loosing oil pressure on new engine

[cblzaccie]

Volvo penta 5.7 on a welcraft excel 26se

This has happened 2 times now, Get a re manufactured engine, I go out, the oil pressure is fine till around 2000 rpm, anything over 2000 -3000 rpm i start to loose oil pressure, it jumps up and down between mid point on the gyuage and about 10 psi then dropps lower and lower, engine develops a knocking sound. this is haddpend on 2 engines now, the mechanic cant figure it out , any ideas? this is costing me much frustration and time

 

[HT32BSX115]

Re: loosing oil pressure on new engine

I'll assume you have verified this with a direct reading (mechanical) oil pressure gage?

Did you change or completely bypass the oil cooler and/or are you losing oil quantity as well?

 

[wca_tim]

Re: loosing oil pressure on new engine

of the two engines it's happened on what stayed the same?

It sounds almost like you're not getting oil returned fast enough and that you pump the pan dry or at least that the pickup is getting out of the oil...

or at least that is consistent with your description. for example if you put a high volume oil pump in a marine engine with a stock sized oil pan, once you run the rpms up for a little bit your oil pump can pump the oil up into the engine faster than it can drain back down... if you're running fast, you lose oild pressure running flat out and everything goes to you know what...

Just something to think about.

 

[cblzaccie]

Re: loosing oil pressure on new engine

I'll assume you have verified this with a direct reading (mechanical) oil pressure gage?

Did you change or completely bypass the oil cooler and/or are you losing oil quantity as well?

no loss of oil and yes direct reading confirms....

 

[Don S]

Re: loosing oil pressure on new engine

Check your oil level. IF it's over full, the crankshaft could be hitting the oil and aireating it. This will cause low oil pressure and the the lifters will start rattling.

 

[n2ostroker]

Re: loosing oil pressure on new engine

One thing I noticed when pulling this old motor apart is the Marine engine came with a windage tray bolted to the mains in the oil pan. This helps catch the oil slinging off the crankshaft and return it to the pan instead of whipping it with air. When I built my 355 my new oil pan had a baffle built in the pan. I would ask and see if the motor you got has either the windage tray or a baffle in the pan. If not and it is just an open pan you get all the oil stuck up in the motor and it's not making it's way back to the pan quick enough. Bring the rpms down as soon as you see the oil pressure go haywire or you'll tear up the crank and rod bearings. That knocking sound is the bearings going dry. If its more of a tick or rattling sound it could be the lifters getting full of air or losing oil and making the valves come out of adjustment. Another thing I noticed is the pan on a marine motor was about an inch deeper to hold more oil.

 

[Bt Doctur]

Re: loosing oil pressure on new engine

Are you "pre-oiling " the engine before the initial start up?
If not you could be tearing up the bearings on a dry start up .You spin a bearing and then spike the oil pressure then once the bearing is torn out oil press drops and the knocking begins