Oil Pressure

[tmea]

I just rebuilt my 1500 SP in my 1968 autostick. All is well and it runs great. After my 100 mi oil change I did some checks: compression was great at 145, 140, 140, 130 cold. When I did an oil pressure check though I have: 33PSI @ 800 rpm and 40PSI at any rpm above and up to 3000. Everything is stock including the single relief piston and spring. Don’t think I have a problem but I have never seen OP so high at an idle before. Usually around 10PSI and 10 more PSI for every 1K rpm. Any thoughts on this?

Tom

[VW_Jimbo]

What oil weight?
_________________
Jimbo

There is never enough time to do it right the first time, but all the time necessary the second time!

[tmea]

10w40

[AlmostHeavenWV_VW]

First thoughts:

Did you re-use your old oil pressure relief springs?
Did you check to make sure the relief bore holes did not have any damages/burrs in the hole?
Any changes to the oil pump? New one?



Gotta make sure to reinstall the oil pressure relief plungers and springs carefully to not have them get jammed up sometimes.
Gotta use the right size geared oil pump to maintain proper pressures.
_________________
1973 Standard Beetle
1600DP AK case
Solex 34PICT3 Carb
Bosch DVDA 205AJ Distributor

[glutamodo]

If you are talking about a hot engine, that idle pressure is too high. However, a cold engine may have much higher oil pressures.

Now, I did run into an issue once mistakenly using a "high pressure" non-stock pressure relief valve spring on a single relief 40HP engine and got way too much pressure.

Many years ago I took these readings on a freshly built (dual relief) engine, but I made sure to include a record of the oil temperature when I did so. 12PSI at 612RPM idle and 39 at 2300RPM and that was at 216°F oil temperature, which I would say is about what one would expect from a decent new build.

_________________
Andy T.


IMAGE NOTE: It has been noted that Chrome based browsers may have issues in displaying my vast image library, which use non-secure links and are on an FTP server. Images should still be viewable if the link is clicked though.
I do not know how to fix this. All I can say is it all works fine for me with what I use, Firefox.

[tmea]

To answer the questions:
- The bore was cleaned by hand with a hone just before being hot tanked. When I assembled, I checked the piston with a magnet stick, it moved in the bore freely.
- Original piston and spring
- Stock pump.. no options as it is an autostick

Looking at the Bentley manual it does not specify a range by RPM just 42psi with a wear limit of 28. I did resurface and deck the pump flanges to remove the circular wear marks. I’m thinking this is how it is supposed to be. The pressure relief valve is definately working and not letting the pump get more than 40PSI build in the system. I am not sure I can think of anything wrong with a little over 30psi at idle. Am I missing something?

Tom

[glutamodo]

And the question about the oil temperature at time of testing?
_________________
Andy T.


IMAGE NOTE: It has been noted that Chrome based browsers may have issues in displaying my vast image library, which use non-secure links and are on an FTP server. Images should still be viewable if the link is clicked though.
I do not know how to fix this. All I can say is it all works fine for me with what I use, Firefox.

[tmea]

Sorry, oil temp was warm. Engine had been run for less five minutes. Just enough to get a warm compression test before checking oil press. It is very hot here in kansas. Judging by the oil temp dipstick it is running much cooler than before the rebuild. The dipstick begins to flicker at 225F. It is not getting close to making contact even at 60mph at 95f.

Tom

[Igpoe]

Is an inaccurate guage a possibility?
_________________
'73 super.

[rayjay]

IMO your readings are great. The “high” idle reading is a sign of excellent bearing clearances and a good oil pump. Low idle pressure is a sign of loose clearances in the bearings and pump.

[viiking]

[tmea]

The rod oil clearances are .0015 for all four. Measured with green plastigauge

[tmea]

[viiking]

If you did it properly and didn’t rotate the crank with the plastigage installed then your clearance is right on the tight limit and in spec.
_________________
1968 1500 RHD Lotus White Beetle since birth. In the hospital for major surgery
1966 Lancia Flavia Pininfarina Coupe - in the waiting room
Discharged: 1983 Vanagon, 1974 1800 Microbus,1968 Low Light,1968 Type 3

[tmea]

I took my time putting this one together. I have everything on the tight side of spec across the board including a CS end play of .003

[viiking]

To tmea.

I'll try and explain my reasoning. It is verbose reply so anyone not interested just skip.

The oil pump is a positive displacement pump. As revolutions increase, the volume of oil pumped and therefore the pressure (due to the resistance in the oil circuit) increases. The increase in volume/pressure is not quite linear i.e. the oil flow almost doubles with double the rpm but close enough for our purposes. The quoted figure that many use of an increase in oil pressure of 10 psig per 1000 rpm is a good indicator.

The flow of a gear pump is controlled by restricting the suction i.e. the oil coming into it from the sump. As there is no valve there and the only restriction on the pump is a blocking oil strainer (small effect due to the size of the mesh) the oil pump runs virtually at full speed and dependent on engine rpm. Hence the increase oil pressure with rpm.

