Low low compression numbers... need some advice please

[dreko]

Hi Forum

So a 82 3B BJ42 that I bought, (Still in Sydney AUS) just got
back from the mechanic's over there....
they did a compression test and here are the numbers
161km on the engine

1: 750 kpa = 108 psi
2: 755 kpa = 109 psi
3: 755 kpa = 109 psi
4: 760 kpa = 110 psi

I know the manual says minimum pressure 280 psi, so are these numbers possible? I've asked the seller if he's burning up the oil consumption. He said no change in oil level... engine runs really strong, his mechanic said 'almost perfect' for this engine

Is it possible the 750 - 760 numbers are in PSI and not kPA??
Would that make it 'almost perfect' ??

I only found a member's compression numbers in the mid 500's here.. no one in the 750 range...

Another member posted that the engine wouldn't even start at very low numbers.

HELP

[amaurer]

If the engine runs then those numbers are impossible, a diesel won't run at those pressures. Given how uniform they are, I suspect you have a bad gauge or are systematically performing the test wrong. 700+ would also be way too high, 450 is about the highest I've seen.

Details?

[roscoFJ73]

I would query the figures pointing out its too high for psi and too low for kpa.
My guess is its psi and his gauge is out of whack.
I would have thought a skilled and knowledgable mechanic would have noticed the figures seem suspect
On the bright side ,the figures are very even.
A 160k/klm is very low for 26 yo vehicle so there is a chance its a real gem.

[mac]

Did he forget to put a 2 in front of 750, as in 2750kpa when he passed on the #'s to you?

Ya, nice and even tho'.

[RufusTheDufus]

A weak battery and closed throttle might lead to numbers like that. Ask him to put a few drips of oil down the glow plug hole and run the test again on each cylinder. If the numbers don't change significantly then he's doing something wrong.

[dreko]

Thanks everyone.. all this information is helpful
I'm going to pass it along to the seller and mechanic...

Good to know it couldn't be these numbers, diesel wouldn't run
I'll ask if there was supposed to be a 2 in front of those numbers

Feeling better...

[dreko]

Ok so the seller spoke with the mechanic who did the compression test. The numbers are in PSI

So a great strong engine!

All is good

[eleblanc]

700 psi????

[amaurer]

Originally Posted by dreko

Ok so the seller spoke with the mechanic who did the compression test. The numbers are in PSI

So a great strong engine!

All is good

Wrong, 700psi is an impossible number.

This mechanic is clueless or hes jerking you around, get a new one. A perfect 3B will not exceed 450psi.

[RufusTheDufus]

Unless the head has been shaved 0.10". That will drive the compression numbers way up. (It may also screw up other stuff though.)

[eleblanc]

anyhow i wouldn't want a 3B with compression of 750psi. It would probably be about to blow. get it done some place else.

[amaurer]

Originally Posted by eleblanc

anyhow i wouldn't want a 3B with compression of 750psi. It would probably be about to blow. get it done some place else.

Well, to be honest, if you measured 425psi on a nice good 3B and then ran 15psi of boost, your preignition pressure is in the neighborhood of 850psi - actually higher, I'm assuming no heating. The internals could probably take it, but I doubt the starter could even turn it over.

Anyway, I don't think you could shave the head enough to bring the compression up that high, this mechanic still doesn't know how to use a gauge.

[eleblanc]

Originally Posted by amaurer

Well, to be honest, if you measured 425psi on a nice good 3B and then ran 15psi of boost, your preignition pressure is in the neighborhood of 850psi - actually higher, I'm assuming no heating. The internals could probably take it, but I doubt the starter could turn it over.

Anyway, I don't think there is enough head to shave to bring the compression up that high, someone doesn't know how to use a gauge.

right but when you pressure test the cylinder the turbo ain't pushing in...

[Tapage]

Hi jack ..

My 2H @ 14 PSI handle 28.8:1 comp ratio ..

Hi jackover ..

The diesel don't burn in less 290 - 300 PSI comp ..

[amaurer]

Originally Posted by eleblanc

right but when you pressure test the cylinder the turbo ain't pushing in...

Yep, I was just saying the engine itself probably won't blow at 750psi compression, but its definitely still a bogus reading.

[dreko]

Ok thanks everyone for the insight and advice. I'm a noob
The compression test is getting redone, so I'll see what happens

Out of curiousity, if a rebuild is in order at some point, what's the best kit
for the 3B? or more importantly, which one to stay away from

thanks

[lostmarbles]

"Engine Australia" supply good kits.

