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Discussion Starter · #1 ·
The Bora started running cold a few weeks ago. I assumed the thermostat had stuck open, & replaced it, but it didn't make any difference. Put a new Coolant temp sensor in too.

I've spent ages trying to clear any possible air locks, but to no avail. I thought that the water pump impellor might have broken, but there's plenty of flow into the header tank.

If I hold the revs up over 1200rpm eventually it will get up to 90 degrees, but as soon as I let it drop down to 900 rpm it starts to cool down again. I can get the bottom hose hot but after a short drive it's stone cold again.

I'm stumped as to what the problem could be.[8o|]

Any ideas anyone ??
 

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NooNoo
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Where did you source the new thermostat?

Is it genuine? If not fit a genuine one, the only control in the cooling system is the stat.

All Golfs/Boras will run at bang on 90°C (indicated) when everything is healthy.

[:)]
 

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there is a thread on here somewhere about it. its coz of the cold weather aparentlly, loads of people were posting the same things up when all that snow hit. mine was running just under 90 in all that snow, but now seems to be ok. i can 100% say there nish all wrong with my car. [Y]
 

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NooNoo
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there is a thread on here somewhere about it. its coz of the cold weather aparentlly, loads of people were posting the same things up when all that snow hit. mine was running just under 90 in all that snow, but now seems to be ok. i can 100% say there nish all wrong with my car.
Your car will run fine of course, but your interior heater may be a little cooler than it could be.

I've fixed this issue on 3 mk4s, twice on my own in the 12 years of ownership.

With a good stat and sender they run at an indicated 90°C with a toasty heater.

[:)]

Edit: The lower hose in this weather will be cold unless 'ragging' the car a bit. WIth the sub zeros the thermostat is almost closed so the lower part of the rad is cold as the coolant spends plenty of time in the air flow.
 

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Discussion Starter · #6 ·
Guage barely moved during the snow, & heater was stone cold!

It now takes about 20 mins of driving to reach 70 degrees. the wife drove the car up to Birmingham last week & after about an hour of cruising speeds it got to 90 degrees.
 

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NooNoo
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Did it appear to get worse after the stat change?

Is it still as bad as it was before?

[:)]
 

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Guage barely moved during the snow, & heater was stone cold!

It now takes about 20 mins of driving to reach 70 degrees. the wife drove the car up to Birmingham last week & after about an hour of cruising speeds it got to 90 degrees.
ahh that aint the undercooling issue spoke about.

Noonoo, im aware of the signs of probs fella[Y].... owned many formed of mk4s. this dudes issue is miles off the thread that was on here.
 

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NooNoo
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My money is on the stat (even though it's new). There's nothing else that can go wrong (as long as the plumbing hasn't been changed).

[Y]
 

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NooNoo
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The problem is that the thermostat is critical to running temps and it's made for 50p. It's hardly an accurately calibrated device, it's made to a cost and therefore I've seen several new ones not work properly.

Once you get a good one though it's a fantastic cheap way to maintain coolant temp, hence manufacturers still use them. Although I believe there are some electronic ones about now.

[:)]
 

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I got this same problem. it gets up to 90 then when you stop moving the engine starts to cool down again, maybe down to 70 or so. I've had a new temp sensor just like you and ive had a new water pump (with a cambelt change) and none of that has made the slightest difference. I will be watching this thread with interest.
 

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NooNoo
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It could be possible that diesels don't generate enough waste heat at idle as the diesel engine is considerably more thermally efficient than a petrol... my experience is with petrols.

Although I doubt designers produce a car that cannot regulate it's temperature properly, at least to -10°C or so. These cars are tested in climatic chambers to -20°C and lower.

I expect all cars to have a positive temperature gain with the interior heater on full, at idle / stationary. Then it's purely upto the thermostat to do it's job!

[:)]
 
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