Crankcase Overpressure but Compression Test OK

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AAVP7
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Crankcase Overpressure but Compression Test OK

Post by AAVP7 » Fri Nov 02, 2018 3:42 am

My 6.2l had a leaking rear crankcase seal. I replaced that, but it blew the new seal again rightaway.I then noticed that the engine was blowing fumes out of the oil filler neck, so I did a crankcase pressure test with a water manometer.

The result is that I have quite some overpressure at idle (+0.6 inch H). According to the TM this should be 0 to a slight vacuum.

When increasing rpm, the overpressure decreases (as it should), but at around 2.000 rpm, the vaccum is only -1.5 inch H (TM says, it should be between -2 to -5 inch H)

I suspected the CDR valve, so put in a new one, but nothing changed. Both CDR valves seem to work (they close when I blow into the DWF water sensor cup line)

So the next suspect would be blowby gases at piston rings or valves. I did a compression test today. The results are all above the threshold from the TM (380 psi), but not much in some cases:

1 - 420 psi
2 - 385 psi
3 - 405 psi
4 - 420 psi
5 - 390 psi
6 - 420 psi
7 - 390 psi
8 - 405 psi

I have no idea which pressure one would get from a new or intact engine ? Is 420 "healthy", or "already deteriorated", and 385 "close to death", or "still OK" ?
The difference between the lowest and the highest value is just 9 %, so less than the allowed maximum from the TM (20 %).

Is there any other possible cause for high crankcase pressure ?

I´m wondering if I need to rebuild the whole engine, or just one or two cylinders, so it would be great if you could give me a feel for "normal" compression numbers.

P.S.: The engine starts and runs just fine. No smoke or something. The vehicle has 36.000 miles, and this looks like the original engine. It´s just leaking oil like mad from the overpressure.
Arnd

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Re: Crankcase Overpressure but Compression Test OK

Post by dilvoy » Sat Nov 03, 2018 10:33 am

What is the rear crankcase seal? are you talking about the oil pan seal that is surrounding the rear main bearing area?
Anyway, if it is over pressure from blow by, it should not build any pressure if you have the oil fill cap removed. Maybe you are talking about the oil pan seal and you did not install it well or let the goop dry before putting it into use or the pan is bent?
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Re: Crankcase Overpressure but Compression Test OK

Post by Kurt Lesser » Sat Nov 03, 2018 10:53 am

Isn't relieving crankcase pressure what the PCV valve is designed to do? Even though you've changed it I think I'd try another one. There's only one thing that will pressurize a crankcase and that's blowby. In gas engines much more than a 10 psi difference between cylinders was cause for concern but I don't know if that holds true for diesels because of their higher compression ratios. Anyone?
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Re: Crankcase Overpressure but Compression Test OK

Post by Ted Gates » Sun Nov 04, 2018 6:02 am

It seems you have a deep water fording cdr, which means you may a dwf valve. Is the valve in the fording position (which pressurizes the crankcase and hubs)? Is it plumbed correctly?

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Re: Crankcase Overpressure but Compression Test OK

Post by AAVP7 » Sun Nov 04, 2018 9:19 am

@ Dilvoy: No, the two-piece seal around the rear crankshaft. When I remove the oil filler cap, you can see (and smell) oily fumes coming out of the filler neck. It´s pretty clearly blowby gasses.
I got a hint from a local HMMWV pro, who told me to run the car really hard for a while, as he suspected the piston rings might be crusted and stuck, and thus no longer sealing well. So I rode my Humvee back and forth on the local Autobahns at 65 to 75 mph to shake my piston rings loose. Measured the crankcase pressure afterwards, and it was down a little bit, but still overpressure (0.4 inch H).

@ Kurt: The TM says the difference between the cylinders should not be more than 20 %, and I´m below that (about 10 %).

@ Ted: Sorry, forgot to mention that. Yes, it´s a Marine Humvee, and it has the DWF kit. The valve is in "Vent" position, but I didn´t check if maybe someone switched the hoses on the valve. Good idea, will check tomorrow rightaway.

