Rover 3500 V8 Oil Pressure Dependant Ignition

[skinnymathew]

When rebuilding my RV8 on a test stand, I added a relay to inhibit the ignition until the oil pressure light goes out. This worked fine but yesterday I ran the engine up to temperature for the first time before turning it off. When I tried to restart it, the oil pressure light would not go out and therefore the engine wouldn't start. I'm wondering whether the hot oil means cranking alone doesn't achieve the minimum pressure. I'll try again later today when the neighbours are awake but I thought someone might be able to confirm they have seen this with their RV8?

[Chris1]

I have a RV8 which is dependent on oil pressure to run fuel pump,
so therefore no oil pressure, no fuel .
The system that I have is just with old style electric oil pressure gauge
driven by the SD1 sender unit .
I don't bother with warning light although it would be easy to employ one.
This is the starting procedure that I use.
Crank engine for a few seconds until I can hear the fuel pump run, do not touch accelerator pedal.
Next pump pedal 4 - 5 times, crank again, engine normally starts .
The other thing is that I don't use a choke plate .
The set up has been like this for over 10k miles and seems OK to me.
Thinkiing about it it is possible that your sender switch is faulty.
Let me know.
Chris.

[skinnymathew]

Cheers, Chris. I actually have two sender switches fitted and one of those is only a day old; neither activated when only cranking when hot yesterday. Your setup is slightly different to mine as I kill the spark until pressure is seen whereas, I assume, yours could start due to the fuel already at the engine?

[Chris1]

Cheers, Chris. I actually have two sender switches fitted and one of those is only a day old; neither activated when only cranking when hot yesterday. Your setup is slightly different to mine as I kill the spark until pressure is seen whereas, I assume, yours could start due to the fuel already at the engine?

You could be right about engine firing up with the fuel still in the Holley, although I have never had the engine start
from cold unless I carry out my start procedure.
Hot start is different for me, all that I do is crank engine with no pedal .
Good luck with the fault finding,
Chris.

[tonym]

Pressure relief valve stuck in oil pump? Try giving the pump a smack with a hammer to try and free it.

[skinnymathew]

Just went out and tried it (with the coil disconnected as my son is not an early riser) and the oil pressure light went out after a few seconds of cranking then came back on a few seconds after I stopped.

This was on the 11 psi switch and I quickly moved the connector to the 5.5-9.9 psi switch and a couple of seconds later that came on too, so both switches seem to be functioning.

I had read about tapping (smacking) the oil pump, Tony, but decided to see if it's a temperature thing first; one change at a time.

This is an interesting one; if it does turn out that I can't achieve minimum oil pressure with hot oil (20W50, btw) just by cranking then I might use the alternator output, too.

That is, the oil pressure switch only inhibits the spark if the engine is already running (i.e. the alternator is charging the battery).

[tonym]

Or, you could run without an "oil up to pressure" relay, after all, millions and millions of cars are out there running without one!

[skinnymathew]

Ah, but if we wanted to be like everyone else, we wouldn't be here.

[rpm015]

Have to agree with Tony, is it a case of reinventing the wheel, once the initial run up/ cam run in is done and you've proved you can get oil pressure it should be ok unless left for very long periods of time, my Rv8 starts first flick of the switch and within a few seconds is up to pressure, have you got an oil pressure guage?, might be worth plumbing one in, disconnecting the relay and see just how quickly the pressure comes up once the engines started. Don't forget the rv8 is a low pressure engine, on a hot day after a hot run my oil pressure at tick over can sometimes go as low as around 15 psi or slightly less

[skinnymathew]

I know what you mean but I didn't set this this up so that it wouldn't start if the oil pressure is low; it's to stop the engine if the oil pressure drops for some reason while it's running.

The fact that it won't start if it can't build pressure was a welcome side effect, until I discovered that it won't start when it's hot (but otherwise fine).