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

Originally Posted by Blknight.aus

thats actually the correct way of doing it. But I got caught out with it once on an air cooled engine TDC it had no leakage but still continued to faf around.

there was a split in the bore running from about 1/3 to 2/3 the length of the cylinder testing at the top showed no problems and it wasnt till we naffed the check and tried to blow the compressed air in with the engine just past TDC and it turned itself over that we found the leak.

re-reading the numbers its interesting that the LH bank are all lower than the right and the 2 pots either side of the dead one are low...If that test was the only one I had to go off of and had no other information Id be tempted to say "warped head/block"

The head would have to have a bloody big warp to have near no compression im putting my money on either valves or piston or rings

Originally Posted by lokka

The head would have to have a bloody big warp to have near no compression im putting my money on either valves or piston or rings

nahh have you seen the "erosion" of an alloy head thats been continued to run once its warped a little and the combustion gasses eat a pathway through the alloy?

The "engine that will not die" project I was part of when the guys in darwin got bored had melt marks through the head you could lay a 1/2 inch extention bar down in.

Sticky rings, valves not seating, either one from being left sitting

Just be confident of oil pressure and run it, really need some load to give it a good work out though, just reving it won't make a lick...either that or a quick strip down

Originally Posted by lokka

Also what are your plans for the hubs,stubs front and back and rear axles i mite be interested if the $ are good

How much you want to pay?? I've got a set pulled from a 95 disco, for tojo conversion, hub to hub, approx 210k

The adjoining cylinders are down suggests that it would be a cylinder head gasket.

Originally Posted by sclarke

I just did a compression test on the 3.9 Disco i bought from auction... i did have it running, and it had a miss

Here is the results, measurements taken from the LH bank, 1st cylinder closest to the radiator then moving towards the firewall, then same on the RH bank all in PSI

LH bank
Dry 70 0 70 110
Wet 100 0 100 170

RH bank
Dry 120 110 120 120
Wet 180 170 180 180

So number 2 on the LH bank has 0, yep nil, squat, nothing.... well it might have about 5psi... as the needle moved..

Took the rocker cover off expecting a stuck valve..
Nope...
No oil in the water and no water in the oil........

Still a blown headgasket or do i expect to find something really nasty in there... like a hole or a nest of mice..lol

What is the Normal readings from a 8.13 CR 3.9V8
The wet test tells me the rings are worn.. wont really know untill i strip it and measure...

Ohh and the oil in the rocker gear was not just oil, but built up carbon crap and sludge... very mild mayonaise on the rocker cover only on the filler cap end...

Can any mechanics enlighten me..

So the fire..... Ha Ha... yeah right.. there was still lpg in the tank, opps it must have just caught fire Mr RACV....
I guess the insurance company would love this one...

Did you have all of the spark plugs out when you did the compression test, if so you may have a blown head gasket leaking to an adjoining bore, replace the spark plugs in the adjoining cylinders and try the comp test again, if no different you may have a hole in the piston, bring that piston up to near TDC and see if you can see a hole or use a bit of wire to feel around, Regards Frank.

Hi Clarkie, for numbers, my 95 V8 runs 130-140psi dry. I had the head gasket blow between cyls 2+4, gave me readings of 130, 0, 0, 140 on the RH bank.
I'd go with Lokkas test, pump air into the cyl and see where it's coming out, hopefully just the gasket or a burnt valve
Then again, you said the heads coming off anyway, so don't waste your time testing, best way to find out for certain

heres my two bob's worth - no matter what the cause if you have 0 compression the head will have to come off Because no matter what the cause you cant fix it with the head on, so bite the bullet and rip it off then you can properly assess the damage.

I guess you were really after an easy fix unfortunately I cant see any way around pulling the head.

Cheers Blythe