Bosch pump 'hunting'

[JohnD]

My Vitesse has a TR6+ engine, with Lucas Pi and a Bosch pump.   On my Drive It Day outing it was running well, except when I slowed down!    Then, intermittent drops in the fuel pressure, some so low that the engine dies.     When stationary, I can hear the Bosch pump 'hunting', revs going up and down, and the fuel pressure following it.

On getting home, the pump was very warm, almost hot!    Is this the problem.   It's mounted inside the boot, but I've never had this problem before and it's been there 5? years.

Any thoughts, please?

John

[Nick Jones]

Check your power supply to the pump first. Measure voltage at the pump terminals and see if that is varying.

I’d also be checking that the suction path of unrestricted (filter/strainer) though if there is a problem there the pump usually signals it’s displeasure with “pumping rocks” type noises.

Presume your pressure dips were synch’d with pump slowing rather than the pump speeding up? The latter would suggest the relief valve acting up.

 

 

[JohnD]

I'm beginning to suspect the PRV!     

I bought from the Bay, because it was adjustable, had a built in pressure gauge and was rated to 150psi.     But the Bourdon tube in the guage leaked and filled the dial with fuel, and tried to fill the boot too!     On investigation, the design of the guage, and workmanship was poor, so I must supect that the valve part is too.    I'll go back to the adjustable Malpassi without a gauge from Malcolm at Prestige.

JOhn

[thebrookster]

John, I understand you correctly in that this happens when you slow down (remove load)??.

If this is right, I would also lean towards your PRV failing. 

My thoughts are it is not suction related, this would affect increasing load, not decreasing usually.

Nick's suggestions of checking power supplies is worth a check, it is possible the pump is seizing. But I would be surprised if this was the case, as again I would expect to associate that with increasing or high loads? That's not set in stone however!

So PRV is first port of call in my opinion. The Bosch pump is supposed to see a constant flow, no matter what the engine is doing which is what the PRV compensates for, so this would explain why the pump speed is fluctuating and also why the pump (or I suspect actually the motor) is hot.

Phil

[JohnD]

Thank you, Phil, good to have two real engineers' opinions! Especially when they agree!

The hestitation isn't on instantly on slowing down, but while driving slowly and then accelerating.  When stopped and the engine idling, the hunting is very obvious.   Can't hear it when driving - too noisy!

 Suction unlikely to be the problem, as the Bosch is fed from the swirl pot - unless the pre-filter has picked up a lot of gunk.      Just a thought, any experience with tank foam?  I filled the tank with some last year, hoping to get to Spa where they insist on it.   Does it shed bits that could block the pre-filter?   But that seems unlikely when flat out and flow greatest, it charges off with no misfires.

John

[JohnD]

The proof is in the pudding, or something.      Carl at TRtraders (running Prestige now that Malcolm Jones has retired) rushed me a Malpassi, which I fitted.   Problem gone!     Fuel pressure rock steady, no hunting, no misfires.    And as said elsewhere, 160bhp on Revingtons rolling road!

Thanks for your advice!

John

[JohnD]

I have dissected the Chinese eBay PRV, and it's so simple.   A diaphragm  clamped between the two halves bears on a disc with a central hole that leads to the exit.  It is pressed to the face of the disc by a spring loaded plate with an adjusting screw at the other end.  That's it.   Difficult to see how it could stick or fail to allow fuel through intermittently, but that's what it was doing!

[JohnD]

 

Here's a pic of the dissected device.

Left, top to bottom: top half of body, adjusting screw is unseen on top, side arm is an air vent;  sealing O-ring; pressure plate that sits below the diaphragm; bottom half of device, output connector that screws into the bottom of that half.     The input was via the threaded hole in the lower half.

Right, top to bottom: Presser pltae for adjsutng screw; spring; pressure plate that bears on the diaphragm;  diaphragm.

It's all so simple, it's difficult to see where the sticking and flow failure could happen!

John

Edited May 9, 2021 by JohnD

[Nick Jones]

Pic no worky.......

.......but I guess you know that, from your other post.

I usually reduce to 640 x 480 and upload as a jpeg (or reduce to “medium” on iPhone).

Is this related to the google drive issue we were discussing a week or two back?

[JohnD]

I gear so, Nick.   I'll try again.

Does this work.  Gearbox and engine out, drawing 'box oil, and more shrapnel on the magnetic plug.

[Jdowson]

Nasty, on a positive, the magnetic plug works.

[Nick Jones]

Pic works 

.... but there is (another) whole tooth on that magnet .

Thought there must be one or two left in there.......

[JohnD]

18 hours ago, Jdowson said:

Nasty, on a positive, the magnetic plug works.

Up to a point!   I changed the gearbox oil in the paddock and got three teeth and a lot of swarf out, but as Nick says there's the rest to come.   Gearbox rebuild booked (thank you, Mike Papworth!) but any trial must await the engine rebuild.

[JohnD]

I've edited the pic of the dissected PRV, above.  Now uploaded from a copy on my HD.  Is it working now?

Edited May 9, 2021 by JohnD

[Nick Jones]

Yep, working for me.

As you say, not much to go wrong. Wonder if there is a system resonance?

Just been messing with dampers on my injector cleaner as that(rather crude on the supply side) was definitely getting resonances at some pulse frequencies and the damper I fitted cured it.

[JohnD]

Resonance - audible whining - is such a problem with Bosch mods on Lucas Pi systems, that some dealers sell 'special' hoses between pump and PRV.     

It would happen when idling, the 'hunting' was audible then, and on accelraion from a low running speed, so a good idea, no doubt from experience.

But an effect that Malpassi avoids entirely.

John