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Author Topic: how to diagnose oil consumption?  (Read 3099 times)
bevans6
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« on: December 15, 2009, 11:54:50 AM »

On my recent trip, I found I was adding a gallon of oil every 500 miles or so to my 8V-71.  Thinking about it, the likely issues are blower seals or bad rings.  If it was a four stroke I would add intake valve seals, but it isn't, so...

I was noticing that it kept smoking at idle even when hot, which I hadn't noticed before.  It would blow a lot of smoke taking off from a stop, but I didn't notice a lot of smoke at speed at all.  It is definitely putting oil out the tail pipe on the road, it made for a very very dirty car in tow...  It seems to be slobbering a fair bit, particularly if it gets idled too much.

A reman Reliabilt blower was installed by Harpers/Detroit Diesel 13,000 miles and three years ago.  New bearings were rolled in about 25,000 miles ago.  It's running the proper SAE 40 oil.

I think it's time for rings, or cylinder kits.  How do I do a better job of diagnosing the problem?

Thanks, Brian
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1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
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wildbob24
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« Reply #1 on: December 15, 2009, 12:29:50 PM »

Brian,

I'm sure some of the more experienced mechanics will chime in, but I'd pull the air horn off the blower and make sure it's dry.

You can remove the air box covers and visually inspect the cylinder through the intake ports.

You can also remove the exhaust manifolds a check for oily ports. If they're all oily, it might well be the blower. If only one or 2, then probably weak cylinders.

You can also run the engine with the exhaust manifolds off to pinpoint which cylinders are smoking.

A compression test would help give an idea of general engine health, but not necessarily excessive oil consumption

Bob

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luvrbus
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« Reply #2 on: December 15, 2009, 01:46:36 PM »

Two strokes have seals on the wrist pin that break loose and will cause oil consuption check the easy stuff  first like overfilling.
A compresson check won't do much good if the engine starts good it is probably ok a compersson check on a 2 stroke is a major deal not a easy task to preform removing 1 injector at a time and with the engine running at 900 rpm
The rings you need to check (oil rings) are not visable through the air box covers.  
Check the easy stuff first then go from there  
good  luck
« Last Edit: December 15, 2009, 01:51:44 PM by luvrbus » Logged

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« Reply #3 on: December 15, 2009, 02:55:49 PM »

If it's a gallon low in 500 miles, I would leave it a gallon low and check it again in 200 miles or so.  If it holds at the one gallon mark, just leave it there.
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« Reply #4 on: December 15, 2009, 03:05:10 PM »

That was my thought too, that it is overfilled and blowing it out.  I can fill mine up to the full mark, drive a couple of hundred miles, and have to add some to get it to the full mark again. Or i can let it seek its own level, which is barely above the add mark, and drive over 1500 miles with no further change in the level. No more dirty car either. Smiley
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« Reply #5 on: December 15, 2009, 03:50:24 PM »

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« Last Edit: July 17, 2010, 02:09:45 PM by Now Just Dallas » Logged

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« Reply #6 on: December 15, 2009, 10:06:46 PM »

Rather then "using"oil I suspect it is leaking from various points.

Fred M

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TomC
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« Reply #7 on: December 15, 2009, 10:32:13 PM »

If it is smoking at idle, it just isn't building enough heat-meaning worn rings.  Does it start alright when it is around 40 degrees outside?  A 2 stroke should not require ether till well below freezing.  If the cylinder kits look good, you could get away with pulling each piston (one at a time-you don't want to get them mixed up), hone the cylinder and install .010 over rings (if necessary).  That will get you at least another 100,000 miles.  Good Luck, TomC
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Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.
bevans6
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« Reply #8 on: December 16, 2009, 05:49:03 AM »

Thanks, guys, about what I thought.  Dumb questions are always the best lead, I find, so here goes:

To do rings, you have to take the pistons out the top, right?  So pulling the top of the engine - blower, injection, heads, etc, is step one?  Is there room to do that on an MC-5C?

If you have the heads off anyway, do you get a valve job done as a matter of course, or do they usually not need that?  I never hear of valve jobs on two strokes...

How long does a job like this take, for a guy who has rebuilt many engines but never a DD?  How do you get the heads off the engine, they are pretty heavy.  Crane of some type?  Do you need access from the floor of the back of the bus, ie take the bedroom apart?

I have a long time to think about this, bus season ended yesterday around my house!  Temps well into the teens this morning, and snow on the ground.  Spring time is just around the corner...

Thanks, Brian
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1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
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« Reply #9 on: December 16, 2009, 06:48:44 AM »

Once you have some kind of dolly or cradle to pull the engine onto, sliding the engine out is fairly quick: inside of a day. It might be easier than trying to it "inframe". Then you have all the freedom to do whatever. The head on my 4-71 is +- 150lbs. I used the skid-steer to lift it and reinstall it. Then you might as well do a complete overhaul. That'll keep you occupied all winter!

JC
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JC
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« Reply #10 on: December 16, 2009, 06:54:03 AM »

I second JC's comment. If I had all winter, I'd pull the engine and trans then do what ever needs to be done.
With everything attached to a sliding cradle the eng/trans assembly can be removed in less than 4 hrs by a single experienced mechanic. (YMMV, but the first time a friend, his 16 yr old son, & I pulled one from a 9 we did it in 3 hrs flat! Grin)
FWIW Grin
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« Reply #11 on: December 16, 2009, 06:55:15 AM »

Shoot even if I didn't have all winter, I'd still slide the engine out!
Grin  BK  Grin
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Busted Knuckle aka Bryce Gaston
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« Reply #12 on: December 16, 2009, 08:08:04 AM »

I don't know how much room you have to do an inframe,but I did pull the heads off our 6-92 the first time while still in the bus. I was able to get the heads of with out using a lift,but you will need the room and a lift to put them back on using the alignment pins, making sure the o-rings stay put while mating the head to the block. So far as access thru the bedroom you will need some room to remove the head bolts and reinstall, torque them down. If time is not the issue, build a lift for the engine, pull the pack out ,it will be a benefit in the long run to see what you are working on FWIW. Grin been there done that. Wink
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« Reply #13 on: December 16, 2009, 09:11:59 AM »


With everything attached to a sliding cradle the eng/trans assembly can be removed in less than 4 hrs by a single experienced mechanic. (YMMV, but the first time a friend, his 16 yr old son, & I pulled one from a 9 we did it in 3 hrs flat!



BK -

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The techs at Greyhound's fleet maintenance shop in San Francisco, working in pairs, could do two complete powertrain swaps on the GMCs in a single 8 hour shift!

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« Reply #14 on: December 16, 2009, 09:18:14 AM »

GMC's V drive were different then T drives.  I believe you with 2 in one shift-since they had spares with fresh engines ready to go. 
Trying to lift a 4 cylinder head off compared to a 3 cylinder head is 1/3 more heavy.  Just pull the engine and git 'er dun!  You'll have a fresh engine for many years after with minimal worries.
I remember being at one of the Cavern get togethers where one of the MCI's had a very worn out 8V-71.  It had gotton to be around 40 during the night. It took a major amount of cranking and then lots of smoke to get that tired engine running.  Compared to mine hitting the starter for about 3 seconds and it running with a bit of smoke for a couple of minutes, then settling down.  Good Luck, TomC
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Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.
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