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Discussion Starter #1 (Edited)
Not sure this is the right place to ask, but here goes.

I have a John Deere 757 zero-turn with a 25 hp Kawasaki FH721D that was showing low oil pressure below 1/2 throttle when run at temps above the mid 70’s for an hour or more. I’ve added an Oil Pressure gauge that told me what the pressure was and changed the oil to higher temp Mobil 1 20W50 motorcycle oil. The oil change at least got me to where I can go to Idle without getting a Low Oil Pressure light, but is within a pound or two of where I did get it with 10W30 oil. I even opened up the air intake area so it could breath easier, which helped, but not enough.

?) Since most machines only have Low Oil Pressure Lights, does anyone have gauge experience that’s willing to share what Air Cooled engine oil pressure should look like? I expected more from the switch to the 20W50.

?) How would I know if the oil pump needs replaced. From looking at a parts breakdown, I can’t see where it could wear out.

Here’s the link to my 757 story.
 

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Increase rpm's from idle slowly to see what the pressure is on the gauge when the light goes out.

I would suspect the pressure sender/switch going bad over an actual problem with the pump though. Looking at your other thread, if you're getting ove 20 psi at idle when hot then I think it's fine.
 

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Discussion Starter #3
Eliminated oil pressure switch, when new one acted the same as the old one.

With the new oil, the light doesn’t come on any more at idle, but the pressure is so close to where it did before. Would just like to have a little more cushion.

Interesting to me when the light flickers. Probably representing the pressure spikes from each of the three lobes of the pump and the LED light responds so quickly that you get to see that happen.
 

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Discussion Starter #4
Not sure this is the right place to ask, but here goes.

I have a John Deere 757 zero-turn with a 25 hp Kawasaki FH721D that was showing low oil pressure below 1/2 throttle when run at temps above the mid 70’s for an hour or more. I’ve added an Oil Pressure gauge that told me what the pressure was and changed the oil to higher temp Mobil 1 20W50 motorcycle oil. The oil change at least got me to where I can go to Idle without getting a Low Oil Pressure light, but is within a pound or two of where I did get it with 10W30 oil. I even opened up the air intake area so it could breath easier, which helped, but not enough.

?) Since most machines only have Low Oil Pressure Lights, does anyone have gauge experience that’s willing to share what Air Cooled engine oil pressure should look like? I expected more from the switch to the 20W50.

?) How would I know if the oil pump needs replaced. From looking at a parts breakdown, I can’t see where it could wear out.

Here’s the link to my 757 story.
?) Since most machines only have Low Oil Pressure Lights, does anyone have gauge experience that’s willing to share what Air Cooled engine oil pressure should look like?

?) How would I know if the oil pump needs replaced.


Okay, I was gone for a couple days. No answers to questions yet?
 

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That manual claims that the oil pressure for a new or newly rebuilt engine should be between 35 and 45 psi, but that the pressure switch for the warning light shouldn't come on until it's below 14.2 psi:

Oil Pressure 241 to 310 kPa (35 to 45 psi)
Oil Pressure Switch:
Detect pressure 98 kPa (14.2 psi)

So if your gauge is showing above 20 psi as you said in your other thread when the light is flickering then the pressure switch is coming on too soon. I'm wondering if adding the pressure gauge to the circuit might be lowering the pressure at the switch. Have you tried putting that back to stock since you changed the oil and changed the switch?
 

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Discussion Starter #7
It flickered before adding the gauge and after. I added the gauge to find out where the oil pressure was. It flickered the same before and after adding the gauge with the 10W30 oil at about the same throttle position. It didn’t flicker after changing to the 20W50 synthetic oil, even at lower engine rpms. I can’t see any variation in the gauge reading that would indicate oil pressure fluctuations, but expect the digit gauge filters that out. Those pulses in oil pressure would cause the LED light to flicker at what appears to be pressure above the 14.2 psi. With the three lobe oil pump, I know there are pulsations in the oil pressure, but no idea how big they are and you won’t be able to see them with a typical analog pressure gauge, test or panel. They have dampening too and the resolution is such that you wouldn’t see 5 psi pulsations. What I don’t know is, what I am seeing normal or something wrong in the pump or elsewhere. I don’t think the pump is bad, but I don’t know if there’s anything else that can be keeping the pressure low or is the engine still starved for air resulting in higher engine temps that are causing the oil to thin causing lower oil pressure. My other thought was should I add an additional oil cooler.

At this point, I’m not planning to make any further changes, but expect it may still be an issue at higher ambient temps.
 

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You could be oil starving from either sludge or something partially blocking the oil pickup. I had that happen to an Onan P220. A fuzz ball from dandilions or the "*wood trees" we have around here and sludge in the pan that covered the ridges in the bottom of the pan. Cleaned it up and has been fine since, but due to all the engine shutdowns (overheating boiling the gas in the carb) I run either 10W40 or 40SAE oil now
 

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Don't know if the Kawasaki engines oil pumps have a relief valve like a car's or not--they can malfunction and let too much pressure bleed off ..

I had a VW Beetle with a 1200cc air cooled 40 hp engine that had a oil pump pressure relief valve that "stuck" in its bore ,it was located near the distributor I think,an old VW mechanic showed me how to fix the problem,the oil light would flash on & off at idle,but go right out just above idle..
There was a slotted head plug you unscrewed and the plunger had a tapped hole in it so you could thread in a bolt and pull it out..

He used a rifle brush to clean up the bore,sprayed it out with brake cleaner,and put some motor oil on the plunger after cleaning the varnish off it good and he also stretched the spring that went under it a little--the oil pressure went up to 15 psi at idle and was 40 psi after that--previously it only had about 5 psi at idle and never went much above 30 at higher rpms..the guy told me since VW's only had a screen instead of a oil filter,the engines often suffered sludge build up--the fact they only held 2.5 qts of oil didn't help either..he claimed multi-weight oils killed a lot of air cooled VW's and told me to only use detergent SAE 30 in warm weather and SAE 20W-20 in winter..
 
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