my car will just shut off after it as been running for awhile. It is a 91z28 tpi. I just replaced the 305 wth a new gm crate 350 motor. I put all new sensors in the motor at the same time. one thing i noticed after the motor change was that after the car is running, the oil pressure drops to zero. A mechanic checked it out and he said there was oil pressure and that maybe the gauge was going bad. It has a new oil pressure switch. Is my car shutting off because the oil pressure gauge is reading zero? is this a saftey thing. what else could this. the motor is pumping oil and there is no knocking at all. if the motr had zero oil pressure i think it would start knocking. There is no codes being set either.

 

The oil pressure switch is connected in series with the fuel pump. So if you have no oil pressure, you have no fuel. Safety feature. IT also works when your fuel pump relay fails, so when the engine cranks long enough to build pressure, it will complete the fuel pump circuit.

There should be an oil pressure sending unit for your gauge and a seperate switch for the fuel pump circuit.

Mark.

 

Yeah, what he said. If the guage reads 0 then it's possible that the fuel pump is being shut off because of that.

The oil pressure sending unit for your guage and the one for the ECM are two seperate ones but are connected. There a lower part that is L shaped that connected into the oil pressure hole in the block just in front of the distributor. There is a 2 wire connector that connects to that. At the "intersection point" of the L there's a threaded hole. A device that looks about the size of the fuel filter sits on top and screws into that hole. On top of this device a single connector plugs in. Anyhow, those are the two seperate parts ... which one goes to the guage and which one goes to the ECM I don't know.

 

Check the oil pressure at the port on the rear ledge of the block, just behind the distributor. The auxilliary oil pressure switch should be located there. This switch maintains power to the fuel pump after the engine is running. Failure of this switch or lack of pressure to it can kill the fuel pump.

 

Vader, i am almost positive there is nothing on the rear ledge of the block around the dist. I replaced the org, 305 with a 350 and there was nothing on the 305 block either. the motor has a threaded plug in the ledge thats all. Can you send me a pic of what it would look like. Do i hook up a line with a gauge at this plug. How do i check for power at the switch. Also how can i tell if the pump shuts off just prior to the engine shutting off. One thing though,. If the pump shuts off, then wouldnt the motr stumble a little on fuel and then die out slowley. My car just shuts off like you turn the key off. The car has oil pressure at start up but goes down after warm up

 

I have that small plug too. No sensor.

 

Vader and I both have '86 models and mine has the sending unit on the edge of the block too. That's odd that yours isn't there. I think the TBI cars don't have that there but as far as I know all the TPI cars should have.

 

you sure tpi car had it i have had 2 91z's and neither of them had anything bolted tot he rear ledge of the block.

 

Maybe GM moved the oil sensor stuff somewhere else in a later years, I'm not sure. I am positive though that on my '86 and every TPI '86 I've seen it's right there on the back ledge of the block. This is not really a subject that's come up much that I've had to put much research into but I'm sure it's worth looking into now just cause I'm curious. If I find something out I'll let you know.

 

I just talked to a friend of mine who owns an '89 Formula that had TBI stock. He said that on his car the oil sensor stuff was on the side of the block just above the oil filter. Maybe that's just for TBI but if I were to make a guess I would say that GM probably switched to that location on TBI and TPI fbodies sometime in the late 80s.

 

there are actually two back by the distributor. but mine aren't hooked up and the car runs.

 

my '88 formula (305 tpi) had both one on top and one above the oil filter. my friend had a 88 305 tbi formula and I replaced the one by the dist. to correct his guage.

If you were missing it, it would peg the guage unless the wire was shorted to ground.

it looks like the one near the filter is just a on/off switch and disconnected would make it shut off. check the terminal ends and wires.

 

can one of you guys take a picture of this thing and post it. i cannot find this thing on the rear ledge of the block. if i peek back there all i see is a big amount of wires running across the back of the block.

 

You can't miss it. If it's there it'll be just to the right (drivers side) of the disritbutor. It's about the size of a fuel filter on our cars and mine is gold. If it's not there look on the side of the block just above the oil filter and see if there's something there with wires connected to it that you don't recognize.

 

Mine is right above the oil filter and the grounds are on the back of the driver side head. This is off a 89 formula 350 TPI

 

The oil pressure sensor for the in-dash gauge should be right under the distributor. These sensors are notorious for going bad.

The sensor above the oil filter is NOT a safety device. It is a backup power supply for the fuel pump should the relay fail. If the relay does not work, the switch by the filter will turn "on" once engine oil pressure goes above approximately 5PSI, and turn on the fuel pump.

Look at the wiring diagram posted above. The switch simply provides an alternate path for 12volts. It does NOT interupt any circuit controlled by the relay.

 

is the oil pressure sensor have a vacuum line hooked up to it or just a plug. I cant seem to find it near the dist. how many wires are going to it.

 

my 90 formula has one oil pressure sender above the oil filter. GM changed this in the later years. It is for the computer, the gauge, and the cutoff. There are no other oil pressure sending units on my block. Your 91Z is most likely the same as mine.