7.4 loses oil pressure at high rpm

[jeffssgtusmc]

Oil pressure drops to zero over 2000 rpm

 

[Bt Doctur]

Re: 7.4 loses oil pressure at high rpm

T in a mechanical gauge to verify that 0 reading

 

[r.j.dawg]

Re: 7.4 loses oil pressure at high rpm

Year of engine would be helpful along with serial number. Carburated?? EFI?? MPI?? Outdrive?? Inboard??

 

[Bondo]

Re: 7.4 loses oil pressure at high rpm

Oil pressure drops to zero over 2000 rpm

Ayuh,.... Probably the gauge, or sender...

If it were truely goin' to 0 psi at 2000 rpms,....

It would only do it Once.....

 

[jeffssgtusmc]

Re: 7.4 loses oil pressure at high rpm

Ok shouldve known id get more questions then answers. It is a 1988 carbed 454 It is rebuilt aboit 20 hrs ago, runs great but loses oil pressure at high rpm i can watch it go down on gauge from about 40 to 50 all the way to zero. Also lifters start tapping so im thinking its not sender or gauge. Oil level is good. Any other questions or ideas let me know.

 

[Bt Doctur]

Re: 7.4 loses oil pressure at high rpm

Now that we have ALL of the information that changes things. What type of oil pump was installed in the engine and how many quarts does the oil pan hold?
If a Hi-Volumn pump was installed vs a standard you running out of oil in the 4 Qt pan.
You need a 10 or 12 quart pan (at least)to run Hi-Vol at hi rpm.

 

[Bondo]

Re: 7.4 loses oil pressure at high rpm

Now that we have ALL of the information that changes things. What type of oil pump was installed in the engine and how many quarts does the oil pan hold?
If a Hi-Volumn pump was installed vs a standard you running out of oil in the 4 Qt pan.
You need a 10 or 12 quart pan (at least)to run Hi-Vol at hi rpm.

Ayuh,.... Or,.... If the oil is over-Full at the hulls runnin' atitude, yer airriating the oil, turnin' it to a foam...

I'd still "T" in a mechanical gauge to double-check...

 

[04fxdwgi]

Re: 7.4 loses oil pressure at high rpm

Now that we have ALL of the information that changes things. What type of oil pump was installed in the engine and how many quarts does the oil pan hold?
If a Hi-Volumn pump was installed vs a standard you running out of oil in the 4 Qt pan.
You need a 10 or 12 quart pan (at least)to run Hi-Vol at hi rpm.

The 7.4 has a 7 qt pan on it. Dropping to zero is like others said, aerating the oil or maybe pumping too much and sucking pan dry (which I doubt because oil would be coming out the valve cover holes / vents. ). 2K rpm isn't much at all.... If the reliefe valve was stuck, the pressure would be low at idle. This is a strange one.... Especially since there is valve train noise, indicating low pressure. Valve train stays quiet down to 7 - 10 psi. Maybe a blockage on the oil cooler or a crappy filter?

I would change oil / filter again and see what happens.

Just rebuilt.... Hoses connected to correct ports on the oil filter remote after rebuild?

 

[jeffssgtusmc]

Re: 7.4 loses oil pressure at high rpm

Bondo i do not understand your language. Im not following the overfull airriating thing. I get the high volume oil pump sucking pan dry but not the airriateing. It is possible it has a high volume oil pump but i really dont feel like pulling the motor out again

 

[Idlespeedonly]

Re: 7.4 loses oil pressure at high rpm

The aerating will happen when the oil level reaches the crankshaft. It turns the crankcase into a big blender.

 

[franktrav]

Re: 7.4 loses oil pressure at high rpm

the oil pickup tube may be dislodged from the oil pump.....if that happens,,,,,,you will lose oil pressure once the oil in the pan drops below the bottom of the pum
a good practice is to tack weld the pickup tube to the pump during the rebuild process

 

[jeffssgtusmc]

Re: 7.4 loses oil pressure at high rpm

It is tacked on and im pretty sure it is a high volume i looked at the the reciept that came with block and thats what it says came with it

 

[jeffssgtusmc]

Re: 7.4 loses oil pressure at high rpm

I am still curious about airriating i think ill do an oil change and see what happens with little less oil

 

[wingless]

Re: 7.4 loses oil pressure at high rpm

+1 for the mechanical gauge. The parts are available from Home Depot and are a cheap easy sanity check.

The Mercruiser wiring harness on my engine used push-on friction connectors for the oil pressure sender. The connector lost its spring force and also got weird at higher RPM. The mechanical gauge showed I was good, so I snipped off the push-on connector and replaced it w/ a ring tongue connector and a nut.

All has been good after the fix.

 

[Bondo]

Re: 7.4 loses oil pressure at high rpm

I am still curious about airriating i think ill do an oil change and see what happens with little less oil

Ayuh,.... Just suck out a quart, 'n Try it...

Was it built with oil control,..?? splash pans, oil traps in the base, crank wipers,..??

 

[jeffssgtusmc]

Re: 7.4 loses oil pressure at high rpm

What about adjusting the lifters maybe another 1/4 turn or something.

No windage trays or anything.

 

[jeffssgtusmc]

Re: 7.4 loses oil pressure at high rpm

Would adjusting lifters do anything to pressure??

 

[Bt Doctur]

Re: 7.4 loses oil pressure at high rpm

lifter would do nothing, but the "no windage tray" has me curious. most auto pans have one welded inside the pan, most marine motors have it off the main bearing studs. without one you will aerate the oil into foam.

 

[sharkin95]

Re: 7.4 loses oil pressure at high rpm

Did you prime the oil pump before starting the engine? on a melling hv oil pump you should be getting 70-80 psi running and 60psi at idle.this engine does take a hv pump and it is impossible to drain the oil in the pan unless the passages back to the pan are blocked. Both solid and hyd lifters have metering valves to limit the flow of oil to the top of the engine.

 

[92rslt1]

Re: 7.4 loses oil pressure at high rpm

Did you ever get this fixed? If you used a melling pump, use the hardened shaft! Ask me how I know.. It will strip the stock one. That being said make sure you have the proper filter and check the obvious first, amke sure there is no clogs and pray one of th emain caps didnt come loose or something. Its possible the pump failed. If I remember they come with different spring weights, maybe the spring broke or something and isnt putting the proper pressure on the gears. Heck the tac weld could have failed too and the pick up miht have came off like others said.. Thats exactly what it would do if that happened.