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#1 Fuel vapour Lock

Posted: Fri Apr 19, 2019 7:51 pm
by Adamski
S3 V12 FHC 1972 Manual. Been very hot today. After a good run the car starts and dies. Impossible to restart. Ignition electrics replaced so I'm sure it's not them. Been using the cheapest fuel which has loads of Ethanol. This boils and vaporises at about 100 deg. C. I suspect the fuel vapour locks liquid from entering the float chambers.
I did introduce a modification recently where I fitted a valve from a tee from the top fuel rail and it discharges downstream of the non return valve on the return line. Today the car misbehaved and I left the valve open for a while to vent out any vapour and introduce a cooler charge of fuel. The car eventually started. My conclusion is that the Non RV does not give enough flow to vent the system and it was not designed for that anyway, only as an overpressure device for the floats. I will now leave my new valve cracked open permanently to enables better circulation of fresh fuel. I have heard of muscle cars having a permanent 3/16" orifice on a return line to have a permanent recirculation. I could of course use premium unleaded which has no or less ethanol but who know what's in there today. I will try it next time as it's just too scary the thought of being stranded.
Any thoughts or similar experiences.

#2 Re: Fuel vapour Lock

Posted: Sat Apr 20, 2019 6:22 am
by MarekH
If you floor the accelerator when hot cranking, then the extra airflow will clear out the manifolds, pull fresh air/fuel mix through and thus force new cooler fuel to be admitted into the float bowls.

The other suggestion is to keep the tank topped up higher. If you have a large reservoir of relatively cool fuel then the recirculating hot fuel will have less influence on the overall temperature. "Cool" in this context means less than under-bonnet temperature.

Another solution that might be worth considering would be to run a swirl pot which always refills with fuel from the tank, but the returning fuel is forced to mix with that of the tank first, before going back around the loop. The default setup is that the return and pickup are in very close proximity to each other.

kind regards
Marek