Hypothetical oil pressure question...

[bpu699]

On cold start up, oil is thicker, so oil pressure should be much higher...right?

If you start your car cold, in the morning, and max oil pressure is 4 bar... gun the motor slightly, still 4 bar.

And later warmed up, you max oil pressure is still 4 bar...gun the motor, still 4 bar...

How can this be?

This was my motor before rebuild. I haven’t put it back in the car but trying to make sure I haven’t missed anything.

In my mind, the only thing that can do that is the relief valves...but those looked fine, springs and shims fine... further shimming made no difference.

Anything else that can explain that scenario? I would think ice cold, the motor should have pegged 5 bar if it was 4 bar hot...

[trond]

most likely the sender is off and your pressure is fine. Can not be 4 bar regardless of rpm and temperature

[boosted79]

Seriously? I see 4B+ all the time on start-up with Valvoline VR1 20-50. And I hear the pressure relief valve in the external t'stat thumping until it warms up.

[bpu699]

But that’s the thing...it was...

Rebuilding the motor due to cam wear, but trying to figure out what was causing the above prerebuild issue...didn’t find anything odd during the rebuild, motor not yet installed or run...

I’ve cold, turn motor on, pressure shoots to 4 bar or a tad over for a split second, then settles at 3 bar idle. Pressing the throttle, went a tad over 4 bar, but no where near 5 bar...

Warmed up, maxed at almost the same...4 bar or a bit over, idle about 2 bar or slightly less.

Idle pressure was fabulous, adding throttle pressure hit 4 bar and plateaued...

No matter what, couldn’t get above that. Again, When cold, I would think it should peg the meter...

During the rebuild bearings on tolerance, pump looks great, etc...

Sender is fine, checked with a manual one...

It was like the pressure relief valve opens a tad over 4 bar, and that’s it. But shimming it made no significant difference...

Clearly it wasn’t a functional issue, as car was tracked and internals looked fine. Just trying to figure out why I can’t get 5 bar, even cold. Perhaps now after the rebuild I will, but can’t figure out why it was happening. Was expecting shot bearing or something, and they looked fine...

[spuggy]

I start the car cold, reads ~120 PSI on a 10 bar gauge for 10-20 seconds fast idle, internal engine thermostat opens, pressure kicks once/twice and settles lower - maybe 80 PSI. Idles @ 60 PSI now.

Hot, idles ~20 PSI. Pretty much anything over 2000 RPM, reads 60 PSI.

Only deviation in the last 15 years was due to the original sender; it started reading higher - hot idle at 60 PSI, 80 PSI over 2000 RPM hot, IIRC.

I'd suggest double-checking the pressure readings with a mechanical pressure gauge and go from there. Sounds like the sender on its way out to me.

[bpu699]

Originally Posted by boosted79

Seriously? I see 4B+ all the time on start-up with Valvoline VR1 20-50. And I hear the pressure relief valve in the external t'stat thumping until it warms up.

???why would the thermostat thump? Could the pressure relief in it cause an issue? I wouldn’t think so, as whether the oil goes through the cooler or around it, the oil still goes to the pressure gauge...

[bpu699]

Originally Posted by spuggy

I start the car cold, reads ~120 PSI on a 10 bar gauge for 10-20 seconds fast idle, internal engine thermostat opens, pressure kicks once/twice and settles lower - maybe 80 PSI. Idles @ 60 PSI now.

Hot, idles ~20 PSI. Pretty much anything over 2000 RPM, reads 60 PSI.

Only deviation in the last 15 years was due to the original sender; it started reading higher - hot idle at 60 PSI, 80 PSI over 2000 RPM hot, IIRC.

I'd suggest double-checking the pressure readings with a mechanical pressure gauge and go from there. Sounds like the sender on its way out to me.

Perhaps I’m misunderstanding the function of the thermostat pressure relief??? Can a malfunctioning thermostat hurt cold oil pressure?

Needle rockets up cold like it wants to hit 5 bar, but immediately falls. Like a valve somewhere opened, and settles at 3 bar cold. It doesn’t stay up for 20 seconds like your does...

Never thought anything in the internal thermostat or internal cooler effected pressure. Saw another thread where someone replaced their cooler and pressure improved...

[boosted79]

"???why would the thermostat thump? Could the pressure relief in it cause an issue? I wouldn’t think so, as whether the oil goes through the cooler or around it, the oil still goes to the pressure gauge... "

Its the pressure relief opening in it when the oil is cold to prevent over pressurizing the scavenge pump. No, it shouldn't have any effect on the engine OP. When you said you shimmed the relief valves, you shimmed both valves in the case? One is a safety valve, the other sets the OP under normal operating conditions. The safety valve is upstream of the engine t'stat on the pressure pump discharge and is there just in case something happens to the engine t'stat that totally blocks the discharge from the pump. The pumps are essentially positive displacement and will pump until the weakest point breaks.