Excessive Blow-by from crankcase vent; how to determine cause.

[Blackjack]

Q. How do I determine the actual cause of my blow by (Rings vs Valves or both) short of pulling off the cylinder head.

I have a 1980 B7100HST-D (D750A engine) that had some blow-by when I purchased it years ago. I don't think it's ever had the engine or head rebuilt.
Now it's increased to the point that you don't want to stay stationary very long, as the smoke coming out of the crankcase vent is pretty annoying, and it drips some oil from that tube.
I've posted a dropbox link to a short video of the blow-by. Sorry for the poor quality camera work.


I'd always suspected rings to be the culprit....and just made certain to keep the oil up in it and not ride it too hard and it has performed like a real champ. Two days of bush-hogging, and she's a beast....except for the blow-by.

While reading the forums....I learned that it's possible for valves, valve guides and valve seats to be a potential cause as well.

It's a fairly significant amount of blow by (and smoke) that comes out of that vent tube. You don't really want to spend a lot of time stationary.

Other symptoms include:

1. Clearly hard to start...that could be rings or valves or both....(Need the battery to be at peak and a solid 30-40 seconds of glow plug to start with confidence).
2. Uses a little oil.... (mostly blow-by....no seal leaks that are readily apparent)
3. Has great power once it's started, and reaches full rpm (that I can tell-no rpm guage) with very little exhaust smoke at high rpm...(There's more smoke coming from the blow-by than the stack it seems).
4. Idles at slow fairly well....but smokes (black smoke not white) more at idle. (Pretty healthy amount of exhaust smoke during the transition of rpms.)
5. Radiator fluid level remains constant through two days of bush-hogging.
6. And the oil get's dirty pretty quickly.

I know in piston aircraft, when doing a compression check, you can listen to the exhaust...and to the oil dip stick area to determine leakage past the rings or the valves when at TDC.

I pulled the valve cover and was tempted to just "spin her over for a few seconds" and see where it's coming from, but thought better of it. Could be pretty messy, plus, I don't have the decompression option and I doubt without actually starting...I'll see very much.

I suspect it's a little of both (rings and valves)...but there's really no way I can see that I can pinpoint it....without pulling the head. Thoughts?

Blackjack

 

[North Idaho Wolfman]

Look at it this way if it's rings then it probably has worn the valves too, and vice versa.

When I torn into my D750 both the rings and valves were very worn! I opted to just replace the complete motor instead of trying to rebuild it. I ended up going to a D950 instead.

 

[Bluegill]

Yep, it's time for a rebuild...Or replace.

 

[Blackjack]

Well Wolfman...I'm sure you're right,
I was just trying to find out if there were some tricks to determining probable areas.
This may be sick, but part of me, after reading the posts of Kubota owners who have gone through that process (rebuild), makes me want to try it. I've rebuilt two gas engines...that turned out great, but my knowledge in the diesel world is sorely lacking.
When I look at the part in the manual that talks about removing the front wheel drive system, and splitting the tractor, holy moley, there's one side of the garage that's tied up for the duration. Not like you can slap the hood back down and push it back and over to the side of the driveway, and slap a cover on the whole thing until you get the engine put back together.

Of course if I do that, I know me, now I've married it, and since I've gone that far, I'll be painting everything and finding more items I'd want to swap out.

But for now....she does everything I need it to. Bush-hog twice a year, and maybe move a trailer or a boat on occasion.

Off the top of your head Wolfman, what were the main issues with installing a D950 in place of the D750 you had to wrestle with (fabrication or purchase new accessories) to make it fit/work?

Blackjack

 

[Rhino1744]

Look at it this way if it's rings then it probably has worn the valves too, and vice versa.

When I torn into my D750 both the rings and valves were very worn! I opted to just replace the complete motor instead of trying to rebuild it. I ended up going to a D950 instead.

Sorry BlackJack, not to high jack your thread, I to am having the same issue with blowby, but i am intristed in swaping out with D950, did you have write up on that, was it plug and play or did it require a lot of parts change outs?

Thanks.
Craig

 

[Rhino1744]

I see Great minds think alike!

