2000 5.7 GSi - How low is too low compression?

[Glevans1]

Had an overheat due to blocked portside riser (two years old, but that's another issue). The engine began to sputter, shut off immediately but major exhaust elbow and rubber overheat. I now have compression readings all around 190 except for cylinder No. 5 which is 150. It goes up approx 20 psi after adding a little oil before test. My question is whether to just ignore this for now and reassemble with new exhaust manifolds and risers - or to get this fixed up via longblock route. Although an old engine, only has about 500 hrs. Obviously a pretty big price difference but I'm not to happy about depending on this engine in case the overheat has caused other damage that's not apparent right now. Is there a "rule of thumb" for compression readings? Thanks

 

[Glevans1]

Further to this post - after talking to my mechanic, it looks like there was no engine overheating issue involved as the coolant in the reservoir is still full. The probable cause of the engine sputtering was the steam and smoke ingested by the engine when the exhaust rubber melted and failed. Still trying for insurance coverage as the teardown shows that besides the blocked riser, I had water intrusion into the engine through the two year old OSCO manifolds. They have a three year warranty. So, hoping for at least some coverage even if I just replace risers and manifolds.OEM this time. Still interested in anyone's thoughts on how dependable the engine will be with the low cylinder.

 

[aerobat]

the rule of thumb is the lowest cylinder should be within 10% of the highest cylinder. saying that - when it would be my engine i would not panic but observe this engine. a new block is surely not needed - in worst case you burned thru a head gasket on this cylinder but you will not damage the block /piston rings that quickly.

it may be you ran with one cylinder low for years without noticing it and this has nothing in common with your overheat - on a 8 cylinder you may really overlook one "weak" easily. so - when the engine feels and sound right , has no water in oil or in this cylinder i would just go boating.

 

[jerryjerry05]

It's possible that one of the valves on that cylinder got dished or bent??
You can do a leak down test and see if it's a valve.
Myself, I'd go crazy not knowing what's wrong.
Some do nothing about low comp on 1 cyl. It's really not that bad.
At 40# low it's still operating.
It could work for years or it could go next time you start.
Get Sea Tow and go have fun.

Oh yea right now it might just need a valve job.
later it could need a block and pistons?