Fault Finding

1. Radiator core clogged with electrolytic sludge (dissimilar-metal corrosion between iron block and alloy heads)
2. Wrong or old coolant, inhibitors exhausted, corrosion accelerating
3. Casting sand still restricting water passages in the block (original factory defect)
4. Failing or cavitating water pump losing flow at higher rpm
5. Missing or removed thermostat masking a deeper cooling fault

• Feel the radiator top and bottom hoses with the engine hot, the whole core should be hot, not just the top tank. A cool bottom means poor flow through the core.
• Look into the expansion tank with the engine running warm: continuous bubbling suggests combustion gases in the coolant, a failing head gasket.
• Check coolant colour and pH. Rusty, brown or clear-with-sediment coolant is exhausted and no longer protecting the alloy heads.
• Verify the fan is cutting in at the correct temperature; a lazy electric fan or a slipping viscous coupling will let the car cook in traffic.

Full cooling-system overhaul: flush and reverse-flush the block to shift casting sand, fit an aluminium radiator of adequate core, renew all hoses, fit a proper high-flow or electric water pump, add an electric fan, refill with a good-quality inhibited (OAT-suitable) coolant and put an annual flush on the calendar. This is the single most important package of work on any Stag.

A competent DIY owner can flush, refill and swap hoses. Persistent overheating after that: especially with bubbles in the expansion tank, is head-gasket territory. Stop and see a Stag specialist; running on will warp the alloy heads past their 0.010″ skim limit.

1. The factory temperature sender reads from one cylinder head only, the other bank can overheat unseen

• With the engine fully warm, use an infrared thermometer on both cylinder heads and both exhaust manifolds. A meaningful temperature split between banks is your proof.
• Confirm the sender itself and the gauge are reading correctly by comparing against the infrared reading on the sensed bank.

Fit a second temperature sender to the opposite head, feeding either a warning switch or a second gauge, so both banks are monitored. This is a standard modern-fixes addition and eliminates the failure mode where the driver sees a normal gauge until damage is done.

Fitting a second sender is within reach of a careful DIY owner. If either bank is already running significantly hotter than the other with the cooling system in known-good order, stop driving and get a specialist to investigate, a hot bank hides warped heads or blocked passages.

1. Stretched single-row timing chains, the V8 uses two, one per bank
2. Worn chain tensioners losing pressure when the engine is off

• Listen at the front of the engine on a cold start. A rattle that clears within a few seconds as oil pressure comes up is the classic worn-chain signature.
• Compare to service history: chains are treated as a service item on a Stag and are typically renewed every 25,000–30,000 miles.

Renew both timing chains, tensioners and guides, always as a set, never one side. Because the V8 is an interference engine, a snapped or jumped chain bends valves and often means a rebuild. Do not defer this job on the strength of it 'quietening down when warm'.

Chain replacement is a serious front-of-engine job on the Stag V8 and is almost always specialist work. If the rattle persists once warm, or the engine has run rough at any point, assume valve/piston contact and get the engine to a Stag specialist before running it further.

1. Coolant emulsifying with engine oil, most commonly a failed head gasket following alloy-head warping from an earlier overheat
2. Persistent short-run/cold-only use causing condensation (much less serious, but must be ruled out)

• Check the coolant level and colour: dropping level, or oily film on the expansion-tank surface, points to head gasket.
• With the engine warm and idling, look for continuous bubbles in the expansion tank, combustion gases in the coolant.
• A specialist sniff test or block test on the expansion tank will confirm combustion gases without removing the head.

Confirm the diagnosis before opening the engine. If it is a head gasket, both heads should be lifted, checked for flatness (skim limit 0.010″ total), and refitted with new gaskets, new studs and a properly overhauled cooling system, otherwise the failure will simply repeat.

This is specialist work from the point of diagnosis onwards. Removing seized head studs from a Stag head is a well-known workshop pitfall and easy to make catastrophic; do not attempt heads off as a first-time DIY.

1. Electrochemical seizure of the steel studs into the alloy heads, aggravated by neglected coolant
2. The factory's two-angle stud design, which makes withdrawal particularly prone to binding and shearing

• Confirm every nut is off and every ancillary is clear before applying any lifting force to the head.
• Penetrating oil down each stud, worked in over days with heat cycles, before any attempt to lift.

