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Old 05-09-2012, 20:09   #16
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Re: 4-108 Oil Pressure Drop After Oil Change.

The Oil Pump, was it "rebuilt" or replaced?
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Old 05-09-2012, 20:44   #17
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Re: 4-108 Oil Pressure Drop After Oil Change.

I looked and the w40 oil bypass is on the oil pump housing, which alas is inside the oil pan. That's going to be a project. I might use a BFH to tap gently on the engine case with the hope of reseating the piston
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Old 06-09-2012, 07:55   #18
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Re: 4-108 Oil Pressure Drop After Oil Change.

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I looked and the w40 oil bypass is on the oil pump housing, which alas is inside the oil pan. That's going to be a project. I might use a BFH to tap gently on the engine case with the hope of reseating the piston
As far as the oil pump, I didn't do much. I gave it a good inspection last spring when i was assembling the engine. I'm not messing with bypass. That includes taking off the oil pan, timing gear cover and back cover have to be taken off as well. Then re-sealing everything, not even to mention having to lift the engine again. I am still well within limits of operating range. I'm not too worried about drop in oil pressure under idle when hot. I'll try tapping. If piston was not seated properly, wouldn't I have the same issue under load?
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Old 06-09-2012, 09:01   #19
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Re: 4-108 Oil Pressure Drop After Oil Change.

The pickup screen on the pump inlet only gets the big chunks. It is not very fine mesh at all. I would think it would be a good time to do an oil analysis to find out if your brand new bearings are turning to flakes. Hopefully it will show no problem, and you can go back to the bypass valve solutions.____Good Luck with it._____Grant.
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Old 06-09-2012, 09:39   #20
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Re: 4-108 Oil Pressure Drop After Oil Change.

My guess is the bypass is not quite seating properly. So you have lower then normal oil pressure at idle and at speed.

You might try cycling the throttle a few times, after the engine been running for 2 minutes. The colder oil will cause the bypass to open more. That's one reason its there after all. So revving the engine a few times might clear any gunk around the valve seat. No idea if that would work, but worth a try.

Hum, If there was a loose bolt on one of the tappet towers (supports for valve rocker arms), Oil could leak out there and cause a low oil pressure reading.

Though since the pressure dropped after the oil change, it's probably not that.
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Old 06-09-2012, 09:39   #21
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Re: 4-108 Oil Pressure Drop After Oil Change.

Actually I looked at you first post again and at Gord's post and I don't think you have an issue. I don't know why there would be a drop after an oil change but you are still well within specs.
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Old 06-09-2012, 10:40   #22
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Re: 4-108 Oil Pressure Drop After Oil Change.

Re-#18, the volume of oil increases exponentialy as the rpm rises. Sailorchic's advice is excellent.
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Old 06-09-2012, 17:57   #23
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Re: 4-108 Oil Pressure Drop After Oil Change.

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Re-#18, the volume of oil increases exponentialy as the rpm rises. Sailorchic's advice is excellent.
Actually volume is a linear relationship to RPM.

The pump is a positive displacement gear pump with no "regulator." Pressure is a function of RPM, oil passage clearances and viscosity with maximum oil pressure limited by the pop off valve which bypasses directly back to the oil pan.

Sailor Chic has a good theory in the non-seating valve but as the pressures are in limits I agree with OP not to rip into the bottom end again to mess with it.

Monitor, change the oil at 20-25 hours and cut open the filter and have a look.
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Old 06-09-2012, 18:29   #24
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Re: 4-108 Oil Pressure Drop After Oil Change.

Do an engine flush. After a rebuild it is not unusual to have bits of crap left hiding in the block and head.
RE your oil pump. Did you measure it to see if was within it's wear limits. A twenty psi drop in pressure between oil changes would seem out of the norm for me.
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Old 12-09-2012, 14:23   #25
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Re: 4-108 Oil Pressure Drop After Oil Change.

I just looked at the old gallon can of oil.
As it turns out I was wrong.
My frist oil was DELO 400 LE SAE 5W40
After an oil change i put in DELO 400 LE SAE 15W40.
So i went from 5W40 to 15W40.
But it doesn't make sense to me that it would change the pressure that much.
Its getting worse. Under load I am still good just over 35PSI, but after motoring when at idle its down below 5psi.

One thing that i noticed is that when i first start the engine it takes longer for oil pressure to rise from 0psi to 40psi. I guess that makes sense as 15w is thicker then 5w.
But if 15w is thicker, I should have more oil pressure when cold.
with 5W I had 55psi when cold. with 15w I have 40psi when cold.
but when its all said and done and i am at operating temperature both oils should be the same. 40 weight. And that's not the case.
Either way if psi keeps dropping I am going to have to re-open the pan and take a look at the oil pump. ouch.
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Old 12-09-2012, 14:48   #26
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Re: 4-108 Oil Pressure Drop After Oil Change.

I had a talk with a guy who was putting his 4-107 back together for the third time. Even with new main bearings, he was fighting low oil pressure. This time he thought he had the problem solved--turned out the injector pump bearing was completely shot, and most of his oil was going through it.

If it was my engine, I'd try a change with straight 40 weight and an OEM filter--if that didn't get the pressure back up, I'd tear it down and replace the oil pump and clean the bypass valve.
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Old 12-09-2012, 16:23   #27
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Re: 4-108 Oil Pressure Drop After Oil Change.

Have you checked the oil pressure sender and gauge? I would check them before tearing down the engine.
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Old 12-09-2012, 19:51   #28
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Re: 4-108 Oil Pressure Drop After Oil Change.

At this point if the pressure is continuing to decrease, I am with DonReadcliffe.

Change the oil now, cut open the filter and see if you are making any metal.

If you are at 5psi pressure at idle something is wrong. Good idea to verify the sender as DeepFrz says but if it was working before that is not likely the culprit.

This is not an oil viscosity problem in my mind.
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Old 12-09-2012, 20:13   #29
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Re: 4-108 Oil Pressure Drop After Oil Change.

Every one but me is assuming your rebuild was good. New crank and bearings or reground crank? Or old used crank? I have seen this drop in pressure on engines that had flat crank journals. At first the break-in oil PLUS the assembly lube makes for thicker oil. Now you have just oil. Flat cranks can pound out bearings in no time. If you have a good round crank and the inserts were plastigauged then this should not be an issue.

Bypass is possible, bad injector pump is possible but not probable. I don't like your analysis of the oil pump suitability though. Oil pump clearance is absolutely critical.

Good luck
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Old 14-09-2012, 08:27   #30
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Re: 4-108 Oil Pressure Drop After Oil Change.

I had an engine machine shop do the clearance measurements, acid tanking, honing, and they did cross stitching of cracks in the head. They checked the crank for balance and clearances as well. I went with standard bearings per their recommendation. In the passed 12 years, I've used them on two occasions with two different atomic 4 engines.
By flat journals you mean not perfectly round. I would think that when they did clearance measurements and while making the decision to grind vs. not to grind, they would have recommended going with oversize bearings and turning the journals. They were pretty thorough. They consulted with two separate machine shops regarding head cracks. Shaving the head vs. not, etc.

Injector pump bearing? You mean the bearing on the IP drive gear. That same gear drives the water pump">raw water pump. I don't think that one has a bearing. But if it lets oil out it would just run down inside the timing cover and back in the crankcase. That check can be done w/o lifting the engine, the problem is I can't really run the engine w/o timing cover to see if oil is gushing out.
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