A misfire, or an engine that will not idle

Question

When does it run badly?

Almost every running fault can be produced by ignition, by fuelling or by mechanical condition, so naming the component first is guesswork. Naming the conditions first is not.

A component that fails only under load, only at idle, or only when hot is telling you something specific about itself, and that is the fastest way in.

Answer this

• Only at idle. It clears as soon as the revs riseGoes to: Bad at idle only. What is the idle doing?
• Only under load or at higher revs. It idles perfectlyGoes to: It misfires under load but idles perfectly
• All the time, at every speedGoes to: One cylinder, or all of them?
• Only when the engine is hotGoes to: It only misbehaves when hot
• Only when cold. It clears as it warms upGoes to: Bad when cold, fine when warm
• It started suddenly rather than getting gradually worseGoes to: It came on suddenly

Question

Bad at idle only. What is the idle doing?

Idle is where small air leaks and small mixture errors show up worst, because manifold depression is at its highest and the engine has least to spare.

Answer this

• It hunts, surging up and down on its ownGoes to: An idle that hunts
• It is lumpy and uneven, but steadyUse the cylinder question, since a lumpy idle is usually one cylinder or a mixture fault.Goes to: One cylinder, or all of them?
• It will not idle at all. It dies unless I hold the throttleGoes to: It will not idle at all
• The idle is too high and will not come downGoes to: The idle is too high

Finding

It misfires under load but idles perfectly

Why this follows. Cylinder pressure rises under load, and a spark has to jump its gap against that pressure. A spark that is perfectly adequate at idle can fail entirely under load, which is why this fault appears and disappears with throttle rather than with revs.

This is the pattern that most often gets misdiagnosed as fuelling, because the car feels starved. The distinction is worth making before anything is dismantled: an ignition fault under load usually feels like a sharp cut or stumble, while genuine fuel starvation feels like a progressive loss of power that recovers when you lift off.

What to do

• Check the high tension leads first. Insulation that has aged breaks down under the higher voltage needed at load, and the leads test perfectly at idle.
• Check the coil, and the ballast resistor arrangement if the car has one.
• Check the distributor cap for tracking, which shows up under load before it shows up at idle.
• Check the plug gaps and the plug type.
• Check the mechanical advance weights in the distributor are free. Seized weights give no advance under load and produce exactly this complaint.
• Then check fuel delivery under sustained demand: the filter, the pump, the hoses and the float levels. Fuel supply that is adequate at idle can be inadequate at load.
• On the Series 3 V12, include the Lucas OPUS amplifier and its pick-up in the ignition checks.

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

Question

One cylinder, or all of them?

This is the question that halves the work. A single dead or weak cylinder is a fault at that cylinder. An engine that is rough everywhere is a fault common to all of them, which means timing, mixture, or general condition.

There is one pattern worth knowing before you start. On a six-cylinder car with three SU carburettors, each carburettor feeds two cylinders, so a carburettor fault shows up as two adjacent cylinders running badly rather than one. Two weak cylinders next to each other is a carburettor pattern, not an ignition one.

Answer this

• One cylinder, or two next to each otherGoes to: Is there a good spark at that cylinder?
• All of them. The engine is rough everywhereGoes to: Rough everywhere. What do the plugs look like?
• I do not know how to tellGoes to: Finding out which cylinder

Question

It only misbehaves when hot

A running fault that appears with heat and clears when the engine cools belongs to the same family as a car that will not restart hot, and the archive treats them together because the components and the tests are the same.

Answer this

• Take me to the hot fault diagnosisThe ignition component test and the vaporisation branch both live there.
• It also affects starting when hot

Finding

Bad when cold, fine when warm

Why this follows. A cold engine needs a richer mixture than a warm one. A fault that clears as the engine warms is usually the enrichment arrangement failing to provide that, or the engine never reaching its proper temperature.

What to do

• Check the choke or enrichment mechanism actually operates, and operates fully.
• Check the fast idle setting associated with it.
• Check the thermostat. An engine running cold because the thermostat is stuck open never gets to the point where it should be running on a warm mixture, and the fault appears to persist longer than it should.
• Check the mixture is not set weak overall, which is tolerable warm and not tolerable cold.
• Confirm the temperature gauge is reading correctly before concluding the engine runs cold, using the gauge check in the overheating diagnosis.
• On the Series 3 V12, check both banks are enriching, since a fault confined to one bank gives a rough cold start that clears unevenly.

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

Finding

It came on suddenly

Why this follows. A fault that appears from one journey to the next is a discrete failure rather than gradual wear, which narrows the list considerably. Wear is progressive. Components fail.

