Thread: V8 3.9 compression test, Question to those in the know

so how long has it been sitting idle?
it should have over 70 in all of course,
how long have you run it for before doing the comp test?
if it has been sitting idle for ages, i would take it for a drive if it's not making serious noises, just gently and see if it improves.

I did a piston in Tassy hooning up a hill in our old 3.5 Rangie. The comp test showed one piston on around 20. When I stripped it the piston had lost all its rings and there was a corresponding amount of metal filings in the sump.
No damage to the piston sleeve at all . Go figure.
Maybe there is a rats nest in yours there instead of a piston.

Originally Posted by harry

so how long has it been sitting idle?
it should have over 70 in all of course,
how long have you run it for before doing the comp test?
if it has been sitting idle for ages, i would take it for a drive if it's not making serious noises, just gently and see if it improves.

Take it for a Drive... ha ha........


As soon as i had it running it was missing, and 0 on the 2nd pot tells me, running it for 5 mins wont improve it...

Hey Steve
Who is the bloke proping himself up on the Disco????
Andrew

ok, so i was right, it hasn't run for a while!
i still suspect a stuck lifter, and would run it for a bit, half an hour or more,
the only damage you run the risk of [if the engine isn't already bad making noise] would be bending a push rod, in my opinion.

Originally Posted by LandyAndy

Hey Steve
Who is the bloke proping himself up on the Disco????
Andrew

1st Wife's old man.... We are still good mates

Originally Posted by harry

ok, so i was right, it hasn't run for a while!
i still suspect a stuck lifter, and would run it for a bit, half an hour or more,
the only damage you run the risk of [if the engine isn't already bad making noise] would be bending a push rod, in my opinion.

Nope... read the 1st post....
took the rocker cover off expecting a Stuck valve...
Nope i even took the whole rocker assembly off to make sure the valve was not open, even gave them some percusional maintenace.....

As per the 1st post... 70 then 0 then 70 psi on the 1st 3 pistons, then the rest were 120psi.


Guys i'd like some positive answers from those in the know... not answers like... Run it for 1 hour and if the busted piston scores the bore and writes the block off then the con rod will snap and render it usless....

If the headgasket was blown between 1-2-3 on the LH bank.. could this cause this??? i cant see how as the 1st and 3rd are 70psi and if the piston had a hole you would expect the sump to pressurise..... hang on.. it cant as it needs to suck it in and if there is a hole how can it create a vaccuum to suck it in????

Damn it... Head comes off on Tues... only one way to find out..

might still be a valve thats not seating or thats cracked and thats not too bad... to prove/disprove a cylinder/piston problem remove the crank case ventilation system and run it if it pumps air with no messing around then you have a rings/piston issue. IT might also pay to do a hot check on it and see if that gets you any raise in pressure. (run at high idle till the thermostat opens) while its doing that check the cooling system for contamination before doing a hot wet compression check.

IF all of that provides no answers do a leakage check. bodge a sparkplug adaptor that will let you put compressed air onto the cylinder via the spark plug and then slowly turn the engine ove with say 15psi of air being pushed into the pot at the apropriate times you should get air out the inlet the exhaust but not out of the breather.

if it passes all that with normality crank the air pressure up to 90psi.

off the top you should hit about 90psi for a dry cold test on an 8.13:1 compression head. (in a perfect world) with a maximum of 10% total deviation from the high pot to the low.

EDIT

My bad since you want to run minimum engine risk do that in the reverse order...

do the compressor leakdown checks FIRST at 15 then 90 PSI then providing that reveales no problems with blowby into the sump go for a more extensive running check.

IF it helps...

Since Im coming down Go no further in the dismantling than removing the exhaust manifold and the vally cover (inlet manifold) as far as removing the heads go and I'll see about borrowing an endoscope from work or albury so we can throw that down the ports and plug hole to see whats what.

Endedit