MEP-803A cranking cylinder pressure

[Ray70]

Hello everyone, I was wondering if anyone here knows what the typical cylinder cranking pressure should be on a typical LPW2 or LPW4 motor?
I've been working on several of these and I'm finding some at about 250psi and some closer to 200-220 psi.
From reading, these motors are supposed to be 18.5 : 1 compression, which should theoretically have a much higher cranking pressure, but I'm wondering if the large swirl dish in the top of the piston allows / causes significantly lower pressure?
I would like to know what others have experienced with cylinder pressures so I can be better able to diagnose combustion problems.

 

[devilphrog]

I'm interested in what set up you used to get the compression tester to fit in the injector hole? I've got an HF diesel compression tester and I couldn't make it work to find my dead cylinders.

 

[Ray70]

Don't laugh, but here's what I had to do.... I used the body of a bad injector, welded the small fuel return line closed and then welded 2 different thread lug nuts together ( one that would thread onto the injector and the other that would accept the thread on one of my compression gage adapters. then used teflon tape to prevent leaks at the threads.
I'm thinking of machining a better 1 piece adapter, but for now this "Jerry Rigged" adapter is better than nothing!
Just clamp it down like an injector and hit the dead crank switch....

 

[devilphrog]

no that works. I was thinking something along the lines of gutting an injector out and adapting the threads. But I was planning on tearing the engine down anyhow, so I ended up bypassing that project and going for new rings and pistons and the assumption that it would make compression upon reassembly, especially as the blowby was such that it was shooting the dipstick out of the tube originally, which is a typically bad sign.

 

[Ray70]

Out of curiosity, were you getting a lot of smoke ( bluish gray ) upon cold start? What I'm finding is that cylinders with 225-250+ seem to run good, but once you drop to about 200psi it seems like the cylinder is right on the edge of insufficent compression temp to fire and will just smoke for a couple minutes ( with cold exhaust temp and a little raw fuel from that cyl. ) then it will begin to fire and the smoke begins to clear as the engine comes up to temp.
On the particular unit I'm working on now I believe the rings are to blame. I used a smoke machine fitted to my compression gage adapter to fill the cyl. with smoke ( after loosening the rocker arm nuts ) and I'm getting smoke in the crankcase. The motor also has a bit of blow by, but by no means enough to push out the dipstick, but definitely not a slight vacuum as Guyfang described in another thread.
As a test I had considered running Kerosene with a little ATF for lubrication in the machine, just to see if it would run without smoke. This is because Kero has a lower auto-ignition temp than diesel. But... I think I might just pull it apart again and check / replace the rings.
Originally this motor was not building compression and when I pulled it apart I found a little rust and all the rings were frozen in the piston grooves, so I honed the cyls. and cleaned up the rings and grooves. Seems not replacing the rings while it was apart may have been a big mistake....

 

[devilphrog]

I was getting smoke of that color all the time. I ended up having 2 cracked pistons so I replaced those and did all new rings. I do have 2 standard size ring sets left over as the new pistons came with rings if you need them.

 

[Guyfang]

If you don't have a vacume on your crank case, you have a problem. Look in the -24 TM for the test procedure. It's simple. It was a very common problem with units coming back from Down Range, and or sets that had been stored for long time periods. This symptom points to ring problems.

 

[Ray70]

Thanks Guyfang. I tore it apart last night and will re-do everything again with new parts this time.... No sense short-cutting anything again!

 

[Jeff803a]

If you don't have a vacume on your crank case, you have a problem. Look in the -24 TM for the test procedure. It's simple. It was a very common problem with units coming back from Down Range, and or sets that had been stored for long time periods. This symptom points to ring problems.

I have crankcase pressure but my compression test came out 250 & 275psi on an 802. I can not find any information where that vacuum is created. There are no tubes I see on the intake or air cleaner but I guess a passage thru the intake is possible. The unit runs a bit rougher than my 803 which runs absolutely amazing & has a no crankcase pressure. I picked another 802 up and same thing. There is a possibility I had a stuck ring since I pulled the injector & shot a ton of penetrating lube down the cylinder for 3 days while I drilled and rigged up an old injector to work with a compression tester. Threads were a PITA to match but nothing epoxy wont make up. Do you have any idea where the source of the vacuum is created so I can check the passage or tube? I hate to tear down a motor with good pressure readings . Thanks Jeff

 

[Ray70]

Jeff, on the 802/803 the crankcase vacuum is created through a small tube in the corner under each valve cover. the tubes connect the intake runners into the rocker area, which then draws from the crankcase through the pushrod tubes and the oil drain passages.
Did you check your crankcase pressure with a manometer or are you feeling pressure when you remove the oil cap?
Removing the cap can be very deceiving because the movement of the crank and pistons often makes it feel like you have pressure.
You need to keep the system closed and test with a manometer connected to the crankcase.

 

[Guyfang]

RTB. There is a procedure for checking vacuum. Use a manometer, or, RTB and make a home made Manometer, that works well.

