Petrol Volvo S40 - Overheating when Idle, Fine when Fast!

fiddlechic

Hi all,

I have a 2005 volvo s40, petrol, and had it serviced and NCTd about 3 months ago now. Only 66000 km on clock. I'm the only owner!

I don't drive it that often - usually up and down from Dublin to Westmeath every second weekend, with a few trips around Dublin in a week.

When I'm driving on the motorway etc - it's totally fine, normal air condititioning, temp gauge at mid way.

But when, for the last month or so, I've been driving in town the air conditioning seems to stop working, the windows fog up, the temp gauge goes up and I've noticed that when the car is idling at traffic lights etc having had similar stop-starts of city driving, it seems to "rev" more, even though rev meter stays the same.
When I then speed up, the temp gauge goes down and the air con seems to work again.

(I have noticed this going out on N11, troublesome in Donnybrook, and then when the "high engine temp" light came on, I accelerated to move into lane to park on Nutley Lane, I noticed the temp gauge went down, so continued driving to see what would happen, and by Foxrock, in freely moving driving, everything back to perfect!)

What could possibly be causing this?
I don't want to walk into the garage as I did when had a problem with my brakes, and ended up being charged nearly 3 grand. I at least want to have a sense of what could be causing it - I know I'm not planning on fixing it myself, but would love any expert ideas on what the problem could be! Thanks!

biko

Speed will cool the engine by the outside airflow flowing over it.
Check the coolant level and top up.

When did you last check coolant level? Is it mixed properly or just water?
Is the heat always working? If not it's a classic low coolant symptom.

fiddlechic

I have never checked the coolant level and have no idea what's in it.
A friend of my Dad's who is a mechanic serviced it 3 months ago - I'll check the coolant tomorrow.

What do you mean by the heat always working?
I have the A/C on all the time to 19deg C.

fiddlechic

Apologies for dumb newbie-ness.
Thanks for taking the time to answer.

biko

fiddlechic wrote: »

What do you mean by the heat always working?
I have the A/C on all the time to 19deg C.

Turn off the AC and just turn up the heat to max. It should be roasting inside the car in minutes (if it's warmed up). If not it could mean the coolant is low.
The reason is that the coolant transports the heat from the engine to the heater blowers in the dash.

Check the coolant level tomorrow before starting car.
The reservoir (#1 below) should not be empty, more like half full (there's min and max markings on it).



You should probably have a read of the general maintenance instructions here too, can't hurt to know a bit about your car.
It's usually the second most expensive thing you'll ever own
http://new.volvocars.com/ownersdocs/2005/2005_s40/05s40_10a.htm

Check operation of the cooling fan (#5). It cools both the a/c condenser and the main radiator. The problem could with with the fan, wiring, temp sensor (normally mounted on the radiator (#4), wiring or fuse. It could also be an ECU problem if the cooling fan is controlled by a 'puter..

fiddlechic

So it was the coolant temperature sensor - sorted!

Thanks so much for the advice - I really felt much less a total greenhorn with an open cheque book ripe for the exploiting when I went to the garage.