Holy Low Compression Batman!

[4felix20]

man i really messed some stuff up this time. after my recent fumbling around with the distributor...i thought i reinstalled it correctly and could have sworn i did. (more detail in my msd 6-btm install topic). i have some really low compression after the short drive i took afterwards.

first of all, i had a 3mm metal headgasket and new pistons and rings. so i think that would account for a bit lower compression than stock but not as much as im seeing now. keep in mind, these numbers were from an unwarmed engine since i couldn't get her to even start for the past few days.

so my number are#1 cylinder - 95-100 PSI#2 - 70-75 PSI#3 - 70-75 PSI#4 - 70-75 PSI

guess i should do a leakdown test now...? i wouldn't think it's my headgasket...

[nissanfanatic]

any blue smoke? Should test on a warm engine just to be sure. Did you forget to open the throttle? Strange that they are all close though. But it doesn't sound good. If you do get it started, def warm it up and check that timing before you shut it off again. Good luck man.

[4felix20]

no blue smoke..

can't warm the engine though, i can't even get her started...and yes i did forget to open the throttle.

time for a retest. *smacks forehead*

[Structure240sx]

plus any fuel sitting in the cylinder will take the oil off the walls and lower the numbers. so it doesnt sound too bad to me

[nissanfanatic]

Warm, run fuel pump out of fuel and then test. See what happens.

I think you coudl even just pull the injector clips and be good.

[Structure240sx]

i always do mine:

full tempunplug injectorsunplug distributor plugsthrottle to the floor crank 5-10 times

[hysteria]

i've always heard you crank it 3 times...

anyone can have good compression if you crank it long enough...

correct me if i'm wrong here though...

i've seen guys with old crappy smoking cars make good compression because they did the test incorrectly (by cranking it for too long). my numbers will just keep climbing if i keep cranking the engine... mine read in the 170's, but if i keep cranking it can get to 200's etc....

i think a lot of people end up thinking they have all even numbers when in actuality their cars numbers are a bit off... not that they have bad compression but they just all approach the same numbers because they crank the car for too long... try minimizing the number of cranks, and do multiple tests because conversely if you didn't crank it long enough and then piston started at the wrong position then you would get a bad number because the compression never happened.

[pr240sx]

If the gauge is working correctly, you can crank the engine all day long and compression will be the same.I just crank until the gauge stops moving

[Jookmasta]

i usually do seven cranks.........

[HolyShiznit]

Doesn't matter how long you crank it. The gauge will eventually stop.

[4felix20]

okay...so i unplugged the injectors and tried again with the throttle at WOT. thanks for reminding me guys.

i got 120psi give or take 5psi through out all cylinders on a cold engine.

on to a different approach now. i'm guessing no spark at the moment...

[DammitBobby]

In order to get a proper compression test you need a good battery and starter. Should be turning 300rpms. Anything below 151 or any difference between cylinders of more then 14psi and its rebuild time.

[4felix20]

well, from what i understand...if the engine were warm and had not been 'washed down' lately from me not unplugging the fuel. i would have seen better results.

also with a 3mm headgasket i'm trying to accept these low but consistent readings as acceptable.

[nissanfanatic]

Just do it correctly. Even if it is a pain to get it started. You have to or you aren't doing it correctly which means you can't compare to the FSM standards. Their standards were taken under different conditions than you are testing. Apples and oranges here.

I count to Five one thousand while cranking. The main thing to look for is how quickly the gauge "jumps up". If you are cranking and it is slowly creeping up, then you are getting a lot of leakage. If it comes up quickly, you have little leakage.

If you can, get access to a leak-down tester. You can listen for where the air is leaking. But test under the same recommended curcumstances as you would compression test.

Check your gauge out too. I used one from a friend that gave me 160psi readings. Bought a new one from advanced auto parts, an "Actron", and it showed 180psi across all four.

Good luck.

[4felix20]

doing it correctly is not an option here...

i'm very aware this isn't a valid compression test. it's just something for me to get an idea of my cylinder's conditions.