Ownership / Diagnostics
Triumph Stag troubleshooting: won't start, overheating and faults
By Jon Cosson
Answer a few questions about what the car is doing and the troubleshooter will point you at the most likely area, then read the reference below for the detail.
Interactive troubleshooter
Step 1 of 2
Question: What is the car doing?
What is the car doing?
Common Triumph Stag problems
The Stag's reputation was made and marred by its 3-litre V8, and most of the faults owners meet are well known and fixable. This page starts with the troubleshooter above and sets out the common problems in more detail below.
Overheating
This is the defining Stag issue. The V8's mixed aluminium and iron construction and its marginal original cooling mean overheating must be taken seriously, because a badly overheated engine can warp the alloy heads. The causes range widely: a tired or undersized radiator, a failing water pump, a stuck thermostat, an old or wrong coolant without the right corrosion inhibitor, retarded ignition timing, a weak mixture, leaking intake-manifold gaskets, or an air leak at the carburettors. The common cures are a good radiator, an uprated or correctly-wired fan, annual coolant changes with the right inhibitor, and correct timing.
Hot starting
A Stag that starts from cold but refuses to restart when hot is usually suffering fuel vaporisation in the Stromberg carburettors over the hot engine, a faulty temperature compensator, or a heat-affected ignition coil or module. Cranking with the throttle held open often gets it going while you address the cause.
Ignition
The Stag left the factory with twin contact points and a condenser, needing a correct dwell of around 26 to 28 degrees, with the condenser a favourite suspect; many cars now run electronic ignition. Timing drift and a heat-affected coil or module are the usual causes of a misfire.
Carburettors
The Stag runs twin Zenith-Stromberg 175 CD carburettors, with the diaphragms, the temperature compensators and the O-rings at the carburettor base the usual points of wear and air leaks. The twin electric fuel pumps live in the boot and should tick when the ignition is switched on.
Electrics and charging
The Stag is 12-volt, negative earth, with an alternator. A charging fault is usually the drive belt, the alternator or the wiring, while a completely dead car is often down to a poor earth.