Fuel Pressure Test Gauge for TR6 PI

[FBM]

Hi,

I'm trying to set up a gauge to measure the fuel pressure just before the MU, to check if the PRV is giving the right pressure.

I've seen this subject referenced before, but can't seem to find those topics via the search function.

So, again, I come here asking for your help for assembling the parts needed for installing provisionally a fuel pressure gauge right before the MU. 

Ideally, the selected "T" would be installed before the MU permanently, to allow the installation of the gauge hose without having to disconnect the fuel line to the MU.

Any other tips to make this work properly would be very appreciated.

Thanks

[BlueTR3A-5EKT]

Does this help?

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

 

The green book gives info on PRV setting.

 

Edited December 2 by BlueTR3A-5EKT

[Tim T]

I made this, which works well, but I do disconnect the existing hose at the MU and substitute this set up.
Cheers,
Tim
 

 

[TR NIALL]

7 hours ago, FBM said:

Hi,

I'm trying to set up a gauge to measure the fuel pressure just before the MU, to check if the PRV is giving the right pressure.

I've seen this subject referenced before, but can't seem to find those topics via the search function.

So, again, I come here asking for your help for assembling the parts needed for installing provisionally a fuel pressure gauge right before the MU. 

Ideally, the selected "T" would be installed before the MU permanently, to allow the installation of the gauge hose without having to disconnect the fuel line to the MU.

Any other tips to make this work properly would be very appreciated.

Thanks

There is Threads on here discussing whats needed but I can’t find them.

[JohnC]

You could try this:

RS Components part numbers:
231-7681 - Inline pressure test point G3/8" (same as 3/8"BSP) Male/Female to M16
257-8915 - Test Point Microbore Flexible Hose M16 to G1/4" 2m long
110-826 - 63mm 0-10 bar gauge (G1/4" fitting)

Edited December 2 by JohnC

[FBM]

8 minutes ago, JohnC said:

You could try this:

RS Components part numbers:
231-7681 - Inline pressure test point G3/8" (same as 3/8"BSP) Male/Female to M16
257-8915 - Test Point Microbore Flexible Hose M16 to G1/4" 2m long
110-826 - 63mm 0-10 bar gauge (G1/4" fitting)

Thanks John. That's it. 

Already had that site on favorites, and the 3/8BSP rings a bell. 

[FBM]

I would like to thanks everyone for the help.

I'm already ordering the parts to see if I can confirm the fuel pressure next weekend.

[FBM]

4 hours ago, BlueTR3A-5EKT said:

Does this help?

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

 

The green book gives info on PRV setting.

 

Hi Peter,

Thanks for that site — it’s a real deep dive into the Lucas system.

My current PRV is this one, which was supplied with a Bosch pump kit around 2015: https://www.moss-europe.co.uk/en-gb/pressure-relief-valve-bosch-type-pump-156167b

So far, I still haven’t found clear instructions on how to set it (or even whether it’s adjustable). I also don’t yet know whether it actually needs setting.

The only certainty at the moment is that the engine will only hold a steady idle above about 1,500 rpm, and it starts to flood below that.

I’ve already checked for air-side leaks that could be affecting the mixture, so before looking into the MU itself, the plan is to confirm whether the fuel pressure at the pump isn’t higher than it should be.

[BlueTR3A-5EKT]

So we presume from that PRV fitment you have a Bosch pump.?

From memory the Bosch pump specific PRV is set to a lower pressure than the one for the Lucas pump.   KMI used to supply Moss.   Might be worth asking them.  
 

 

[stuart]

59 minutes ago, BlueTR3A-5EKT said:

So we presume from that PRV fitment you have a Bosch pump.?

From memory the Bosch pump specific PRV is set to a lower pressure than the one for the Lucas pump.   KMI used to supply Moss.   Might be worth asking them.  
 

 

Correct, 90/95 is often sufficient depending on what model Bosch pump you have, any more causes over fuelling.

Stuart.

[FBM]

Good afternoon Peter and Stuart,

Yes, I have the Bosch pump.

I can’t see any model reference on the pump to help identify it, but I’ll attach a photo of the setup.

Do you know whether this Bosch style PRV is adjustable?

If this flooding issue can be solved by adjusting the PRV, that would be great. I’d prefer to replace the PRV first, before having to touch the MU.

I wasn’t aware that the Bosch setup should be set to a different pressure, so that piece of information is very important for sorting this out. I’ll need a pressure gauge to check this properly.

Given the characteristics of the Lucas PI system, I do expect it to run on the rich side across the rev range. However, from other owners’ experience, it seems that this level of flooding at low rpm, and the inability to hold a steady idle below 1,500 rpm, is not normal.

I also suspect it may be over-fuelling even at around 2000rpm. If i drive too long at around 2000rpm, like in the city and even on the road when going downhill for several miles, just using engine braking, and then press the accelerator to pick up speed, the plugs seem to foul. The car then runs roughly for several miles while I try to clear it. Between 2500 and 5000rpm the car runs great to me, but it's hard to always be driving that way, mainly around town.

Am I heading in the right direction by focusing on fuel pressure, or are there other, more obvious suspects I should check first?

