mjg86

I am trying to trouble shoot oil pressure problem with my 86.

I think it is a heat problem with the oil pressure sending unit and will be changing it this weekend. During some reading on the subject I see there is an oil pressue switch that will cut the fule pump OFF if pressure is ZERO.

Can anybody tell me if there is a time limit on this? How long does zero pressure exsist before the fule pump is turned off?

Thanks

Hendej

As soon as the oil pressure switch opens, the fuel pump relay opens, since it is grounded through the oill pressure switch.

The ECM also has a hand in controlling the fuel pump through the same circuit, but if I understand correctly, the ECM only turns on the fuel pump 2" at key on and during cranking, then turns control over to the oil pressure switch.

jfb

The oil pressure switch keeps from allowing the entire exhaust system to be saturated with properly mixed fuel/air so that if the engine finally catches after long cranking, you won't get fined by the FAA for not filing a flight plan for your C4.

Ray Quayle

The ECM actually energizes the fuel pump relay whenever it is receiving distributor reference pulses. This means during cranking or running. As you said the ECM energizes the fuel pump relay for 2 seconds to prime and will deenergize it if it receives no reference pulses, like if you just switch ignition to on, not start. The fuel pump also receives power from the oil pressure switch as you said, but this is actually a back up to the fuel pump relay. Like if you are on the road and pulled out it traffic just as your fuel pump relay failed, without the oil pressure switch to keep the fuel pump running, you'd be dead. This adversely affects GM's bottom line. The same holds true if the oil pressure switch failed; the ECM keeps the fuel pump running through the fuel pump relay.

Hendej

Thanks for clarifying, Ray. I didn't describe that circuit very well from memory. It's obvious from your explanation (and verified by the service manual) that the engine will chug away even with zero oil pressure. Aside from a common fusible link, the circuits are completely separate and run in parallel. The ECM is clueless to oil pressure status and will continue pumping fuel as long as the distributor is turning (not long w/o oil pressure).

Ray Quayle

Absolutely true. In the event of a crash, the fule pump will stop when the distributor reference pulses stop and oil pressure drops to zero. In the event of a failure here, where the fuel pump relay fails closed or the oil pressure switch stays closed, you will probably burn to death, but at least you wern't hit! :confused: