Lost compression cylinder four

[boredwild]

2005 envoy with 190,000 miles, suddenly misfiring. Pressurization of valve cover, resulting in oil pushing out between head and valve cover. Lots of oil intake tract (resonator).

Compression test results:

1 - 170
2 - 130
3 - 190
4 - zero
5 - 195
6 - 190

Any chance this could be a head gasket issue?
Opinions based on experience appreciated.

 

[mrrsm]

Always that Chance...

Some Suggestions for Follow Up Diagnostics:

(1) Avoid running the Motor... PERIOD.
(2) Drain the Engine Oil... Pull the Oil Filter and perform an Oil Filter Autopsy with a Magnet.***
(3) Look into the Oil Drain Pan for any signs of Water-Coolant in Quantity & Floating Alum Flakes.
(4) Turn the Engine over via the Crankshaft Bolt (in a CW Direction Only) to TDC on Cylinder #4.
(5) Pull the #4 COP & Spark Plug and Poke a 7-9mm Fiber-Optic Bore-Scope down inside.
(6) Any Bright and Shiny Clean Piston Top Areas will reverse vector to the Head Gasket Leak.
(7) THIS Observation proves a Head Gasket Leak and a HOT Coolant Spray onto the Piston.
(8) Open the Radiator Cap when Ice Cold and look for Oil Droplets Floating on the Coolant.
(9) Open the Coolant Reservoir and look for Oil Droplets floating on the Coolant.
(10) Look underneath the Vehicle for signs of a Con-Rod Cracking-Thrown THRU the Block.

Consider that if the Piston Top in the #4 Cylinder is Scoured CLEAN that either the Engine has Over-Heated and the Aluminum Engine Head Warped... or that the Water Pump and Coolant Thermostat failed and the engine was run HOT.

Ordinarily... with the Head Gasket being of an MLS Design of Multi-Layered-Steel (Coated in a Ceramic Like Paste when Brand New around the Upper Rims of the MLS Cylinder Holes)... it will be very unlikely that IT failed from any Hot Gas Erosion. Broken Head Bolts may cause damage from a Turbo-Charged or Super-Charged System causing Detonation. NON-NA (Natural Aspiration) style Intakes will tend to suffer in this way.

Another possibility is having a Melted Through Piston Top caused by Very High Secondary Voltage coming from a durable COP Electrically Arcing High Voltage into the #4 Aluminum Cast Piston or around the Valve Seat(s).

Lastly... if NO Oil is found in the Coolant and the #4 Piston Top is still covered in Black Carbon ... a Broken Rocker Arm in the #4 Intake and Exhaust Valve location(s) of the Valve Train might be involved as this HAS happened on several prior occasions.

Pulling the Valve Cover allows for total access to the Valve Train to investigate this possibility. The very last possibility is either a Burned Exhaust Valve, Bent Valve Stem... or a Loosened Valve Seat which HAS happened on some of the early Model GMT360 2002-2004 Engine Heads.

*** How To Perform an Oil Filter Autopsy

How To "Autopsy" an Oil Filter

We have been having a fairly steady increase in the number of Engine Problems with the GM Atlas LL8 Motors that all eventually leave the Member Owners wondering about whether the problems involve the Lubrication System and prompt the Head Scratching Question of “I wonder what the Hell is Really...

gmtnation.com

 

[Mooseman]

Only thing I could add is that it could also be a burnt valve. It's pretty rare to see 0 compression, even with a blown head gasket or a poor sealing valve. A broken valve head or dropped valve would have caused horrible carnage in the cylinder. To be able to see the valves, you would need a borescope with dual cameras (one forward and one rearwards).

And your #2 compression doesn't look too healthy neither. I'd look into that one too before investing a lot of time and money into it.

 

[boredwild]

Thanks for your insights fellas. Borescope revealed number four piston broken.

 

[mrrsm]

Just a "Nickel's Worth of FREE Advice..." The GM Atlas LL8 4.2L Engines and the other 4 & 5 Cylinder Variants are NOT the Engines to begin a "How To Re- Build My Trailblazer Motor" Project for any Novice Mechanics. Browse around at GMT Nation for the few Major Engine R&R Re-Build Threads for confirmation of this unfortunate reality. THIS Engine is Complicated and Touchy to Work On, once you dig deep inside of it ...and it will become VERY Expensive to Rebuild "From Scratch", which in your present situation, would almost certainly become a necessity.

As an Alternative... Many other Members with similar experiences would instead suggest that you either purchase a "CRATE ENGINE" @ $3,500 to $5,000... or Locate a Wrecked SUV that has an "Engine that was RUNNING at the time of the Wreck" rather than buying a doubtful Pulled Motor off a Salvage Yard Motor & Transmission Inventory from an SUV that sat for 10 Years and was Towed Away for Pennies:

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The Later Model Motors will present you with even greater difficulties due to significant differences in their Engine Timing Designs and certain components that will NOT Mix & Match well with the Early Model GMT360s. This includes the issues with requiring different Engine Harnesses and the additional PCM-BCM issues. Find a reasonably Low Mileage Motor within your present Vintage while it is STILL inside of the Wreck ...and really look things over B4 you Buy IT.

Plan on spending between $1,000 to $1,500 on a Motor that has a Decent Warranty and one that shows NO SIGNS of a Front Head-On or Front Side Near Head On Collision damages that can add insult or injury in the Engine/4L60E and Drive Train. Believe me... I am NOT trying to discourage anyone from doing this work on their own. But... The Learning Curve is STEEP and your Pockets WILL Need to Be DEEP.

 

[boredwild]

Thanks for sharing your wisdom. I will not be looking to rebuild this engine. Maybe I will look for a complete unit with less miles. No pressure at this point as we have replacement vehicle.