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I am trying to diagnose an issue with my Mk1 Golf. The engine heats up when the car is idling or in traffic but cools down almost instantly when I start moving again. I would presume this is the electric thermo fan not kicking in but it is working. I'm assuming the pump is working too as the car cools when driving, so I can believe liquid is circulating.

Could it be a clog in system that limits flow when the cars idling? Would anyone know what could be causing this?

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    Is the fan shroud in place and fully against the radiator (no big gaps)? – Pᴀᴜʟsᴛᴇʀ2 Sep 4 '14 at 1:38
  • Is the fan spinning at full speed? If the connections are poor or dirty it might be running slower than it ought to, and so still appear to be working, but not pushing enough air through... Also, is the radiator clear and in good condition? – Nick C Sep 4 '14 at 9:17
  • @Paulster2 sorry for the late reply, yes the fan shroud is in place. There is a 8mm or so gap running along the top where the shroud meets the top of the radiator but it looks like its meant to be there. – MeltingDog Sep 8 '14 at 1:39
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The two main things that would happen to engine with results as you describe. Easiest to check is the thermostat. The thermostat has a pin coming out of it which locates into the thermostat housing. The location of the pin breaks off and allows the thermostat to float, causing erratic temperature control. The second item is the water pump impeller. They are made of plastic and sometimes come adrift, or slip on their shafts, or even break up entirely causing poor water circulation. One tip, when filling the system with coolant, remove the slightly higher heater hose and fast idle the engine. (You can catch most of the water into the removed hose.) Keep the hose off, as such, until the water comes out of the heater clear of all air, then refit the hose. In passing my son went to Uni at Griffin Cold Coast.

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