B7100 Low Compression

[Schmotown]

I have an 1980's B7100 HST-D, D750 AH engine. It is new to me and it's in fairly good condition for it's age. The engine starts up without prolonged cranking but it smokes way more than I believe it should. After it warms up it still smokes but not nearly as much as at start-up. The breather also smokes but very small amount. Otherwise, the tractor operates to my satisfaction.

I know all the "smoke" talk is very subjective, especially on a tractor of this age and with no working hour meter. I would like to reduce most of the smoking if possible and to that end if have performed the following tests:

Compression Test: Cyl #1: (Dry 240#); (Wet 250#), Cyl #2: (Dry 200#); (Wet 200#), Cyl #3: (Dry 220#); (Wet 240#).

Leak Down Test @ 100 PSI: Cyl #1 = 12%, Cyl #2 = 18%, Cyl #3 = 15%. These tests were done with the push rods removed. I hear quite a bit of air escaping from the intake ports, the intake assembly has been removed. The exhaust is much more quiet and the valve cover vent is also very quiet.

Cranking oil pressure = 40 psi with glow plugs removed.

What has me confused is why does it have such low compression but a very acceptable leak down test. I have also tested the gauges used in these tests to make sure my results were accurate. Unless I am not understanding my results (very possible) and with good oil pressure I think I will have the head reconditioned because of the very noticeable air leak at the intake valves.

My findings don't make a lot of sense to me, low compression but good leak down test, and I am hoping someone can shed some light on why my test results seem to conflict with each other and if starting with reconditioning the head makes sense.

Thank you in advance. Larry

 

[North Idaho Wolfman]

If you have smoke you have bad rings!
And your compression is in the dumpster, it should be much higher.
Doing the head is not going to help that one bit.

 

[Schmotown]

If you have smoke you have bad rings!
And your compression is in the dumpster, it should be much higher.
Doing the head is not going to help that one bit.

Thanks for the reply it is much appreciated. Can you help me understand why adding a bit of oil and redoing the compression test did not improve the sealing of the rings? Is it because they are so out of tolerance that the oil can't improve the seal at the rings?

 

[85Hokie]

whoa............ DO not place oil in the ol' cylinder on a DIESEL. Think about it - once that 20-1 compression hits that oil - it will try to ignite.

With those low compression numbers - I find it hard to believe it starts!!!!!!!

IF it starts "easily" = and that is the typical problem, low compression does NOT want to start easily.

It may been the valves are not sealing 100% too.

SO, it is new to you .........maybe it has not been run much then in the past.

what I am about to say may strike you funny................. RUN it hard and long at or near WIDE OPEN THROTTLE.

See if over time the smoking slows down a bit.

 

[85Hokie]

 

[North Idaho Wolfman]

Thanks for the reply it is much appreciated. Can you help me understand why adding a bit of oil and redoing the compression test did not improve the sealing of the rings? Is it because they are so out of tolerance that the oil can't improve the seal at the rings?

I don't do "wet" tests on these engines, 50% of the time it will give you false results.

I don't think your getting accurate results because at 200 to 240 it should not fire.
Like 85hokie pointed out that motor is looking for a minimum of 327PSI to start and optimally it would be 412PSI to 469PSI.

I had one in the 250PSI range that would only fire if you spun it up really fast.

I'm not saying that the valves are not seating correctly and that is not contributing to your low compression numbers.
Although that is very odd for that engine, and even more odd that all the valves would not be seating good enough to give you consistent low numbers.

I would bet when you pull the head you will find the cylinders are worn down.
I would also bet it needs a complete rebuild, with new sleaves, pistons and rings.

 

[Schmotown]

I don't do "wet" tests on these engines, 50% of the time it will give you false results.

I don't think your getting accurate results because at 200 to 240 it should not fire.
Like 85hokie pointed out that motor is looking for a minimum of 327PSI to start and optimally it would be 412PSI to 469PSI.

I had one in the 250PSI range that would only fire if you spun it up really fast.

I'm not saying that the valves are not seating correctly and that is not contributing to your low compression numbers.
Although that is very odd for that engine, and even more odd that all the valves would not be seating good enough to give you consistent low numbers.

I would bet when you pull the head you will find the cylinders are worn down.
I would also bet it needs a complete rebuild, with new sleaves, pistons and rings.

Thank you both for your insight. As I have the front end removed for sealing an oil leak I am not able to run it "hard" at this time as 85Hokie suggests. Perhaps this is a good time to pull the head for a preliminary look at the valves, cylinder walls, and measure piston protrusion above the block. From there I could drop the pan, push the pistons out and check tolerances if necessary.

Does the above sound like a decent plan to get some answers to my problem? Thank, Larry.

 

[North Idaho Wolfman]

Thank you both for your insight. As I have the front end removed for sealing an oil leak I am not able to run it "hard" at this time as 85Hokie suggests. Perhaps this is a good time to pull the head for a preliminary look at the valves, cylinder walls, and measure piston protrusion above the block. From there I could drop the pan, push the pistons out and check tolerances if necessary.

Does the above sound like a decent plan to get some answers to my problem? Thank, Larry.

Yep you should be able to find out a lot when you pull the head.

 

[Schmotown]

Yep you should be able to find out a lot when you pull the head.

Okay, off with it's head!

Thanks, I report back with my findings.

 

[85Hokie]

Okay, off with it's head!

Thanks, I report back with my findings.

with pictures please!

 

[Schmotown]

with pictures please!

Yes Sir....

 

[Mark_BX25D]

Does the above sound like a decent plan to get some answers to my problem? Thank, Larry.

A tractor with those numbers and it still runs? That does not sound like a problem I would try to solve.

That would look a lot like, "If it ain't broke keep your mitts off of it!" to me.

Smoke? Meh. It's an old diesel.

If I just had to do something, I'd start with running those compression tests again to make sure those numbers are real.

 

[Runs With Scissors]

I'm with @Mark_BX25D

If my 40'ish year old "diesel" tractor ran, I could put up with a lot of "smoke".

My personal belief, is this is a "solution in need of a problem" type of scenario.

Is it possible that it has been smoking like that since 1985?

I had a 1977 Buick Regal that I had to carry a 1 gallon jug of used oil in the back seat so I could fill it up every time I got gas........Ran it like that for probably 30K miles and it still ran fine when I sold it.

I had to put plugs in it every 10K or so, but........just sayin