The BIG Question!! MF 135

[abeav]

I posted a couple weeks back with questions about main and rod bearings on my MF135 diesel losing oil pressure after 15 to 20 minutes. Today I bought a new Clevite oil pump. 40 bucks! I have decided to roll the dice and just replace the pump. I am hoping that the relief valve may be sticking open. This engine runs and sounds great. Very little if any blow by and no smoke. The oil pressure hot at idle is about 5-6 PSI. At higher RPMs the pressure is a tad over 20 PSI.

Well here is my BIG question! I was told that I can shim the relief valve spring and cause higher oil pressure. How do I do that? With my pressure just below the recommended minimums a little more pressure and a known good relief valve my put me where I need to be until I have some time and money to do a complete overhaul. I know that I am just patching things until I can do things right. What do you good people think? Is it worth a try?

Thanks for any ideas or advise!
Allen

 

[Jack in NB]

Hello Allen -

I'd guess that it's not a sticking relief valve. If stuck partly open, you probably woudn't have much pressure cold - or - hot.

That pressure dropoff is consistent with worn bearings.

Since you have to remove the pan to replace the oil pump (or even shim the relief valve - it's part of the oil pump) - it would be a good chance to check the rod and main bearings while you're in there.

If they're worn, or the crank needs grinding, you have 90% of the work done at this stage, so it makes every sense to do it right!

And with no smoke or blowby, the top end sounds good - with the bottom end done, the engine should last for another 2 or 3 decades!

Probably we'll be out of diesel fuel by then anyway!

My 2 cents!

 

[Jerry/MT]

Here's how the lube system works. With normal bearing clearances and cold oil, the relief valve would be open at start up and would start to close as the oil warms, trying to maintain a constant oil pressure. With the hot oil and spec clearances, it would probably always be open a little. As the bearings wear, the relief valve will be open cold and would be more closed hot until finally it will only be open cold and the bearing clearnaces now are controlling the hot oil pressure. The clearnaces will continue to open and the oil pressure will drop when the oil warms up and oil pressure will be a function of oil temperature and engine speed. You are may be at this later stage with your 135. Of course if your releif valve is stuck open, the combination of it being open and wear in the bearings will have the same effect.

Shimming the relief valve means you put something like a small washer(s) under the spring to increase it's tension. But if it's not the controlling factor, it'll be ineffective.
My TO-30 with about 400 hrs on a rebuild maintains constant oil pressure hot ot cold, idle or max rpm indicating that the clearances are still tight and the relief vave is working correctly.

If this were my tractor, I would Plastigage the mains when I opend the pan to replace the pump and check the relief valve to make sure it was functioning properly. If the mains were worn, I'd replace them and the rod bearings.

Replacing the pump is a roll of the dice but if you were to rebuild the lower end, it's good practice to replace or rebuild the pump at the same time so if you find your mains are worn you're not out anything but your time and maybe a pan gasket if a pump change doesn't fix the problem.

 

[Frema]

Hello.
Jerry is right. You never can tell whether or not. After having let out the possibility of wear in bearings by checking them, you just change the pump/relief. Putting in the pump first - and finding that it was to no avail, means you have to pull the pan another time to fix the bearings.
Frema.Dk.