Starting in Extreme Heat
On a scorching day an Aceca can crank without firing after a short stop, not a fault, but the petrol boiling before it reaches the engine. With today's volatile fuel it is increasingly common on cars of this age, and it is very manageable once you understand it.
Why it happens
Two mechanisms turn fuel to vapour in the wrong place, and it is worth knowing which one you are dealing with, because they present quite differently.
- Vapour lock: fuel boils on the supply side (the pipes, an engine-bay filter, the pump), so the pump moves vapour instead of liquid and the engine is starved. It cranks but will not fire. For how that side of the car is arranged, see the fuel system article.
- Percolation: heat boils the fuel in the carburettor float chambers, flooding the engine over-rich, with a strong petrol smell to go with it. The Aceca's induction depends on its engine, a Bristol car's triple Solex carburettors, or the SU or Solex of an AC-engined car, but the float chambers sit close to a hot engine either way, so percolation is the usual culprit.
Modern ethanol-blended petrol boils far more readily than the fuel these cars were built for, so a car that never did this when new can do it now.
The heat-soak trap
The worst moment is not while driving. It is ten to thirty minutes after you switch off. With the engine stopped there is no coolant flow and no airflow, so stored heat soaks up into the fuel system and boils the petrol sitting in it. Start straight away and it is usually fine; wait an hour and it is fine; it is that middle window, at the pub, the fuel station, the show field, that catches you out.
The symptoms
- Runs perfectly, but will not restart after a short stop on a hot day.
- Cranks but will not fire (starvation), or fires, stumbles and reeks of petrol (flooding).
- A rough, hunting idle and stalling in slow, hot traffic.
- Always comes good once cool, the sign that it is heat, not a fault.
The roadside cure
- Cool the fuel system. Park in shade and open the bonnet. If you have water, pour or wipe it over the fuel pump, any engine-bay filter and the fuel line to condense the vapour back to liquid.
- Give it a few minutes with the bonnet up.
- Clear a flooded engine by holding the throttle wide open while cranking; keep it running once it catches (a puff of smoke is normal). Do not sit endlessly cranking.
Preventing it
Physical fixes work; additives rarely do.
- Know your set-up: which engine and carburettors you have, and whether your fuel pump is mechanical or an electric SU-type pump. An electric pump near the cool tank pushes fuel forward under pressure and resists vapour lock far better than an engine-mounted pump, the fuel system article sets out the difference.
- Insulate and re-route the fuel line away from the exhaust, and use modern ethanol-rated hose.
- Fit a heat shield or insulating spacer at the carburettors to slow heat into the float chambers.
- Keep the cooling system healthy and the engine bay as cool as possible; see the cooling system article.
- Choose ethanol-free or low-ethanol super unleaded (E5) over E10, the fuel, oil and additives guide covers the choice in detail.
Do not misdiagnose it
It is easily mistaken for a failing fuel pump, worn carburettors or an ignition fault, and money gets spent chasing the wrong thing. The give-away is the pattern: only when hot, after a short stop, always right once cool. Rule the heat in first.
Common questions
- Why won't my AC Aceca restart on a hot day?
- Almost always because the petrol has vaporised before it reaches the cylinders, either vapour lock in the supply side, or percolation boiling the fuel in the carburettor float chambers. The car will start again perfectly once everything has cooled.
- What is the difference between vapour lock and percolation?
- Vapour lock is fuel boiling on the supply side, the lines, an engine-bay filter or the pump, so the pump moves vapour instead of liquid and the engine is starved. Percolation is fuel boiling in the carburettor float chambers, which floods the engine over-rich and leaves a strong smell of petrol.
- Why is it worse ten minutes after I stop?
- With the engine switched off there is no coolant flow and no airflow, so stored heat soaks up into the fuel system. That heat-soak window, roughly ten to thirty minutes after shutdown, is when the petrol sitting in the pipes and float chambers is most likely to boil.
- How do I get going again at the roadside?
- Park in shade, open the bonnet and let the fuel system cool for a few minutes. If you have water, wipe or pour it over the pump, any engine-bay filter and the fuel line. If the engine is flooded, crank it with the throttle held wide open and keep it running once it catches.
- Does ethanol petrol make it worse?
- Yes. Modern ethanol-blended petrol boils more readily than the fuel these cars were designed around, so a car that never suffered when new can suffer now. Ethanol-free or low-ethanol super unleaded (E5) is generally kinder than E10.
About the author
A lifelong classic-car enthusiast in North Yorkshire, near Thornton-le-Dale, who owns and maintains an AC Aceca, writing the single-model reference he always wished existed. Read the full bio.