EggH6:OilPressure

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Oil Pressure too High?

I hooked up a mechanical oil pressure gauge this afternoon to 1) see it the resulting values match my EIS numbers and 2) see how high the pressure really is (EIS only shows up to 99). Well the result is interesting. My oil pressure at engine start is ~125 psi which is higher than I thought it would be. Then after taxiing for a while to get the temperatures up it came down to about 85 at idle and as soon as the rpm goes over ~1300 jumps up to 120-125 psi again. It might have gone lower than that if I kept on taxiing (I've seen as low as 60 psi on the EIS), but didn't think that was necessary.

Question: why does the oil pressure stay so high at engine start? It obviously doesn't do that on a car. Is it because we use an external oil cooler on our engines instead of the small stock liquid cooled oil cooler that sits behind the oil filter on a car installation? Is the pressure also higher because of the 3/8" ID oil cooler lines running from the engine? I.e. if I would replace them with 0.5" ID lines, would that help lower the pressure?

Question: what is the safe oil pressure range on our H6 engines? I couldn't find that information in the service manual?

Tomorrow I plan to remove the oil cooler and lines from the engine to see if they are clogged at all. Question: what is the best way to test the flow through the oil cooler? Is there a process for this?

Thanks Andre


Andre, Oil pressure is high with cold oil because its viscosity is high and flows thru the engine passages, bearing clearances and misc. orifices more slowly. The pump is of positive displacement and will pump pretty much the same volume of oil no matter what the oil temp. Thus more pressure occurs in the system. There is a bypass valve in the system that limits oil pressure to a safe level. In your case it seems to be 125 psi. Sometimes, even this valve can be a bit overwhelmed if the oil is very cold or the engine RPM is too high. In your car it happens all the time if you live in the frozen north, usually with no ill consequences. If cold enough though, I've seen oil filters leak around their base a couple of times.

Depending on where your oil pressure sensor is located, lines you feel might be too small may or may not have any effect on indicated oil pressure. If the sensor is located upstream of the filter, maybe in the input side of the oil cooler adapter, you might read a high pressure. If located downstream of the cooler and filter, you will possibly read a slightly lower pressure. I feel the best and most honest place to measure oil pressure is at the main bearing oil gallery. This is the pressure the engine actually sees.

There should be no reason to remove your cooler and lines to check for pressure drop. To see how well your oil cooler and lines are effecting flow, measure pressure at both of the ports in your oil cooler adapter. One of the ports is upstream of the cooler and the other is downstream. With cold oil there will probably be several psi difference. As the oil warms up, the difference will probably be 5 psi or so.

By the way, I use the factory oil/water cooler and it has a pressure drop of about 5 psi across it. I use a standard cooler too.

If you have the factory service manual for your engine, there should be a set of specs for the oiling system. In it there is a value called the Relief Valve Operation Pressure. It's in the section LU. For the H-6 it's 85 psi. For the newer engines it's higher, but I don't know how much. For my H-6 engine, I see over 100 psi pretty much every time I start it from cold even though the limit is supposed to be 85. Pretty typical, I'd say.

Mike T.