How To - Understand Fuel Tanks

[HarryM1BYT]

Originally Posted by Mike Noc

It could well be. What I couldn't understand when my car stopped with a dead ITP was that putting 5 litres of fuel in didn't get it going but 15 litres did. Adding 5 litres would easily immerse the inlet to the ITP, so it had to be something else.
Mike

That makes some sense to me...

Once the UBP looses it prime and stops, it will need quite a bit hgher a fuel level, to be able to reprime itself.

So the best fix is to do away with the ITP and fit a pick up which reaches all the way to the bottom of the bottom of the pick up dish?

I would really like to know what changes Rover made (if any) to the ITP system, which enabled them to do away with the UBP.

[James.uk]

What a terrible design!! Another one in fact, as sadly there are quite a few ludicrous design items on our cars innit.. Makes one wonder if they were deliberate coz the Co was being sold.. ----

And I thought Jags idea of two separate pumps and tanks was mad!!!!!!
...

[T-Cut]

Originally Posted by Mike Noc

- - - I think that when the tank level gets down to the priming valve then adding 5 litres may not be enough to cover it again as the cross section area of the tank at that height is quite large.

Sounds like the priming valve is located at a higher point than the top of the swirl cup. If so, then presumably the system will not function using the UBP alone until the ITP valve is flooded to a significantly greater depth than the cup. It's a circumstance that can only arise if the ITP stops working and where the quarter tank regime is based. It means the UBP isn't especially level critical, but needs a hydraulically locked ITP. I assume that with all systems/pumps fully functional, you could effectively drive the car until the tank is completely empty.

TC

[Mike Noc]

Originally Posted by HarryM1BYT

So the best fix is to do away with the ITP and fit a pick up which reaches all the way to the bottom of the bottom of the pick up dish?

Now that's an interesting thought. It's often stated that with a failed ITP the UBP has to do more work and will fail sooner or later. But it could be that the design life of both pumps is more or less the same so after one fails the other will probably follow whether the first pump is replaced or not.

Who knows how long a new UBP would last running on its own?

I do know I got at least 20k miles from my UBP running on its own. It had already covered over 100k miles and it was still going strong when I replaced it.

Mike

[Mike Noc]

Originally Posted by T-Cut

Sounds like the priming valve is located at a higher point than the top of the swirl cup. If so, then presumably the system will not function using the UBP alone until the ITP valve is flooded to a significantly greater depth than the cup. It's a circumstance that can only arise if the ITP stops working and where the quarter tank regime is based. It means the UBP isn't especially level critical, but needs a hydraulically locked ITP. I assume that with all systems/pumps fully functional, you could effectively drive the car until the tank is completely empty.

TC

Yes TC that sounds about right, and I agree if the system is working as it should then you can run the car to as good as empty.

It is a bit of a complex system for supplying fuel, but it is also regulating fuel temperatures, both ensuring a fast warm up and then stabilising the temperature of the fuel, and regulating the levels in both sides of the saddle tank. The more you look at it the more it ticks all the boxes.

Shame they didn't build the pumps to last longer, as if they had the LP fuel supply wouldn't be a problem.

There are recommendations never to run the tank below a quarter full even with both pumps running but I can't see a problem with it and I've nearly always run down to the low level light before refilling and had no issues.

Mike

[HarryM1BYT]

Originally Posted by Mike Noc

Yes TC that sounds about right, and I agree if the system is working as it should then you can run the car to as good as empty.

It is a bit of a complex system for supplying fuel, but it is also regulating fuel temperatures, both ensuring a fast warm up and then stabilising the temperature of the fuel, and regulating the levels in both sides of the saddle tank. The more you look at it the more it ticks all the boxes.

Shame they didn't build the pumps to last longer, as if they had the LP fuel supply wouldn't be a problem.

There are recommendations never to run the tank below a quarter full even with both pumps running but I can't see a problem with it and I've nearly always run down to the low level light before refilling and had no issues.

Mike

Just a wild thought and probably wrong....

The 52PSI regulator, rather than a regulator, if that were instead an air release system, then it would make much more sense to me.

We have already arrived at a conclusion that the ITP can start sucking air at below a quarter tank, which means some air will be pumped along with the fuel and the air would need to be released before the fuel is sent down the car to the UBP and HP pumps.

[T-Cut]

Originally Posted by HarryM1BYT

Just a wild thought and probably wrong....

The 52PSI regulator, rather than a regulator, if that were instead an air release system, then it would make much more sense to me.

We have already arrived at a conclusion that the ITP can start sucking air at below a quarter tank, which means some air will be pumped along with the fuel and the air would need to be released before the fuel is sent down the car to the UBP and HP pumps.

The references do call it a pressure regulator that's shut under normal operation. Exactly what condition opens it isn't clear. I think that sort of pressure could only come with the feedback from the HPP/Injectors. Maybe it's some sort of relief valve?

Of course the petrol pump delivers at 50psi on its own.

TC

[Mike Noc]

Agree TC both UBP and ITP have their own pressure relief valves, the UBP being external to the pump and the ITP built into it.

The pressure relief from the HP pump returns with the injector leakback fuel to the thermostatic valve for the heat exchanger. When the fuel is hot and the thermostatic valve is open it can vent to the tank, but with the fuel cold and the thermostatic vave closed the returning fuel joins the outlet from the fuel tank to return to the UBP.

There is a restrictor in the line also going back to the tank but this looks like it vents off any oversupply from the ITP when it is only topping up the supply fuel when the thermostatic valve is closed.

During this warm up period pressure could build up in this loop, so it makes sense to have a regulating valve in the circuit to relieve it if needs be.

Mike

[T-Cut]

Originally Posted by Mike Noc

The pressure relief from the HP pump returns with the injector leakback fuel to the thermostatic valve for the heat exchanger. When the fuel is hot and the thermostatic valve is open it can vent to the tank, but with the fuel cold and the thermostatic vave closed the returning fuel joins the outlet from the fuel tank to return to the UBP.

I thought the HP leakback is always hot?

TC

[StuBlake13]

What a valuable thread Great insights to be gleaned here and being new to diesel it has me intrigued as I've never heard of this Quarter Tank Syndrome and other than this thread and one regarding Dodge vehicles there is nothing more Google can offer me. Can someone elaborate as to where this term is derived from?