Thanks everybody for your replies so far !
Arnd

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Re: Crankcase Overpressure but Compression Test OK

Post by Retired War Horses » Sun Nov 04, 2018 10:22 am

AAVP7 wrote:
Sun Nov 04, 2018 9:19 am
@ Dilvoy: No, the two-piece seal around the rear crankshaft. When I remove the oil filler cap, you can see (and smell) oily fumes coming out of the filler neck. It´s pretty clearly blowby gasses.
I got a hint from a local HMMWV pro, who told me to run the car really hard for a while, as he suspected the piston rings might be crusted and stuck, and thus no longer sealing well. So I rode my Humvee back and forth on the local Autobahns at 65 to 75 mph to shake my piston rings loose. Measured the crankcase pressure afterwards, and it was down a little bit, but still overpressure (0.4 inch H).

@ Kurt: The TM says the difference between the cylinders should not be more than 20 %, and I´m below that (about 10 %).

@ Ted: Sorry, forgot to mention that. Yes, it´s a Marine Humvee, and it has the DWF kit. The valve is in "Vent" position, but I didn´t check if maybe someone switched the hoses on the valve. Good idea, will check tomorrow rightaway.

Thanks everybody for your replies so far !
The Valve setting can be confusing, the pointed side is where it should point to vent, not the handle side...FYI
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Re: Crankcase Overpressure but Compression Test OK

Post by dilvoy » Sun Nov 04, 2018 4:27 pm

You might have to drive as much as 25 miles at higher speeds to clean out gunked up piston ring grooves so don't give up yet. I had a really bad smoker stop smoking after just ten miles. Was an airport vehicle that had a huge amount of idle hours on it.
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Re: Crankcase Overpressure but Compression Test OK

Post by AAVP7 » Mon Nov 05, 2018 3:57 am

I rode for almost two hours on the weekend, all of it above 65, so I guess I have covered at least 100 miles.
Was interesting to see how the Humvee shakes at such speed. With the unbalaced runflats, everything was vibrating like mad. Someone here compared the feeling to entering the earth athmoshere in the space shuttle, and I can only second that... :lol:
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Re: Crankcase Overpressure but Compression Test OK

Post by AAVP7 » Mon Nov 05, 2018 1:47 pm

Checked the tubing on the DWF valve today. Everything like in the manuals. The center outout is connected to the vent line system. Top connection goes to the air cleaner intake. And the bottom connection has a thicker tube leading to the CDR valve.

The little arrow on the valve handle points upwards towards "Vent".

At the moment, I´d say it looks like too much blowby gases due to wear, either at the engine valves or the piston rings, despite the compression test results being still within the limits.

Is just replacing the piston rings worth a try, or will I probably need a complete engine overhaul ?
Arnd

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Re: Crankcase Overpressure but Compression Test OK

Post by dilvoy » Tue Nov 06, 2018 6:19 pm

The rings wear, but the cylinder walls wear also so just changing rings is not good enough and honing them will make the bore too big for the oil ridges on the piston skirt to do their job well. You might try a leak down test for each cylinder and see what that tells you.
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Re: Crankcase Overpressure but Compression Test OK

Post by wooody1 » Wed Nov 07, 2018 6:15 am

There are many humvee owners in Germany a couple I can put you in touch with that maybe able to assist hands on.

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Re: Crankcase Overpressure but Compression Test OK

Post by AAVP7 » Wed Nov 07, 2018 12:43 pm

Thanks very much ! I bought that Humvee almost two years ago from a well reputed Humvee dealer here in Germany, and today he agreed to give me a low-mile replacement engine for a very very fair price. That company really knows customer service....

Once I put in the new engine, I will then see if I can do further checks on the old one, and then maybe overhaul it. Apart from that overpressure issue, that engine was running great, after all.

Thanks again to everbody for your advice !
Arnd

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