Increasing Oil Pressure

[dvd]

I went on a trip in early October and another this weekend both required long hours of motoring. What I have noticed is my oil pressure starts out ok about 35 at startup but as the engine gets warm it goes up to about 40. However, after extended periods of motoring and at cruising speeds, I have noticed the oil pressure goes up to about 50 lbs and stays there.

I suppose I could adjust it but is it normal to start out ok and then for the pressure to increase over time??

dvd

[JOHN COOKSON]

Electric or mechanical gauge?
If electric you could confirm the readings with a second mechanical gauge. Also if electric maybe there is a bubble or something with the sending unit.
The sending unit is compatible with the gauge?

TRUE GRIT

[joe_db]

My old engine did that. It started like 25-30 and worked up to 40-45 over about 30 minutes.

[thatch]

One Theory

Although "increasing oil pressure" seems to be "counter intiuitive" when compared to other engines, where oil pressure tends to decrease as they warm up, I feel that there is a reasonable answer which lies in the area of the oil pump. One of the fittings, a 1/8" street elbow, that connects the filter screen to the oil pump, has a relatively small opening compared to the rest of the oil passages in the oil pump and block assembly. When the oil is cold and at it's thickest, there is a natural tendancy for it "not" to want to flow, kind of like trying to drink a thick milk-shake through a thin straw. As the oil temp increases, it's ability to flow through this restriction increases also, providing increased volume and potential pressure on the "bearing" side of things. A partially plugged filter screen would have the same effect, but I feel that that small elbow is probably the reason for this mystery.
Tom

[dvd]

So can I fix it without taking the whole engine apart?? Or is it not a problem?

[thatch]

DVD, It seems to be a very normal occurrance, so I wouldn't worry about it. I actually believe that Hanley inlarged that "restriction" during a rebuild, so he may know if that cured the problem. I feel that the bottom line would be to, adjust the oil pressure (hot) to 40 psi and let the warm-up pressure stabilize on it's own. As with any engine, don't put the "pedal to the metal" until things have warmed up completely.
Tom

[sastanley]

thatch, that explanation makes perfect sense to me and is how mine behaved as well, prior to the Indigo filter addition. It is less noticeable now, cold oil pressure is in the 55-60 PSI range, and it settles to 37-40 when hot.

[hanleyclifford]

Thatch is right. During a rebuild I changed that to a 1/4" NPT street 90. It makes a huge difference in oil flow and should be standard at rebuild time.

[dvd]

Oil Pressure Adjustment

I know where the oil pressure adjustment is and I believe I know how to adjust it but should the oil pressure be adjusted underway. Or do you just start making small adjustments at the dock until its right. ( I do have the Moyer Manual)

The only thing I'm worried about is to much oil pressure can cause problems with seals and create oil leaks. So I suppose I could just leave it alone.

[hanleyclifford]

Before doing anything I would install a second oil pressure gauge and do another diagnostic run. If the two gauges agree the pressure should be adjusted underway after reaching and holding desired operating temperature. I like 40 psi. It is useful to remember that pressures can vary substantially throughout the system.

[sastanley]

hanley, Good point. When I installed the aftermarket oil filter system, I also moved the oil pressure sender (since the feed line for the filter used the 1/8" NPT fitting just aft of the flywheel which was the original location of the sender)...so the location of the sender unit could certainly alter my perception of pressure throughout the system!

[hanleyclifford]

hanley, Good point. When I installed the aftermarket oil filter system, I also moved the oil pressure sender (since the feed line for the filter used the 1/8" NPT fitting just aft of the flywheel which was the original location of the sender)...so the location of the sender unit could certainly alter my perception of pressure throughout the system!

Yes, the point nearest the "oil dump" hole is likely to show the lowest pressure. When the standard oil relief valve is "two blocked" (as they say) and the oil relief is moved to a remote location the pressure just downstream of the rear main bearing and 3 and 4 connecting rod bearings and the rear cam bearings is bound to rise and the front bearing and 1 and 2 connecting rod bearings are going to lose a little. Fortunately the differential is not great enough to cause a problem but I do like to have a gauge at each end of the "system".

[sastanley]

Hanley, I am not following you directly...I've also never torn apart an A-4 to see how the oil system flows inside the engine.

It is my understanding that the oil pressure adjustment system is the last point before the oil is dumped back into the oil pan..thus, by screwing in the adjuster you effectively increase back pressure and pressurize the rest of the block, like kinking the end of a 50' hose so the rest of the hose is pressurized. I do not know where the stock oil pressure sending unit location (near the flywheel) is in relation to this layout, but it appears to just be a hole in the block.

On the Indigo remote filter design, which is fed from the stock pressure sender 1/8" NPT fitting, and I assume is fed thru crankcase oil pressure via the oil pump, and then dumped back into the oil pan thru the large oil pan fitting on the port side, which I think eliminates the stock oil pressure fitting...hence why it is two-blocked..the pressure is regulated at the oil filter fitting.

Do I have that right?

[hanleyclifford]

Pondering the Answer

Look at page 5-3 of the MM service and Repair Manual. In the stock configuration oil is "dumped" just as the oil enters the gallery feeding the cam and forward bearings; in the modified version the pressure is relieved way forward before and near where the oil reaches the front main bearing. Every bearing is a pressure loss as is the relief valve and the pressure can vary throughout the system. The question becomes: where is the optimal point to relieve pressure so as to achieve greatest uniformity of pressure throughout the system?

[sastanley]

Hanley, thanks for directing me to the A-4 bible..it is on the boat, so I'll have to reference it next time I go to visit.

It is about time to bring it home, and put it on the bedside table for some review over the winter...