XUD run-on

[Dollywobbler]

This has brought back memories. While I owned this car, if you started it cold OR after it had been left for an hour or more, it would be entirely normal. Start it after ten minutes of inactivity and it would rev up to 2000rpm, then eventually settle to idle. Always a bit disconcerting! Does it still do that?

There must be extra fuel getting in somewhere. Sounds like it's developed a new symptom to whatever this problem is. Hope you find it! I never did.

[Thread Bear]

The pressure within the pump AND gravity stop the fuel draining back and it's not hydraulically complete as there is a breather on the tank.

T

But does the return pipe open at the bottom of the tank, under the normal levels of fuel in the tank? Do not no what the surface tension of diesel is but if this feed back is positioned so and there were negative pressure I would have thought that the fuel could back down this return if there is no resistance. That situation requires that there is not pressure continually to empty into the tank from the pump, as you say, but also a physical check in the return. A flow to consumption would be a inducement for an open system to syphon. If the outlet is below liquid level the breather on the tank makes no difference. Note ball bearing in one return for LHM for the same principle.
If the above garbage is true a blocking breather of the fuel tank and expanding gas would provide a negative pressure on the return rather than just a neutral one. Sounds unlikely but strange things can happen. Just a thought as I have no idea on the plumbing.

[Defender110]

There is a one way valve built into the filter head to prevent fuel return to the tank from the feed.

[BX Bandit]

I don't fully understand what you mean TB but I'll clarify my understanding of the return.
There will be a low pressure (pump internal) and high (injection) pressure. What ever is not physically injected into the engine will return, under pressure to the tank.
There is no in line check valve
The return goes into the top of the tank which is always at atmospheric pressure as there is an open ended vent pipe.
So pressure wise, it is impossible to feed the pump via the return pipe as the tank pressure would have to be higher than internal pump pressure.
As far as gravity and siphoning is concerned, the return pipe (which is just under the rear seat) would need to be higher than the fuel pump. I'm not sure if its possible even on a steep incline but again the head of pressure would not be any where near sufficient to overcome pump internal pressure.

Fingers crossed that's all correct

[Thread Bear]

These cunning designers think of everything don't they? I knew there would be check to the eventuality somewhere. It was if it were active or passive. Active within the pump being a darned good idea. Thank for that, it makes perfect sense.

Syphoning can occur if there is flow demand and the height to be overcome is less the the surface tension (probably wrong term) of the liquid in question.

Example of wet flannel touching sink full of water and dangling below the sink on the outside. Most of the water will end up on the floor to the level of the side dangled flannel in the water. Use a towel on a bath and the water has to far to climb against gravity eventually. Trees use this principle to get water a hell of a long way up their trunks as surface area is proportional to the height of the unsupported column of liquid. By using chambers with permeable membranes the effect is enhanced as it can be osmosis effects. Water has a high surface tension. I forget the equation. Fascinating and used to play with this sort of thing as a kid, about two years ago!

Anyway that's splitting hairs, the answer is found.

[Willy]

OH DEAR! The dream machine now has a new MOT but has come out of the garage not switching off (fail).

Stop solenoid in the post for tomorrow, but just in case it's something daft not to do with that, where is the stop lever on the Lucas CAV?

ta

[rich_tzd]

OH DEAR! The dream machine now has a new MOT but has come out of the garage not switching off (fail).

Stop solenoid in the post for tomorrow, but just in case it's something daft not to do with that, where is the stop lever on the Lucas CAV?

ta

I'm pretty sure on the Lucas pump it's labeled with 'stop' and an arrow. In any case it's the lever that the idle speed screw rests against on the top.

[Willy]

Good news... found the stop lever (although it's not particularly effective)

Bad news... ebay solenoid number one has PHAILED to make a difference, just makes the revs drop by about 100. Nb I did accidentally drop the spring and plunger, so I had to use the originals from the old solenoid. Both old and new passed the "click" test, too, puzzingly.

Solenoid #2 in the post...

NB how fast should a 1.8TD spin at idle?

[themildbunch]

air in the fuel lines will cause a high idle with a Lucas pump.... mine started doing it out of the blue recently - quickly got worse and lost prime altogether... leak in the fuel filter housing

[Willy]

When I say fast idle, I mean about 1050/1100. I just can't remember what it should be, my 19TXD is the only other non-electronic diesel I've driven and that settled at a smidge over 1000 too (Bosch).

[BX Bandit]

Cold idle should be about 1100, hot idle about 800 - 850

[MULLEY]

Mine has never idled that low, its usually about a 1000 when its hot & about 1100 when cold, seems to run fine & is great on fuel, so not sure why mine runs much higher?

[BX Bandit]

Could be wrongly adjusted accelerator cable, seized/sticky wax stat or incorrectly adjusted idle on the pump. Doesn't really make much difference anyway.

[MULLEY]

Good

[Dollywobbler]

I don't think I've ever owned an XUD that idles anything lower than just below 1000rpm.