An E-Type that will not start when hot

Question

What exactly happens when it is hot?

Heat-related starting faults fall into three families: the starter itself, the ignition, and the fuel. They produce different symptoms and the difference is easy to observe.

Note the timing as well. Under-bonnet temperature peaks some minutes after the engine is switched off, not while it is running, which is why the worst case is often a short stop rather than a long one.

Answer this

• It cranks very slowly, or not at all, when hotGoes to: Starter heat soak
• It cranks normally but will not fire when hotGoes to: It cranks properly but will not fire. Is there a spark when hot?
• It starts, runs badly for a few minutes, then clearsGoes to: It starts, runs badly, then clears
• It floods, and smells strongly of petrol when hotGoes to: It floods when hot
• It will not start at all, hot or coldStart from the general starting diagnosis instead.

Finding

Starter heat soak

Why this follows. The starter sits close to the exhaust. As its windings heat, their resistance rises and the current it needs rises with it, so a starter that is perfectly adequate cold becomes marginal hot.

This is a real and well-known characteristic rather than a sign of neglect, but it is made much worse by any resistance elsewhere in the circuit, and that part is fixable.

What to do

• Check the whole starter circuit for voltage drop while cranking hot, including the earth return. A circuit with a little too much resistance works cold and fails hot.
• Check the engine to body earth strap in particular.
• Check the battery under load, since a marginal battery and a hot starter combine.
• Check whether an aftermarket or non-standard exhaust has brought heat closer to the starter than it originally was.
• Consider heat shielding between exhaust and starter. Owners also fit geared high torque starters for this, which many report as effective, though it is a modification and the archive does not rate suppliers or products.

Read further: Earths, wiring and the loom, The charging system

Question

It cranks properly but will not fire. Is there a spark when hot?

Petrol and sparks. Keep the test plug away from the fuel system.

Test for a spark while the fault is present, not after the car has cooled. A component that breaks down with heat will test perfectly once it is cold, which is why this fault gets misdiagnosed so often.

Answer this

• No spark when hot, good spark when coldGoes to: An ignition component breaking down with heat
• Good spark when hotGoes to: Fuel vaporising before it reaches the engine

Finding

It starts, runs badly, then clears

Why this follows. The engine is running on a mixture disrupted by vapour, and once the fuel system has purged the vapour and refilled with liquid, normal running resumes. The clearing is the diagnosis.

What to do

• Treat this as the same fault family as vaporisation and work through those checks.
• Note how long it takes to clear, and whether it clears faster when driven than when idling, since airflow through the engine bay is what removes the heat.
• Check the float chamber levels, since a level disturbed by boiling settles differently.
• Rule out an ignition component that recovers as it cools, which can look similar. A spark test during the bad running separates them.

Read further: Modern fuel and E10, Triple SU carburettors

Finding

It floods when hot

Why this follows. The opposite of vaporisation starving the engine: here too much fuel reaches it, either because heat has driven fuel out of the float chambers into the inlet, or because the starting technique is adding enrichment the hot engine does not need.

What to do

• Try starting hot with the throttle fully open and no choke or enrichment at all. If it then starts, the mixture was too rich and the technique was the fault.
• Check the float chamber needle valves. A valve that does not seat lets fuel continue to arrive.
• Check float levels and float condition. A float that has absorbed fuel sits low and floods.
• Check for fuel percolating out of the float chambers into the inlet after switch-off, which leaves the engine flooded before you even turn the key.
• Check the choke or enrichment mechanism returns fully when released.

Read further: Triple SU carburettors, The four Zenith Stromberg carburettors

Finding

An ignition component breaking down with heat

Why this follows. A part that gives a spark cold and none hot is failing thermally. Cooling it and finding that the car starts is the test that confirms which one.

What to do

• Cool the suspected component with a damp cloth while the fault is present and try again. If the car starts, you have identified it. Do this one component at a time or the test tells you nothing.
• On a six-cylinder car, the coil is the usual candidate, followed by the condenser and the high tension leads, which break down internally when hot.
• On a Series 3 V12, the Lucas OPUS amplifier is the known thermal failure and should be first on the list.
• Check the ballast resistor arrangement if fitted.
• Check the distributor cap for tracking, which can appear only once the cap is hot and has expanded.

Read further: The ignition system, Lucas OPUS ignition on the V12

Finding

Fuel vaporising before it reaches the engine

Why this follows. Petrol boiling in the line or the float chamber gives vapour rather than liquid at the jet. The pump cannot pump vapour, so the engine is cranking on nothing even though the fuel system is full.

Modern petrol is more volatile than the fuel the car was developed on, and ethanol content lowers the boiling point further, so this fault is more common now than it was in period. That is a general point about modern fuel rather than a measured figure for this car.

What to do

• Check the routing of fuel lines relative to the exhaust and the manifold. A line that has been rerouted during a restoration is a common cause.
• Check that heat shielding between the exhaust and the carburettors and fuel lines is present and intact.
• Check the float chambers for heat, and whether insulating spacers or shields that should be there are missing.
• Consider the fuel itself. Higher volatility fuel worsens this, and a car that developed the fault without any mechanical change may simply be on different petrol.
• Confirm the diagnosis before spending: if the car restarts readily after a long cool down but not a short one, and there is a good spark throughout, vaporisation fits.

Read further: Modern fuel and E10, Triple SU carburettors, The four Zenith Stromberg carburettors