Low Compression problems

[DoriftoTSi]

I compression tested my car tonight and i got really dissapointing numbers.
I let the car crank over 4 or so times before i let off. It read at 125 for all as its peak, and it wasn't able to sustain it for long, it would just drop very quickly. Then we put some oil into the spark plug hole and tried it again in cylinder 1, it hit 160 and dropped again. So is it the piston rings that are going bad? or problem with the valves....

 

[NOSLO2PT0]

should crank it 6-7 times until the gauge quits going higher. The gauge will hold the highest # that is read. It shouldn't fall down. If the #'s went up with oil. your rings are going bad.

 

[way2slow4g63]

also make sure your car is at normal operating temp

 

[DoriftoTSi]

It's at normal temp, but why is the compression leaking like that, could it be because of the rings, i thought it was a problem with the valves.

 

[rcarrier]

same thing i just went through, if you put oil in the cylinder and the #s rise, you have got bad rings, the oil is filling in the gap between your worn out rings and cyliner walls!

 

[DSMatMSU]

just to throw a loop into things.... i was experiencing low compression in my middle two cyls. on a newly built motor.... I was afraid the rings didnt seat, or a bad headgasket seal...

same thing, i would get low compression... put a little oil in... and the compression would shoot back up.... and then drop down again after awhile...
so i take the head off and find out my valves aren't seating correctly, I get the the valves re-seated.... now perfect compression across the board now.... so yea... weird stuff.... wasnt the rings in my case.... i guess the oil was coating the valves somehow or another...

 

[johnk]

If one of your valves isn't seating right then you will most likely lose all compression in the cylinder with the bad valve. If it's seating bad it's eventually going to melt on one side because it's not losing its heat through the seat on the head. Once that happens it will feel like your driving a 3 banger instead cause all compression in that cylinder will be gone.

If compression went up in that cylinder after adding oil I would say it's your rings.

 

[DSMatMSU]

nope... i had like 130 compression... put in some OIL and get 180 190 (something like that)

it seems odd, it gave me all kinds of hell trying to figure that out... i went as far as pulling a piston to check the rings, and the end gap...

but its something to check out, cuz you gotta have the head off to check the pistons anyway.... so you might as well check it out before going all the way through the block... just giving my experience, but most cases would lead to the rings...

 

[johnk]

I'm not saying your compression will be bad today if it's not seating perfect. I had good compression and a valve burnt up within a weak and I lost all compression in the affected cylinder. Millage may vary based on your setup how you drive and how much you drive but you'll get there in a short time period if the valve isn't seating.

 

[InjenTalon96]

ya man take the head off and check that out first, that could save u some money also

 

[johnk]

If your timing belt is off you'll get bad compression across the board like you've got. Doesn't mean you've bent your valves if it's only 1 tooth you'll notice a big difference in power and your compression will drop. If you have a boost guage the idle vacuum will drop too. If your belt didn't slip just take the head off and see what the problem is for your self.

When I melted my valve I could tell soon as I took the exhaust manifold off the runner was much darker than the others.