Erratic idle speed, fuel drains when engine off?!!!


Hi Guys,

Wondering if someone can help me, I'm having a couple of problems with my 1994 F78.

For about a week or so now, every time I turn the engine off the fuel seems to drain out of the engine, and needs about 6 pumps on the primer before I can start the engine as normal (within two turns of the starter).

Then recently I seem to be having another, perhaps related problem, which is that the idle speed varies erratically, from normal to stall. Today I was sitting in traffic after a motorway run and it almost stalled. I turned up the idle speed to 900RPM yet it still varied erratically from 900 RPM to 400 RPM and would have stalled had I not kept touching the accelerator every two seconds when the engine was at idle.

Under power the car seems to run OK, although just before it started wanting to stall I did notice a loss of power under acceleration from the lights, but after that even though it wanted to stall at idle it seemed to behave as normal under load in town traffic.

So does anyone have any ideas what could be causing these two strange problems? Any chance it's related to the recent cold weather?

Any advice would be much appreciated.

Many thanks.




I can tell you exactly what it is, it is the fuel pipe non return valve defective.

Located in the fuel filter housing is a non return valve, this ensures that the filter stays full of fuel, and prevents it syphoning back to tank. If this is defective it syphons all the fuel from the filter back to tank, making starting difficult and causing erratic idling as it reprimes the system, often sucking air up as well, or having trapped air in the fuel lines.
During normal operation the overly large filter houses enough fuel to start the engine and run it, this allows it enough time and power to fully fill the fuel pipes if they leak back to tank.

To test: rempve the feed pipe from the filter, drain the filter of fuel and fit a piece of rubber pipe onto the feed pipe stub, suck the pipe and if you get air into your mouth the valve is defective. On some housings this stub pipe is cast into a nut, this can be removed and the non return valve cleaned or replaced, on others it cannot, this means a good secondhand fuel filter housing.

Also check that the fuel pick up pipe in the fuel tank is intact, these can pin hole or simply snap off meaning the vehicle will only run correctly with a full tank of fuel. While this is out, check the strainer on the end of the pipe is intact, these can snap and a flap will cover the end of the pipe, thus acting as a valve and preventing or restricting fuel entering the pick up pipe.
Also check the tank breather, these often become clogged up or rusted, these are simply a plastic body with a ball bearing which sits between two springs. The ball can move either way to let air into the tank as fuel is used, or air out of the tank as the tank warms up and the fuel expands. If you suspect the tank breather, run the engine with the fuel filler cap off, if its tick over normalises it is the breather.

Hi thanks for your reply.

Hi thanks for your reply. It's interesting you mention the breather as I have noticed that every time I fill her up with diesel i hear a sucking sound as I remove the filler cap. I had assumed this was normal as it has always done it and these problems have only started recently, but does this mean the breather is blocked?
I'll do all the checks you recommend and post back the results.
Thanks again,

Out Of Breath

No, you are confusing the filler breather with the tank breather, the filler breather pipe is a much larger pipe designed to expel air at speed as the tank becomes full. This connects from the top of the tank into the main filler pipe, just below the fuel pump nozzle input hole.

The tank breather is designed to compensate for tank pressure differentials as fuel is used, or allow air to escape as the fuel tank warms up, thus causing pressure. This is connected to a 4mm bore rubber pipe and is generally mounted on the body, depending on model, and is a plastic housing about 30mm in diameter and 60mm long.
Due to the use of these vehicles, the outlet hole often becomes blocked, or internal corrosion and dirt sieze the internals, this causes a vacuum in the tank as it sucks fuel up.

Found it

Hi guys,
Tested the one way valve as suggested and it wasn't that so checked the fuel line and found it weeping diesel at the back above the rear axle just after it leaves the fuel tank. Replaced both the inlet and return with rubber fuel hose and it starts after one turn of the starter, no more priming required. Obviously air was being sucked in to the system causing the engine to run over lean and the power to fall off.
Thanks for your advice!