And by-th-way, here's some good info from them for the B engine:



Note that it gives compression figures based on how many rpm (250) that the starter motor is turning the engine over at.

This confirms what others have already said here - That a poor startermotor/battery/etc will give low figures. ----- Which is one reason (I suspect) why people are now preferring to use "cylinder leakage testers" rather than "compression testers".

Incidentally - if my conversions are correct, 3MPa is about 450psi and 2MPa is about 300psi. (And 200kPa = 30psi)

[RufusTheDufus]

BTW, that document is for the B, not the 3B.

[lostmarbles]

Originally Posted by RufusTheDufus

BTW, that document is for the B, not the 3B.

thanks (- Forgot to mention that but I've edited that information into my previous post now)

[dreko]

Ok the compression test numbers

1. 410 psi
2. 410 psi

[dreko]

whoops

1. 410 psi
2. 410 psi
3. 350 psi
4. 390 psi

So other than sorting out cylinder 3 when it arrives, I think the numbers look pretty good!

[amaurer]

Originally Posted by dreko

whoops

1. 410 psi
2. 410 psi
3. 350 psi
4. 390 psi

So other than sorting out cylinder 3 when it arrives, I think the numbers look pretty good!

Valve adjustment might clear up #3. If it doesn't I might be a little concerned.

[roscoFJ73]

Originally Posted by dreko

whoops

1. 410 psi
2. 410 psi
3. 350 psi
4. 390 psi

So other than sorting out cylinder 3 when it arrives, I think the numbers look pretty good!

Did they give any reason for the 1st set of compression figures? I take it you have bought the vehicle .
The figures are not a disaster but #3 is out of spec. Im not sure how you would fix it without a tear down,
It seems strange that the original figures were close by a margin of a few % and 3 was equal 2nd highest and now is way down

[tasnolan]

Sorry to ask my own question but thought I was asking similar advice to dreko. My compression test numbers came back as
1 2750
2 2750
3 2750
4 3000
What would these figures be in PSI and I am looking at installing a turbo, would these compression figures indicate a strong enough engine to play with? cheers in advance for any help

[dreko]

Hey Tasnolan

1. 2750 kpa = 398 psi
4. 3000 kpa = 435 psi
All strong numbers, except for the difference between 1-3 vs. 4

Similar to my problem. I guess a valve adjustment, maybe there's a gasket leak... I have a feeling this
isn't a critically serious problem, but should be fixed at some point

Rosco: I did buy the rig. Yes the numbers looked very consistent in the 1st test, but obviously something was done incorrectly... this could explain the almost identical numbers

[tasnolan]

Thanks Dreko, so being within 10% of each other I would be alright to whack a turbo on it, or, should I be adjusting valves etc to get these numbers the same before mod's?

[amaurer]

Originally Posted by tasnolan

Thanks Dreko, so being within 10% of each other I would be alright to whack a turbo on it, or, should I be adjusting valves etc to get these numbers the same before mod's?

Well you should definitely do the valve adjustment since that might be a free fix. Apart from that, however, everything gets expensive. Those numbers are just fine for playtime, do what you want and run it, it'll last a while.

dreko, not to be a downer, but I'm not sure I share your confidence about your low cylinder. You should spring the $30 for harbor freight's compression tester and see if valve adjustment brings it up. Then do the oil-in-the-cylinder test to see if you have worn rings. If its the rings, well, that sucks, but there is also not much you can do about so just run it till it dies. If its not the rings then it can be probably be fixed by popping the head off (head gasket/valve seat perhaps), which you should do right away before you kill what might be an otherwise trusty motor.

crushers/rosco/brownbear or other Diesel Tech gurus may have better ideas, but thats the plan i'd use.

[dreko]

tasnolan: everything I've read seems to indicate that the 3b does quite well with a turbo. More power... so my answer would be got for it

amaurer: I hear you, I'll buy the compression tester. I"ve never done a valve adjustment. Does the field manual explain the process well? Hopefully this is something I can adjust easily... I will try the oil in the cylinder test. Ok, I'll take care of this problem as soon as it arrives. Thanks for helping
How's your BJ42 doing? Any problems with it?

[dreko]

Ok here's a noob question

If the compression rings are messed up... one can replace the rings right? or the piston head?

Thanks

[canucksafari]

Originally Posted by dreko

Ok here's a noob question

If the compression rings are messed up... one can replace the rings right? or the piston head?

Thanks

Yes, but you might as well do a full rebuild.