On the discharge side of the oil pump there is a pressure relief valve. At higher than normal pressure, the valve is forced open and allows the oil to bypass the oil cooler and supply the bearings directly. This is particularly true of a cold engine where the oil is thick and the oil pressure high. As the oil heats up, the pressure relief valve slowly closes and allows more oil to the cooler and then the bearings. But the total oil flow will not change i.e. direct + bypass is constant at constant rpm. The oil from the cooler then starts to supply more and more of the cooled oil to the bearings.

A higher oil pressure than is required to open the pressure relief valve means that more oil is bypassing the cooler. So if your pump is developing more and more pressure and if it is above 42 psig (according to Bentley) the pressure relief valve is open and all this excess oil flow is going direct to the bearings and not through the oil cooler. So at high speed at high temperature, your engine may not cool sufficiently. So just having high oil pressure at higher revs doesn't mean that all this oil is being cooled. The best situation would be that at normal operating temperature the pressure relief valve is NEVER open.

How much needs to be cooled? I would expect that at any warmed up engine temperature you'd want all of your oil being cooled. So I guess the engineers at VW determined that the developed oil pressure in excess of the pressure relief valve was wasted. Knowing how much the oil pressure increases with rpm and not wanting to have too much more oil pressure than the pressure relief valve opening (42 psig) they would have determined a minimum and maximum oil pressure knowing what is called the "pump curve" and specified the oil pump gear clearances to achieve that result. This is a standard engineering practice.

As you have a Bentley, I'm sure you have looked at Fig 100 of Part 1 Engine which shows the oil flow circuit.

The problem is that the oil pressure switch is in the wrong location. I'm assuming that you are taking the pressure at this location.

It tells you the pressure that the oil pump has developed before the pressure relief valve and before the oil cooler. High oil pressure here does not indicate anything about what oil flow or pressure your bearings are actually seeing. In fact high oil pressure can also indicate low oil flow. If there is a blockage in the oil cooler, or the oil galleys to the bearings, you may have good pressure at the switch but not necessarily sufficient oil at the bearings. This is a problem that every engine builder will have. Luckily you don't need much oil pressure at idle anyway so long as there is a flow. I guess that is why VW said it was OK at idle and hot to have a flickering oil pressure light. So at idle 2-3 psig was sufficient to lubricate.

So I would think that you shouldn't discount that there may be blockages in the oil circuit because your oil pressure at idle is so high. I don't think that it is due to an excessively efficient oil pump. It could be due to a pressure relief valve that doesn't open. It could be a bad gauge. It could be blockages after the oil pressure switch.

This is not to frighten you. You look like you've done what I have done on my rebuild, but I would be looking for reasons for the high idle oil pressure if it were my engine.

As a final analogy.(Hooray the audience says).

You go to your Dr and they send you for a treadmill heart test. As you exercise your blood pressure increases. This is normal. The pump is working as designed.

When the Dr discusses your results it indicates that your resting and non-resting blood pressure is too high. Why? The arteries in your body are blocking up. High systolic pressure at idle, high systolic pressure at high intensity. What's the relief valve? A haemorrhage. What's the danger? A stroke.
_________________
1968 1500 RHD Lotus White Beetle since birth. In the hospital for major surgery
1966 Lancia Flavia Pininfarina Coupe - in the waiting room
Discharged: 1983 Vanagon, 1974 1800 Microbus,1968 Low Light,1968 Type 3

[tmea]

Viiking:

Thank you for taking time to write a most helpful reply. I am going to have to think about it for a while. In the interim I am going to take a reading with a hot engine. If I get the same result I’ll go buy and try one of those small cheap gauges at the parts store just to eliminate a bad gauge.

I am not completely on board with your thesis of an obstruction for a few reasons: First, I blew out all of the passages before assembly and the case was also hot tanked. Second, only oil that has been through the cooler goes to the bearings. If I had a flow problem I would also have heat and presumably some Babbitt material in the sump on my 100 mile oil change. It was clean. Third, After the 100 mile OC I drove it about 35miles in 98F temps. Most of this drive was varying between 55 and 65 mph. At no time did my 230 degree dipstick sensor come close to a flicker. I did test and calibrate it so I know it is accurate. It also used to come on prior to the rebuild and I would have to slow to 50th to get the temps down. Bottom line, I am not getting any heat that would indicate a problem. Finally, my thinking is that if I had an obstruction, as you point out a great deal of oil would be bypassing the filter and just thrown back into the crank case with no cooling. In this condition I should see very high oil temperatures over a short time but the temp is fine.

I temper all of this with your other point on flickering oil light at idle. VW wrote that this is not a problem (I think the cut-out is 6psi). My engine did this prior to the rebuild. For the final conundrum I can find now VW spec that says anything about a range of pressure - RPM. Just a single oil pressure.

Thank you for your insights. I’m going to do the things I mentioned this weekend and will report back. In the meantime, I welcome anymore thoughts from anyone. Much at stake for me here.

Tom

[rayjay]

There are lots of misconceptions about the oiling of an ice. Here is some info.

http://www.engine-misfire.com/how-oil-pressure-works/

[viiking]

Thanks for taking the time to read my “thesis” or as others will unkindly say my diatribe!

[rayjay]

Again,,,, there is nothing wrong with the OPs motor.

The VW specs about oil pressure at idle is all about CYA. It's like the industry standard oil comsumption CYA of a quart every 1000 miles is acceptable.