M18 Oil Pressure issue
 

Hello all. I just bought my first 1872 super. The M18 has an issue with oil pressure after it heats up to operating temperature. I'm running 30wt oil. When I first start the engine when it's cold my oil pressure at idle is 25 psi. When I begin to mow the pressure rises to 50 psi. As I continue to mow the pressure begins to drop to around 12 psi. When I idle the engine down the pressure drops to near zero and the pressure light flickers. I've mowed with it 3 times and the above has happened each time.

This deeply concerned me so I pulled the engine and removed the closure plate to inspect the oil pump. The oil pump appears to have no problems. The pump gears appear to have no wear and the shaft is tight and turns when I turn the crank.

The pressure relief ball and spring are in place. Has anyone ever experienced the spring loosing its spring tension over time allowing the ball to open and dump oil pressure. I considered stretching the spring to see if this would help the low pressure issue. I've heard of putting in a stronger spring to increase oil pressure in certain oil pumps in cars.

Anywhere else the engine could be dumping oil pressure? The sleeve bearings seam to be tight and there is no rod knock. I've read in another thread that talked about the pickup screen clogging, but wouldn't it have low oil pressure when it was cold as well?

Thanks for the help!

I assume it has fresh oil and filter?

Seems I had to pull the switch/sensor off of my m18 to clean some debris--probably stirred up during a ring job. Anyway, the idiot light has stayed off for a year now. Never hooked up a gauge to see the real pressure.

Yes.

Thanks

I had an intermittent oil light problem with my M18. I looked into the oil pump and relief spring but they looked fine. I was worried it needed a rebuild. Then I followed advice I received on this forum to check for a restriction in the oil lines. Remove the oil filter and blow air through both lines to see if it is restricted. Double check the filter number.

In my case someone had collapsed one of the oil lines with a hose clamp.

I also had a mag 18 with oil issues and it turned out to be a collapsed oil hose. When I took them off I blew air through them and one would barely let any air through.

I'll check my oil hoses and oil lines. Thanks so much!

if you look @ the oiling system diagram, the M-18 pressurizes the cam on the pto end and the crank, and rod journal.
it also pressurizes the hollow cam to lube the other end of the cam as well as feeds the other end of the crank & rod journal.(flywheel end)
A loss through excessive clearance in each & every journal will cause low pressure when the oil is hot.
They can run as high as #80 when cold with tight clearances.
#10 @ idle when hot is as low I'd want to see on a high hour machine.
But check your oil lines as mentioned.
I don't use cooling lines/remote filters, I use a block mounted filters, but some don't like them because they are messy when changing filters.
Now as far as a plugged screen, it is possible because you are restricting the flow (amount) of oil.
The pressure relief ball is just that, it relieves the pressure in the system that is in excess of the rated spring pressure.
If the flow is not there, you are not going to build pressure.
To make you feel better, a lot more engines are running without oil pressure gauges than are using them.
No gauge = no worry.
Ya I know I like a gauge also, but my main mower don't have one.
And it has a lot of hours on the rotating parts.
If it goes "boom", I just slip in a spare engine and fix the old one.

Thanks ol'George for your insight to my problem.

I'll share some history that I have discovered as I have progressed with my investigation of what's going on.

First of all I bought this super as a 2072 from a person I have known for over 40 years. He told me it had the M20 engine in it and I had no reason to doubt him. 20 years ago he had bought an 1872 and shortly after he bought it, a screw that holds the air filter assembly to the elbow on the carburetor came loose and got sucked into the # 1 cylinder. This screw got embedded into the top of the piston. He took the head off and found the screw and removed it. As of last summer, he was still using this engine in that same 1872, according to him.

When I pulled the engine out of this 2072, I discovered it had an M18 in it. I really didn't think much about it because a previous owner could have switched hoods or replaced the engine. The engine ran good with no smoke and no knocking.

While the engine was out I removed the heads and guess what I found on top of the number 1 piston? A nice dent left by a screw!

Moral to the story, Even someone whom you may trust, will still put the screw to you!

The engine only has 75 psi of compression in each cylinder. Both pistons are loose in the cylinders. I'm convinced this engine is wore out and I'm just going to put it back together and use 40 or even 50 weight oil in it and run it till it runs no more. I will be looking for a command engine to put into my 72 series super this winter.

Thanks for the help!

Why not get a brand new vanguard from small engine warehouse? I plan to order one for the 782 come income tax time.