Fuel Pressure Problem

[Kevo_6]

2 hours ago, Waldi said:

That is how I do it Kev.

Sorry Waldi, not with you. Which do you use, the metering unit or brake servo/booster?

Cheers Kev.

Edited June 13 by Kevo_6

[TR NIALL]

2 hours ago, Kevo_6 said:

Sorry Waldi, not with you. Which do you use, the metering unit or brake servo/booster?

Cheers Kev.

If it’s the Tongue Test it’s the one from the Manifold to the MU.

[Kevo_6]

21 minutes ago, TR NIALL said:

If it’s the Tongue Test it’s the one from the Manifold to the MU.

Done the tongue Niall and all good there. It’s for checking overall vacuum.

Edited June 13 by Kevo_6

[TR NIALL]

13 minutes ago, Kevo_6 said:

Done the tongue Niall and all good there. It’s for checking overall vacuum.

You’ll have to get a T piece fitting for the pipe from the Manifold with a Gauge attached and read Vacuum from there. 

[Mike C]

I just put a vacuum gauge directly on the brake booster hose, the brakes won't be used while the car's stationary.

[TR NIALL]

Something like this maybe,never tried it but presume it should work.

[Kevo_6]

Thanks everyone, I shall crack on and let you know how I get on.👍

[Waldi]

19 hours ago, Kevo_6 said:

Sorry Waldi, not with you. Which do you use, the metering unit or brake servo/booster?

Cheers Kev.

Hi Kev,
and I fully understand, my reply was vague, sorry.

I measure vacuum from the MU hose, because that is what influences the MU.

Hope this helps.

Waldi 

[Kevo_6]

8 hours ago, Waldi said:

Hi Kev,
and I fully understand, my reply was vague, sorry.

I measure vacuum from the MU hose, because that is what influences the MU.

Hope this helps.

Waldi 

Cheers Waldi 👍

[Kevo_6]

I’ve checked the flow from the tank and seems to flow ok.

Checked the fuel pressure again and it is back to about 85/90psi, tried adjusting it for the last 2 hours and I can’t increase it. Very #%**ing annoying!

Started the car no problem and ticking over at about 800rpm after a short while.

Checked the vacuum and it is between 81/2 and 91/2 ins HG.

I don’t know what else to check, cars running ok but not tested on the road as I’m nearly empty.

A near empty tank wouldn’t make the pressure low would it? 
Only things I can think of is put the old fuel filter back in as before the car was running ok but a bit rich, buy a new pressure gauge or replace the PRV.

Sorry for this long post but I’m becoming to be at the end of my tether.

Cheers Kev.😥

Edited June 16 by Kevo_6

[TR NIALL]

1 hour ago, Kevo_6 said:

I’ve checked the flow from the tank and seems to flow ok.

Checked the fuel pressure again and it is back to about 85/90psi, tried adjusting it for the last 2 hours and I can’t increase it. Very #%**ing annoying!

Started the car no problem and ticking over at about 800rpm after a short while.

Checked the vacuum and it is between 81/2 and 91/2 ins HG.

I don’t know what else to check, cars running ok but not tested on the road as I’m nearly empty.

A near empty tank wouldn’t make the pressure low would it? 
Only things I can think of is put the old fuel filter back in as before the car was running ok but a bit rich, buy a new pressure gauge or replace the PRV.

Sorry for this long post but I’m becoming to be at the end of my tether.

Cheers Kev.😥

I’d be putting the old Filter back in and see how the Car behaves,they all run a bit rich at tick over but should be all good when cruising along.

[Kevo_6]

3 hours ago, TR NIALL said:

I’d be putting the old Filter back in and see how the Car behaves,they all run a bit rich at tick over but should be all good when cruising along.

Thanks Niall, I’m planning putting the old filter back in tomorrow.

I know they run rich on idle but my last trip it was quite bad. I do quite like to thrash it too.
Cheers Kev.

Edited June 16 by Kevo_6

[Waldi]

My car runs ok with a near empty tank (say 3-5 liter) Kev, although when you drive it could be different as air can be drawn in.

Waldi

[Kevo_6]

9 minutes ago, Waldi said:

My car runs ok with a near empty tank (say 3-5 liter) Kev, although when you drive it could be different as air can be drawn in.

Waldi

👍

[roy53]

On 6/11/2026 at 4:31 PM, Kevo_6 said:

Well I by passed the fuel filter and got an increase to about 80psi. So my next thought was that maybe the pump wasn’t providing enough umph so I swapped it to another one which was working a few years ago but that just screamed like a banshee. Put the other one back on and also removed and refitted the feed to the metering unit to check the filter in there, buttoned everything back up and then I had 90psi. (Air lock maybe) I then went to start the car which started on about the second turn, which is usual and the pressure is now between 100 and 105psi. The engine seems to be running sweet enough. 
Now to sort out my running rich problem which i will start with timing, new points etc.

Thanks for everyone’s help and suggestions so far.👍

Cheers Kev.

I would say that shows a voltage difference so slightly lower battery power when not running 

Roy

[Kevo_6]

29 minutes ago, roy53 said:

I would say that shows a voltage difference so slightly lower battery power when not running 

Roy

Thanks Roy, I did consider voltage drop but now I can’t get it any higher than 85psi.

 I’m also going to blow through all the fuel lines and strip the PRV down again and polish up the insides as well tomorrow.
Cheers Kev.

