Quick Tech

Ther­mo­stat swap: it’s a sim­ple and easy DIY task.

Classics Monthly - - Contents -

What does it do?

As its name sug­gests, the ther­mo­stat in a car en­gine opens and closes ac­cord­ing to tem­per­a­ture and reg­u­lates the flow of wa­ter around the en­gine. In its most ba­sic op­er­a­tion it al­lows the en­gine to reach op­er­at­ing tem­per­a­ture quickly by not pass­ing wa­ter through the en­gine radiator when the en­gine is cold. As the en­gine warms up, it opens grad­u­ally, al­low­ing wa­ter to cir­cu­late through the en­tire sys­tem.

What goes wrong?

A tra­di­tional ba­sic ther­mo­stat works us­ing heat and the prin­ci­ple of ex­pan­sion. A wax can­is­ter heats up with the coolant and pushes against a metal disc in­side the unit which blocks the flow of wa­ter. A spring in­side the unit al­lows it to fail ‘safe’ in the open po­si­tion. This pre­vents over­heat­ing but also means that the en­gine will take for­ever to warm up. Also, in many cases the ther­mo­stat will open and close dur­ing reg­u­lar runinng, es­pe­cially in the Bri­tish win­ter so if it fails the en­gine never reaches full op­er­at­ing tem­per­a­ture. They can also fail the other way and jam shut with the re­sult that the en­gine starts run­ning hot.

How do I know I need a new one?

If the heater doesn’t seem to be kick­ing out the warmth it used to, then check the tem­per­a­ture gauge if you have one: is it sit­ting slightly lower than it usu­ally does? Also feel the radiator hoses by hand for heat.

How hard is it?

It’s one of the sim­pler jobs in car main­te­nance. In most older cars the ther­mo­stat is ac­ces­si­ble and is lo­cated in­side a hous­ing which can be un­bolted to re­move the unit. You’ll lose a bit of coolant do­ing the job, so you’ll need some­thing to catch it and some fresh anti-freeze to top up, but it’s a sim­ple job in most cases.

How long will it take?

If ev­ery­thing goes well it re­ally can be a five-minute fix. Even with a few prob­lems, it’s usu­ally a 30-minute task at most.

Any tips and tricks?

Take it easy if there’s an al­loy cast­ing – you don’t want to be snap­ping bolts and studs. Spray plenty of pen­e­trat­ing fluid on them first. And some cars like ’90s and newer BMWs, will have an elec­tronic ther­mo­stat which in­ter­faces with the en­gine management. This means you need to swap the en­tire plas­tic hous­ing. When top­ping up the sys­tem, be care­ful not to get air­locks - the work­shop man­ual will tell you how to bleed it.

Above left and cen­tre: modern elec­tronic 'stat and hous­ing. Above right: the unit from a 4-litre Jaguar en­gine.

Tra­di­tional ther­mo­stat is usu­ally to be found in­side a hous­ing with a re­mov­able cover. It's a ba­sic DIY job.

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