4.2 L oil pressure measurement-cold to hot

zaid3ssaf

Well-Known Member
Since we don't have a real pressure gauge, I used the method in this video to connect an oil pressure gauge and measured the oil pressure from cold start (75F) to almost operating temperature (190 F). Once at operating temperature, the reading is shown at idle (630 RPM), 1500 RPM, 2500 RPM, and finally 3500 RPM.

Here's the video:

Too long didn't read (TL;DR):
0:00 - Idle cold: 65-70 PSI
0:00- 11:30 warming up
Not shown - Idle hot (1 hour driving): 10-20 PSI

11:28 - 1500 RPM-hot: 50 PSI
11:40 - 2500 RPM-hot: 55 PSI
11:55 - 3500 RPM-hot: 65 PSI

02 Oldsmobile Bravada 4.2L, 235K miles
 
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budwich

Well-Known Member
I guess the question would be: is that typical for an XXXX miles engine. If it is, I guess maybe that is why the "pressure gage" that gm put in, isn't "real". People probably would have gotten really "upset" seeing the pressures drop like that.
 
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zaid3ssaf

zaid3ssaf

Well-Known Member
I added the mileage to the post. The service manual calls for a minimum oil pressure of 12 PSI at 1200 RPM (IIRC). I think they faked the oil gauge to not freak people out by how low of oil pressure do these engines run on. Or maybe to save few bucks so they can send some big wif at GM to 3 week vacation in the Maldives.
 
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TJBaker57

Well-Known Member
Another factor is "how hot is hot?". I added an aftermarket mechanical oil pressure gauge to my 2002 4.2 a few years ago. Sat it right up on the top of the dash near the VIN plate where I can see it without effort.

Some may know that the 2002 models had an oil temperature sensor. I have that PID so I can see that data as well using an OBD adapter and a smartphone app.

What I have seen is the engine oil temperature lags waaay behind the engine coolant temperature. I would say it takes a good 45 minutes or so at highway speeds to get the oil temp truly up to its higher operating temps. And that is when I see really low idle oil pressures. Like 10 psi.

Some day I would like to take a new oil pressure switch and test it to see exactly where does it activate.
 
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zaid3ssaf

zaid3ssaf

Well-Known Member
Another factor is "how hot is hot?".

What I have seen is the engine oil temperature lags waaay behind the engine coolant temperature. I would say it takes a good 45 minutes or so at highway speeds to get the oil temp truly up to its higher operating temps. And that is when I see really low idle oil pressures. Like 10 psi.
Correct. The engine oil temperature lags behind the coolant temperature. The video shown was taken right when the coolant first reaches operating temperature, hence, the idle pressure is 20 PSI. After an hour of interstate driving the idle oil pressure would be at 12 PSI. However, when reved up, the pressure still gets 50-65 PSI.
 

TJBaker57

Well-Known Member
If I am remembering correctly, it is when my engine oil temperature goes higher than 195 degrees Fahrenheit or so that my idle oil pressure drops below 10 psi.

I will add that I have my doubts as to the accuracy of the oil temperature sensor though. When the overnight temps are in the 30s and the trans temp sensor agrees with that my oil temp sensor reads maybe 0 to 5 degrees. I once put a thermocouple down the dipstick tube in an attempt to check the sensors accuracy,, but I have forgotten the outcome!
 
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zaid3ssaf

zaid3ssaf

Well-Known Member
If I am remembering correctly, it is when my engine oil temperature goes higher than 195 degrees Fahrenheit or so that my idle oil pressure drops below 10 psi.
Makes sense as oil thins at higher temps leading to lower resistance to flow i.e pressure

I will add that I have my doubts as to the accuracy of the oil temperature sensor though. When the overnight temps are in the 30s and the trans temp sensor agrees with that my oil temp sensor reads maybe 0 to 5 degrees. I once put a thermocouple down the dipstick tube in an attempt to check the sensors accuracy,, but I have forgotten the outcome!
Can't expect them to be as accurate as a thermocouple. What's the PID for the oil temperature?
 

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