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Steve, Motorcycle Mechanic
Category: Motorcycle
Satisfied Customers: 6980
Experience:  30+ yrs. experience, MMI, HD technician, HD early, engine rebuild/performance, bike builder, kit bike program manager, CCI 2001-2004
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How's it going I have an 89 evo. at idle it has 15 lbs oil

Customer Question

how's it going
I have an 89 evo. at idle it has 15 lbs oil pressure. I rev it up a little and the oil pressure drops to almost 0
Submitted: 1 year ago.
Category: Motorcycle
Expert:  Steve replied 1 year ago.

tell me everything you can about when this started.

we can continue then.


Customer: replied 1 year ago.
I just purchased the bike used. I don't know any of the history on it. The person I purchased it from moved. He acquired it as part payment on a debt.
Customer: replied 1 year ago.
1989 Harley Davidson FXSTC, 1340, 13,000 miles, 20/50 v-twin motor oil,
Expert:  Steve replied 1 year ago.

oil pressure does not go down when the engine speeds up. you have a problem with INDICATED pressure, not actual oil pressure.

the best thing you could do is remove the oil pressure gauge and throw it away. if your engine needed one, Harley would have installed one before they sold it. that's up to you. if the low oil pressure warning light is on, you do not have low oil pressure.

the factory spec on oil pressure for your engine is 25-35 psi at normal operating temperature and highway speeds.

that said, you most likely have an issue with the engine breather.

there is a hose on the back of the engine, just inboard of the oil pump that connects to the back side of the air filter.

-ensure it's not kinked or restricted in any way.

- make sure the oil tank level is between 1/2 and 3/4 full with the oil at operating temperature (typically checked after a ride of 30 miles or more)

- never change the oil or top off the oil tank without first starting the engine. again, the best time to do this is after a ride.

check all those things and let me know what you find.


Customer: replied 1 year ago.
Thank you for getting back with me. I'm going to pick up a new oil pressure sending unit and the relief valve spring this afternoon and take a closer look at the check valve ball and ball seat also. It pukes a lot of oil after sitting a few days.
It came with an S&S super E carb with an after market air filter that doesn't have an engine breather vent line option. The case breather hose was left open and routed under and to the back of the engine. I have a filter to put on the end of the breather line. Do you think that the lack of positive engine case venting might be part of the oil pressure drop?
Expert:  Steve replied 1 year ago.

I don't think you have an oil pressure drop at all, or you would have opened your question complaining of loud tappet noise. just ensure you have indication of return flow to the oil tank.

-remove the filter on the breather. it's unnecessary and when it fills up with oil and dirt it will clog and cause breather problems.

- you can replace the check ball and spring if you want, but the problem is the seat, no the ball. you can lap the seat with an old EVO push rod, but it seldom fixes the issue. the reality is you need a new oil pump. the problem with sumping will continue until you replace it.

- the relief valve and spring does not affect oil pressure in the way you are describing it. it just just bleeds the over pressure to the rod bearings.

let me know if you need more help.


Expert:  Steve replied 1 year ago.

let me know if you need more help.

if no, please leave a POSITIVE RATING.

thank you,


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