 

[Rob]

you will find the d850 & 950 are a compatible unit, if you are looking to swap make sure you get as close as spec unit to your own as possible.
eg, if yours is thermo syphon cooling system it is preferable to get a unit with the same setup, if you get a pumper and want to swap it with a thermo item you will have issues with hose sizes, possibly electrical components and aux fittings etc all can be overcome , but , its a bit more work to be honest.

been there,done it

rob

 

[North Idaho Wolfman]

There are four main areas that you will run into hiccups on a swap.

Cooling system, Charging system, Pilot bushing, and Throttle linkage setup.

I got a D950 out of a Jacobson mower.

The original D750 had a thermo siphon water system, (large hoses, no water pump, no thermostat, and water heater is inline on the bottom hose), it had dynamo charging system, (it sits behind the fan where you would think be a water pump), the throttle linkage was a pull to stop type of setup (shutting off key would not stop motor), had a bushing in the end of the crank to support the output shaft.

The newer D950 had a small hose water pump and thermostat, and will need a heater installed into the block or thermostat housing, It has a remote alternator setup, the throttle was set for the type that uses a solenoid to shut off the fuel, different flywheel and no pilot bushing installed.

Removal and install was quick, easy day's work did it.
I soldered new hose ends on the radiator, installed a Chevy self exciting/ regulating one wire alternator, bought a dual coil solenoid to shut off fuel.
The main install was an exact fit, remember to install the pilot bushing before you install the motor.

I can give more details, if you need them.

 

[Blackjack]

No problem Rhino, we're all on the same side here. (You might want to look at Wolfman's and Robs posts about 11 pages back on their posts.....they are awesome helping us figure out the mods necessary.)

I'm just really glad this site exists...

last night, I was perusing the forum, and it's amazing to me that two of the gents who were really persistent at documenting their projects well with photo's and dialogue...answered my post here.
Thanks ...I'm humbled.

Seriously...I think Rob's teardown was what...last year at this time...and I remember the pic of his engine balanced on two jack stands...and the drive shaft balancing on the bell housing unit turned vertical...I smiled. Looked great.

And Wolfman, with his engine install, similarly a while back...I remember the pics of the led lights, and making the special wrench to get that huge bolt off. Now that's a can do spirit..where there's a will..there's a way.

Both of you guys can definitely git er done! You are skilled, but it's that think through the problem, solve it that you showed, that makes me almost feel like.."Yeah Buddy, I can do this!"

I only have one question at this time...
Q. How is it, that after you all have completed all those improvements to your tractors...that you are still listed as an apprentice on this site. There ain't no way. In any event you guys have earned my respect.
Thanks for taking the time to document those projects, for us to learn.

Blackjack

 

[Blackjack]

One More time...Excessive Blow-by from crankcase vent; how to determine cause.

I'm going to ask this question another way.
(Can you tell I really don't want to split the tractor?)

Is it possible....that sometimes people just overhaul the top end?...and if so, why? Would it be for valve seats, valves, springs,...or is that mostly for a gasket /cooling system failure....and to resurface the heads.

Couldn't the valve seats contribute to the excessive blow-by I'm experiencing? Or similarly...be a major contributor to the hard start?

I know I solve my problem by just dropping in a new engine...

but would I be able to determine anything by shutting off the fuel, pulling the valve cover and spinning it over...to see if and where the excess blowby is occurring? (I'm sure it would create a little mess.)

It seems like a compression test in itself doesn't point to whether it's rings or valves.....unless its a static test on tdc. I have a diesel compression tester....but I have to spin the engine to get the readings...it's not something I can hook an airhose up to to see where the blow by is being produced from.

Blackjack

 

[North Idaho Wolfman]

Put it in a spot that you don't care if it gets a little oil on it, Pull the valve cover and run it, if you see the bypass (smoke is coming from the bottom end or (highly unlikely) from the valve seats.

 

[Blackjack]

Put it in a sop that you don't care if it sets a little oil on it, Pull the valve cover and run it, if you see the bypass (smoke is coming from the bottom end or (highly unlikely) from the valve seats.

Thanks Man....that's all I was looking for....just someone to say "go for it if it makes you feel better".

Will let you know after I clean up my mess.

Blackjack

 

[Blackjack]

OK...here are the results of the valve cover off find the source of the by-pass test.
(Drumroll....)

You guys were right.(again). But I had to learn.

