Oil pressure saftey switch issue

[jkenan]

My engine starts fine when starting cold, and will run without interruption until I turn it off (over this last weekend motoring down the ICW, it ran for 7 hours straight). The problem is that when I turn the engine off, it will not start up again, and I isolated the problem to the oil pressure saftey switch by bypassing the switch, then the hot engine started up immediately.

Before bypassing the switch and while cranking the engine to no avail, I did notice the oil pressure was not registering on the guage, which I thought may be why the switch was closed, but after I bypassed it and the engine started, oil pressure was fine. If low oil pressure upon starting causes the switch to close, then I would expect the same issue when starting cold, but it starts cold no problem. Hmmmm....

Any thoughts? I'd like to have the switch in place and not bypassed.

John

[Don Moyer]

John,

Very strange indeed. I'm not as sure as you are that the oil pressure safety switch is the culprit. It sounds very much as though there is a holding circuit created through the oil safety switch which drops out when the engine is shut off. Is the wire from the "R" terminal on the starter solenoid connected to the fuel pump side of the oil safety switch? In fact, is the terminal on the fuel pump side of the safety switch connected to anything else?

I'd remove the jumper wire from around the safety switch and the next time the engine shuts off and will not start, connect a jumper wire from the positive terminal of the coil directly to the big battery cable on the starter solenoid and see if the engine starts.

Lastly, are you sure your oil pressure really is OK. Is it registering normally prior to your shutting down with the engine hot?

Don

[jkenan]

Oil pressure is registering normally (40psi) while the engine is running. There is no fluctuation at all while running. There is nothing else connected to the saftey switch besides the + coil and the fuel pump, and right now, those wires are bridged, bypassing the safety switch so I could start the engine while it was hot.

I have not connected anything to the R terminal on the starter, but understand it is designed to overcome the voltage drop that happens during starting. I thought the connection was supposed to be between the R terminal on the starter and the + terminal on the coil, not the fuel pump. I've been meaning to connect it but havn't yet since I normally have no issue starting the engine, even when hot (until now).

A connection directly from the + terminal on the coil to the fuel pump (by means of a bridge around the safety switch) succeeds in starting a hot engine. Does this not point to the safety switch as the culprit? Could it be a heat related issue since the switch allows starting when cold?

Thanks for your feedback.

John

[JimG]

Warm oil is thinner than hot, so your oil pump or the switch may be marginal. I had a sports car that I rebuilt but failed to rebuild the oil pump - I had to use 60W oil to get it to maintain pressure once hot (and it had a pressure cutoff switch).

[Don Moyer]

You make a strong case against your oil safety switch; I simply don't understand why it would continue working until you shut the engine off. The straightforward way to check the switch is to disconnect it and put an ohm meter across the terminals while someone cranks the engine over for a second or two. The switch should be open with the engine at rest and closed as soon as you start to crank the engine (as oil pressure climbs above 10 psi).

Don

[jkenan]

As JimG mentioned in the previous thread, could hot oil affect oil pressure while starting? I did mention that oil pressure was not registering when the engine was hot and while cranking, but once started it was a steady 40psi. Next time I'm on the boat, I will check OP during cold cranking and then again during hot cranking. Perhaps the particular oil I'm using is too low of a viscosity when hot to generate enough pressure to open the switch when cranking, but enough viscosity to generate the required pressure while running. It's SAE 30, but perhaps viscosity varies from brand to brand.

Thoughts?