Help! Oil Pressure Drop

[thr3333333]

I have a 92 Vette with about 85K miles that starts and runs at speed with normal oil pressure (around 40 PSI). However, the oil pressure sinks (down to about 10 PSI) when stopped in drive after the car warms up. The pressure goes back up to 40 when I get back to speed. I have taken the car to the local Chevy Dealer and they called GM and they tell me it is within specs...but it never used to sink that low, and it now makes me nervous to go for long drives. Any advice would be greatly appreciated...I'm not a mechanic and don't have a regular mechanic I can trust, so I'm at the mercy of service departments. Thanks - Thr3333333;help

 

[keremarsan]

Hi !

I am using metric system , maybe you can check following indicators:

When the oil is hot ( 220 - 230 fahrenheit ) , approx 100 degre celcius , the oil pressure can drop to 130-150 ( I think kpascal in metric system ) , it drop to 150 from 400 ,

If the outside temp is very high and oil temp reach 240-260 fahrenheit ( 110-120 celcius ) , I can see ( rare ) , 100 - 110 kpasc.

Summary : you can see your pressure divided by 4 in certain condition .

Imp : if your car is automatic and if you are waiting on " Drive " , the RPM will drop to maybe 600 tr/min from 700 tr/min, this can also drop the oil pressure. In this case, if oil is very hot , you can put on " Neurtral " which can raise RPM a little and a little more oil pressure you will get.

Please convert kpascal data to PSI to see if your 10 PSI is good , I think it is OK .
Check the oil level ( near mximum mark ) also

Bye

 

[keremarsan]

surely ,
you can see 110 - 170 kpas oil pressure , only when the engine is on idle

 

[John Robinson]

Keremansan is correct. When the oil is cold it has a higher viscosity and you will see higher oil pressure readings. As the oil gets hotter it becomes thinner and the oil pressure will be lower.

Oil pressure is regulated by RPM's. The higher the RPM's the higher the oil pressure, and the lower the RPM 's the lower the oil pressure.

What you are describing is what my readings are on my 93 with 90K

 

[Hib Halverson]

First, know that the analog gauges in C4s are very inaccurate.

Next, if you think you have a low oil pressure problem, I'd install a known-good gauge to the engine and check the hot oil pressure with that as a verification of the OE gauges accuracy.

If you do have 10 psi at idle, that's a little low but not a real problem as long as when you rev the engine the pressure comes up to 40 or so right away.

Lastly, if you're running the engine on a 5W30 oil, because the location of the car is Hawaii, I'd change to Mobil 1 10W30.

 

[John Robinson]

First, know that the analog gages in Cs are very inaccurate.

Hib

Thanks for reminding us that gages can be faulty. I sometimes think that when the auto industry went to idiot lights we created a whole new generation of drivers.

In the old days the cars were not all that durable and required constant attention. I can remember that it was expected that the engine would need a rebuild before 50K and you absolutely did not grind valves without replacing pistons and rings. In order to keep track of the vitals we put extra gages in the cars. The manufacture accepted oil usage of 200 miles to a quart as acceptable oil mileage.


With the advent of idiot lights we also got much better engines and they would last 100k +. Now we start to get nervous if the oil usage is 1000 miles to a quart and expect to not have to add oil between oil changes.

Sometimes too much knowledge will create imaged problems. I am as guilty as anybody else about over analyzing. When I first joined the CAC and looked at the threads and posts I started worrying about having to replace all sorts of parts. After several years of owning this car I have learned to just drive it and have fun in it.

 

[Hib Halverson]

Oh...it's not that the OE late C4 analogs are inherantly faulty its that, even when they are working properly, they display inaccurately. It's a quirk of that specific design. In short, the scales on those gauges are not linear.

It is certainly possible that the person who owns the 92 and started the thread may have a problem with low oil pressure at idle but it's also possible there's no problem at all. The only way to check is with a known-good gauge which reads in a more accurate, non-linear manner.

 

[John Robinson]

Oh...it's not that the OE late C4 analogs are inherantly faulty its that, even when they are working properly, they display inaccurately. It's a quirk of that specific design. In short, the scales on those gauges are not linear.

It is certainly possible that the person who owns the 92 and started the thread may have a problem with low oil pressure at idle but it's also possible there's no problem at all. The only way to check is with a known-good gauge which reads in a more accurate, non-linear manner.

Your point is well taken. Information is only as good as the device providing it. I also would not want to downplay the need for the owner of the 92 to throughly check out his oil pressure.

 

[JAKE]

Also, that specific issue is written about in the owner's manual for my 96 saying the pressure could be as low at 6 psi under the conditions you mentioned and that it's normal..

If you want to see a higher pressure at idle, move up one step in oil viscosity. Move up from, say 5W30 to 10W30; you're hot idle pressure will be higher and it's still within the GM accepted viscosity range according to the owner's manual for my 96.

The manual also informs that the gauges aren't linear.

Jake