Edited June 16 by Kevo_6

[Steve-B]

11 minutes ago, Kevo_6 said:

I’m also going to blow through all the fuel lines and strip the PRV down again and polish up the insides as well tomorrow.

I’d be curious to see what ends up it catch pot end of the line  Kev. Anything other than clear fuel means more work 🤞

Edited June 16 by Steve-B

[Kevo_6]

2 minutes ago, Steve-B said:

I’d be curious to see what ends up it catch pot end of the line  Kev.

Me too Steve.🤞

[Waldi]

Hi Kev,

I diid read this topic again.

I do not think a low fuel pressure can cause running rich since less fuel will be injected with each pulse, but the low pressure is not ok.
The original PRV is not designed by Lucas to fully close, there will always be some flow, but if the shuttle (plunger) stays in a “cracked open” position, this will result in low fuel pressure.
I once had this after the winter. I opened it, did not see any dirt, polished the internals with car wax (not with abrasive products as this would increase wear and unwanted leakage). All was fine since then. And I added a fuel stabilizer every fall (tip from MikeC).

To check the leakage, you could clamp the PRV return hose with a hose plier (just a few seconds), and see if the pressure rises.

I use a plier like shown in the piccy.
Make sure the hose is ok and properly connected to the PRV outlet pipe.

The running rich issue on your car will likely be something else, but the pressure should be corrected anyway (first). My humble opinion.

Waldi

 

[Kevo_6]

1 hour ago, Waldi said:

Hi Kev,

I diid read this topic again.

I do not think a low fuel pressure can cause running rich since less fuel will be injected with each pulse, but the low pressure is not ok.
The original PRV is not designed by Lucas to fully close, there will always be some flow, but if the shuttle (plunger) stays in a “cracked open” position, this will result in low fuel pressure.
I once had this after the winter. I opened it, did not see any dirt, polished the internals with car wax (not with abrasive products as this would increase wear and unwanted leakage). All was fine since then. And I added a fuel stabilizer every fall (tip from MikeC).

To check the leakage, you could clamp the PRV return hose with a hose plier (just a few seconds), and see if the pressure rises.

I use a plier like shown in the piccy.
Make sure the hose is ok and properly connected to the PRV outlet pipe.

The running rich issue on your car will likely be something else, but the pressure should be corrected anyway (first). My humble opinion.

Waldi

 

Hi Waldi, the fuel pressure problem seems to be a consequence of changing the fuel filter which I was doing as part of a service to then get down to the running rich problem, which I think is the butterflies.

I have the exact same pipe clamps and had been using them when adjusting the PRV and left it clamped up one time and the fuel pressure did rise to about 100psi. 
I assume that is supposed to happen?

Kev.

[roy53]

If you did bypass the filter and no charge then that must be ok

Whilst I keep mine as left the factory I have now fitted the Revington modern PRV and pipe which is so easy to adjust with no mess

would recommend this

if your 2pumps are both giving the same readings then they seem ok

 

best of luck have fun

Roy

Edited June 17 by roy53

[Kevo_6]

1 hour ago, roy53 said:

 

Whilst I keep mine as left the factory I have now fitted the Revington modern PRV and pipe which is so easy to adjust with no mess

would recommend this

 

best of luck have fun

Roy

Hi Roy, I think that’s my next step.

[Kevo_6]

An update, no progress!

I blew through the fuel lines and nice clean fuel came out and no bits. 
I swapped the fuel filter, no change. 
I checked the voltage of the pump, about right for a slightly discharged battery 12.9v and current pull was 4.6amps, so within spec I think. 
I removed the PRV again, stripped it and polished, cleaned it, some new dowty washers and re assembled it. 
I still can only achieve about 80psi. I kept winding it in and got to 80psi and the more i turned it in from there the pressure would drop.

Such a bloody faff, clamp pipe, undo pipe, adjust PRV, refit pipe and undo clamp. I’m sure I’ve done this now about 50 times.

It’s got to be the PRV that’s knackered surely, isn’t it?

As Roy says above I think I will buy a new modern diaphragm PRV, much easier to adjust.

Kev…

[TR NIALL]

13 minutes ago, Kevo_6 said:

An update, no progress!

I blew through the fuel lines and nice clean fuel came out and no bits. 
I swapped the fuel filter, no change. 
I checked the voltage of the pump, about right for a slightly discharged battery 12.9v and current pull was 4.6amps, so within spec I think. 
I removed the PRV again, stripped it and polished, cleaned it, some new dowty washers and re assembled it. 
I still can only achieve about 80psi. I kept winding it in and got to 80psi and the more i turned it in from there the pressure would drop.

Such a bloody faff, clamp pipe, undo pipe, adjust PRV, refit pipe and undo clamp. I’m sure I’ve done this now about 50 times.

It’s got to be the PRV that’s knackered surely, isn’t it?

As Roy says above I think I will buy a new modern diaphragm PRV, much easier to adjust.

Kev…

Get the Modern PRV and no more faffing about you can adjust it without taking anything apart,Shop around as the prices seem to differ drastically.

[Kevo_6]

16 minutes ago, TR NIALL said:

Get the Modern PRV and no more faffing about you can adjust it without taking anything apart,Shop around as the prices seem to differ drastically.

Will do Niall, I the ones Fred Milturn parts do seem to be the cheapest. I just need to check with them.👍