Fuel problems

[pfield69]

I keep having problems with my van's fuel.

It will drive for 100s of miles with no problem but then randomly fails to start. It looks like the fuel pump fails to draw fuel up.

I have 2 fuel filters, added a non-return valve and changed the engine bay fuel hoses. I have also changed the fairly new fuel pump and drive pin.

Any ideas?

my thinking is perhaps something wrong in the fuel tank or perhaps a worn cam that drives the pin.

Re-MoT on Wednesday (as last time in failed was on the MoT ramp last week) so a quick fix would be good.

[AdrianC]

Petrol or diesel? Carb or injection? Mechanical or electrical pump?

[pfield69]

Oops.
1.9 DG engine petrol mechanical pump with carb (not webber)

[kevtherev]

Firstly the engine does not need two filters, the one by the tank is enough.

It looks like the fuel pump fails to draw the fuel

have you investigated this and found it to be true ?
or are you just guessing?

[Dazco]

Does that carb have the filter in the inlet? If so might be getting clogged up

[pfield69]

I have disconnected the fuel pipe to the carb and cranked the engine with no output (new pump has output).

Last time it happened (it had been standing for winter) we put a electric fuel filter on it to draw up fuel. Once 'primed' it started fine.

I have 2 filter as when I purchased the van I kept getting blocked jets. I was under the impression it would not hurt having 2.

[AdrianC]

I have 2 filter as when I purchased the van I kept getting blocked jets. I was under the impression it would not hurt having 2.

It won't _hurt_. Nor would a third, or a fourth, or a fifth, or a sixth. But they won't add any benefit over one filter. So why not just change that one?

As it is now, to get to the carb, the fuel has to pass through both filters. If the first filter is working fine, the second one is pointless. If the first filter's letting gunk through, then the second filter does something - but why not just change the first one? Filters rarely fail "open", though - far more likely is that they block and restrict flow... That's why changing them is a service item, as with any other filter.

Same with adding non-return valves to the pipework. Yes, if the pump is losing it's prime, an extra valve'll stop it doing so. But think about why it'd lose it's prime. The pump has two non-return valves in it already. It can't work without them. The pushrod moves a rubber diaphragm back-and-forth, increasing and decreasing the size of the chamber within. As it increases, the exit valve closes, so the pump pulls fuel in from the tank. As it decreases, the entry valve closes, so the pump pushes fuel to the carb. For that prime to be lost, the valves have to let the fuel run back to the tank. But if/when either valve fails completely, the pump'll just wobble the fuel within it about. So, by adding that non-return valve, you're just masking the first symptoms of a failing pump.

[pfield69]

Thanks.

You describe the fuel pump as I anticipated it to work.

What confuses me is that it doesn't act like this. If I blow own the pump output fuel is blown backwards to the tank. I would have expected to blow against a stop valve.

I put the non return valve in after I fitted the last new pump. Perhaps the pumps are just rubbish?

[kevtherev]

Does your engine blow fuel back to the tank?
no
would an air/fuel leak allow fuel to return back down the pipe? (either diaphragm or a joint, or a filter)
yes

it's a difficult one this, but the physics of it says that fuel would not drop back without a leak.
The only places for this to happen are..
floatbowl (float height)
leak

[pfield69]

Arghhhhhh!

Float chamber

The float chamber casket is damaged. So if float drops air can get in causing fuel to flow back?

I would have thought the non return would stop this though.