What to do

• Check for a high tension lead that has come off or chafed through.
• Check the plugs, since one can fail outright.
• Check the condenser, which fails suddenly and completely rather than gradually.
• Check the rotor arm, which can crack or track without warning.
• Check for water in the fuel or in the distributor cap, particularly after the car has been washed or driven in heavy rain.
• Check for a blockage in the fuel system that has moved into place, such as sediment disturbed by a low tank.
• Check anything that was recently worked on, however unrelated it seems. A fault that appears immediately after work is usually related to that work.

Read further: The ignition system, Modern fuel and E10

Finding

An idle that hunts

Why this follows. Hunting is a mixture that is marginal at idle. The engine leans out, slows, draws more fuel, speeds up, and repeats. It is characteristic of an air leak or of carburettors that are not balanced against each other.

What to do

• Check for air leaks systematically, since this is the commonest cause. Manifold and flange gaskets, the servo hose, and the vacuum advance pipe.
• Check the throttle spindles for wear in their bores.
• Check the carburettors are balanced against each other. On three SUs, one carburettor doing more work than the others gives an unstable idle even when each is individually set correctly.
• Check the throttle linkage returns to the same place every time, and that nothing binds.
• Check the idle mixture setting on each carburettor, after balancing rather than before.
• Check the ignition timing at idle, including whether the vacuum advance is behaving as it should at idle.

Read further: Triple SU carburettors, The ignition system

Finding

It will not idle at all

Why this follows. An engine that runs with the throttle open and dies when it closes has either too little fuel, too much air, or a spark badly timed for that condition.

What to do

• Check the throttle plates are actually closing to their stops, and that the linkage is not holding them open or shut.
• Check for air leaks as in the hunting branch.
• Check the idle mixture is not set far too weak.
• Check the jets are not blocked.
• Check the ignition timing, and whether the vacuum advance connection is on the correct port, since a vacuum advance fed from the wrong side of the throttle behaves the opposite way at idle.
• Check the points gap and condition, since a closed-up gap gives a weak spark that fails first at low speed.

Read further: Triple SU carburettors, The ignition system

Finding

The idle is too high

Why this follows. A high idle is almost always mechanical: something is holding the throttle open, or air is bypassing the throttle entirely.

What to do

• Check the choke or enrichment mechanism has fully returned, including its cable and the fast idle arrangement.
• Check the throttle linkage and its return spring for binding, and check the cable is not too tight.
• Check the throttle stop screws have not been wound in to mask a different fault, which is a common find on a car with an unknown history.
• Check for an air leak large enough to raise the idle, which is the same fault family as hunting but further along.
• Check the ignition timing is not over-advanced, which raises idle speed.

Read further: Triple SU carburettors, The ignition system

Question

Is there a good spark at that cylinder?

With the cylinder identified, there are only three things it can be: spark, fuel, or compression. Establish the spark first, because the test is quick and it is free.

Answer this

• No spark, or a weak one, at that cylinderGoes to: No spark at one cylinder
• A good spark at that cylinderGoes to: Good spark at one weak cylinder

Question

Rough everywhere. What do the plugs look like?

When every cylinder is affected, the fault is common to all of them. The plugs are the cheapest evidence available and they read the mixture directly.

Answer this

• Sooty and black, and it smells of unburnt petrolGoes to: Running rich on every cylinder
• White, bleached or blisteredGoes to: Running weak on every cylinder
• They look about rightGoes to: Plugs look right, engine still rough

Finding

Finding out which cylinder

Pulling plug leads with the engine running carries a shock risk, and on a Series 3 V12 it can damage the Lucas OPUS amplifier. The archive does not recommend it. Use the two methods below instead.

Why this follows. You cannot diagnose a misfire without knowing whether it is one cylinder or all of them, and there are two ways to find out that do not involve pulling live high tension leads.

What to do

• Read the plugs. Run the engine to temperature, stop it, and remove all the plugs, keeping them in cylinder order. A cylinder that is not firing properly gives a plug that looks clearly different from its neighbours, wet, sooty, or unusually clean where the others are coloured.
• Measure the exhaust manifold branches. Run the engine to temperature and measure the temperature of each branch close to the head with an infrared thermometer. A cylinder that is not contributing runs its branch noticeably cooler than the others. This is a comparison between branches on the same engine, not a figure to work to.
• Note which cylinders are affected and whether they are adjacent, because adjacency points at a shared carburettor or a shared inlet tract rather than at ignition.
• On the Series 3 V12, note whether the affected cylinders are all on one bank, which points at something serving that bank rather than at an individual cylinder.

Where next

• It is one cylinder, or two adjacent onesGoes to: Is there a good spark at that cylinder?
• It is all of themGoes to: Rough everywhere. What do the plugs look like?

Read further: The ignition system, Triple SU carburettors

Finding

No spark at one cylinder

Why this follows. A spark that is good at every other cylinder and absent at one has failed somewhere between the distributor cap segment and the plug, because everything upstream is shared with the cylinders that are working.

