75ZT Community

My mate Wesley has just asked me a question which I was unable to answer definitively:-

Received wisdom has it that one should never allow the fuel level to go below 1/4 of a tank in order to maintain the life of the fuel pumps, owing to the "saddle" shape of the tank.

I must admit that I've always subscribed to that philosophy but I was wondering whether it has any basis in fact?

Any ideas please?

I don't stick to it myself. I believe the theory is that if your in tank pump is weak, letting the tank go below 1/4 then puts extra strain on the underbonnet one, causing early failure of that. Mind you I don't believe a lot of what I read.

There's one electric pump in the tank. It sits inside a sort of large jam jar called a swirl pot. Some of the fuel pumped is sent to the other side of the tank where it is used as a venturi pump to take fuel from that side, over into the swirl pot where the electric pump sits. Now this happens all the time, so I'm not sure how dropping below 1/4 tank affects this process, or therefore the wear rate of the pump. Now I'm not saying it doesn't make a difference, just that I don't see how.

Incidentally, its a VERY common system so I can't see as a system that it has any intrinsic flaws.

A comprehensive answer Duncan

Anybody else care to comment please?

raistlin wrote:Received wisdom has it that one should never allow the fuel level to go below 1/4 of a tank in order to maintain the life of the fuel pumps, owing to the "saddle" shape of the tank.

The fuel level has no effect on the service life of the pumps. The quarter tank mantra is a practical consequence, a work around for a failed ITP. The ITP is centrifugal and in twin pump systems tends to wear out the commutator. It's due apparently, to the brushes being too hard. When this happens, the ITP stops running. This is where the quarter tank level becomes important. The UBP is a positive displacement type and is actually able to draw fuel from the tank unassisted by the ITP. It's working overtime in doing so, but some manage it for thousands of miles. However, the fuel comes via the now stationary ITP. There's a pressure activated priming valve in the ITP which remains open when the pump's not running. The UBP will pull air into the line via this valve unless the valve is submerged. It needs - you guessed it - around a quarter tank level to do that. The overtime often causes premature failure of the UBP.

Fuel in the RH side must be available to the ITP and is maintained by two venturi (static) pumps. These transfer fuel from the LH side to RH side and are driven by the high pressure backflow from the injector pump. This schematic might explain it.



This diagram only shows the liquid flows and vapour handling is an added complication. It also omits the fuel cooler to keep it simple. Everything shown here is actually inside the tank.

If there's any interest, I can post the write up that goes with the drawing.

TC

Thanks TC, so you're saying, in essence, that it is good practice never to let the tank level drop below a quarter?

Thanks TC. I'd missed the fact that the venture pumps are supplied from the return flow, the other system I know uses feed from one of the two in tank pumps. It may be a returnless system, I don't remember.

raistlin wrote:Thanks TC, so you're saying, in essence, that it is good practice never to let the tank level drop below a quarter?

If you know the ITP has failed, then yes. Keeping the level up will help you to remain mobile. If the ITP is OK, you'll gain nothing from following the quarter tank mantra. I suppose the presence of diesel around the commutator and brushes might offer an element of lubrication, but it would need to be demonstrated to be true.

If you have a single pump system (ITP only) then no, there's no evidence that it 'saves' the pump. The second generation ITPs are not prone to commutator wear so seem to be as robust as petrol pumps. And no, you can't replace a worn out ITP with the later more reliable type.

TC

Duncan wrote:Thanks TC. I'd missed the fact that the venture pumps are supplied from the return flow, the other system I know uses feed from one of the two in tank pumps. It may be a returnless system, I don't remember.

The petrol tank uses part of the electric pump's output to drive the venturi pumps. There's only the single in-tank pump in the petrol system and of course, no fuel return loop.

Here's the petrol diagram.



Again, vapour handling is omitted from this to keep it simple.

TC

Thanks for posting your diagrams TC. I'll add them to the Know How Database for ease of access.

Thanks TC, both my pumps were replaced last year and running perfectly. Mind you, having said that, I tend to re-fill the tank having driven approx 350 miles anyway but at least it's good to have an understandable explanation

Well there you have it, a R75 myth found to be truth lurking in the depth of the fuel tank...

Cheers

Mike