low oil pressure

[thxbaby]

I sold my 91 C4 to my son in 2005 with 35000 miles. The car now has 67000 miles and he has had this problem just start. When you start it up cold the oil pressure is OK. If you drive it for a while and pull up to a stop the pressure will drop to 0 on the gauge. The next time you stop it normal. I am thinking it may be a sending unit or a lose ground.
We just change the oil and filter and have always used Castrol GTX 10W-30 and PureOne filter.

Any help would be greatly appreciated.

Thanks!


Jim C

[c4c5specialist]

HI there,

Check your oil pressure with a mechanical oil pressure gauge.

15 psi minimum at idle.

Allthebest, c4c5

[NORTY]

JIm, it does sound like a guage or sending unit problem, or you'd hear lifter noise along with overheating. Speaking of overheating, what oil temps is your son seeing with the drop in oil pressure?

[thxbaby]

We were able to get a gauge on it last night.
When it first started the pressure was just over 50 PSI.
Then as it wormed up it drop but never went below 30 PSI.
The sending unit had oil inside the connector so I am sure its on its way out. The water temp runs 190-205 deg and the oil is 200-205 deg. Never had any lifter noise at all. I am sure the new sending unit will fix the problem.
I do have one other question.
The GM manual list a sensor and a switch, both are shown on the top left by the distributor. As I read it the one with two wires is the Fuel shut off and the one with the single wire is the oil pressure, is this correct? Is that the only pressure switch?

Thanks!

Jim C

[thxbaby]

I have been looking at the wiring diagram for the oil pressure sensors. It look like the one that the gauge work off of is the one with 3 wires. It shows number 31 gone to the gauge and the other two are for the fuel shut off. The switch with a single wire must be for the light. To change the one with 3 wires it looks like I may need to pull the distributor to get enough room to replace it. Has anyone replaced this sensor before? If so any input you have would be helpful.

Thanks!

Jim C

[c4c5specialist]

HI there,

The SENSOR sending unit is the ONE PIN canister shaped one.

Allthebest, c4c5

[thxbaby]

Thanks! The other one look to be a little bit of a bear to get to.

Jim C

[thxbaby]

c4c5specialist

The oil pressure by the gauge in the car is always at 75 or more.
Even when the plug to the sending unit is disconnected it does not change.
If the plug is grounded out still reads 75+ but the check engine light comes on. We put a 40 ohm resister in the plug and then to ground with no change to the gauge.

It looks like we may have a problem with the instrument cluster?

Any thoughts?

Jim C

[c4c5specialist]

HI there,

#1, what is the color of the wire your grounding??

#2, what kind of sending unit did you use ?

When you disconnect them, they should be pinned high.

Allthebest, c4c5

[thxbaby]

The plug has a single wire and the color appear to be faded, it may be a tan or lite brown? The oil pressure sensor is located just to the left and behind the distributor. The replacement unit was a Borg Warner.
When disconnected the gauge does not move. As I understand as the resistance increases the oil pressure gauge should read higher.

Thanks!

Jim C