Compression test

[dhirocz]

Anyone ever run a compression test on an 871? I just finished replacing my compressor (noisy), rebuilding the P/S pump and a number of gaskets. I'm still getting some signs of blowby in the crankcase, such as visible oil vapor out of the breather tube at hot idle and oil vapor smell while driving.

730k miles, had @m*m^2, 6 dimes and a bearing roll when the flat cam was fixed at 685k. It's also running the maintenance free breather element on it.

It seems to be doing it less since swapping compressors, which tells me the compressor might have been pressurizing the crankcase perhaps...but I am still getting heavy oil stains out of the breather tube anytime I'm loaded. It's no longer bleeding oil from everywhere like it was before.

I just want to make sure I'm not overlooking something and driving it like that unnecessarily. I expect there to be signs of wear, and am bringing to back to Savannah to do a complete inframe this summer.

A compression test will at least allow me to verify that it isnt something else I've overlooked. If not, would increasing the amount of gear oil I'm running help? I've also considered that the rings could be caked in junk.

[Wiseman]

To me sounds like classic example of an 871 digesting soot for 681k miles clogging the piston rings channels and not only .
Then you did very good thing, making it burn very clean . Shortly after that engine got clean and here we go . Blowby ... .
Nothing to worry about for now . But fix the gases/fumes getting into cabin ASAP !!!

My solution is turning on the recirculating air every time I sense anything suspicious. Not perfectly safe but if you want put an carbon monoxide detector for your sake.

[dhirocz]

I figure the rings might just be gummed up. Haven't decided if I want to soak them in a solvent or not to clean them up or even if it's worth it. I'm trying to verify that it is, in fact, rings before shooting in that dark at solutions when the true solution is a complete overhaul.

Just trying to make sure I didn't overlook something, as well as make sure I make it to my planned teardown.

Only odor is the smell of oil, only when in stop and go, under certain conditions. Most noticable when I crack the window. I never smell it if I idle the truck, which I hardly do unless it's well below freezing. I've considered extending the breather tube outside the body to vent it more outside until I solve this.

[Hammerhead]

Realistically, what will a compression test tell you?
If there is a dramatic drop off on one or maybe two cylinders it may tell you you have a glazed cyl or possibly a broken ring.
Not trying to be a jerk, I'm being serious. On a big diesel you have to pop the rocker cover. If you don't have external air cleaners, it's say 15 mins to pop the top. Then you have to relieve the pressure, or pull a vacuum on the coolant so you don't leak coolant into the cyl's w/o injectors. Then you have to pull the injector rocker shafts, then pull the injector hold down clamps, then pop all 6 injectors (or 1 at a time, but I'd pull all 6 to avoid compression issues on the "not in use cyl's"), then install a plug in one (unless you have 6 compression test adapters, doubtful) cyl and THEN finally crank the engine to do a compression test...
You're now what? Honestly what? Hour forty five? Two because everything is hot as hell and I haven't even talked about evacuating residual oil from the head yet, so now the engine is realistically cooled down to what? 135-140° at best? How reliable is a compression test at this temp?
For all this hassle, f$%k it, let it cool, don't burn yourself, pull the injectors and boroscope the cylinders, it literally will tell you just as much with waaaaaay less hassle and hot oil burns...
But I am a lazy beer drinking bastard...
Once again, not trying to be a jerk, but at 135 to even to say 160°, you're still trying to do a compression test, narurally aspirated, on a forced induction engine. There is no reliable, that I know of, way to compression test a forced induction Diesel engine, not that I wouldn't like to, I just don't have access to the equipment required to properly do it...besides engine manufacturers, I don't think anyone does.
If you do or have a better plan/idea than me, please share it. I'd love to know, I've thought about this extensively before and couldn't figure it out.

[Wiseman]

I’m willing to bet my @ss that your ring channels are so clean right now , rings are playing golf inside.
Off course there is inevitable liner glaze to some extent .
The smell isn’t only oil ! Its combination of combustion gasses and oil vapor! Don’t underestimate the harm to your health they can do!!!

[dhirocz]

The idea is to just have a better idea on the condition of what is going on. For all I know I have two cylinders that are weak and leak down quickly, or one that hardly has any compression. Or maybe they are all a little low. Maybe a leaking injector cup in the head glazed a cylinder. Might even detect a head gasket starting to go. Who knows?

