Oil Pressure Light

[haydenm315]

I've noticed that since switching to the amsoil 10w30 from mobil 1 synthetic 5w30, the oil pressure light doesn't flicker at low engine revs. When I say low I mean around 500. Is this because the amsoil is a little thicker and gives higher oil pressure?

 

[oldsho]

I don't have experience with Mobil 1 in an SHO, but I had a VW GTI new in '84 and put Mobil 1 in it and the low oil pressure light and buzzer came on when it got warmed up. I was told not to use synthetic in it because it might damage the engine. I switched back and they never came on again. I just assumed that the sensor was not calibrated correctly for the synthetic. Maybe the same in the SHO.

 

[stevetatro]

Why are you ever, ever at 500rpms? Do the cams create that lopey of an idle? Sure, the oil pressure is lower than it would be at a more "normal" idle.

"oldsho", there are many, many people who have used synthetic, especially Mobil1, in 5W-30, and have never had oil pressure problems.

<small>[ January 29, 2003, 10:43 AM: Message edited by: stevetatro ]</small>

 

[oldsho]

My response had more to do with the ignorant VW mechanics than with Mobil 1. I switched oils because I didn't want my warranty to be voided by VW. That car had enough issues before I traded it on an 87 Mustang GT Convertible that I didn't need another one. I had been running Mobil 1 in my other cars at the time and had no ill effect. My point here was that it is probably sensor related, and maybe the sensor is just calibrated for a higher idle. I suspect it has everything to do with the low idle and not that it is without pressure. I do wonder if the light comes on with non-synthetic oil.

 

[SHOZ123]

On the viscocity scale Mobil 1 is in the low range for 30 weight.

 

[Mike Kopstain]

oldsho:
My response had more to do with the ignorant VW mechanics than with Mobil 1. I switched oils because I didn't want my warranty to be voided by VW. That car had enough issues before I traded it on an 87 Mustang GT Convertible that I didn't need another one. I had been running Mobil 1 in my other cars at the time and had no ill effect. My point here was that it is probably sensor related, and maybe the sensor is just calibrated for a higher idle. I suspect it has everything to do with the low idle and not that it is without pressure. I do wonder if the light comes on with non-synthetic oil.

The sensor turns the light on at 6 lbs of pressure, regardless of the viscocity. Now the thinner oil may have a harder time building that pressure, but it is still under the 6 psi mark.

<small>[ January 30, 2003, 11:51 AM: Message edited by: Mikeys_Taurus ]</small>

 

[stevetatro]

I always wondered what the magical PSI was for the oil light. Thanks Mike.

Don't SHOs typically see about 9 PSI at idle?

 

[AutoSHO]

stevetatro:
I always wondered what the magical PSI was for the oil light. Thanks Mike.

Don't SHOs typically see about 9 PSI at idle?

No, the bare Minumim for a SHO is 12 psi. They typically run 15-18 hot on a motor with some miles. On a motor with all new bearings they run closer to 20-25 psi.

 

[jthomas68]

I`ve known multiple SHO`s(V6)running Autometer electrical guages,none of them run near 15-20psi.They are closer to 11-13psi,and i don`t believe a brand new motor would even run 20-25psi at a hot idle,that`s a bit high.

 

[Ishodu]

Like JT said mine runs around 12 at hot idle.

 

[DavidT]

Mine is usually 14 at hot idle... I've seen it as low as 12 though.
BTW, can someone explain to me how reading your oil pressure aids in diagnosing worn bearings, etc...

<small>[ January 30, 2003, 05:26 PM: Message edited by: dkautumna ]</small>

 

[projectSHO89]

The clearing between bearing surfaces acts as a restriction to oil flow. The oil pump attempts to move a constant (per given engine RPM) volume of oil through the system. This creates a pressure difference across the bearing (multiple times). The tighter the clearance, the greater the resistance to flow volume, therefore a greater pressure difference is seen across the bearing surface.

A hydraulic system like this has a very close corollary to electrical systems. Using Ohm's Law where E(voltage in volts)=I(current in Amps) X R (resistance in ohms) as a guide, voltage, the electromotive force equates to hydraulic pressure, current is the volume of flow (electrons in electrical system), and resistance, which is the opposition to flow or a "restriction" in the circuit.

If you have a known constant (at a given RPM) volume of oil (I) being forced through a restriction (R), then the pressure (E) is generated.

This is a simplified explanation. Real world variables in an engine include viscosity changes of the oil due to remperature changes, etc..

As a matter of practicality, as bearing surfaces wear, the restriction to flow decreases, and the pressure (at a constant flow rate) drops.

Gotta go to work, may expound further later.

Steve

 

[haydenm315]

stevetatro:
Why are you ever, ever at 500rpms? Do the cams create that lopey of an idle? Sure, the oil pressure is lower than it would be at a more "normal" idle.

"oldsho", there are many, many people who have used synthetic, especially Mobil1, in 5W-30, and have never had oil pressure problems.

I have a 5 speed. When I go to the JCC to hit the weights I have to drive up a hill with speed bumps on it. I tend to go very slow over these bumps and I've let the revs go down to 500 before. I really don't notice a whole lot of a lopey idle. It's a bit lopey but not like a dragster or anything. The main problem I have is keeping the wheels from spinning around 5000rpms in first and second gear if the conditions aren't good enough (wet/cold tires/bad road/etc). I've got brand new dunlop 225 width z rubber on all 4 paws. The car idles somewhere between 800-1000 when warmed up. I've bogged the start in the past by not timing things right with my feet or my TPS sensor just getting a bit old.

<small>[ January 31, 2003, 05:39 PM: Message edited by: haydenm315 ]</small>

 

[haydenm315]

Looks like a thread I started actually produced some good information about oil pressure and wasn't deleted. I guess I'm not 100% bad after all

 

[Ishodu]

I think maybe the better way of checking the oil pressure may be at a given RPM or Speed. Idle speed may be a bit different.

 

[ricksmol]

Oil pressure is measured as PSI @ RPM.
Example 45 PSI @ 3000 RPM. Not at idle.

Rick

 

[AutoSHO]

ricksmol:
Oil pressure is measured as PSI @ RPM.
Example 45 PSI @ 3000 RPM. Not at idle.

Rick

Thats all fine and good except for the fact that our engines only have a specified MINIMUM oil pressure at Idle from the factory (12 psi I think). They do not give a specific oil pressure at 3000 rpm. However, the oil pressure at 3000 rpm should be approximately 40-50 psi Hot.

 

[DavidT]

ok, so with worn bearings, what would you expect to have (psi) at idle or at 3000 rpms? Would there be a significant loss of psi?