replacement but I am aware that if a valve dropped and hit a piston, I am looking at a new engine. I'll keep you guys posted as I find out more. Thanks.

 

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Doug_Scott Senior Enthusiast

Doug_Scott

Posted
Posted

Two cylinders cannot lose compression from one broken valve spring. Each cylinder has 2 valve springs, neither of which is shared with another cylinder.

 

Personally I would be asking which valve spring was broken? Along with show me the broken spring. Should be very obvious with the valve cover removed. Would also want to see why the cylinder nexxt to it was also showing 0psi compression. What was the cause of that? Given todays technology, there should be pics available showing the damage.

bobejones Enthusiast

bobejones

Posted
Posted

Loss of power coupled with traction light??? Something smells off in the loss of compression diagnosis. ABS sensors use a tach feed back to the PCM to determine wheel slip and retard timing until slip condition resolves.

 

If the you are down on 2 cylinders, the engine will run VERY rough. Not just loss of power, but ROUGH!!! Like shaking because the engine is no longer balanced. You will also have abnormal sounds from the engine.

 

If your engine idles smooth, go find another mechanic and tell that dealer to refund you for wasting your time.

 

Also, you can run a leakdown test on those cylinders at TDC for those cylinders to determine where the leak is. Not rocket science.

vettelovralexand Enthusiast

vettelovralexand

Posted
Posted

If it really is a broken valve spring, it is a super easy fix if you are willing to do it yourself and buy a spring compressing tool. The hardest part is getting setup so you dont drop a valve in a cylinder. They have tools, but some people push rope into the cylinder through the spark plug hole and then bring the cylinder toward tdc to hold the valve up while the spring is swapped.

Crf450r420 Enthusiast

Crf450r420

Posted
Posted

The L92 did have some valve drop issues at one point, but didn't hear a lot of it happening. bobejones is right, if it was not running very rough at idle then you were not down two holes and you need to make sure your not getting screwed.

Jsdirt Senior Enthusiast

Jsdirt

Posted
Posted

If GM is making 100k-mile valve springs now, I'm putting my truck up for sale TOMORROW. :throwup:

Crf450r420 Enthusiast

Crf450r420

Posted
Posted

Valve spring do fail from time to time. Even the insanely expensive ones on top fuel motors. The valve springs are made in Germany and they are the only not expensive beehive spring I'll put on a motor I build.

Jsdirt Senior Enthusiast

Jsdirt

Posted
Posted

Yeah ... but your talkin' mega-lift with a camshaft that wouldn't even fit in our engines. My engine is 100% stock. I've seen engines with 300 -400k+ miles with all original internals - granted, not one was built in the 21st century ...

 

No excuse for breaking a valve spring on a stock engine, let alone at 100k miles! I've seen them wear out and get weak .. but never break. There's cars out there 100+ years old with original valve springs, still running!

 

This is just a blatant example of a company trying to save a buck by nickel-and-diming things they shouldn't.

Jsdirt Senior Enthusiast

Jsdirt

Posted
Posted

Doesn't matter where they came from - they shouldn't be breaking on a STOCK engine.

Jsdirt Senior Enthusiast

Jsdirt

Posted
Posted

Thanks for the tip pal. Good to see another sucker on here with his head up GM's ass. Throw another $60 grand at 'em - they love ya! :lol:

 

Proof positive - there really is an ass for every seat .... :seeya:

jlcms2007 Enthusiast

jlcms2007

Posted
Posted
Loss of power coupled with traction light??? Something smells off in the loss of compression diagnosis. ABS sensors use a tach feed back to the PCM to determine wheel slip and retard timing until slip condition resolves.

 

If the you are down on 2 cylinders, the engine will run VERY rough. Not just loss of power, but ROUGH!!! Like shaking because the engine is no longer balanced. You will also have abnormal sounds from the engine.

 

If your engine idles smooth, go find another mechanic and tell that dealer to refund you for wasting your time.

 

Also, you can run a leakdown test on those cylinders at TDC for those cylinders to determine where the leak is. Not rocket science. [/quote

 

 

Reread his original post, and yes for whatever reason when ecm picks up misfires it shuts traction control down, tcc engagement and some other functions to somewhat "mask" horrible drivability.

 

 

Sent from my iPhone using Tapatalk

gmtech117 Enthusiast

gmtech117

Posted
Posted

Correct, when the ecm sets a code for misfiring, it also sends a message to abs module And will set the traction light on.

 

A broken valve spring is obviously a very easy thing to visually see, so if you doubt the dealer get them to show you. Broken springs are not uncommon. It's a moving part, parts fail, weather you maintain or not. I would be hoping all that is wrong is the spring, will cost you some labor but atleast it shouldn't be any real expensive parts involved.

 

Good luck, let us know what they come up with.

Doug_Scott Senior Enthusiast

Doug_Scott

Posted
Posted

Extremeley easy to change a valve spring, without using rope. Take your compression tester adaptor, screw into plug hole, loosen both rockers so valves are not being held open, and connect air line to adaptor. Quality compression testers have a standard air line fitting on the adaptor.

 

The only time I would expect to see a stock engine have a valve spring fail by breaking is on first run in. A defect that causes a break will be in manufacturing, and will show up right away. Stock valve springs are one of those parts that is not where there is any money to save. Cost per failure is far too high.

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