Low compression? Cummins

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I have a 2003 5.9 cummins I've completely taken the truck apart just fixing everything and making it a nice truck and i have the engine out fixing oil leaks and cleaning up the oil. Before i removed the engine if I parked on a hill in first gear the truck would slowly creep down the hill, i pulled the head and had the valve seats reground because I figured that was the problem since I didn’t have any blow by and with the head off there is no scoring in the cylinders at all and all the pistons look to be in excellent shape too except fo i can wiggle them I believe it was about .020”, i also had no knocking or worn spots in the cylinder
Problems:
Slightly higher egts than expected
Creeps down hills
Wiggle pistons
Turn engine over fairly easy
Put head back on had injectors pop tested with new crush washers and still turns over easier but it ran good before
 

crash68

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First off you'll probably have a few people chime in lamb basting you for not using the parking brake.
You say the truck creeps down the hill, are you sure the engine is allowing the creep? Mark the drive belt and let the truck move on its own to see if the crank is turning.
 
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Yes I know the parking brake should be used haha but, it didn’t work at the time and i did open the hood and watched the the engine turning over from the front while it was creeping
 

Fast69Mopar

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Yes I know the parking brake should be used haha but, it didn’t work at the time and i did open the hood and watched the the engine turning over from the front while it was creeping
First gear is a very low gear itself. Did the truck roll while in 2nd or 3rd?

How steep of an incline are we talking about?
 
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I haven’t tried any higher gears and I still haven’t reassembled the truck yet do to the fact that im concerned about low compression and also the fact that i can wiggle the pistons and the hill honestly isn’t that steep the tires are probably about 2 ft lower on one end than the other.
 

Smokeybear01

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If it rolls in 1st gear it will only roll easier in higher gears. Why not get a compression check and then you'll know for certain what you're dealing with?
 
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I will do that unless someone could provide some insight on some things,
Is there supposed to be that much slack with the pistons in the cylinders? There was no scoring at all, had no blow by checking with the oil cap off, and no knocking of any kind. If you had to guess how many ft/lbs would it take to overcome compression?
 

CorDog009

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Pistons are tapered. They heat from the top down. They’re larger at the bottom than the top. If it passes the compression test, I wouldn’t worry about it.
 
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How high of psi should i be getting and where can i get a compression tester, and should i worry with a leak down tester or only if I have lower than expected compression?
 

LoneWolf3574

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I will admit that I don't know very much about diesels, but I do have an interest in the 5.9L Cummins, spent a bit of time looking for a truck with one before deciding that it was more than I needed. A quick Google search turned up that a 5.9L 24V Cummins has a compression ratio of 17.2:1

PSI = X:Y*P = (17.2 / 1) * 14.696 = 252.7712 psi

I hope this helps you 09megacabcummins. Anybody else, please feel free to correct me if I am wrong, I am no expert and a professional shadetree only in my own little realm ;P
 

Smokeybear01

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You should have a minimum 375# on all cylinders but 425# is average for a used Cummins. The important thing is that all the cylinders are in the same pressure range and that you don't have a low one or two. You can buy a cheap compression kit for about $40. But I would talk to an auto parts store and see if they rent the kit or maybe even a loaner. There are some good videos on Utube that show which port you connect them to and a lot of other good information. Just make sure you watch or study Cummins specific stuff. Also you want to remember that the pistons are nothing more than holders for the oil and compression rings. If you have no scoring in the cylinder you (probably) aren't going to find any major problems with the engine, short of it being used. I've seen lots of engines grow tired, but they still run fine.
 

CorDog009

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Pretty sure on the Cummins you just check blowby pressure instead of compression. Remove the oil fill cap with it running and see if you feel any pressure. I’ve seen engines blow the cap off when unscrewing it be of all the blowby.
 

Smokeybear01

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Pretty sure on the Cummins you just check blowby pressure instead of compression. Remove the oil fill cap with it running and see if you feel any pressure. I’ve seen engines blow the cap off when unscrewing it be of all the blowby.
That's a great way to check a Cummins in a truck you're thinking of buying, but I think the OP has the engine out so starting it might be a tad tricky, depending on his setup.
 
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Yes haha the engine is out of the truck on a stand and as soon as i can get a compression tester i will test it, I just hate removing the injectors again but it will be a load off my mind before I reinstall it. I had checked before i removed it for blow by and it didn’t have any, just a very slight haze you can barely see but no pressure.
 
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