Fuel Feed Problem 420 Sedan

I’ve been having a weird thing happening with my 420. If I fill both tanks and drive on the right tank at around half tank unbeknownst to me it somehow is feeding off the left tank at the same time. To my surprise when I switch to the left tank it was just above half. I had not used it since topping off and only running on the right side. I made sure each pump is running independently and checked the lines.

Fuel system was fully gone through, new pumps, filters in tanks new hoses and I have vented caps. I’m planning on topping the right only and use the left to see what happens.
Has anyone else experienced this?

Hi Tim,
I’ll stop siphoning gas out of the left tank. LOL I would trying to use a stick/rod to see if the LH tank is really only 1/2 full.


I did a visual as well. This is strange. A few weeks ago I drove 98 miles each way to a car show in WI and thought maybe I forgot I used the left tank. Since I’ve been watching this and now know something is happening. Car is always in my garage.

Just keep driving and if something develops you will know. Probably nothing. Were it leaking you could smell it.

Hi Tim,
A wild ass guess - I would check that the tanks are vented correctly. I was wondering if the LH tank’s vent hose is plugged or pinched. If it is then when the fuel warms up (being over the exhaust) it could become pressurized and force fuel out of that tank.

Good thought. I have vented caps. One of my vent tubes is plugged when I had my tanks out for cleaning. I spent hours trying to open it up. I finally settled for vented caps.

hard to give a silver Bullitt answer. I assume its not a daily driver and spends a lot of time parked up. i also assume you have external fuel pumps? i would sagest that the tanks are leveling out and that the non return valves may be shot.

I drive it regularly. Two new SU pumps. I completely rebuilt every mechanical part and system. Oddly if I feed left tank first the right stays up. It doesn’t really bother me anymore. I don’t care to spend any more money on this car. I have $24k in a $9 to $12k car.

I understand what you mean about how much one gets tied up in a car. I’m way up there on the Mk10 which is nowhere near done, certainly more than I could sell it for. However, we can’t back up and do it over, so best to just enjoy what we’ve got. You have a really nice car.

Very true. Thank you! The 420 is a blast to drive, especially with the transmission swap.