Hi,

I have 1992 Z-28 with a 5.7 TPI motor in it. Just got finished putting in new injectors and EGR valve. The car was surging and throughing an EGR system code. Two of the injectors tested very low on the ohm test so I changed the whole lot out.

Now that's done. No more codes...no more surging. Car purrs.

Now, however, the engine temp is staying higher that prior to the work, and the oil pressure gauge in the dash bottoms out at stops (when fully warmed up). If I slip it into N, the RPMs come up and the oil pressure does as well. This wasn't happening before.

1. How do I verify if the oil pressure sensor/sending unit is working? Obviously, I don't want to run the car without oil pressure. When driving the oil pressure is fine, but directly corelates to the engine RPMs. Lower RPMs, lower pressure readings on the dash.

Any help would be appreciated!

 

You will always have lower oil pressure at lower rpms - that's a given. Sender should be above oil filter - not sure how to "test" it - might just replace and see what happens with the gauge. Know these gauges were never accurate from day 1 - they are simply "pretties" installed by GM to give a general indication - every time you change a sender the gauge will read different, regardless of anything else being touched - it's just a fact of life.

Last month, my oil pressure was always 40 at idle, then the connector broke off the gauge. This month, nothin gat all changed except for the sender (and I installed the same brand/part number as I had before), and now my oil pressure is 20 at idle.

As long as you see the gauge moving at all, you are good. Like a fuel gauge - if it stops moving at all, you're out of gas!

 

Thanks for the reply. Is there a way to hook up an external pressure tester to verify a number. Sitting at a light wondering if the engine is going to seize is a bit nerve racking. By the way, the car doesn't smoke or burn oil either.

 

Also, I don't get an oil"light" either. I understand that there is some electronic communication between the fuel pump related to oil pressure as well. No drivability issues, just that gauge reading zero. Also, went for a ride tonight. The car did not heat up as much as during the heat of the day. The oil pressure gauge stayed above the "red".

 

Like I said, as long as you see it move from some position to another position, then you know you have oil flowing. For piece of mind, you can remove a valve cover and watch the oil shoot out of the pushrods (grab some cardboard or something to place between the tops of the heads and the fender, the oil really does shoot out - cover the fenders as well with something non-absorbing).

You will not get any oil light if you have a gauge - there was either a gauge or a light, but not both.

The relationship between oil pressure sender and fuel pump ... it's just a "backup" circuit for the fuel pump. In the event that the ECM or the fuel pump relay fail, the car will still start and run, because the oil pressure will send electricity to the fuel pump directly from the battery. That's the only "relationship" between the two.

If it makes you happy, you can install a mechanical oil pressure gauge and see oil pressure on a different gauge - for the money/time involved in all that, I'd try a new stock sender first. But if interested,, there is a hole on top of the block china wall, just behind the intake, and just to the driver side of the distributor - it's a small hole, with a plug in it. You can get connectors from any hardware store (NPT brass fittings in the plumbing section), and using teflon tape on the threads, make a port for a mechanical oil pressure line to run to a mechanical gauge. Note that a mechanical gauge uses plastic tubing to literally run oil into the car to the gauge.

 

not really. its common for a bad sender to work cold and die off hot.

 

I thought if the oil pressure got too low (5psi) the ECM would cut power to the fuel pump, not just the other way around.

 

I had a oil sender go out on me at a stop light with the old 350 TPI... no oil pressure at all. I pulled over immediately shut the car off and checked the oil. All looked good fired it back up no tapping or etc. Figured it was the sender that had gone bad. I bought a new autometer gauge and the new oil sender fixed it. So yes a car will run without the oil sender working to the gauge.

My Bonneville had a bad sender but it would read fine on the gauge. It blew oil out all over. It finally sprayed 3-4 quarts out over a 20 mile drive before I finally parked it and bought a new sensor. Apparently that's common on the newer cars for the sensor to fail and leak oil like crazy. I'm not sure if that also happens to the older ones. I'm just lucky I like to watch gauges at all times.

 

The one that failed on my '89 350 poured oil out when I disconnected the electrical connector. I could tell it failed because it still moved the same but all the readings were about 40 psi lower than before.

 

It's a fail safe to the FP relay. Once you get 4psi oil pressure, the switch closes powering the FP circuit along with the relay (wired in parallel). If the relay were to fail, the OPS still give the FP power.