Loss of compression multiple cylinders

[Playa]

Ok here is what i obtained from the mechanic yesterday, hopefully someone more knowledgeable will be able to help better understand what is going on and potential causes.

Affected cylinders are:
#7- 20 lbs compression
#5- 90 lbs compression
#4- 100 lbs compression

Mechanic says minimum compression should be 150 lbs, but anything under 170 is considered problematic.

Computer code was PO300 Random Cylinder Misfire

Educate me

[texanatc]

.

[Low Fence]

I still think it's lifters failing

Known problem in that engine

[Bill]

Figure it out?


μολν λαβ

[BTLowry]

I would definitely get a second opinion

This new stuff is the devil, had a Ford that acted like transmission was slipping. Turned out to be a speed sensor
Computerized stuff can send you on a wild goose chase

Good luck with it, maybe it will turn out to be something cheap

[sailor]

Ask the mechanic what the compression was, he's probably guessing. Could even be a plugged catalytic converter. Get another opinion

This........

[Playa]

Figure it out?


μολν λαβ

Not yet. Holidays were looming and had travel plans out of town. Getting a second opinion early next week.

New engine cost $8,000 for parts and labor. Blue book value of truck is $7,000. That math doesent really work out. My wife has told me to quit fretting about having a car payment for the 1st time in 4 years and to start enjoying the thought of having a new truck LOL!

There are few things I hate worse in the world than buying a vehicle!

[muzzlebrake]

Sounds like a valve issue. Cam chain, lifters collapsing are possibilities. The 5.3 engine cylinder deactivation feature causes the engine to suck oil past the rings on the cylinders that are deactivated and after lots of miles the carbon build up on the valves can lead to the exhaust valve not closing good. A good valve job may fix the problem and is not all that expensive to do. Once fixed disable the cylinder deactivation. The cylinder deactivation sill also cause the engine to use oil more than normal, sometimes as much as a quart every 500 to 1000 miles.

[muzzlebrake]

The reason the cylinder deactivation cause oil consumption is when the cylinder is not firing and is acting like an air pump there is no combustion pressure on top of the compression ring to keep it expanded against the cylinder wall. Once this happens the crankcase pressure will push oil up past the oil and scraper ring and is sucked into the combustion chamber during intake stroke. This oil is burned and leave slots of carbon deposits especially if using dinosaur oil(non synthetic). Over time it will load up on the valves and valve seats and require a valve job to clean it up. It can also lead to failure of the catalytic converters and O2 sensors in the exhaust. Cylinder deactivation was a not so smart idea from GM IMO and most engines get better mileage without it since the engine is working at max efficiency with all holes lit up.

[acearrow]

The reason the cylinder deactivation cause oil consumption is when the cylinder is not firing and is acting like an air pump there is no combustion pressure on top of the compression ring to keep it expanded against the cylinder wall. Once this happens the crankcase pressure will push oil up past the oil and scraper ring and is sucked into the combustion chamber during intake stroke. This oil is burned and leave slots of carbon deposits especially if using dinosaur oil(non synthetic). Over time it will load up on the valves and valve seats and require a valve job to clean it up. It can also lead to failure of the catalytic converters and O2 sensors in the exhaust. Cylinder deactivation was a not so smart idea from GM IMO and most engines get better mileage without it since the engine is working at max efficiency with all holes lit up.

obviously someone who has turned a wrench or two. Sounds like expert information there and a reason to get a second opinion.

[muzzlebrake]

On this link you can see the spring loaded lifters of the Gen IV engine that has cylinder deactivation. Scroll down. These are the lifters that have been failing after high miles. IMO the Gen III was a better engine except for some issues they had with heat treating of the cam lobes and knock sensor locations that would get corroded terminals.

http://www.superchevy.com/how-to/engines-drivetrain/1603-ls-engine-tips-gen-iii-vs-iv/

I do prefer the locations of the knock sensors and cam position sensor of the Gen IV engines since it makes them accessible without having to remove the intake manifold.
Even with the few shortcomings mentioned the modular 5.3L Gen III, IV and now the V are arguably the best V8 engines ever produced by any manufacturer both for fuel economy and longevity.

These are real Hemi Killers that can produce well over 400hp naturally aspirated with just a few mods.