Thermostat

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eddypeck
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Thermostat

Postby eddypeck » Tue Oct 11, 2016 11:02 am

The thermostat controls the coolant temperature by regulating coolant flow to the radiator. A thermostat stuck open will cause the engine to warm up slowly and run below normal temperature at highway speed. A thermostat stuck closed will restrict coolant flow to the radiator and cause overheating.

Replacement is only necessary if the thermostat is faulty. In addition to the tools required for draining the coolant, a wrench or a socket is required to remove the thermostat housing. Use a new O-ring, Volkswagen Part No. 059 121 119, when the thermostat housing is reinstalled.

Be prepared to change the water pump and thermostat housing too.

The water pump is aluminium and by design spends its life filled with water. Aluminium may not rust but corrosion is still an issue, especially when aluminium is in contact with steel. There's a strong chance of at least one of the steel housing bolts shearing due to chemical reaction between the corrosion of the aluminium and steel - I swear this combination is stronger than welding!

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At time of writing the new water pump is about £30 so it's hardly worth the effort of trying to remove the bolt.

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The thermostat housing itself is usually plastic (on some models you may be luck enough to find an aluminium housing) and is likely to warp from prolonged exposure to heat. It usually poses no problem when until you come to remove it, as it's unlikely it will seal properly once refitted.

It's recomended that genuine VW parts are used for the thermostat and housing, and always fit with new seals. Check the seals come with the new parts and if not make sure you get them at time of purchase, it's a pain to have it all apart on the drive only to discover you're missing a crucial o-ring.
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mk2daz
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Thermostat

Postby mk2daz » Tue Oct 11, 2016 11:57 am

I'm wondering if I have a stuck open thermostat.. in my case it doesn't take a huge amount of time to warm up but the running temp (according to the on board MFA) peaks at 92, it will push over that a tiny bit during motorway driving. Also my temp needle never hits past the white block on the gauge. I have a 1991 8v digifant. The engine never stopped overheats but I'm worried it's 'over cooling' slightly. Are the thermostats and housing etc available from VW still?


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eddypeck
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Re: Thermostat

Postby eddypeck » Tue Oct 11, 2016 12:06 pm

Certainly sounds like your thermostat isn't working properly, and yes you should be able to get genuine parts from VW or TPS.
Euro car parts, GSF and others will also sell non-genuine parts which should be OK if you stick to reputable OE quality brands (Febi, Meyle, Lemforder, Hella etc.) and not the budget option.

VW Spares http://www.vwspares.co.uk/g2cooling.php
and VW online http://www.vwsonline.co.uk/car_parts_br ... ne/cooling
are also both worth a look.

It's not a bad job to do. Slightly more involved if you'be got power steering.
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mk2daz
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Re: Thermostat

Postby mk2daz » Tue Oct 11, 2016 12:37 pm

Thanks Tim, I presume there is no way of telling it is actually stuck open without physically taking it out and inspecting?


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eddypeck
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Re: Thermostat

Postby eddypeck » Tue Oct 11, 2016 1:04 pm

mk2daz wrote:Thanks Tim, I presume there is no way of telling it is actually stuck open without physically taking it out and inspecting?


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In theory, with the engine running hold the bottom rad hose. This should remain cold until the car is up to temp and when the thermostat opens the bottom hose will suddenly get hot. So if the bottom hose gets warm gradually I suppose that would indicate the it's stuck open and if the bottom hose isn't warm in 10 mins then I'd say it's stuck shut.
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TonyB
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Re: Thermostat

Postby TonyB » Tue Oct 11, 2016 1:07 pm

A thermostat and a new housing is about a tenner from VWH (and you'll get a spare o-ring as both parts come with one)...

Testing it does involve taking it out, sticking it in water and heating it up (and then measuring its dimensions at different temps). For a tenner I just put in new kit...
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eddypeck
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Re: Thermostat

Postby eddypeck » Tue Oct 11, 2016 1:12 pm

TonyB wrote:A thermostat and a new housing is about a tenner from VWH (and you'll get a spare o-ring as both parts come with one)...

Testing it does involve taking it out, sticking it in water and heating it up (and then measuring its dimensions at different temps). For a tenner I just put in new kit...



My thoughts exactly, and a very worthwhile job once done. Always worth testing the new one by boiling it in a pan before you fit it, occasionally you can get a dud.
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