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Engine fault on Mazda 6

Mazda 6 55 plate 140k miles full service history. On longer drives 80 miles+ towards end as I speed up 75+ engine temp gauge falls towards cold. Slow down 65-70 temp gauge rises back to normal, speed up drops again. Any ideas why it is doing this please?

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Cold?

Hi, Matt; That symptom is very odd, I have to admit. Just opposite of normal. LOL. Only a guess, here, but could the thermostat be sticking open? If so, though, I would think that it would take a long time for the engine to reach normal operating temp?
Good luck.
Loren

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Reply to Loren

Thanks Loren. Yes it makes no sense as naturally expect engine to get hotter not colder as speed increases and it only starts happening after engine has done a decent amount of miles so towards end or final third of journey. Checked oil again after trip back from Ipswich and oil level was fine so had not burned oil which I suspected was happening. and temperature was fine on a shorter drive with average speed closer to 70 than 80 for most of the journey due to roads and traffic, when stuck in lots of stop start traffic for an hour due to road closure temp gauge did not alter from normal only when I got free of traffic and started to speed up to 80+. One idea is a problem with the air filter or even carberetta as maybe air is getting in or not so cooling the engine too much. Car is getting a bit older but is fully serviced every year and was done in November. Just having to monitor the situation I guess and maybe wait until at even lower speed temp gauge drops right down to cold and have to stop and wait for the AA to come and sort it out. If not trip to garage to ask them to scratch their heads and have a think. Thanks for you reply.

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Cold

Hi, Matt; Thanks for the reply. Your Mazda should be fuel injected, but regardless, air entering the engine shouldn't have any effect on the temp. Running too cool will ruin the fuel economy and in long term, could cause internal engine damage from running too rich air/fuel mix. Are you in a cold climate, or just because it's not summer, yet? You could try covering the lower 1/4-1/3 of the radiator with cardboard like the truck drivers do in the winter time, but you'd have to watch the temp like a hawk! Once you find the answer, I'd sure like you to let me know. Very curious on this one.
Loren

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Reply to Loren

Hi Loren I am not in a cold climate but in the South of England. Do not like the idea of covering the radiator as temp is fine on normal journeys it seems at present at least. One other post said check rad fluid so will do this I guess either engine is just getting old or maybe thermostat is developing a fault, will keep an eye on fluid levels, oil levels and temp gauge as a result. Next long run is at the weekend two journeys over 150 miles each with lots of motorway driving so another chance to see if fault is still present and if so will have to take more actions

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The cover on the radiator is

Done as a test to see what happens. I like the ideas so far (thermostat) and would get that changed out first as it's cheaper than most other repairs.

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(NT) Check your radiator fluid

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