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[BettyBlu]

Well of course the current lovely weather starting to expose some flaws in the cooling system.

Recently fitted new thermostat to deal with engine running hot (95C - 100C). Fresh coolant mix, all hoses checked and where necessary, replaced.

Since then temp. has sat nicely in the middle with a 1C to 2C variation either way depending on situation. Great, I thought - sorted!

However out comes the sun, on goes the air-con and engine temp starts rising to 100C+. Switch off the air-con (completely, not ECO) and temps go back to normal.
So a pretty conclusive connection between air-con and engine temp.
AC does pump out cool air, no funny smells or noises. At times I have noticed a change in tone from the compressor upon starting especially when the engine is hot but that's the only thing I've noticed.
Any hints much appreciated, MOT next month and I'm having a major service beforehand so this may discover the problem. But just in case its something obvious and known I'd thought I'd give a shout.

[rancido64]

As a temporary fix you could put the fan running constant - if you do bridge the connection with a 30amp fuse it won't burn the fan out

[jonathanh]

well, when air con goes on, it should also start the fans up in front of the rad on slow speed.  this should take your engine temp down.  First check this is happening. 

second, omega's run hot and I would expect my fans to kick in/ heat to get to 100 particularly doing town driving where there is a lot of heat to dissapate and not much airflow

[BettyBlu]

Thanks for the tips. Today, Sunday 31st is just about the best weather to test these ideas!

[JamesV6CDX]

If it's only doing it on one particular heat setting - how old is the HBV?

[omega trev]

hi all
i am having similar troubles with my mig, when the temp gets to 97ish the small fan on the left of the two at the front of the aircon condenser comes on but this is when the eco mode is on, when eco off the same fan comes on intermittantly all the time but the temp is getting up to 100, i have never seen the right hand fan or the larger one behind the rad come on at all, does anyone know what the sequence is for these fans?

[BettyBlu]

... - how old is the HBV?

Hope this isn't something blatantly obvious but what's an HBV?

[jonathanh]

heater bypass valve:  its a plastic valve in that diverts hot water from the heater matrix when the temperature controls call for cold

[BettyBlu]

Ah, cheers! I racked my brains out on that

AS afar as I can tell the HBV is original - nothing in the service history mentions it and I've not replaced. So it's 10 years and ~88000 miles old. Worth checking this then?

[Andy B]

.....
 So it's 10 years and ~88000 miles old. Worth checking this then?

Most definitely! I'm on my 2nd (genuine) in the last 5 years.

[BettyBlu]

OK, I'll def. give that a go then. Thanks. Does anyone have a guide to HBV replacement?
Can't find any reference in my Haynes, which is a surprise. Unless I'm just missing it?

And for what it's worth - it took me nearly 7 hours to replace the thermostat. Yes, I'm a noob  :-?

[Mike Collins]

I would not be surprised at seven hours for a thermostat change, separating the housing from the cross pipe is a challenge.

[BettyBlu]

Thanks for that Mike - I thought it was just me

To be honest I was nervous every step of the way - kept thinking I was going to trash my beloved V6...

[mathewst]

hi all
i am having similar troubles with my mig, when the temp gets to 97ish the small fan on the left of the two at the front of the aircon condenser comes on but this is when the eco mode is on, when eco off the same fan comes on intermittantly all the time but the temp is getting up to 100, i have never seen the right hand fan or the larger one behind the rad come on at all, does anyone know what the sequence is for these fans?

Check the relays immediately, your car is in serious danger of overheating (esspecialy if you get in a teraffic jam). the fan behind the radiator must kick in when the temp id a little over 90 degrees. Do the fans work when you switch on the A/C?