lucxy

Just had my tech take oil pressure measurement with a mechanical guage at the oil pump and it consistently reads 15-20 psi higher than what I see at in the cabin or via OBDII interface.

Any reason for that? Do I have a bad oil pressure sending unit, oil sensor or is that normal? Or something else is going on?

I am running Mobil 1 0W40, oil pressure @190F showing 24psi via OBD and around 40psi by mechanical guage. The pressure differential is maintained throughout the RPM range or around +15-20 psi by the mechanical guage.

Any thoughts?

Dan_the_C5_Man

Where exactly did he connect his mechanical gauge? There are only a few choices (side of the block, top of the block (where the stock pressure sensor lives).

Either way, it's most likely a difference between what his gauge thinks 25psi is vs. the sensor calibration of what 25psi is.

lucxy

Measurement was taken on the side of the block.

olddragger

I will always believe a mechanical gauge over an electronic one--- if they are reading from the same site.

lucxy

I am trying to figure out under what circumstances there would be such a big difference in pressure between the side of the block and the top of the block where the sending unit is? Can the oil cooler cause such a big drop, I am running an external oil cooler B&M.

Cold oil pressure is around 60 psi via OBD but drop to around low 20 when hot yet the mechanical gauge is reading a much higher pressure when hot.

olddragger

oil cooler itself will drop oil pressure a little--when I removed mine( I don't track the car) I did notice my oil pressure go up just a little, but not much. Maybe 5psi?
The pressure difference between those too sites should not be that great.
Another thing to consider. What is your idle rpm? Mine seems to be a little lower than some. I idle dead on 600rpms. It seems that most others idle at 650?
If I increase my rpms by 500 ( hard to do lol) my oil pressure changes by a fair amount--at least 5psi.

stefuel

I would not be surprised to see lower pressure away from the pump. Tell your tech to rig up a "T" between the block and sensor so he can attach his mechanical gauge to the same port and then compare notes. Let us know what you find then.

lucxy

Mine idles around 850 rpm, I have a built motor with k501 cam. But I am not sure why it is important. At idle the mechanical gauge reads around 40 psi on the side of the block vs 24 psi at the gauge when hot. I would expect mid 30s at the gauge. I hope it is a bad oil sending unit.

My tech will check it out next week.

Undy

What's to say the mechanical gauge isn't off??

lucxy

Anything is possible but I doubt it.

lucxy

So had the oil sending unit replaced and T to a mechanical gauge and there is still a significant difference. The mechanical gauge is at 34 psi while instruments show 26 psi @ idle/hot. Now measured in the same spot.

How can this be? Why such a big difference in mechanical vs car gauge?

Ideas?

Dan_the_C5_Man

You need to consider the path the oil takes to get to the back of the block (but in this case you've eliminated that variable).

You should also consider that GM's version of 30 psi might not be the same as everyone else's.. And don't get me started on GM's version of accurate oil temps displayed on the DIC.. -ds-

lucxy

So I am taking mechanical gauge as the "truth" and basically 26 GM PSI=34 mechanical PSI. I hope it is a linear difference meaning I can add +8 psi for any given DIC oil pressure reading. Not sure what else to do.

Do you mean to say oil temps are also off?

Dan_the_C5_Man

I plan to validate with an external temp probe (when I get so much free time that I am REALLY bored), but to summarize, when I watch my oil temps rise from cold start up to full temp on the DIC, no, I don't buy into the numbers, or the rate of change reported.

The temps might be accurate (or should I say about as accurate as your pressures are) when within the normal operating range, but at either extremes - I think they've played with the algorithms, and there are multiple reasons they would have done so.

stefuel

A analog gauge is your best bet.. A digital gauge relies on pressure/resistance (ohms readings) AND how a programmer deals with the information provided As long as the engine is running, the analog gauge will be working, even during the worst electrical malfunction

Vette @ 71

Doesn't the sending unit have to be calibrated??