OT: Compact Tractor diesel compression test
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Lead Driver
Joined: Jun 2013
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From: north of Minneapolis, MN
OT: Compact Tractor diesel compression test
Best place for display info. Here goes.
Working on a mid 80's Kubota L4150, 50hp 5 cylinder diesel. I have the workshop manual, so I know what the numbers are supposed to be hot. But, I really don't want to fire it up, remove a bunch of parts (to gain access to either the GPR hole, or remove the injectors) and have it be all cooled down again.
So, if the hot numbers are supposed to be in the ~400 psi range, what effect will doing it cold have on the numbers?
Working on a mid 80's Kubota L4150, 50hp 5 cylinder diesel. I have the workshop manual, so I know what the numbers are supposed to be hot. But, I really don't want to fire it up, remove a bunch of parts (to gain access to either the GPR hole, or remove the injectors) and have it be all cooled down again.
So, if the hot numbers are supposed to be in the ~400 psi range, what effect will doing it cold have on the numbers?
Senior User
Joined: Jun 2015
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As long as all cylinders are within 10% of the average you are good, if you have a low reading then do a wet test to see if it is the rings or valves. Which does not really make a difference since the head has to come off if you have a issue.
Hotshot
Joined: Jan 2011
Posts: 18,849
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From: Puget Sound
...or go through the miserable exercise I just endured. More on that in a sec.
First off... you can't run the engine with everything exposed and ready to pop a GP? We can run our 7.3Ls with the valve covers off. Second... I don't know about thee Kubota, but our rigs don't cool very fast.
Leak-down test allegedly tells far more than a compression test. The book says to do that one warm too, but I had a good cold compression test, followed by a good cold leak test with a block on the stand. I used an OTC leak-down test tool. I tried the Harbor Freight tool first - totally epic fail. I can hear precisely where the air is going, or if you are hard of hearing - you can use smoke near the exhaust, intake, or a crankcase vent to see where blowby is occurring.
In direct answer to your question - I've seen about a 25 PSI difference (at the 400 PSI range) between cold and hot compression tests, with cold being lower. I don't think temperature is as much a factor as a recently-ran engine has a little more lube at the rings.
First off... you can't run the engine with everything exposed and ready to pop a GP? We can run our 7.3Ls with the valve covers off. Second... I don't know about thee Kubota, but our rigs don't cool very fast.
Leak-down test allegedly tells far more than a compression test. The book says to do that one warm too, but I had a good cold compression test, followed by a good cold leak test with a block on the stand. I used an OTC leak-down test tool. I tried the Harbor Freight tool first - totally epic fail. I can hear precisely where the air is going, or if you are hard of hearing - you can use smoke near the exhaust, intake, or a crankcase vent to see where blowby is occurring.
In direct answer to your question - I've seen about a 25 PSI difference (at the 400 PSI range) between cold and hot compression tests, with cold being lower. I don't think temperature is as much a factor as a recently-ran engine has a little more lube at the rings.
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Lead Driver
Joined: Jun 2013
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From: north of Minneapolis, MN
I can run it, get it warm I suppose, then (to access the GP holes) remove injector lines, the intake manifold, then the GP's. I think it's a fuel issue, but the comp test will point me where to go. If the numbers are good, then the injectors come out, along with the injection pump and off to the rebuilder they go.
If the numbers are bad....well, then there is a decision to be made.
If the numbers are bad....well, then there is a decision to be made.
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Lead Driver
Joined: Jun 2013
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From: north of Minneapolis, MN
It's a mid/late 80's Kubota L4150....I hear it spent it's life on a golf course. It's been rode hard from what I can tell. It starts without any aids such as starting fluid or application of the GP's. That kinda tells me the compression is OK.
But I hauled this thing from west of Tugly (Ellensburg WA) to MN, to work on it for a friend. And I, by god, am going to work on it.
I'm hoping for an injection issue, more than a compression issue.
Lead Driver
Joined: Apr 2016
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From: Bahstun
when above 70 I don't wait for GP at all. just crank and it will start even my 95 F350 will start right up at 70 with out waiting
Why do you think it has low compression?
What symptoms do you have?
Why do you think it has low compression?
What symptoms do you have?
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Elder User
Joined: Sep 2011
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From: Vancouver
You don't say how many hours. That said, if it starts easily, the odds are pump. Have the pump and injectors tested. They can quote you a test/rebuild cost over the phone. Even if compression is an issue, you would still be advised to rebuild the injection system if doing any other mechanical work to the engine.
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Lead Driver
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From: Bahstun
OP didn't say whats wrong with it yet. no need to jump to conclusions just yet
please define "any other"
I doubt anyone would rebuild the inj pump for a GP, water pump, head gasket or items not related to fuel
please define "any other"
I doubt anyone would rebuild the inj pump for a GP, water pump, head gasket or items not related to fuel
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Lead Driver
Joined: Jun 2013
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From: north of Minneapolis, MN
First things first.
It's not mine. It starts, it runs with a defined miss, and choking white smoke.
Hours? Good question. The hour meter is gone, somebody put an accessory tach on it, that hour meter shows 5900. I don't believe that one as I suspect they were using the tach to obtain proper pto rpm.
When I got home, after visiting with the diesel injection shop (who recommended a compression test to start), started it..backed it out. I let it run until it felt good and warm. I removed the oil cap, which is on a 8" riser attached to the valve cover.
It appears that there is no blow-by...in fact, it took a thin zip lock bag and placed it over the standpipe, and sealed it with my hand......it could barely create enough pressure to get the wrinkles out of the bag. I think I could have put a ping pong ball on it and it would have easily stayed.
That however doesn't rule out a valve, but it is a good sign.
It's not mine. It starts, it runs with a defined miss, and choking white smoke.
Hours? Good question. The hour meter is gone, somebody put an accessory tach on it, that hour meter shows 5900. I don't believe that one as I suspect they were using the tach to obtain proper pto rpm.
When I got home, after visiting with the diesel injection shop (who recommended a compression test to start), started it..backed it out. I let it run until it felt good and warm. I removed the oil cap, which is on a 8" riser attached to the valve cover.
It appears that there is no blow-by...in fact, it took a thin zip lock bag and placed it over the standpipe, and sealed it with my hand......it could barely create enough pressure to get the wrinkles out of the bag. I think I could have put a ping pong ball on it and it would have easily stayed.
That however doesn't rule out a valve, but it is a good sign.
Lead Driver
Joined: Jun 2004
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From: Amarillo, TEXAS!
How about cranking the engine over while not allowing it to start, either by keeping power from the fuel solenoid or jumping the starter solenoid? A relative compression test, of sorts. If there was a compression problem significant enough to cause the problem it has, you should hear a distinct difference in the cranking when the cylinder is on the compression stroke.
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Lead Driver
Joined: Jun 2013
Posts: 5,647
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From: north of Minneapolis, MN
How about cranking the engine over while not allowing it to start, either by keeping power from the fuel solenoid or jumping the starter solenoid? A relative compression test, of sorts. If there was a compression problem significant enough to cause the problem it has, you should hear a distinct difference in the cranking when the cylinder is on the compression stroke.
One more question. I'm trying to remove the injector lines from the pump, how does one break them loose? The 5 lines are so close together on the pump, I can't get a backup wrench on the fitting.
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Lead Driver
Joined: Jun 2013
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From: north of Minneapolis, MN
Pictures? I would if I knew how.