Patience, heat, penetrating oil and, where studs shear, careful drilling and helicoiling in the head. On rebuild, fit uprated studs of a better material and correct grade, and never reassemble the engine on the original coolant regime.

Sheared studs deep in an alloy head are a specialist recovery job. A snapped stud pulled crookedly out of the head can scrap it. If a stud will not move with reasonable heat and penetrant, stop and take the head to a specialist rather than force it.

1. Worn cam followers (tappets) and their bores in the heads, a well-known Stag top-end wear point
2. Valve clearances out of adjustment, the cheaper, more benign cause; rule it out first
3. Worn camshaft lobes on a high-mileage or poorly-oiled engine

• Listen at the top of each cylinder head at warm idle. A tapping that stays once warm, as opposed to the front-of-engine chain rattle that fades, points to the top end.
• Have the valve clearances checked and set to the workshop-manual figures first; a surprising amount of 'tappety Stag' noise is simply clearances.
• If the noise remains after clearances are correct, suspect the followers and their bores.

Set the valve clearances correctly first. If the tapping persists, the heads come off to renew the cam followers, and the camshaft too if the lobes are worn. Parts are available; the labour is the cost. Sort the cooling system at the same time, as heat is what wears these engines.

Clearance adjustment is within careful DIY reach with the manual to hand. Heads-off cam-follower or camshaft work is specialist territory on the Stag V8, with the same seized-stud pitfalls as any heads-off job.

1. Laycock overdrive not engaging on the manual car (electrical, solenoid or hydraulic fault within the overdrive unit)
2. Automatic car doing what it does, the Borg-Warner 3-speed has no overdrive, and the car simply runs at higher revs than a modern equivalent

• On the manual car, confirm the overdrive switch, relay, inhibitor switch (only engages in top gears) and solenoid all operate. A click on switch-on and a noticeable drop in revs at a steady throttle is the confirmation.
• Check the oil level in the gearbox/overdrive unit; low oil starves the overdrive's hydraulic operation before the gearbox itself complains.
• On the automatic, verify this is simply the character of the 3-speed BW auto rather than a fault before spending money.

For a manual with a non-engaging overdrive: work the electrical circuit first (switch, relay, inhibitor, solenoid), cheap and often the answer. If oil and electrics are known good and the unit still won't engage, the overdrive itself needs a specialist rebuild.

Electrical fault-finding on the overdrive circuit is well within DIY. Anything inside the overdrive unit itself, solenoid replacement excepted, is specialist work.

1. The sliding-spline driveshafts seizing when the splines have not been kept greased, the classic cause of the Stag 'twitch' with worn universal joints making it worse
2. Worn rear suspension bushes contributing to rear-axle steer

• With the car safely raised, grasp each half-shaft and check for radial and axial play. Any perceptible play in a UJ is a fail.
• Inspect UJs for dry, rusty needle rollers and for the tell-tale orange dust around the caps.
• Inspect rear suspension bushes for cracking, distortion and voiding.

Keep the driveshaft splines greased, that alone holds the twitch at bay. Renew any worn universal joints, and the accepted modern upgrade is to convert to CV-jointed rear half-shafts, which removes the twitch entirely. Refresh the rear suspension bushes at the same time while everything is apart.

UJ replacement is within DIY with the right press. The CV half-shaft conversion is best fitted by, or with parts specified by, a recognised Stag specialist to ensure the correct part is used for the differential and hub combination on the car.

1. Worn or maladjusted contact points and a tired condenser
2. Ignition timing incorrect, note the Stag is timed on number-two cylinder, not number one, which catches people out
3. Perished HT leads, a cracked distributor cap or a worn rotor arm
4. A tired coil breaking down when hot

• Set the points gap / dwell and the timing to the manual figures, remembering the Stag's timing reference is cylinder two.
• Inspect the cap, rotor and HT leads for cracks, tracking and corrosion, and read the plugs for a cylinder or bank running wrong.
• If the misfire appears only when hot, suspect the coil or condenser breaking down with heat.

