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On the streets the temp is normal: it has new radiator, head gaskets and brand new thermostat. What is causing this change in temperature. It gets so hot it's almost to the very top close to the H, but as soon as I slow down and drive on the regular streets it starts to cool down to normal.

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    What kind of car? Make/brand/model? – Pᴀᴜʟsᴛᴇʀ2 Aug 9 '14 at 11:19
  • Are you losing any coolant? – DFord Aug 9 '14 at 12:49
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Check your main coolant hoses going into and out of the radiator. Do this by allowing the engine to be fully warm, then while looking at these hoses, rev the engine up to around 2000-2500 RPM (or have someone do it for you while you observe the hoses). What you are looking for is to see if one of them collapses. What happens is, as hoses get old, they weaken and can collapse under higher engine speeds. The water pump has enough of a draw on the fluids it can collapse the weak hose, causing a lack of fluid flow through the engine, and thus overheating. As soon as the engine speed decreases, the hose goes to normal and cooling resumes.

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    Good call - I'd suspect the lower radiator hose before the upper, since the upper returns water TO the radiator while the lower draws water FROM the radiator to the water pump's intake. – TDHofstetter Aug 9 '14 at 14:43
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You could have a cracked head or block on your engine. You may have a faulty water pump. Are all of the hoses fitted correctly? From all the work you have had done it is obvious you have a continuing cooling system problem which has not been diagnosed. Take the vehicle back to the garage that done the repair work and explain to them the vehicle is still running hot and when it does so.

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