There is a definitive free test here. Swap the plug and then the lead with a neighbouring cylinder that is running properly, one at a time. If the fault moves with the part, you have found it. If the fault stays with the cylinder, the part is innocent and the cap or the cylinder itself is the suspect.

What to do

• Swap the plug with a good neighbour and see whether the fault moves.
• Swap the high tension lead with a good neighbour and see whether the fault moves. Old leads break down internally and often only under load or when hot.
• Inspect the distributor cap segment for that cylinder, and hold the cap up to the light looking for a hairline carbon track between segments.
• Check the plug gap and the plug's condition, and whether it is the correct type for the engine.
• On the Series 3 V12, check the lead routing, since leads that have been rerouted so they run alongside one another can cross-fire.

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

Finding

Good spark at one weak cylinder

Why this follows. With spark and fuel arriving, a single weak cylinder is usually a compression problem, and on this engine there is a specific and unglamorous cause worth checking before anything expensive.

The XK engine sets its valve clearances with shims under bucket tappets, which means adjusting them requires removing the camshafts. Because that is a real job rather than a roadside one, clearances get neglected. A clearance that has closed up holds its valve slightly open, the valve cannot shed heat into its seat, and it burns. A single burnt exhaust valve is a common end point of a neglected clearance.

What to do

• Carry out a compression test on all cylinders and compare them against each other. What matters is the spread between cylinders, not any single figure.
• Repeat the test wet, with a little oil in the cylinder. If compression rises markedly, the loss is past the rings. If it does not, the loss is past a valve or the head gasket.
• Check the valve clearances if there is any doubt about when they were last done, and be aware this means removing the camshafts.
• If two adjacent cylinders are affected on a six-cylinder car, check the carburettor feeding that pair rather than the cylinders, including its jet, needle, float level and throttle spindle.
• Consider a head gasket failure between two adjacent cylinders, which gives two weak neighbours with no carburettor fault.
• On the Series 3 V12 the carburettors each feed a group of cylinders rather than a pair, and the archive has not established the exact grouping, so confirm it against the workshop manual for the car before reasoning from adjacency.

Read further: Head gasket failure, Timing gear and top-end noise, Triple SU carburettors

Finding

Running rich on every cylinder

Why this follows. A mixture that is too rich on every cylinder points at something shared: the enrichment device, the fuel level in the float chambers, or a restriction in the air side rather than the fuel side.

What to do

• Check the choke or enrichment mechanism returns fully. A cable that has stretched or a linkage that binds leaves partial enrichment applied permanently.
• Check the air filter. A blocked filter richens the mixture across the board.
• Check the float levels and the float chamber needle valves. A high level richens every jet fed from it.
• Check for worn jets and needles, which pass more fuel than they should.
• On Zenith Stromberg carburettors, check the diaphragms. A punctured diaphragm changes the way the piston rises and upsets mixture.
• Check the ignition timing before condemning the carburettors, since retarded timing produces unburnt fuel and sooty plugs without any fuelling fault at all.

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

Finding

Running weak on every cylinder

Why this follows. A weak mixture everywhere is either fuel that is not arriving in sufficient quantity, or air that is arriving where it should not.

Air leaks are the more common of the two on a car of this age, and worn throttle spindles are the classic example. The spindle wears its bore in the carburettor body, air is drawn in around it, and the effect is worst at idle when manifold depression is highest.

What to do

• Check for air leaks at the inlet manifold gaskets, the carburettor flange gaskets and any spacer.
• Check the throttle spindles for wear by trying to move each one sideways in its bore.
• Check every vacuum connection, including the brake servo hose and the distributor vacuum advance pipe. A perished servo hose leans the mixture and is easy to overlook.
• Check fuel delivery can keep up: the filter, the pump and the hoses.
• Check the jets are not partly blocked, and that the needles are the correct ones for the engine.
• Check the fuel itself if the car has stood, since stale fuel behaves like a weak mixture.

Read further: Triple SU carburettors, Brake hydraulics and the servo, Modern fuel and E10

Finding

Plugs look right, engine still rough

Why this follows. If the mixture is reading correctly on every cylinder and the engine is still rough, the fault is in when the spark happens, or in the mechanical condition of the engine.

What to do

• Check the ignition timing, both static and with the engine running, and check the mechanical advance weights in the distributor are free rather than seized.
• Check the vacuum advance is working and that its pipe is intact.
• Check the points, the condenser, the cap and the rotor.
• Check the distributor drive and shaft for wear. A worn shaft lets the points gap vary as the engine turns, which upsets timing differently on each cylinder.
• Carry out a compression test across all cylinders. Compression that is uniformly low points at general wear or at valve clearances, rather than at any one cylinder.
• On the Series 3 V12, check the Lucas OPUS system before assuming a mechanical fault.

Read further: The ignition system, Lucas OPUS ignition on the V12, Timing gear and top-end noise