 

Edited December 2 by FBM

[stuart]

Cant see what PRV you have as it would be up and out of shot by the looks of it and I can only tell you have a Bosch filter but the pump is below with no markings on it.

Stuart.

[Paul Hig]

I got a pre calibrated new style PRV from Prestige injection (TR Trader) when I fitted my bosch type pump set up

[FBM]

1 hour ago, stuart said:

Cant see what PRV you have as it would be up and out of shot by the looks of it and I can only tell you have a Bosch filter but the pump is below with no markings on it.

Stuart.

Stuart,

The pump has engravings that say: Bosch 0 580 254 xxx (the rest I not visible from this angle).

I don't have a photo of my actual PRV, but it's this model of the picture below. I'll soon get some photos of my real PRV.

[FBM]

1 hour ago, Paul Hig said:

I got a pre calibrated new style PRV from Prestige injection (TR Trader) when I fitted my bosch type pump set up

That’s probably the Malpassi unit with the diaphragm.

It’s definitely an option.

[PodOne]

Just for info I run a Malpassi unit with the diaphragm set at 105- 108 with a Bosch Pump/filter from Moss and she runs fine nice colour on the plugs.

Andy 

[FBM]

13 hours ago, JohnC said:

You could try this:

RS Components part numbers:
231-7681 - Inline pressure test point G3/8" (same as 3/8"BSP) Male/Female to M16
257-8915 - Test Point Microbore Flexible Hose M16 to G1/4" 2m long
110-826 - 63mm 0-10 bar gauge (G1/4" fitting)

John, 

RS Components were great to deal with.

I've sent them your references and told them what was for, and although they informed me that the 2 Hydrotechnik parts are discontinued, they found equivalent alternatives in current references.

Order is already on its way.

Thanks again.

[Mike C]

After 50+ years of modifications nearly every PI system is different.

For what it's worth,  I have an old Bosch pump and I've always  set my PRV to fully open on a bench at 680 Kpag ( approx 99 psig) with compressed air. For my system I know this works.

[stuart]

13 hours ago, PodOne said:

Just for info I run a Malpassi unit with the diaphragm set at 105- 108 with a Bosch Pump/filter from Moss and she runs fine nice colour on the plugs.

Andy 

As I said earlier it depends on the model of Bosch pump as to its best settings.

Stuart.

[stuart]

17 hours ago, FBM said:

Stuart,

The pump has engravings that say: Bosch 0 580 254 xxx (the rest I not visible from this angle).

I don't have a photo of my actual PRV, but it's this model of the picture below. I'll soon get some photos of my real PRV.

Thats an original Lucas style PRV and its possible for them to stick wide open.

Stuart.

[FBM]

5 hours ago, stuart said:

Thats an original Lucas style PRV and its possible for them to stick wide open.

Stuart.

Good afternoon Stuart,

After the other tests I’ve already done into the cause of the over-fuelling, that does seem quite likely.

If this valve were stuck wide open with a Bosch pump, would the car run at all?

One thing I’ve noticed is that, contrary to what most owners report, this car has never been able to start with any choke applied. I can only start it using the accelerator.

Is this PRV adjustable, or even serviceable?

I’ve read somewhere that a PRV of this type can be adjusted with shims, but I’m not sure whether that applies to this one.

Francis

 

 

[Mike C]

The Lucas PRV has an internal adjustment screw- the adjustment procedure is covered in the Brown Book.

Just as important to me as the absolute setting  is making sure the PRV holds this setting at all flows and doesn't stick - I polish mine internally with cutting compound or similar then check the  pressure control valve holds a back pressure  from zero to full output of air from my average sized garage compressor.

[JohnC]

On 12/3/2025 at 12:11 PM, FBM said:

I've sent them your references and told them what was for, and although they informed me that the 2 Hydrotechnik parts are discontinued, they found equivalent alternatives in current references.

Great to hear they still give good service. Please would you post here (or PM me) the alternative part numbers? Just in case somebody else asks…

Cheers
JC

[FBM]

5 hours ago, JohnC said:

Great to hear they still give good service. Please would you post here (or PM me) the alternative part numbers? Just in case somebody else asks…

Cheers
JC

Yes, of course.

RS Components part numbers:

231-7681 – Inline pressure test point, G 3/8" (equivalent to 3/8" BSP) male/female to M16 -> replaced by 432-5498 – Stauff hydraulic test point G 3/8 male to G 3/8 female (3/8" M/F) + SMK 20 R 1/8 VC ASS.

257-8915 – Test-point microbore flexible hose, M16 to G 1/4", 2 m -> replaced by 432-5656 – Stauff hydraulic test-point hose SMS 20/M1000A, connection A M16, connection B G 1/4, 1m (a 2m option was not available in stock at this moment).

110-826 – 63 mm pressure gauge 0–10 bar (G 1/4" fitting) – unchanged.

 

 

[Chris Hale]

23 hours ago, FBM said:

I’ve read somewhere that a PRV of this type can be adjusted with shims, but I’m not sure whether that applies to this one.

No shims involved, it can be adjusted in position.

Take out the boot back board out

Disconnect the pipe you can see going into the PRV (indicated on one of Harry Trnutter pics from another post)

You can now access a cross head screw which adjusts the pressure.

 

Edited December 4 by Chris Hale