Note: I have to thank my spouse for coming out to film the test...(it was funny, she had an old shower curtain up in front of her...because I told her I had no idea how much oil would spray out of that thing with the valve cover off...and how much pressure that vent displaced). She's pretty gutsy....I was at least partially hidden by the dash.

Anyway....it seems that the smoke is being generated from the areas surrounding the journals around the push rod areas (where it goes down to the tappets.....and not from the valves.....

For those mechanical "freshmen" ..like me ...and didn't know what it looks like running with the valve cover off...the attached/link to the movie is provided for a limited time.

https://www.dropbox.com/s/kxadyffnvrlb0sn/IMG_1470.MOV

so now I will be either looking to trade in, or, overhaul or purchase an engine.

Thanks for the assist,
Blackjack

 

[Rous]

Hello ppl. Old post I know but it came up in my google search and gave me an excuse to create an account.
Blackjack, the best way to determine valves, rings or both on a 750 (imo) is to pull all of the glow plugs (so you can spin engine by hand at front pulley) attach a compression tester into glow plug hole and check compression. Record numbers then remove glow plug, use an eye dropper or similar and put a few drops of clean motor oil into one cylinder at a time, reattach compression tester and retest. If you dont have a compression tester you have one reliable chance to make the judgement yourself. Put your finger over glow plug hole and have someone crank engine by hand and judge the compression for yourself. Add the oil and repeat, if you notice a considerable increase in compression your rings are suspect. I rebuild alot of these engines and I can tell you that bad fuel, lack of oil changes, filter changes, long low rpm operation result alot of times in stuck rings rather than worn ones. I won't get in detail about that here but a Google search should give you plenty of info on freeing stuck rings (a touch of wizardry and sometimes a ouija seance are helpful) although sometimes it's easier to just drop the pan, and push the pistons out the top of the block to free. You probably dont need to deck your head, it is most likely valves/seats/guides in which case bring head to machine shop. I also find that if rings are stuck alot of times you will notice dipstick tube showing high oil when running, valves usually make a hissing noise while cranking.

 

[GeoHorn]

There is another method (which is used with aircraft engines) called a “differential compression test”.

Place a cylinder at top-dead-center and, thru the spark plug or glow plug/injector threaded-hole, feed compressed air and listen for escaping air from the cylinder.

Example: Feed a regulated 80 psi of air from the shop compressor into each cylinder.

If you can hear air escaping at the exhaust... it’s a leaking exhaust valve.
If you can hear air escaping at the air-filter/induction.... it’s a leaking intake valve.
If you can hear air escaping at the oil-filler cap area.... it’s leaking piston rings.

It’s normally done at TDC in order to assure that BOTH valves are closed simultaneously. With a tractor it might be necessary to block the engine from rotating such as placing a manual transmission in-(high)gear and locking the parking brakes, etc.

Hope this helps.

 

[Dave_eng]

Re: One More time...Excessive Blow-by from crankcase vent; how to determine cause.

I'm going to ask this question another way.
(Can you tell I really don't want to split the tractor?)

Is it possible....that sometimes people just overhaul the top end?...and if so, why? Would it be for valve seats, valves, springs,...or is that mostly for a gasket /cooling system failure....and to resurface the heads.

Couldn't the valve seats contribute to the excessive blow-by I'm experiencing? Or similarly...be a major contributor to the hard start?

I know I solve my problem by just dropping in a new engine...

but would I be able to determine anything by shutting off the fuel, pulling the valve cover and spinning it over...to see if and where the excess blowby is occurring? (I'm sure it would create a little mess.)

It seems like a compression test in itself doesn't point to whether it's rings or valves.....unless its a static test on tdc. I have a diesel compression tester....but I have to spin the engine to get the readings...it's not something I can hook an airhose up to to see where the blow by is being produced from.

Blackjack

For a crude test to see where the blow by is originating and without the gauges to determine the % of leakage, turn your compression tester into a cylinder leakage tester by connecting an air hose to it.

Put cylinder at TDC, inject air and see where it is coming out.

Likely have to remove a schraeder valve from the compression testing screw in fitting as it acts like a one way check valve in the wrong direction for injecting air. Slowly increase the air pressure and expect at some point the engine may try and turn over.

https://www.youtube.com/watch?v=hGv70RyiGGc

Dave