Original PRV - can they be rebuilt ?

[Jonny TR6]

I don't have any issues with my original PRV, however, whilst half the car is apart and the fuel tank is out and empty, I'm wondering whether some preventive maintenance is worth it on the PRV ?

Being unfamiliar with the internals etc I don't want to strip it down in case I upset something in there and alter the fuel pressure. I'm using it with a Bosch pump and pre/post filters.

Are the originals a fixed pressure or adjustable inside ?

Done the Google thing but have gone down a lot of rabbit holes and drawn a blank.

 

[Brent C]

If it ain’t broke don’t fix it

 

Brent

[Jonny TR6]

Thanks Brent - but if it contains rubber o rings etc which will potentially have degraded over time due to E5/E10 etc. I'd much rather ask the question than put it all back together and find it leaking further down the line.

I'm not familiar with the internals and would rather be educated.

[Andy Moltu]

Think it's  a plunger and a spring.

Can gum up.

[Waldi]

It consists of a spring loaded steel plunjer in a brass(?) orifice with a very narrow tolerance.

No rubber parts inside.

If you have 2 large copper washers, I recommend to replace these by Viton-grade B dowty seals; I do not know the dimensions but maybe others can maybe tell that.

There are 2 different versions I know off. 

If you decide to dismantle it, you will have to check and possibly adjust the pressure.

I did send mine to a specialist for inspection and adjustment with all the other injection parts and that is my recommendation to you too.

Waldi

[Nigel Triumph]

Years ago Malcolm Jones at Prestige told me that adjustment of the PRV is critical, and only achievable with a proper test rig including an accurate pressure gauge... So I did as he suggested and bought a reconditioned exchange PRV from him when I fitted a Bosch pump conversion.

As Waldi says, it's a good idea to leave this job to a specialist.

Nigel

[Jonny TR6]

Thanks all - exactly the answers I was looking for.

Now - where’s that specialist…

[wheeler]

Neil Ferguson 07977001571

[John L]

There is one o ring in the end inside, my pump would alter the flow if the electrics volts were increased, this was down to the o ring not making a good seal, I contacted KMI who sent me another o ring, now all fine. Even if the volts change a bit the flow stays the same.

John

[Waldi]

Hi John,

my PRV was seized recently, so I had to open it. There was no O-ring in mine. Or did I overlook it somewhere?

Thanks,
Waldi

[barkerwilliams]

if the plumbing is apart I would fit a shut-off tap in the fuel line. With a high tank and a low metering unit, pump and filters - just so many jobs need the tank emptying before commencng so a tap is very handy at little cost.

Alan

Edited July 12, 2022 by barkerwilliams

[Jonny TR6]

12 minutes ago, barkerwilliams said:

if the plumbing is apart I would fit a shut-off tap in the fuel line. With a high tank and a low metering unit, pump and filters - just so many jobs need the tank emptying before commencng so a tap is very handy at little cost.

Alan

Brilliantly simple suggestion Alan. I will do just that !

[Andy Moltu]

Adjusting the pressure just requires a fuel pressure test guage. Plumb it in to the high pressure line (ideally at the metering unit end and adjust the pressure to 105 to110 psi.

This does not require a specialist.

[Jonny TR6]

18 minutes ago, Andy Moltu said:

Adjusting the pressure just requires a fuel pressure test guage. Plumb it in to the high pressure line (ideally at the metering unit end and adjust the pressure to 105 to110 psi.

This does not require a specialist.

Plumbing in the gauge is easy - adjusting the pressure is also easy.

If you know how…

So - how do you adjust the pressure on an original PRV ?!

[Mike C]

I set mine to open at 90psi using compressed air on a bench. But that's just mine- after 50 odd years of owner's fiddling  most PI systems today have individual foibles.

The Lucas Red Book would be a good place to start. Available here;

http://vitessesteve.co.uk/LucasStuff/Lucas_Petrol_Injection_Manuals.htm

 

Edited July 12, 2022 by Mike C

[zelrik]

Very informative & timely posts, thankyou everyone who contributed.

Jonny - only yesterday I was looking at my old hoarded PRVs,  as my fuel pressure is indicating higher than it was about 6 months ago - before my TR was stored.

(Was 107 now 125 +psi. I think the high pressure is "flooding" the sparkplugs making starting hard.)

On one stored PRV I found some rust internally. So this shows that internal corrosion CAN occur.

My problem might/could be blocked filters? So I must undo the unit & investigate.

The Books say to adjust the PRV pressure gradually. Turn the internal plastic screw, only a quarter of a turn, & then recheck.

[Jonny TR6]

Aha ! Thanks Zelrik 

just what I was looking for - thanks Mike also. I’ll look up the Lucas book.

[Andy Moltu]

The adjustment screw is accessible by removing the  return to the tank off the top of my head. Essentially turning one way compresses the spring more (increasing the pressure)  and the other reduces it.

