XK150 Fuel Bowl empties when shut down

After a complete engine overhaul and 2 new fuel pumps I am experiencing a strange thing with the fuel. After shut down as the engine cools the fuel disappears from the glass fuel filter bowl. When I start the car the bowl is empty, key on bowl fills in a few seconds, key off, bowl empties almost as rapidly. It is not overflowing as there is no smell of gasoline. It must be going back thru the pumps. However the fuel bowl both fills and drains from the top cap so it cannot be gravity feed. There is no indication of fuel backflow.
Has anyone experienced this/ have a suggestion?

Check your gas cap for a plugged vent.

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Unlocked gas cap and left open with same results.

Do you mean you put in a new pump, that failed, and then tried another new pump, or does your car normally have two?
Are we talking about the standard SU LCS pump?
Are they installed upside down? They are marked UP.

In the SU the two one-way valves E & F inside are not perfect seals, they are just flat brass discs held in place by a clip, and shut by gravity against another flat metal surface, so there will always be some slight backflow leakage past them when not pumping.

Do you have a glass filter something like this? Is it installed upside down?

If the filter is higher than the pump and the tank, gravity will pull the fuel back by the siphoning effect, until the carb bowls are empty and pulling air, then the gravity effect will stop, and the remaining fuel will flow in both directions from the high point.

Those of us without glass filters are not aware of this and simply accept the fact that the SU has to pump a few seconds before we are ready to start.

Hi sailor,

Do you have a paper filter inside the bowl?


Rob, Sorry for not getting back earlier. Thanks for the response. You really know your pumps.

Yes it is an XK150S which had two SU pumps mounted in parallel (I put new ones in) and feeding a glass bowl filter with a paper filter installed (mounted upright) and the filter bowl is higher than the pumps and tank.

Since the feed to/from the filter bowl is in the top it is hard to understand why the gas drains from the bowl. I can understand the gas draining from the upper feed line back thru the SU pump if the pumps check valves leak but why drain from the bowl?

Hi Clive, yes I have a paper filter inside the bowl. As I recall I used a different paper filter last time changed. Before I used a filter that did nothing because it was not sealed against the top (using the regular paper filter recommended by XK’s Unlimited)

I talked with a Jag machine shop in Seattle and they suggested a different filter with a rubberized top that seals so that the gas is forced to go thru the paper filter. I was amazed to discover that with the XK’s filter it wasn’t filtering. Just had a thought I wonder if that may have an effect on the likely siphoning back of the fuel.

Remove the paper filter and see what happens.

I suspect the filter is choked on inside, effectively turning it into a tube.

I run a filter before the bowl, the transparent type so you can see if there is accumulated debris.

Without the paper filter debris falls to the bottom of the bowl anyway.


I was going to ask if yours has tube going to the bottom?
You’ll notice that in my picture above, mine has just a screen in the lid. I thought it once had a tube but am not sure about that.
In the old days these were known as sediment bowls.

The filter is new and I only use ethanol free gas. The reason all this came to light is a result of doing an engine overhaul and getting things back together. All this means little as the car runs fine when running. I was just curious from a science standpoint. How the heck can the fuel flow backwards from the sediment bowl if the pickup is at the top of the bowl? I am going to forget it I guess.

Thanks for the tips.

Rob, No tube going to the bottom in the sediment bowl. When I first got the car there was significant rust particles in the bottom of the bow. Tank was fairly new so I figured it was the old steel fuel lines and problems from using ethanol gas. The alcohol would attract moisture and sit for weeks months and years in the bottom of a slight bend creating rusted lines. Changed all the lines out for copper and new pumps etc. No sediment any more but cannot understand how fuel can flow back from the fuel bowl that has a pickup in the top cap. Think I will give up trying to understand as the car runs fine when running.

Thanks for the suggestions and help.


The answer is that it cannot drain or fuel be sucked back as long as the filter is free flowing.

The bowls were not designed to have the paper filters, and and sediment is trapped on the inside of them - out of sight. Having had blocked bowl filters, I now do not use them - instead preferring to have filters in other parts of the system where their condition can be observed.

I have installed a filter before the pumps - i.e. between the tank and pumps, and another just before the bowl.

It is a five minute job to see what happens if you remove the filter. My concern is that at WOT and high revs you may get fuel starvation if there is a partial blockage.


Thanks Clive, its worth a try. I must admit tho’ I have had it running smoothly at 80-90 mph on a 200 mile trip to Canada just awhile ago. I would think if the filter was blocked even partially it would show up with lack of power or sluggishness. Also I have a video clip pof the bowl filling after draining. Turn on the key and the pumps give about 6 or 8 clicks and the bowl is full. Turn key off and the bowl drains at about 1/2 the rate of fill.

I will pull the filter and try it.

Thanks again