Service the ignition as a set, points (or fit electronic ignition), condenser, cap, rotor, leads and plugs, and set the timing correctly on cylinder two. Many owners fit electronic ignition (such as a Lumenition kit or a 123 distributor) to remove points maintenance and sharpen running; it is one of the standard reliability upgrades.

Ignition servicing and electronic-ignition fitting are well within DIY. If the engine still misfires with a known-good ignition system, move on to fuelling or compression before spending further.

1. Twin Zenith-Stromberg 175 CD carburettors out of balance, or with worn needles and split diaphragms
2. Ethanol (E10) petrol degrading rubber seals, diaphragms and hoses, and drawing in water
3. Incorrect float levels or a sticking float needle
4. Fuel vaporisation from engine-bay heat soak after a run

• Balance the two carburettors and check the diaphragms for splits and the needles for wear.
• Confirm the fuel hoses and seals are ethanol-resistant specification; perished or swollen rubber is a giveaway of E10 damage.
• Check float-chamber levels, and investigate hard hot-starting as heat soak / vaporisation before condemning the carbs.

Balance and, where worn, overhaul the Zenith-Strombergs with new diaphragms, needles and seals. Fit ethanol-resistant fuel hose throughout, and use an ethanol-protection additive or grade for a car that sits, see the E10 fuel guide for the full picture.

Carb balancing and overhaul are satisfying DIY jobs with the right gauge and kit. A fuelling fault that survives a proper overhaul may be a worn spindle or an air leak better traced by a specialist.

1. Bad earths, the classic Stag culprit is a corroded rear light-cluster earth, but any fifty-year-old earth point can be the problem
2. Aged, brittle wiring and corroded bullet and spade connectors
3. Tired switches and a fusebox with corroded contacts

• Work every circuit methodically, lights, indicators, wipers, heater, horn, and note whether failures share a common earth.
• Clean and remake suspect earth points back to bright metal; a huge proportion of 'Lucas gremlins' are simply earths.
• Measure voltage drop across old connectors and switches rather than assuming the component has failed.

Clean and remake the earth points first, it fixes more Stag electrical faults than any other single action. Renew brittle wiring and corroded connectors, and where the loom is beyond help a good-quality replacement harness is available and transformative.

Almost all of this is DIY with a multimeter and patience. An intermittent fault that resists systematic checking, or any sign of heat-damaged wiring, is worth an auto-electrician's time before it causes a fire.

1. Seized or sticking front caliper pistons after long standing
2. Perished flexible brake hoses, ballooning or restricting flow
3. Old, moisture-laden brake fluid reducing performance and corroding internals
4. Leaking rear wheel cylinders, or a tired brake servo

• Check for even braking and for a caliper or drum running hot after a drive, which indicates a binding piston or shoe.
• Inspect the flexible hoses for cracking, bulging and age, they perish internally where you can't see.
• Note the fluid's age and colour; dark fluid is overdue a change. Look behind the rear drums for weeping wheel cylinders.

Overhaul or renew seized calipers and leaking wheel cylinders, fit new flexible hoses, and flush through fresh fluid. Confirm the servo holds vacuum. Brakes are not the place to defer work on a heavy, quick classic.

Caliper, cylinder and hose work is within competent DIY. If in any doubt about your own brake work, have it inspected, this is the one system where a mistake is unforgiving.

1. Age and trapped moisture in the Stag's known rot spots, inner and outer sills, A-post bases, inner and outer wings, the scuttle, the boot floor and the chassis outriggers
2. Blocked drain channels letting water sit where it shouldn't
3. Late (post-1974) cars are often said to rust harder, a change of steel supplier usually blamed

• Inspect the structural areas first, inner sills, A-posts and outriggers, with a torch and, where sound allows, a light tap; filler and fresh underseal are questions, not answers.
• Lift the carpets and boot mat to see the floors directly.
• Clear and test the scuttle and other drain channels so water runs away rather than pooling.

Structural rust must be cut out and repaired with correctly-shaped panels and proper welding, never filler. Treat inside the cavities with a corrosion inhibitor afterwards and keep the drains clear. Budget realistically: bodywork is where Stag restorations overrun.

Non-structural patch repairs suit a competent welder. Sills, A-posts, outriggers and floors are structural and MOT-critical, have them done properly, by someone who knows the Stag, if you are not confident.