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914World.com _ 914World Garage _ Compression

Posted by: meares Aug 1 2005, 12:08 PM

two cyl. are at 130, one is at 125, and the other is at 105. Where should they be? Between 130 and 125? What should i do about the one at 105?

Posted by: SirAndy Aug 1 2005, 12:15 PM

QUOTE (meares @ Aug 1 2005, 11:08 AM)
Where should they be?

is this a fresh rebuild? if not, how old (milage?)

what size? engine specs?
Andy

Posted by: ArtechnikA Aug 1 2005, 12:17 PM

it'd help us a lot to know what kind of engine you have; we can presume you are near sea level.

absolute numbers aren't very important, especially in the absence of any real engine type data.
they should vary between cylinders as little as possible. 10 psi difference is where people start to become concerned *assuming* all 4 were tested with identical conditions. (plugs out, WOT, fresh battery...)

QUOTE
What should i do about the one at 105?

find out why and either fix it or ignore it.

Thing Number One - adjust your valves. has a MAJOR impact on low-speed compression.

could be a broken ring, could be a leaky valve. could be it just REALLY needs a valve adjustment.

other than valve adjustment or a defect in your testing technique, there are no cheap answers to low compression...

Posted by: meares Aug 1 2005, 12:34 PM

73 2L with +- 85000 miles...4 new injectors,new plugs,new wires, points are fine, it's getting fuel and spark, but seems to be hitting on 3 clynders.....i'm in Atlanta, the mech did a vavle adj. on it, but i don't know if this was before or after the comp test......i guess he tested 1-4 in succession.. they seem REALLY reputable.....they do A LOT of cup cars for events

Posted by: Travis Neff Aug 1 2005, 01:07 PM

Leak down test would help on that bum cylinder. That can help pinpoint if it is a valve or ring problem

Posted by: SirAndy Aug 1 2005, 01:27 PM

QUOTE (meares @ Aug 1 2005, 11:34 AM)
73 2L with +- 85000 miles...

130 is a very good number for a 85k-mile engine ...

i would test it again. i had the same trouble, 3 cyl. in the 160/165 range, one < 100 ...
took 3 valve adjustements and 4 tests to get equal numbers, now all four are ~160.
the key is not to panic when the first numbers come in.
like it was said above, maybe a glitch during the test, maybe the valve adjustement was off.

try again and if the same cylinder keeps coming back way lower than the others,
it's time to take the top end off the engine ...
Andy

Posted by: ArtechnikA Aug 1 2005, 01:38 PM

something that's fairly common in the 6-cylinder cars is for a bit of carbon to fall down and hold the exhaust valve open a teeny bit. so i suppose it's possible that a similar thing could happen in a /4 even though the valves are in different places.

like Andy says - when hearing scary news, first, get a second opinion. definitely get another check, and if at all possible keep a battery charger on the battery during the tests to minimise its influence.

some books suggest adding a spoonful of motor oil to the cylinder to help isolate the problem to rings as opposed to valves, but that really only works well in upright engines. when you get down to fault isolation, a leakdown tester (or other way to apply pressure to a cylinder, such as a valve spring replacement kit) can be helpful.

if you *don't* have some kind of combustion chamber leakage issue, it tends to point to the valve train...