If it's reading 127 PSI it is likely stuck - dissasemble, clean up with something very fine like TCut to get rid of gum/corrosion. rinse and reassemble and set the pressure.

[JCP]

Because of erratic pressure with my Bosch system, I replaced the Lucas-style PRV  with a diaphragm type.  Wishing to see if the original PRV had any problems I dismantled it.  Despite much research, I have been unable to find a component diagram for the Lucas PRV, but this is what came out of mine; note the mangled O ring - where did it fit? Other than the obvious components, the important parts are:

The brass housing.   This has 3 small radial holes just above the "step".

The inner "valve".  This is a tube, open at the spring end, with an annular groove near the top with 4 opposed holes, and 2 further holes near the spring end.  The spring bears on a shoulder on the valve, forcing it on a seat in the brass housing;  this in turn pushes on the brass housing  - and the aluminium spacer.

The O ring would fit best between the aluminium sleeve and the step in the brass housing   - but this would partially block the 3 radial holes.

I have been trying to fathom out how the system works.   My best guess is that:

• fuel under pressure enters the aluminium spacer.
• when the pressure exceeds the force of the spring, the valve depresses until the annular groove is level with the 3 holes in the brass housing and fuel can pass into the centre of the valve and thence into the overspill.

But why does the brass housing not move as it is held in place by the same spring?

And where does the O ring fit?

 

 

[john.r.davies]

Rimmers have a parts diagram.  It looks nothing like your pic (which is unlike any other PRV I've seen) but does show the O-ring:

https://rimmerbros.com/Item--i-GRID600052

 

Lucas almost certainly developed this valve from diesel kit, so this may help:

https://www.google.co.uk/url?sa=i&url=https%3A%2F%2Finjectionpumps.co.uk%2Fproduct%2Fcav-dpa-dps-transfer-pressure-regulating-spring-7123-033e%2F&psig=AOvVaw143T0Eprkrrsv6jGhxcwKD&ust=1697106587945000&source=images&cd=vfe&opi=89978449&ved=0CBIQjhxqFwoTCIjrquzk7YEDFQAAAAAdAAAAABAJ

Edited October 11, 2023 by john.r.davies

[JCP]

Thanks John.  My CP was restored before i bought it, so i don't know the origin of the PRV, but the rubber O ring definitely was inside it - although whether it should have been there is another question!  The PRV was definitely new, so perhaps it was configured to work with the Bosch set-up? I will probably send it away to Prestige to have it refurbished as a spare. John P

[Andy Moltu]

On 7/12/2022 at 4:43 PM, Nigel Triumph said:

Years ago Malcolm Jones at Prestige told me that adjustment of the PRV is critical, and only achievable with a proper test rig including an accurate pressure gauge... So I did as he suggested and bought a reconditioned exchange PRV from him when I fitted a Bosch pump conversion.

As Waldi says, it's a good idea to leave this job to a specialist.

Nigel

I think that he might have been talking through his rear end.

The PRV is a very simple construction as has already been stated on this thread.

Adjusting is best done on the car because the fuel pressure reading is taken at the metering unit entry.

A t piece a bit of pipe and a pressure guage is the "Specialist kit". Remove the return union fro the PRV to tank and adjust the pressure with a screw driver.

[JCP]

Following on from my earlier posts I think that I have now resolved the assembly and operation of the Lucas PRV and I attach a rough schematic diagram of how it goes together.

Working from the input end, the spacer tube buts up against the brass body (red) and locks it against the step inside the main body of the PRV so that, when held in place by the circlip, no movement is possible.  I suspect that the spacer tube is a necessary addition in the change from short to long-body PRVs.  I also suspect that the original Lucas PRV uses a steel tube (as Waldi's), but the aftermarket versions use aluminium.

The operation of the PRV is as suggested in my earlier post:  pump pressure operates on the top of the valve and when this exceeds the opposing pressure of the spring/adjuster, it moves down to allow the 4 radial holes in the brass body to line up with the groove and holes in the valve, allowing excess fuel to escape down the centre of the valve and thence back to the tank.

My aftermarket PRV has 3 “optional” components not seen in all versions:

- a small washer between the nylon adjuster and the base of the spring  to spread the load.

- a washer against the shoulder of the valve to provide positive location of the top end of the spring. 

- and the mysterious O ring!  There will obviously be some leakage between the brass body and the bore of the main body of the PRV and the purpose of the O ring is to block this leakage.  It is not present in all PRVs and it is either there as a modern "improvement ", or to compensate for the lower machining/assembly tolerances of the aftermarket PRVs. In either case, it is not a very satisfactory solution as it is trapped on a narrow lip against the narrow circumference of the brass body under very high pump pressure  - hence the failure.

Apologies for the long post.  John P