Low fuel pump pressure... I think!?

[Richiep]

Hi All,

 

I recently had my TR6 regularly stall if it was left idling for 10 minutes or so.. After a few suggestions, such as possible blockage in the fuel lines, or tank, or fuel pump (Lucas) problem, or PRV.. I've since done the following..

 

Had the fuel pump reconditioned

Drained and removed, dried, and cleaned out fuel tank

Disconnected each fuel line from the tank as far as the metering unit

Replaced fuel filter

Bought and fitted fuel pressure gauge test point at metering unit inlet

 

Now... It doesn't seem to stall after say 20 minutes idling, which is great..

... But.. Having checked the fuel pressure with the gauge I get a reading of approx 75 psi at idle!

I can get it up to about 100 - 105 psi if I rev the engine.

 

I've checked the voltage at the battery & fuel pump..

12.7 volts at the battery, and

12.5 volts at the fuel pump, so I don't think I've got resistance issues?

 

As I say, the car runs.. But the pressure reading seems low from what I've read on the forum! Everything I've read suggests I should get 100 - 105 psi at idle, or with the ignition on, but engine not running?

 

Just trying to figure out if I've got a problem.. And if so, what it might be..!

 

Any thoughts would be appreciated!

 

Thanks in advance

 

Richard

[TR NIALL]

Faulty PRV maybe.

[Richiep]

Hi,

 

Thanks for the reply.

I guess the PRV could be stuck half open?

 

Is it straightforward to dismantle, clean and reset?

 

Richard

[Peter Cobbold]

Did you check the voltage at the pump at idle? or with ignition on but engine not running.

[kcsun]

Try running a temporary thicker power wire direct from the battery to the pump and see if that makes a difference (maybe use a long "jump start" cable).

kc

[Richiep]

Thanks for the further replies..

 

Voltage was checked with ignition on, but engine not running, which have the 12.7v at battery, and 12.5v at pump connections. That made me think the wiring may be ok? But the suggestion to run a thicker wire sounds worth a try.

 

Most of the rest of the injection system has been reconditioned apart from the PRV.. But then I don't want to dismantle it unless necessary!

 

Thanks

 

Richard

[Kevo_6]

Hi Richard. There is strainer/filter in the PRV, worth cleaning it.

[Agile]

Hi Richard,

You mentioned that if you rev the engine, the fuel pressure raises to what it should be. Brown book: "The valve (PRV) is set to provide a constant pressure of 106-110 p.s.i. (7.45 - 7.70 kgf/cm2)"

Suggest to check the voltage across the pump terminals when you rev the engine. Typically the voltage should go up 13-point-something. Does this higher voltage a the pump terminals coincide with the correct fuel pressure?

If so, what voltage is this? Is the grounding wire for the pump OK?

I would btw expect the pump to be able to deliver the right pressure already at 12.5 volts. If it only does at the higher voltage, are you sure the pump is OK?

Also, a (simple) DVM might not give you the right voltage measurement due to the "electrical noise" created by the pump itself. Cross check with another voltage measuring device.

You can do a basic check on the PRV: if the pump is delivering the right pressure and the PRV works, you should be able to note a decent flow of fuel from the PRV back into the fuel tank. In my case it does so with just the ignition on / engine not running.

Improving the wiring to the pump: good idea. Since the pump is controlled via the ignition key, the pump current travels various connectors, fuse, contacts, (safety) switches as well as a relatively long wire. Consider therefore to use a relay. Use the existing circuit (including the safety switch) to activate the relay and use a dedicated new (thick enough) wire directly from the battery via a fuse (!) via the relay to the pump. Also upgrade the pump grounding wire.

PRV: the brown book describes how to check it (its filter) / overhaul it.

Success!

[Richiep]

Hi,

 

Many thanks for the additional suggestions and advice. I have a brown book, so will check that to overhaul the PRV. Will recheck voltages and consider adding a relay.

 

Thanks again guys

 

Richard