Maybe I'm being anal. Anal works for me. The whole idea is to verify if this is actually a condition caused by wear or something else. The cylinders still looked pretty good when I last had it apart, which at this point doesn't mean much. I did take a look around with a boroscope. A compression test would give me a better basis in which to make an educated guess.

I want to know if one or two stand out. If they are all a bit low, which wouldn't surprise me, then so be it. Good point on the injectors though, it would take a couple hours to run the test...which would be a pain and pretty much commit me to an overhead afterwards. If someone was going to be in it doing injectors though, it wouldn't necessarily be a bad deal!

Anyway, the idea was to pull all 6 injectors and disable the fuel system, take another peek with the scope, then hook up the tester and give her a few cranks...on each cylinder. Record the data and look for the outliers.

The leakdown might be the hard part without the air pressure. Even with the valves closed and the cylinder aired up to say 30 psi, there's that missing dynamic of the piston actually moving as the cylinder is filling (as if the engine were running) compared to testing it stationary that would lead me to question it. Still could provide some useful data.

I also considered blocking off the air system as if I was doing a leak test and monitoring the crankcase for pressure past the rings into the crankcase. Problem is, it would also loose air past valve guides as well.

Anyway, I don't have it all figured out yet either. That's why I'm asking questions. Some of what I know of engines applies to these, and some doesn't. What I do know is it's almost 1am and I just saw my wife in a nightie so adios for now!

[dhirocz]

(01-16-2018 )Wiseman Wrote:  The smell isn’t only oil ! Its combination of combustion gasses and oil vapor! Don’t underestimate the harm to your health they can do!!!

...very good point.

[Hammerhead]

(01-16-2018 )Wiseman Wrote:  The smell isn’t only oil ! Its combination of combustion gasses and oil vapor! Don’t underestimate the harm to your health they can do!!!

(01-16-2018 )dhirocz Wrote:  ...very good point.

After the "wife in a nighty" comment, you'd better be done before you posted this!

[Hammerhead]

(01-16-2018 )dhirocz Wrote:  The idea is to just have a better idea on the condition of what is going on. For all I know I have two cylinders that are weak and leak down quickly, or one that hardly has any compression. Or maybe they are all a little low. Maybe a leaking injector cup in the head glazed a cylinder. Might even detect a head gasket starting to go. Who knows?

Maybe I'm being anal. Anal works for me. The whole idea is to verify if this is actually a condition caused by wear or something else. The cylinders still looked pretty good when I last had it apart, which at this point doesn't mean much. I did take a look around with a boroscope. A compression test would give me a better basis in which to make an educated guess.

I want to know if one or two stand out. If they are all a bit low, which wouldn't surprise me, then so be it. Good point on the injectors though, it would take a couple hours to run the test...which would be a pain and pretty much commit me to an overhead afterwards. If someone was going to be in it doing injectors though, it wouldn't necessarily be a bad deal!

Anyway, the idea was to pull all 6 injectors and disable the fuel system, take another peek with the scope, then hook up the tester and give her a few cranks...on each cylinder. Record the data and look for the outliers.

The leakdown might be the hard part without the air pressure. Even with the valves closed and the cylinder aired up to say 30 psi, there's that missing dynamic of the piston actually moving as the cylinder is filling (as if the engine were running) compared to testing it stationary that would lead me to question it. Still could provide some useful data.

I also considered blocking off the air system as if I was doing a leak test and monitoring the crankcase for pressure past the rings into the crankcase. Problem is, it would also loose air past valve guides as well.

Anyway, I don't have it all figured out yet either. That's why I'm asking questions. Some of what I know of engines applies to these, and some doesn't. What I do know is it's almost 1am and I just saw my wife in a nightie so adios for now!

For whatever my opinion is worth, I think you're onto something here...
A leak down/cylinder differential pressure test would be a better way to go to start. They should also be done at as close to operating temp as possible, so all the same time to access of a compression test apply, but you could actually trace the leak with this test as opposed to "just a compression psi number" of a compression test.
You'd have a much better idea of where the pressure is escaping than how much psi each cyl creates statically.
I have used a leak down tester on gas engines for extended periods of time both seeking answers and demonstrating to someone that he was "barking up the wrong tree", so IMHO this is a much better idea..,that you'll read tomorrow.