Testing fuel pressure

[Horsebox]

Hi guys,

I checked the technical manual and it says fuel pressures are checked with the engine off. I was thinking, won't the primary pressure be somewhat higher when the engine is actually running, due to the higher voltage across the fuel pump? It's not a small difference either, at least not in 6042s case, as once the alternator kicks in system voltage goes from 12V nominal to about 14V.

Any thoughts?

Mark

[Chris4099]

Once the system reaches a set level (can't recall at this moment as I'm away from the manual), any additional pressure is bled off by the PPR. So as long as the fuel pump can reach that required pressure at 12v, any additional pressure applied by the pump at higher voltages will be removed. Now if you blocked off the return line, then you would probably see a difference, but that wouldn't be recommended. The pump needs to continuously pump fuel in order to cool itself.

[DMCMW Dave]

Hi guys,

I checked the technical manual and it says fuel pressures are checked with the engine off. I was thinking, won't the primary pressure be somewhat higher when the engine is actually running, due to the higher voltage across the fuel pump? It's not a small difference either, at least not in 6042s case, as once the alternator kicks in system voltage goes from 12V nominal to about 14V.

Any thoughts?

Mark

The pressures are regulated down from a much higher dead pump pressure so it won't make any visible difference. (The pump can hit close to 75 PSI and you are regulating it to closer to 50, so if the voltage is low and the pump is only capable of, say, 60, you would not see it. And the car will still start. It has to be this way to deal with voltage sag when starting with a weak battery, for example).

It's not easy to do a full system pressure test with the engine off BTW. You can test parts of the system but to see what the warmup regulator is doing it really needs to be, er , warming up.

Actually thinking about it you could simulate warmup if the electronics and the regulator heater are powered and you have a small hand vacuum pump. But it's easier just to run the engine. Unless of course the engine doesn't run.

[Horsebox]

That's really useful info, thank you.

I have a fuel pressure testing kit on order, but I've never used one before so I haven't got my head around the plumbing. Dave I get your point about wanting to see the CPR do it's thing in a running engine, particularly as I've replaced mine recently. I was thinking the same regards the primary. Hence doing all tests with the engine running, not just the pump.

Can the testing equipment cope with checking the pressures on a running engine?

Mark

[DMCMW Dave]

That's really useful info, thank you.

I have a fuel pressure testing kit on order, but I've never used one before so I haven't got my head around the plumbing. Dave I get your point about wanting to see the CPR do it's thing in a running engine, particularly as I've replaced mine recently. I was thinking the same regards the primary. Hence doing all tests with the engine running, not just the pump.

Can the testing equipment cope with checking the pressures on a running engine?

Mark

Sure. Just make sure nothing leaks. THAT gets exciting.

[Horsebox]

Noted, thanks. Fire extinguisher at hand....!

Whilst you're here, can I ask your thoughts on the fuel pump priming thing when switching to ignition II?

Another poster on another thread says there's no reason the RPM relay should do this, due its internal workings, but that's way above my head. Also there's no mention of it in the Owners Manual regarding how to start the car. But most cars seem to do this. Why?

Spurious tach signal to that input of the RPM relay? I see that there is a capacitor in this circuit that presumably is meant to stop DC signals triggering the relay to power the fuel pump...at that point my brain fades and I have no clue.

Thanks again.

[Chris4099]

Depends on the RPM relay. My factory original one would run the pump before starting, but when I replaced it years ago, the new one didn't. My car started fine both with and without the priming, so it's not a requirement. Pressure builds up so fast I would think priming the system only reduces engine cranking time by a fraction of a second.

[DMCMW Dave]

Depends on the RPM relay. My factory original one would run the pump before starting, but when I replaced it years ago, the new one didn't. My car started fine both with and without the priming, so it's not a requirement. Pressure builds up so fast I would think priming the system only reduces engine cranking time by a fraction of a second.

What he said. I think it's a non-issue.

[Horsebox]

I agree it's not a problem. The car starts instantly after all. :)

So nobody has got to the bottom of the fuel pump prime or nobody wants to share?

Mark :)

[Bitsyncmaster]

Primary pressure is 75 psi. Control pressure is about 50 psi in the summer with the engine hot. I ran my pump down to 8 volts and it still had 75 psi primary pressure.