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Birdseye

Lucas Fuel Pump and PRV

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All,

I know this topic has been done to death but I cant find any reference on the forum to my particular symptoms and associated problem with the lucas fuel pump on my TR6 1973 CR

1.       Ignition on engine off the fuel pump runs perfectly albeit with the usual whine and will continue to run perfectly until switched off

2.       Ignition on engine running – the pump runs perfectly for about a minute and then starts to hunt. The engine continues to run (tick over) with the pump continually hunting.  Soon after shutting down the engine and restarting the pump it resumes its normal whine. Note that before the pump starts to hunt the engine revs through its full range.

3.       Take the car for a spin and the engine runs well and runs ok even with the pump hunting but will then start to splutter and cut as the pump appears to die and then resume.

**The pump is NOT getting hot just warm to the touch. There is 12.5 volts and 4.9 amps at the pump with the engine running.  There is over half a tank of super unleaded.  I don’t suspect the MU as the engine runs too well.  The MU return pipe isn’t blocked.

The pump is delivering plenty of fuel and I assume the pump delivers the same pressure and flow rate whether the engine is running or not.

To double check the pump wiring I connected new cables directly to the fuse box and earth. It didn’t make any difference; however, I realise this doesn’t test the tails but I don’t really suspect the wiring. 

The pump and MU were rebuilt about a year ago and were then checked over by Neil F.  Neil also checked over the return valve and found all to be satisfactory.  All of the pipe work is new as is the delphi fuel filter, (no leaks anywhere).  The car has only done 150 miles since the rebuild and was running well before the above symptoms started.

Unfortunately, I don’t have a pressure gauge but suspecting the PRV rather than the pump or the MU I started the engine, waited for the pump to start hunting and then clamped off the PRV return pipe.  Restricting the return flow (cant fully clamp it off) almost stops the pump from hunting.   With the pump switched off and the return pipe removed from the PRV I noticed that fuel constantly dribbles past the adjuster screw.  Having now removed the PRV I can also blow into the PRV and feel a trickle of air past the adjuster screw – is this normal or is there excessive wear between the valve body, the distance piece and the valve.  The valve and plunger are clean and appear to be airtight and not sticking; the spring is OK and the strainer is free from dirt.

Grateful for any ideas before coughing up yet more cash on a new valve.  (P.S.  I am going to look for a pressure tester)

Many Thanks

Ross

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If Neil checked it over then it was fine at that point.

perhaps check earthing and for something loose in the tank or filter thats occasinally blocking flow?

steve

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Hi Steve,

I checked the earth and wiring in general and all ok. I did wonder about the filter but the pump works Ok when the engine isn't running. I guess the same would apply to something loose in the tank. The more I think about it its got to be associated somehow with the PRV. Or am I being hopeful.

Ross

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Ross, Revington can sell you a pressure tester, you get the deposit back if you return it, else you can just keep it.

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I'd strip, clean, inspect and recalibrate the PRV. I set my PRV opening pressure using a certified pressure gauge and compressed air. 

The PRV is basically a spring force vs pressure force gadget and I don't like petrol spraying around the garage.  I  have no problems using this methodology.

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Hi Mike, do you know if its normal to get some fuel leakage past the adjuster screw when the PRV is not pressurised. The small valve inside the PRV seams to be fine.  I assume the leakage occurs between the valve and the valve body and the distance piece.  Also, do you know if a poor PRV or poor setting will cause the hunting I'm experiencing ?

Ross

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I've never noticed any leak past the adjuster screw, but I've only set the pressure with air. From the Lucas PI Red Book I get the impression that a fuel leakage of up to 11/2 gph is acceptable past the adjuster screw.

A hunting PRV will impose a varying pressure/flow loading on the pump and could cause it in turn to hunt by changing the operating point on the pump curve.

Are you sure you don't have a blockage of some sort in your pump suction line - like silicon form the fuel cap/tank  joint or similar loose in the tank?

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Mike, thanks for the info on the PRV and associated leakage.  It is possible there is a partial blockage in the gravity fed line and or the filter, however, I ran the pump dumping the fuel output into a container and it flowed really well.  Because it flowed freely it diverted my attention away from this possibility.  But as you and Steve have both mentioned this I will check and measure the flow rate properly.  In the meantime, how did you set the PRV with air - did you use a compressor setting the compressor gauge to the 100 - 110 psi. I assume it works the same as setting an injector, I will give this a try too.

Ross

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Hi

Last year I had a similar problem and I eventually found that it was the little nylon mesh inline filter to the pump collapsing and blocking the fuel line. Bought a second hand one from Revingtons and now all fine. There is usually a nylon filter inline to the PRV and the metering unit as well.

