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Oil pressure drops after 10 minutes driving 

 

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 Post subject: Oil pressure drops after 10 minutes driving
Posted: Wed Oct 18, 2017 10:47 am 
Getting Side Ways
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I have just replaced my 346,000km EF engine with a freshly rebuilt AU bottom end with EF head, and have started breaking it in. It runs great - smooth and grunty. But the oil pressure has me baffled.

When starting from cold, the pressure is 5-6 bars on the gauge and stays there for about 10 minutes - about 5 minutes after it reaches full operating temp. Then the pressure drops to 1-2 bars in normal driving, not much higher when revs increase, and into the orange zone at idle. It has done this both times I have driven it since the engine went in. The temperature is rock steady at the bottom of the normal range from about 5 minutes driving.

It has a brand new JP oil pump and the engine builder checked the bearing clearances. I am running Royal Purple 10W30 non-synthetic break-in oil for the first week while I run it in. This is what is specified for the AU, but thinner than the 15W40 specified for the EF. I have some Royal Purple full-synthetic 15W40 going in on the weekend.

Should I be concerned about the weird pressure readings? I will be drag racing the car in a couple weeks, getting close to 6000 rpm, so don't want to destroy the engine through low oil pressure. There are no other symptoms, and the pressure does stay high for several minutes after reaching operating temp, so it shouldn't be the oil thinning. The viscosity of synthetic oil doesn't change much as it warms up, so maybe it will be better after the thicker 15W40 synthetic oil goes in - but I'm not hopeful.

Can anyone tell me what's going on here? Thanks.

 

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 Post subject: Re: Oil pressure drops after 10 minutes driving
Posted: Wed Oct 18, 2017 12:39 pm 
Getting Side Ways
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10W-30 is way to thin, even for a fresh engine. And be careful with Royal Purple - they tell you to run less clearance, but it cant handle high compression applications, and you can get bearing damage.

What clearances did your engine builder use? Were they "measured" or just Plasti-gauged?

Does the oil pressure drop as soon as the revs drop, or does it take a minute or so? And is it a standard or hi-pressure oil pump?

I would get hold of a proper oil pressure gauge and get some good readings before you go changing too much.

And when you drop the current oil, remove the filter and cut it open and check for metal, etc.

Good luck. 8-)

 

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 Post subject: Re: Oil pressure drops after 10 minutes driving
Posted: Wed Oct 18, 2017 12:56 pm 
Getting Side Ways
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Thanks for the quick feedback.

I would have thought that 10W30 would be fine for the bearings since that is what the AU engine runs from the factory. I assumed the thicker 15W40 in the EF was due to the different valvetrain.

I'm pretty certain he miked the clearances, but will check.

It's like the oil pump changes gears: after about 10 minutes, the pressure drops at all rpms. It's a standard pressure JP pump.

I'll see what happens when I put the 15W40 in this weekend and take if from there. And checking the contents of the filter is a great idea.

Thanks for the help.

 

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95 EF XR6 wagon, 17" FTRs, DBA, QFM, KYB/Koni, AU block, ported EF head, SS Inductions, Territory intake, MLS gasket, milled, Franklin RH24B cam, Coby headers, no cat, Tickford 2.5", 2800rpm stall, J3 chip

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 Post subject: Re: Oil pressure drops after 10 minutes driving
Posted: Wed Oct 18, 2017 5:04 pm 
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efxr6wagon wrote:
It's like the oil pump changes gears: after about 10 minutes, the pressure drops at all rpms. It's a standard pressure JP pump.



that probly when the oil has warmed up ...... then thins out?.
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 Post subject: Re: Oil pressure drops after 10 minutes driving
Posted: Thu Oct 19, 2017 1:46 pm 
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efxr6wagon wrote:
Then the pressure drops to 1-2 bars in normal driving, not much higher when revs increase, and into the orange zone at idle.
Any sign of big end bearing knock at idle when the pressure is low ?
Check the oil pressure sender with a torch for oil leaks - old senders can get brittle, and you can inadvertently damage it refitting it to the AU engine. Usually though if it is damaged. or the membrane has ruptured you'd be reading low oil pressure all the time.

 

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 Post subject: Re: Oil pressure drops after 10 minutes driving
Posted: Thu Oct 19, 2017 4:53 pm 
Getting Side Ways
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Nope. No knock.

And the oil pressure sender is visually fine. But I did use teflon threadtape to prevent leaks. I wonder if that is enough to cause a weak or intermittent earth to the block.

 

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