Why would a properly sealed fuel tank help with cold starting?

Question

Weird question I know.

For a long while my Mk1 Golf had trouble cold-starting. It would cough a few times but not catch until maybe the 6th attempt.

Over the years I tried a number of thing to try and fix it, including but not limited to:

• Adjusting timing
• Checking for vacuum leaks
• Replacing fuel pumps and filters
• Replacing injector seals
• Replacing IAC valve
• Adjusting idle screws.

None of these seemed to make a difference.

However, recently I filled the tank and noticed a bit of fuel was coming out of the top of the tank where the fuel sender unit is. The O ring seal there has always been a pain to seat properly so this time I used aviation gasket sealant as well.

And now not only is there no fuel leak but the car starts perfectly every time. Further more it seems the car is a bit more peppy than before.

I'm greatful this has made a difference, but I'm just not sure why it's made a difference. I mean, surely the fuel tank is not fully sealed otherwise air could not come in to replace the fuel that's being sucked out. Could anyone share some light on this?

Answer 1

I'm greatful this has made a difference, but I'm just not sure why it's made a difference. I mean, surely the fuel tank is not fully sealed otherwise air could not come in to replace the fuel that's being sucked out. Could anyone share some light on this?

Your impression, while reasonable, is incorrect. This is not a 1953 vehicle.

Gasoline has a modest vapor pressure. Gasoline vapor contributes very seriously to regional smog (made famous in Los Angeles). As such, since at least the 1970s, the fuel tank is required to be sealed, and there is a charcoal canister which absorbs fuel vapors and keeps vapor pressure low in the tank. The engine then purges this canister when you drive the car (by intaking the fumes into the engine).

Indeed, for states which do functional smog checks, fuel tank pressurization is part of it for 1980s and 1990s cars. For 2000 and newer cars, the engine control system checks for this automatically, and will light the Service Engine Soon / Check Engine light if the pressurization is not working.

If your habit is to ignore that light, don't - This isn't your father's smog controls, maintaining those systems will help your engine perform.

As far as the root problem of starting, I think the system does not like having the fuel tank non-pressurized. Perhaps the pump was having trouble sustaining a prime.