Compression Test Results - Need Info

[ViPER1313]

I tested the compression on an SHO I am buying for my buddy today - the car is not currently running. All cylinders were 210 w/ the exception of the far left one on the front bank, which was at 150. The cylinder's compression went up to 190 after a cap of oil was dumped into it. I know that the rings are probably shot, I just need to know:

-Other than some smoke, how badly will this low compression affect drivability?
-Is it possible to replace a piston / rod from the bottom of the engine?

 

[Off Road SHO]

I know that the rings are probably shot, I just need to know:

-Other than some smoke, how badly will this low compression affect drivability?
-Is it possible to replace a piston / rod from the bottom of the engine?

It'll feel like you have an extra drag on the motor, kind of like having the AC on all of the time. It will be okay, but how did this happen? The SHO motor is VERY tough.

No, there isn't enough room to get a piston out the bottom, too much bracing and the crank is in the way. Also, you need to compress the new rings into their respective grooves (with a piston ring compressor) and then carefully slide the rod and piston into the cylinder. This has to be done from the flat side (head side) of the block, not the crank side, which is very bumpy.

Tom

 

[ViPER1313]

I have no history on this car, other than that it is a 89 with a 93k actual mileage title (my guess is 193k by the seat wear though) and that the car's engine seems well maintained (no oil in plugwells, Protech sticker on the intake.)

Could this be something else other than the rings? I would think a HG problem would affect 2 or more cylinders and / or putting oil in the cylinder would not improve the results. Would a cracked piston hold any compression? I doubt a bent valve would allow any compression either.

I have no idea why the rings would fail - guess I'll have to get it started to find out

 

[Mike Jones Who?]

i thought to give an accurate comp test you need to warm it up, so that way all the metal expands and seals up correctly

 

[AREA 91]

Engine up to operating temperature, DIS disconnected, throttle body at WOT, and your ready!

 

[Off Road SHO]

Engine up to operating temperature, DIS disconnected, throttle body at WOT, and your ready!

That is basically true, however if the engine won't start and stay running a cold test will tell you what you need to know. In this case, he has one cylinder much lower than the others and it comes up quite a bit with a little oil. This points to the rings since oil will temporarily seal bad rings, but it doesn't seal bad valves. Sorry about your discovery. You could always find someone with a borescope to take a peek inside that cylinder and look for the dreaded scoring.

Tom

 

[SonicRiot]

A wet compression test on a V engine is innacurrate, at best. The oil settles in the lowest point in the cylinder and can create poor, unrepeatable results.

A cranking compression of 150 can easily jump to 190 just by a few starts. The important numbers are comparing the first "puff" to the third "puff" and figuring out the maximum difference amongst the cylinders.

To more accurately find the condition of the engine, run a cylinder leakdown test.

MOST LIKELY, if the rings were shot, ALL of the cylinders would exhibit low cranking compression from wear.

One cylinder showing low cranking compression could indicate a serious problem. It could be as simple as a leaking headgasket or the flip side, however is that there was detonation and a ring, ring landing, valve, or piston broke.


210 and 190 are not excessivley deviant numbers to obtain. The engine might just need a valve adjustment. I certainly would not condemn the engine at hand for numbers such as the ones above without further investigation.

 

[Off Road SHO]

Maybe I mis read his first post; I thought he got 210 and 150.

Tom

 

[SonicRiot]

At first, but a few cranks could easily bugger that number, which is why I indicated a leakdown test should be performed.

The rest of the crap in my post was supplementary to help interpret the numbers at hand.

 

[ViPER1313]

I'll know by the end of the week how well it runs. I'm not to worried - I would be happy if it was a head gasket rather than the rings. Leakdown test was not possible given the time constraints to look at the car, the oil is not milky, and the radiator was not connected so I couldn't see if there was oil in the antifreeze. Thanks for the help though, hope this thing runs alright

 

[ViPER1313]

I have no idea why the compression numbers for that cylinder were so low - the car only burns a tiny bit of oil when you get on and off the throttle repeatedly and quickly (valve stem seals) and the head gasket is fine Runs like a bat out of **** and gets a 9 on the cylinder balance test