Minimum fuel level on 1948 3 1/2 Mark IV saloon

A friend came back to his home and pulled into his garage with the thought he was low on gas in his 1948 Mark IV 3 1/2 saloon. The fuel gauge is not operational, he uses a stick in the tank (which just barely showed a wet tip when placed all the way to tank bottom through the fill orifice at this time). When he tried to start the car a week later, the fuel pumps clicked but no start. On phone call to me I suggested putting a couple of gallons of gas in the car. No start and fuel pumps still click continuously. He then removed a fuel line at the pumps and tried putting a couple of cups of gas from that opening into the line heading back to the gas tank in hopes of filling the line going back towards tank and up to pumps. No improvement. I now have recommended he go to gas station a few times with his 2 gallon tank and get at least gallons in the car.

I’m not familiar with the inside of a Mark IV fuel tank.
1.) Does fuel enter above the tank bottom for the line feeding the pumps?
2.) Are there baffles preventing a 2-gallon fill from reaching the feed line?
3.) What is the minimum amount of fuel needed to restart a car which ran out of gas? Presumably the fuel sloshing when driving to empty could take the fuel level well below a minimum needed to restart the car when stationary?


IIRC the original MKIV 3 1/2 Litre setup the two fuel pumps ”pull” the fuel, which means if there is air in the system it may not get fuel up very easily.

On the MKV the fuel pump sits low in the chassis and pushes the fuel up the line.

So if a MKIV fuel line was allowed to get dry it may take some priming to get the fuel moving again.

I would manually fill the line with fuel from the ”wrong” end, the carb side and put more fuel in the tank and perhaps try with some low pressure if possible (on a MKV easy by entering some air pressure via the fuel tank breather pipe) until the carb bowl are full and the fuel pumps are pumping fuel instead of air.


I don’t know if Mark IV is the same, but my '38 SS has a reserve side of the tank, which I think holds about 3 gallons, so if you run out on the main tank you pull a knob under the filler cap and it opens the valve to the reserve tank. The reserve tank gets filled first before any fuel gets to the main tank, so if both were empty you would need 4 or more gallons to get any to the main tank.
I don’t think the fuel pump is not capable of pulling up fuel, at least I have not had trouble with it. It pumps quite a while until the carbs are full and it stops pumping. Then it is ready to start.

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Hullo Roger,

You note that you are not familiar with these tanks so I’ll go through some details.

The MkIV tank has a low height baffle across from front to back, a bit left of the centreline, to separate the left and right sides to create a reserve section of about 5 litres (not sure of the true amount) in the left part. This quantity forms part of the total capacity of about 65 litres, and is not additional to.

There are two suction pipes inside the tank, one to draw from the ‘main’ side and the other to draw from the ‘reserve’ side. The ends of these pipes extend into the spigots that house the gauze filters which are integral with the drain plugs. The side supplying fuel is determined by the fuel tap switching position under the left rear wing. The knob in the ‘down’ position is for the main tank and ‘up’ is for the reserve. So both pipes draw up from the bottom of the tank. When you have a completely empty tank, i.e both sides dry, the reserve part has to be filled before it overflows to the main part.

The driving habit and standard practice should be that you always set the tap to the main position - knob down - and switch to the reserve if you run out, but reset the knob to the main after refilling. (Some people incorrectly think that each pipe is set at a different level to create the reserve.) In effect, either side can be considered the reserve, because if you run on the reserve position (knob up) until dry, the main side on the right of the baffle will still be wet.

A few comments about your enquiry:

1). If the tank has been sucked dry, there will only be an eggcup amount left in the bottom of the drain plug spigot.
2). If the tap is set to the main position with a dry tank, you need to pour in enough fuel to overflow into the main side. So yes, 5 litres plus. Say 2 gallons U.S. +.
3). As above. Maybe set the tap to the reserve position to start on the least amount of fuel.

Has the car been run with the tap in both the main and reserve positions? Does the tap work or has it been cannibalised with the internals missing or made ineffective?

There should be no need to prime the fuel lines from the pump ends if the pump valves and diaphragms are good. A good pump will fill the carbs in about ten seconds, a poor one in maybe 20.

The function of the main versus reserve depends on the integrity of the seal of the baffle to the tank bottom and edges, a condition not easy to confirm.


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To add to Peter’s thorough description, the level gauge only shows the level in the main side, not the reserve.
These marks show the location of the reserve baffle or wall, with holes at the top so the fuel spills over into the main side.
There is another anti-sloshing baffle on the right side with holes at the bottom.

Here is the switching valve; yours may have a different type but the idea is the same.

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Just an interesting characteristic when driving. The internal baffling doesn’t do much to slow up the sloshing about. Typically, with a half tank and and some driving situations like roundabouts, the gauge will show anything between near empty and full. Has anyone added a voltage stabiliser into the instrument circuit and was it worth the trouble? I won’t be doing this but I am curious.

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Hi Peter,

I certainly recognise the characteristic but I’m quite happy to let my brain do the averaging.


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After a long time I drove my MK IV yesterday and realized the gauge was indicating “empty”. I was on the Autobahn and feared to run totally empty before reaching a gas station. Luckily I reached a gas station and filled completely: 61,33 liters. By the way I remembered the reseve knob and tried to draw the knob, but it was not possible at all.
Also after a long time I today joined the forum to find wiper blades for my MK IV (I’ll seach later on for it). Unbelievable that the first post I found here dealth with the problem I had yesterday. I just took a photo from under the left rear wing (added) but couldn’t find a reason for my problem. I never used the knob within the last 12 years. Perhaps there is a possibility to make it movable again.
By the way: the former owner of my car from Australia advised me that there are 2 recesses in the tank, which should be unscrewed and cleaned every few years. Did you know / ever heard of it?

Thanx for your answers

Yes, he undoubtedly means the two drain plugs, one for the main tank on the right and one for the reserve on the left side.

They have a filter screen in them like this.

The fiber sealing washer is the same as for XK120 and later Jaguars and is sold by some of the parts vendors such as Moss.

The remote control valve will most likely have to be taken apart and cleaned with new cork seals.

Thank you Rob for your explanation. I’ll look for the drain plugs and clean the filters.
Do you have an idea how to move the reserve knob?

There are three fuel line compression nuts to be loosened, and the operating extension rod, then one nut holding the valve to the bracket.
Then there is a tiny screw holding the plunger inside the housing. There is another long screw holding the seals in place.

Many thanks, Rob! I’ll try to fix it.

I have had success repairing these valves/taps by replacing the cork seals with Tygon (gasoline/petrol) resistant tubing. Not original I know however the valves now work properly and do not leak.

Hullo Uwe and David,

I also used fuel hose. I had one that was a slide fit in the bore, but not tight. Just a few tips for making these - cut them neatly to length and sand the ends to be square. The reason for this is that the assembly is tightened enough to ensure there is adequate side pressure on the bore to make a good seal. If the ends are not square, the seals will skew as you tighten the assembly. It has worked perfectly now for ten years.

Note the miniature screw near the end of the ferrule. It is to tighten on the thread of the spindle to lock the adjustment and must be loosened before disassembly.

You will find there is a fine adjustment between too loose and too tight and you may have to tighten it up after installation if you see some leakage. To make this easier, I did not install the travel limiting screw on the top, nor did I tighten the little thread lock screw, and had a low petrol level. Because you need to remove the spindle assembly to make an adjustment, have a plug handy to reduce fuel loss. When I was satisfied, I tightened the lock screw and installed the limiter screw.

A hint for the future is to operate the tap control back and forth a few times occasionally to make sure the internal seals don’t develop a ‘grip’ in the bore.

Good luck.