 

[Light in the Dark]

Jeff, on the 802/803 the crankcase vacuum is created through a small tube in the corner under each valve cover. the tubes connect the intake runners into the rocker area, which then draws from the crankcase through the pushrod tubes and the oil drain passages.
Did you check your crankcase pressure with a manometer or are you feeling pressure when you remove the oil cap?
Removing the cap can be very deceiving because the movement of the crank and pistons often makes it feel like you have pressure.
You need to keep the system closed and test with a manometer connected to the crankcase.

I've got an 802 here that is a leafblower of oil out of the oil fill. Im wondering if I am dealing with a bent push rod, bent exhaust valve, demonic possession all on the table at this point.

 

[Ray70]

If it starts up on its own during a lightening storm, ( especially without a remote start installed ) then I'd lean towards Demonic Possession... otherwise maybe just stuck rings?
How's it sound when cranking? does it crank abnormally fast?

 

[Light in the Dark]

I haven'trun it for a few months, but I gave it a multi hour load bank run with ATF in the case to try to unstick anything... didn't seem to change anything. Just haven't gotten back to it. I don't recall anything abnormal with it in terms of starting. It actually ran very well, and made rated power with ease.

 

[Jeff803a]

I have crankcase pressure but my compression test came out 250 & 275psi on an 802. I can not find any information where that vacuum is created. There are no tubes I see on the intake or air cleaner but I guess a passage thru the intake is possible. The unit runs a bit rougher than my 803 which runs absolutely amazing & has a no crankcase pressure. I picked another 802 up and same thing. There is a possibility I had a stuck ring since I pulled the injector & shot a ton of penetrating lube down the cylinder for 3 days while I drilled and rigged up an old injector to work with a compression tester. Threads were a PITA to match but nothing epoxy wont make up. Do you have any idea where the source of the vacuum is created so I can check the passage or tube? I hate to tear down a motor with good pressure readings . Thanks Jeff

I've got an 802 here that is a leafblower of oil out of the oil fill. Im wondering if I am dealing with a bent push rod, bent exhaust valve, demonic possession all on the table at this point.

Jeff, on the 802/803 the crankcase vacuum is created through a small tube in the corner under each valve cover. the tubes connect the intake runners into the rocker area, which then draws from the crankcase through the pushrod tubes and the oil drain passages.
Did you check your crankcase pressure with a manometer or are you feeling pressure when you remove the oil cap?
Removing the cap can be very deceiving because the movement of the crank and pistons often makes it feel like you have pressure.
You need to keep the system closed and test with a manometer connected to the crankcase.

Will look into making my manometer, when I pulled the dipstick I defiantly had some back good pressure there. What is good average and low compression numbers. I had calculated 18.5 x 14.6 atmosphere pressure and came up with 270psi. I will check those tubes under the valve covers for any obstruction of the tubes coming from the intake. With compression numbers like 250&275 I would not think I have a piston/ring issueThis unit has 230hrs so its not even broke in. Thanks again for the reply I do really enjoy working on these & have bought 3 in the past couple months.

 

[Ray70]

Jeff, it sounds like you have decent compression. I think in the past I found anything over 250 is fine. Once you get down around 200-225 you start having starting issues, like smoking for 30 seconds to 1 minute until heat builds up a bit. If she starts good and doesn't continue to smoke ( gray ) other than that initial black puff from , I'd say you're good.
Rig up a quick water manometer and see where you're at before you bother taking the valve covers off. You may not have an issue at all!
The 802's will always run much "Rougher" than the much better balanced 803, because both pistons are cycling in phase together, this also causes a TON of air disturbance in the crankcase due to both pistons coming down together. This makes it feel like you have a ton of pressure / blow-by, but it's usually just the air being pulsed in and out due to the piston motion.
In an 803 you have 2 pistons going up and 2 coming down, so the motor is much smoother and the air pulses are canceled out by the opposing piston motions.

 

[Ray70]

L.I.T.D, I believe most of your Gen business has typically been 803's right?
Is your Leaf Blower unit acting much worse than other 802's you've seen or are you comparing it to a typical 803? There is a night and day difference between what happens when you open the oil cap or pull the dipstick on the 2.
I'm not sure you can pull the oil cap on a running 802 without taking an oil spray shower!

 

[Light in the Dark]

Id say more 803s than 802s, but thats because they have historically been more well known/popular/sought after... but plenty of each. If I think about it though, its probably more than 2:1.

I honestly only pulled the cap on this one as a customer request and the oil blew out 10' behind me (as well as over/on me) and I decided it was too much. Perhaps I need to recheck... the dipstick stayed in place just fine under running conditions (which in units that have had pressure issues before, that works itself out of its position).

Will reevaluate soon, as I've got some here that need to go find a new home

 

[Zed254]

I was convinced I had pressure in my 802 because the fluid in the manometer would disappear. I then decided to make an air chamber to help reduce the pressure swings in the oil sump out of a peanut butter jar. I've got ~2" vacuum.