Coolant temp up and down... but new gen stat

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M7R
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Joined: Sat Jul 14, 2012 1:55 pm

Coolant temp up and down... but new gen stat

Post by M7R » Thu Mar 12, 2020 11:58 am

Had a fun issue with the Polo, it started to show a stat failure so I changed the stat... all good in the hood, temp holds steady at 90....fast forward about 6 months, and it starts again with the temp dropping on longer higher speed runs, (50mph or so cruise and if the outside temp was down around 0-5deg the temp gauge would head for the bottom again, and it got to the point the car was starting to run rough as it was over fueling so much.

Fitted a new stat couple of weeks back that was meant to be for GTI, but turns out its was 87deg not the 80 deg of a GTI... lasted a week and then I took it out as it wasnt right, the car never got warm and ran like a dog due to over fueling.

Last weekend I fitted a new gen thermostat (the bridge in the bottom hose fixing was all OK, made sure I checked that). filled up with coolant and ran for about 30 mins till the fan kicked in, all good. Left to cool, checked the level and it was still ok. This was with the heater in the car on full hot, and it did get nice and warm, and some running at 2000 rpm or so for 20-30 seconds to get things flowing.

Day or so later other half took the car on a 30 mile trip, she said the temp was steady at 90 after a short while, then had a wobble down and up a couple of times to 70, and back to 90... then steady again, and was steady on the way back too.

Checked the coolant level and it had dropped a little so topped back up. All good I thought.

This morning on the 5 mile trip to work, after a mile or two of 30mph-40mph up and steady at 90, deg... a mile or so later there is a short 50 section, and as soon as we are on that temp drops to 70 again, and then goes back up as we slow down for the island about 800m or so away, after that it held at 90 again down the next mile of 50 and into works site.

Is it likely it is still clearing the air locks?, The coolant that has come out has always been clear, and both top and bottom hoses get hot and pressureised. I don't think it is the sensor at fault as the temp only seems to drop when going from slower speeds to higher speeds, and now seems to be more 90... drop to say 70.. few seconds later climb back up again..but all on 50mph roads.

Cheers

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937carrera
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Posts: 395
Joined: Sat May 17, 2014 8:19 am
Drives: 2000 Polo GTi
Location: North Yorkshire

Re: Coolant temp up and down... but new gen stat

Post by 937carrera » Wed Mar 18, 2020 7:30 pm

Definitely sounds like a stat problem......... or a stat housing problem.

I had a similar issue with temp varying with load / speed, except I wasn't driving the car. It was only after I collected the car after it dumped all its coolant and diagnosed a failed head gasket that I knew what had happened:

Failure of slow speed on the coolant fan plus, failure of thermostat housing > incorrect control > overheating > warped head.

It won't be an airlock, so look at replacing that thermostat with a good quality one, and the thermostat housing too, check the operation of the cooling fan, otherwise you may find yourself with a much larger job.

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