Why won't my Jaguar Mk2 restart on a hot day?

Petrol can boil in the fuel lines, pump or SU carburettor float chambers after a short stop, causing vapour lock or percolation. Once the fuel system cools, the engine will usually start normally.

Is it vapour lock or percolation in a Mk2?

The Mk2's electric SU fuel pump is mounted at the cool rear of the car, so supply-side vapour lock is less likely than percolation, where fuel boils in the float chambers and floods the engine.

How do I fix a hot-flooded Mk2 at the roadside?

Open the bonnet to let heat escape, cool the fuel pump and lines with water if available, then hold the throttle wide open while cranking to clear the flood. Keep it running once it catches.

Can a restomod Mk2 avoid hot-start problems?

Yes — electric cooling fans, electronic ignition, modern fuel lines and a healthy cooling system all reduce heat soak and make hot restarts far more reliable.

Knowledge base

Starting in Extreme Heat

By Jon Cosson — classic-car owner, North Yorkshire, near Thornton-le-Dale

Classic Jaguar Mk2 saloon at the roadside on a hot summer day with its bonnet raised — Placeholder image — owner to replace with a photograph showing the car on a hot day.

On a scorching day a Mk2 can crank without firing after a short stop — not a fault, but the petrol boiling before it reaches the engine. It is common on carburettor classics with modern fuel — and it is one of those problems a well-sorted restomod quietly solves.

Why it happens

Two mechanisms turn fuel to vapour in the wrong place. Vapour lock is when fuel boils on the supply side — pipes, filter and pump — so the pump moves vapour instead of liquid and the engine is starved. Percolation is when heat boils the fuel in the SU carburettor float chambers, flooding the engine over-rich and producing a strong petrol smell.

The XK's engine bay gets hot and the SU float chambers sit right beside it. But the Mk2 has one built-in advantage: its SU fuel pump is usually electric and mounted at the cool rear of the car near the tank, pushing fuel forward under pressure. That resists vapour lock far better than an engine-mounted mechanical pump. So on a Mk2, percolation in the float chambers is the more likely culprit than supply-side vapour lock. Modern ethanol-blended petrol, which boils more readily than 1960s fuel, makes it worse.

The heat-soak trap

The worst moment is not while driving — it is 10 to 30 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, filter and 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.

The restomod advantage

Here is where a fast-road Mk2 pulls ahead: several upgrades owners fit for other reasons also happen to cure hot-fuel trouble.

An electric cooling fan keeps the whole engine bay cooler, so there is less heat to soak into the fuel.
Electronic ignition gives a stronger, more consistent spark, so a hot, slightly flooded engine catches far more readily.
Modern ethanol-rated fuel lines, insulated routing and a healthy cooling system round it off.

So a well-sorted restomod Mk2 — like 873 EUT, which runs an electric fan, a modern fused electrical system and updated fuel plumbing — tends not to suffer the way a standard car can. It is one of the quiet, everyday benefits of the fast-road recipe.

Preventing it on any Mk2

Choose ethanol-free or low-ethanol super unleaded (E5) over E10.
Insulate and re-route the fuel line away from heat; fit a heat shield or insulating spacer at the SU carburettors.
Keep the cooling system healthy and consider an electric fan.
Make sure the ignition is strong — electronic ignition helps hot restarts.

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.

It is the heat, not the car

A Mk2 that runs beautifully in cool weather but refuses to restart on a hot day is usually telling you the fuel is boiling, not that anything is worn out. Cool it down, start it once, and plan the preventive upgrades.

Set-ups vary by car, so confirm any figures and procedures against your workshop manual. Take care around hot components and petrol.

Common questions

Why won't my Jaguar Mk2 restart on a hot day?: Petrol can boil in the fuel lines, pump or SU carburettor float chambers after a short stop, causing vapour lock or percolation. Once the fuel system cools, the engine will usually start normally.
Is it vapour lock or percolation in a Mk2?: The Mk2's electric SU fuel pump is mounted at the cool rear of the car, so supply-side vapour lock is less likely than percolation, where fuel boils in the float chambers and floods the engine.
How do I fix a hot-flooded Mk2 at the roadside?: Open the bonnet to let heat escape, cool the fuel pump and lines with water if available, then hold the throttle wide open while cranking to clear the flood. Keep it running once it catches.
Can a restomod Mk2 avoid hot-start problems?: Yes — electric cooling fans, electronic ignition, modern fuel lines and a healthy cooling system all reduce heat soak and make hot restarts far more reliable.