Low oil pressure at full RPMs

[quicksandfarmer]

An update:

The only explanation that made sense to me was that the engine was running hot which was causing the oil to thin. I topped off the coolant -- and it took 3 quarts! Sunday is my mowing day, and I mowed for six hours today. Oil pressure stayed at 0.2 MPa or higher. Interestingly, the temperature was quite a bit higher -- it had been steady around 60C, today it was around 80-85C.

My hypothesis is that the coolant was low enough that the sensor was dry, thus it was reading low, although that's really just a guess. Nominal capacity is 7.4 quarts so there was still something in there.

Now I have to figure out why I'm losing coolant. I have never liked the design without an overflow tank, particularly since it makes it so hard to check the level. In the back of my mind the words "head gasket" are forming...

Thanks to everyone who responded.

 

[Rich Waugh]

The low coolant would cause the errant temperature readings. A diesel engine should definitely NOT be operated for extended periods of time at such a low temp as 60C, either. They are designed to operate at a temp of 185 -190F (90-97C), which is where they have maximum thermal efficiency. In reality, your engine was probably overheating somewhat and thinning the oil. Keep a close eye on the coolant level to ensure this doesn't repeat.

It is easy to install a coolant recovery system on these tractors, if you can only find a place to mount the bottle. It is a good idea to do it.

Check your oil to see if there is coolant in it and vice versa. If you open the drain bung in the sump very slowly with the tractor on a level surface you should be able to tell if any coolant comes out before the oil does. Any oil in the cooling system should result in an oil film at the top of the radiator tank. If you haven't ever re-torqued the head gasket and set the valve lash, now would be a time to do it. It should be done at 50 hours and then at 200-hour intervals thereafter. Fortunately, since the tractor is running okay, it isn't likely that you have warped the head or cracked the block and the worst you may have to face would be replacing the head gasket. That isn't too difficult.

The issue of low coolant creating false temp readings is one good reason to add an oil temperature gauge. Another easy task and inexpensive, too. All diesel engines should be equipped with oil temp gauges at the factory but these Chinese tractors don't ever seem to be.

Don't delay on determining the status of your head gasket. Delay could result in serious engine damage if a bad gasket is not replaced.

 

[Bob Rooks]

The low coolant level would have been caught during your "pre-flight" inspections. Always do a walk-around inspection before climbing into the seat. Running engines with air in the cooling system is just begging for trouble, that's the reason virtually all modern day manufacturers have coolant recovery systems on their engines and equipment.

Your hypothesis about the immersed sensor is correct.

 

[greg_g]

I stand corrected. In all the years over which I've owned 4 different Chinese tractors, I apparently kept my coolant at acceptable levels. I'm sometimes a "show me" kinda guy, so never realized that low coolant could actually result in diesel temperature readings to err on the cold side.

Anyway. I had acceptable results with simple $10 coolant recovery kits from JC Whitney, attached them with cable ties to the upper radiator hoses and supports.

//greg//

 

[Bob Rooks]

I stand corrected...... so never realized that low coolant could actually result in diesel temperature readings to err on the cold side.

No problem. However it applies to all closed system liquid cooled engines - gas, diesel, etc. Hot air is actually "cooler" than hot coolant when measured with a capillary bulb or sensor.

 

[quicksandfarmer]

So I torqued the head gasket bolts last night. A couple of them budged a bit but nothing dramatic. I drove the tractor today and couldn't notice any difference in the way it ran. I've been looking for signs of a leaking head gasket but so far nothing -- no oil in the coolant, no bubbles, no suspicious pressure. Overall the system holds pressure, even if I have left it for a few days it will still hiss when I open the cap. If there is a leak it is sublte. I have a pressure tester but no adapter for the tractor, so I ordered the adapter. So we'll see.

I did have a bit of a scare today, though. I used the tractor to move some stuff, then turned it off. When I went to move it again, it wouldn't start -- it would turn about 3/4 of a turn and then stop with a horrible noise. I was sure that I had somehow ruptured the head gasket and one of the cylinders was full of coolant and locking up. I was already thinking of how I'm going to spend the rest of the summer without a tractor while I work repairing the engine when I realized what the problem was. When I stopped, I didn't like the way the hoses for the FEL were routed, so I disconnected them and re-routed them. I didn't properly seat the quick-connects and they weren't connected, so the hydraulic pump was dead-heading. A firm push and I was on my way.

 

[erj]

So I torqued the head gasket bolts last night. A couple of them budged a bit but nothing dramatic.

To retorque the headbolts, they need to be loosened and then retorqued. This should be done one at a time to avoid disturbing the head gasket and in the sequence specified for tightening them.
After hours of running, heating and cooling - the bolts "stick" and you can't get a true torque reading without loosening them.
RonJ

 

[Bob Rooks]

I've been looking for signs of a leaking head gasket but so far nothing -- no oil in the coolant, no bubbles, no suspicious pressure. Overall the system holds pressure, even if I have left it for a few days it will still hiss when I open the cap. If there is a leak it is sublte.

When the coolant gets hot (operating temperature and beyond) it expands and forces the excess past the radiator cap and out the overflow tube - lost to the earth. The radiator cap also has a vacuum break at about 10" Hg. When the engine cools down the coolant contracts, and since there is no coolant recovery system installed, will draw air back in to the radiator. The air drawn back in will displace the coolant and eventual overheating may result. This is the explanation for the loss of coolant when there is no internal or external leak.

 

[quicksandfarmer]

Another update: I noticed today that my fan belt was loose, quite loose. I tightened it and we'll see if it has any effect on operation.

 

[Bob Rooks]

Another update: I noticed today that my fan belt was loose, quite loose. I tightened it and we'll see if it has any effect on operation.

That will play a big part in keeping the engine at normal temps. Assumed you already checked that. Hmm.