Worth a check.

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Thanks Kev, the filter inside the PRV looks ok and is clean but your right its worth a thorough check, or are you referring to the mesh filter inside the pump intake ?  Also, did your pump hunt (dramatically vary its pitch) with the engine running but was stable when the engine wasn't running?

 

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2 hours ago, Birdseye said:

Mike, thanks for the info on the PRV and associated leakage.  It is possible there is a partial blockage in the gravity fed line and or the filter, however, I ran the pump dumping the fuel output into a container and it flowed really well.  Because it flowed freely it diverted my attention away from this possibility.  But as you and Steve have both mentioned this I will check and measure the flow rate properly.  In the meantime, how did you set the PRV with air - did you use a compressor setting the compressor gauge to the 100 - 110 psi. I assume it works the same as setting an injector, I will give this a try too. 

Ross

Looking at my old notes and rough sketch-I connected a compressed air  line to the pump discharge with a needle valve on the compressor supply  and the gauge in the line. All CA pipe diameters matched the pump discharge line.  The PRV return to the tank was left in place.  I then undid the fuel tank cap opened the needle valve and  pressurized the pump discharge  line by opening the needle valve. I then adjusted the PRV screw  to get  680 kPag in the pump discharge with the PRV discharging all surplus air back into the tank. Some might consider 680kPa  too low  low-but that works for my engine. When the PRV was set I opened and closed  the CA supply valve several times to make sure the system stabilized at  680 kPa on the pump discharge for all flows. 

I also checked to make sure had no possibility of sonic velocity in the throat of the PRV -looked OK with the checks I did  at the time.

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1 hour ago, Birdseye said:

Thanks Kev, the filter inside the PRV looks ok and is clean but your right its worth a thorough check, or are you referring to the mesh filter inside the pump intake ?  Also, did your pump hunt (dramatically vary its pitch) with the engine running but was stable when the engine wasn't running?

 

Hi, yes the mesh filter in the pump intake and yes as you describe but my car would cut out every 10mins of driving and then take another 10mins or so to start. I think as the plastic collapsed it would take time to open up again.

Edited by Kevo_6

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Mike, I think I follow your method and will give it a try as soon as I can.

Kev, as per your suggestion I have just pulled the filter out of the pump.  Its not perfect by any means. 3 of the 4 bracing strips (for want of a better description) are concave and I guess could be collapsing as the pump sucks fuel.  However this doesn't follow the logic of the fuel flow because the fuel flows into the filter and out through the mesh and therefore the filter would expand and not collapse. But who knows with fluid dynamics.   That said it cured your problem so its still worth swapping out.  The one in the PRV is the same so I may just to test it while I find a replacement. I guess they are not available off the shelf.

Ross

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Hi Mike,

you use an interesting method for adjusting the PRV. I would not have thought this is possible.

Would it officially not be done with a test liquid (fuel)?

Air will give sonic flow with the pressure ratio we have in the PRV, and this will restrict flowrate, but not (upstream) pressure.

But if your car runs ok, I guess the setting is about ok?

Waldi

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I would like to thank everyone who helped with this problem.  Its fixed, at least for now but unfortunately I cant explain what fixed it as I took apart and cleaned most of the critical parts (excluding the MU) but including blowing through the pipework with compressed air.  Whatever it was has now gone. I suspect crud in the pipework as suggested by Steve as it favours the symptoms as everything else looked fine.  The one gain in this short saga, the pump has never sounded so quiet.

Once again - thanks all.

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Result Ross, well done.;)

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Excellent news Ross!

fuel can be funny.....

earlier today i took my Honda CB360 based cafe racer out for a spin, not been out in a couple of weeks.

started fine but then running rough.....

Now as i’ve rebuilt this bike in it’s entirety apart from the gears, it has to be my fault....

and it was, i’d run low on fuel :-)

new fuel and all good again !

steve

Edited by Steves_TR6

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9 hours ago, Waldi said:

Hi Mike,

you use an interesting method for adjusting the PRV. I would not have thought this is possible.

Would it officially not be done with a test liquid (fuel)?

Air will give sonic flow with the pressure ratio we have in the PRV, and this will restrict flowrate, but not (upstream) pressure.

But if your car runs ok, I guess the setting is about ok?

Waldi

I'm happy it works. I did not want to get involved in testing with petrol unless I had to.

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Hi Mike,

Thanks, that makes sense off course and as you say, when the car runs fine, its ok.

Waldi

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