zero compression!

[Quark]

ok, so my gf's car ( 90 240sx) started to run really really rough, like one cylinder wasnt firing or something. I really suspected a blown head gasket. The car was loosing coolant a week before it started to run rough, but there wasnt milky stuff in oil and such.

did a compression test on the motor and I came up with 170, 160, 160, 0

the zero one is the 4th cylinder, closest to the firewall.

does zero compression mean blown head gasket? stuck valve? or something else?

if its a blown head gasket, i was going to replace the motor with another one, though Im sure the motor was running but its out of the car now, and I dont know how to check if its working well, how do i compression test the motor? or should i find a way to leak down the motor?

please, help. I want to fix her car in time for christmas.

[navysnail]

it verry well could be the head gasket

[s13sr20chris]

that does sound like a blown head gasket. could be cracked head or block too.

[Quark]

alright, I guess out goes that motor,

but is there a way to test compression on a motor that has been pulled out? cause I have 2 motors but i have no idea if one of them runs and the other one supposed to run, but I wanna try compression testing it before i drop it in or before i hook up the motor completely, maybe I just drop it in, connect it to the engine harness and see what happens?

[s13sr20chris]

spin that sucker really fast or buy a leakdown gauge($200 from snap-on).

[navysnail]

all you really need to do is spin the flywheel with the starter, thing is, that the compression is measured with the rpm from the starter motor turning the engine, so any faster or slower and it will be off. leakdown gauge would work, but if you can get the starter to turn it you should be good to go.

[Quark]

hmm... so what, hot wire my starter to 12v battery? and how much should I compensate for the cold motor if I do it this way?

[s13sr20chris]

mabe 5%(or less) lower due to coldness.

[Quark]

cool, thanks so much you guys.

[Altiman94]

a leakdown test is the only true way to tell where the compression is going if ytou are losing any. That's the method I use when having problems with compression. Much more accurate. I compression check first, then leakdown.

[s13sr20chris]

its true, leakdown is better. i use it when i can afford the time and effort(read: when its my car!j/k)