no start question -- help

[fixitman4710]

hey guys or gals im thinking of buying a 95 z71 4x4 with 180,000 miles
on it. the guy wants 1000.00 dollars for it. he said it needs a starter
and it will start and run for 30 seconds and shut off.
he said oil pressure is good and has bad oil pressure switch.
can the switch be bypassed to run the engine.
i appreciate any help

steve n.carolina

[JohnC]

If the bad switch is what's making the engine quit...

The OPS doesn't shut the engine off, it only controls the fuel lift pump. If the fuel system is in good shape and the filter isn't clogged the engine should run fairly well even if the switch fails. Most common symptom is misfiring under load.

But isn't the starter bad, too?

[fixitman4710]

If the bad switch is what's making the engine quit...

The OPS doesn't shut the engine off, it only controls the fuel lift pump. If the fuel system is in good shape and the filter isn't clogged the engine should run fairly well even if the switch fails. Most common symptom is misfiring under load.

But isn't the starter bad, too? yes sir he said the starter is
bad. but when he and a friend were trying to get it started it only
ran for 30 seconds and shut off. could it be a bad lift pump?
thanks for helping

steve

[rameye]

bypass the OPS...run solid/constant 12 volts to the pump to debunk or prove his theory...negotiate from there.

[DmaxMaverick]

If it won't run without the lift pump, it has a significant fuel system restriction/leak, or the IP is bad. A marginally healthy IP is capable of maintaining moderate power levels without the lift pump, all else being healthy.

On the 1995 models, the lift pump should run ONLY with key-on-start, or with oil pressure. You can test the pump (independent of the OPS): set E-brake or chock wheels, turn key to "run" (no start, engine off), shift to D, turn key to start. The engine should not crank unless in P or N, but the lift pump should run. You should hear/feel it running, and you should have pressure/flow at the water drain (valve located at the thermostat housing). To test lift pump with engine running, open the valve (hose routed to a container). You should have fuel flow, and the engine should not die. If it dies and/or there is no flow, the pump is not pumping sufficiently (for whatever reason).

In any case, the OPS is a known weakness and they commonly fail. When in doubt, replace it. At your soonest convenience, wire in a relay so the OPS doesn't carry the lift pump load. This will help the OPS live indefinitely, as well as improve lift pump reliability.

[fixitman4710]

If it won't run without the lift pump, it has a significant fuel system restriction/leak, or the IP is bad. A marginally healthy IP is capable of maintaining moderate power levels without the lift pump, all else being healthy.

On the 1995 models, the lift pump should run ONLY with key-on-start, or with oil pressure. You can test the pump (independent of the OPS): set E-brake or chock wheels, turn key to "run" (no start, engine off), shift to D, turn key to start. The engine should not crank unless in P or N, but the lift pump should run. You should hear/feel it running, and you should have pressure/flow at the water drain (valve located at the thermostat housing). To test lift pump with engine running, open the valve (hose routed to a container). You should have fuel flow, and the engine should not die. If it dies and/or there is no flow, the pump is not pumping sufficiently (for whatever reason).

In any case, the OPS is a known weakness and they commonly fail. When in doubt, replace it. At your soonest convenience, wire in a relay so the OPS doesn't carry the lift pump load. This will help the OPS live indefinitely, as well as improve lift pump reliability.
thanks alot for the help. i needed to be educated on this one.
have a good weekend