Thread: 3.9 v8 oil pressure

Doesn't the oil pump gears hold the oil up to a certain degree creating a bit of a seal after the engine is off and if there were gaps between gears or the bottom base plate then it would take longer to get the pressure to build up again?

Doesn't the oil pump gears hold the oil up to a certain degree creating a bit of a seal after the engine is off and if there were gaps between gears or the bottom base plate then it would take longer to get the pressure to build up again? 9th July 2012 11:59 AM

This is usually accomplished by the anti drain back valve in the oil filter. I don't think there would be much volume of oil between the pump and filter, and oil cannot drain out of the pump itself as it is lower than the feeder gallery.
If the anti drain back valve is not working , all the oil above the filter ( or the gallery attached to the oil pickup) drains back through the pump, but the pump should remain full.
Regards Philip A

Hi Macman,

Is this low oil pressure at startup on the rebuilt motor? I haven't been watching the forums much recently, so sorry if it's covering old ground.

I had a very similar issue after putting my motor back together. To cut a long story short, it turned out that regardless of what i considered careful attention, I put one of my new rocker assemblies together, with the rocker shaft oil holes facing upward and outward, instead of toward the valley. With valve springs and lifters pushing the rockers, the clearance on the upper side allows oil to flow too freely. I pulled the assembly apart, flipped the shaft, and oil pressure is that of a new engine....which it should be.

Is their a chance this could be you issue?

Cheers
Andre

Andre,

Interesting thought. I used my old rockers and shafts, and didn't disassemble them. They were kept clean marked for left and right banks, cleaned with shellite and compressed air and lubed with moly paste before being reinstalled. It's relatively easy to get to them so I will check that all looks good.

I forgot to mention that my cam bearing clearances were middle of the road with about 0.004" on all journals - new bearings and new cam. Again, I feel safe in assuming that considering the age and mileage of the original components the clearances of the new parts should be at least as good.

RPI or someone like that reckons that those rockers and shafts can wear quickly when they get bits of metal embedded in them. I would just replace the whole lot as a matter of course. And Des Hammill reckons they get too much oil and this lowers how much the rest of the motor gets. He says that putting a restriction in the block or heads is the way to go. I'm going to do that with the heads I'm working on now.

Some more developments...

I have come to the regrettable conclusion that the engine needs to come out. I've just been through a bunch of hassle with the oil pump. I decided to rule it out as the source of the lower than ideal hot oil pressure and the lack of pressure on startup. I ordered a new set of gears and gaskets and noted two things. The new pump gaskets were all around 0.23mm thick. I hadn't taken the new gears out of the packets before disassembling my oil pump. This becomes significant in a moment! The gasket I fitted during the rebuild came from a Payen gasket kit and measured 0.42mm. The "old" gear to housing clearance measured at 0.07mm, so the total clearance when built was considerable at 0.35mm.

I went to measure the new gear to housing clearance and it was only then that I unpacked the new stuff and remembered why I should never order Britpart gears. They were absolute rubbish. Burred edges, casting inclusions on the shaft of the drive gear and they didn't mesh properly - just the thing you need in your oil pump. Back they went (more delays and another trip into the city) and they were exchanged for a Bearmach drive gear and unbranded "A" grade small gear. Better finish, but when I set about fitting them yesterday they didn't mesh smoothly. Close inspection revealed that the tooth profile is completely different and were obviously not from the same manufacturer. I compromised with the new drive gear and the old small gear once I'd established they meshed properly. Final clearance with this arrangement of gears was about the same with the 0.07mm.

Installed and primed up, hot idle pressure is now 25 to 30 psi and 2700 rpm pressure is just under 40 psi. The installed clearance of 0.15mm (half of what it was) boosted the pressure back to where it should be, but I was disappointed to find this morning that start-up oil pressure still takes too long to raise and there's still a clattering lifter.

I want to remove the intake and valley gasket and observe the cam bearings and rocker gear to see how long it takes for oil to bleed past the cam journals and rockers. The sump also needs to come off so I can check the pickup. I COULD do this with the engine in the car but I'm so sick or working under it and it's very difficult to keep everything clean. There's something in there that's not right and I'm not prepared to lunch the engine after all this work. I also can't rule out the timing cover gasket until it's apart and I really don't want to have to do it in the car.

I'll probably throw new main bearings at it if there's any sign that the low startup pressure has harmed them.

Other trivia: oil is currently HPR Gas 20W-60. I had previously run HPR Gas 10 10W-50 which gave lower idle pressure but about the same high speed pressure. Pressure relief valve has a new spring, and the valve moves freely in the housing.

Any other suggestions folks? And please, nothing to do with matches or arson.

Jeez - no good mate. Unfortunately I have no suggestions. Sing out if you need an extra pair of hands if you end up having to pull the engine.

Thanks Gav. I might have to take you up on it. You about next weekend?

I've been looking slitty-eyed at the Disco all day while splitting firewood.

It WILL be something simple. I'm considering trying to pull the timing cover to replace the gasket with the engine in place because I really don't want to take the whole lot out again. If that's not the cause then the only remaining things it could be are a leak somewhere in the oil pickup tube, an excessive clearance somewhere in a rotating assembly.

I am going to check the old main bearings that came out just to see what sorts of oil grooves were in the shells.