PRV Problem

[John L]

I fitted a new PRV from Moss, pn 156167B not the diaphragm type.

I've fitted now with a relay and direct wire to the battery with a bosch pump mounted under the boot by the prv valve, using a cav filter.

I've adjusted the pressure to 105psi at the MU with the key on and engine not running, but when the engine is started and running the pressure varies up with the increase of battery voltage from the alternator to something like 112-115 psi, if the engine speed is dropped and so the voltage it will go back down to around the 105 psi.

Surely this means that the regulator valve is not working as it should, if the pump is responding to more power and so pumping more, the PRV should still maintain the pressure 105 psi at the MU?

I see there are 2 PRV's 156167 is for a Lucas pump, can anybody explain the difference between the two?

Can anybody explain why this is happening and a possible solution please.

John

[barkerwilliams]

Is the return/discharge pipe from the PRV back to the tank, large diameter, unkinked and freeflowing when you blow through it?

 

I am just thinking that higher voltage implies greater thoughput from the pump and it has to go somewhere.

 

 

Alan

Edited August 17, 2017 by barkerwilliams

[Mike C]

Check for a kink or blockage in the line from the PRV to the tank.

[John L]

Thanks for the help on this, I still have the original CAV filter setup, the return from the prv goes back to the filter housing, there is another pipe from the top of filter to the top of the fuel tank, where the return from the MU is teed in.

What would be the best amendment to this system?

Should the return from the PRV go straight back to the fuel tank rather than going into the filter?

Should then the return from the MU go into the top of filter housing?

I think some rethinking of the pipework is needed?

John

[aardvark]

John. I think the TR5 had the return going to the filter whereas the TR6 had the return going into the tank. Someone will correct me if I'm wrong. My cav is plumbed with one pipe from the tank in and one pipe out to the pump. Cheers. Dave

[barkerwilliams]

John,

 

Could I make the comment that it does not really matter where the pipe goes as long as the flow is sufficient and does not cause the delivery pressure to rise to the metering unit. If the Bosch is delivering more litres per minute than the Lucas then re-plumbing might be called for.

 

Would it be possible to temporarily block off (clamp) the existing return pipe to the filter and run a temporary long pipe from the PRV back into the fuel filler cap and see if the problem goes away.

 

Then you can act on the result and re-plumb if necessary.

 

I always like to prove a problem before I try and cure it.

 

Alan

[Peter Cobbold]

When drivng the fuel take by the engine willl rise with horsepower and the psi may well drop to 105. Tickover places the greatest demand on the PRV as it has to pass almost all the fuel back to the tank except for a couple of hp-worth.

So check the psi while driving it may well be correct.

Peter

[Mike C]

I run my PRV return to the fuel tank on the passenger side. The fuel pick up feeding the pump is from a deep pocket under the tank on the drivers side. Returned fuel has to cross the tank before getting back to the suction line. The intention is to pick up cool fuel from the tank , not heated fuel that has been recently through the pump, which can happen if it's returned from the PRV to the filter housing..

[John L]

Thanks guys Alan, Peter and Mike

That would make perfect sense, in the cooler weather its fine, so will have a think now how to tackle the issue, There is only one return pipe into the tank which perhaps is still not big enough.

I'm gradually ticking things off the to do list, now I have the MU setup where it seems happiest with the engines increase in manifold vacuum, which I still have to find out where its coming from.

John