Heater blowing cold air on motorway runs and warm air on city driving

**AJB75** · 30-11-2012,10:31 AM

I bought a 2004 mk5 golf in March. During a full service, water pump & timing belt change 2 weeks ago the mechanic discovered that the fan had been disconnected by previous owner. Since it was reconnected the heater blows cold air when I'm doing motorway driving (60/70 mph) but works ok on shorter / slower drives. The water temperature remains at 90 so don't think it's a thermostat issue. Anyone experience similar or have any ideas before I take it back to the mechanic? Money is tight and don't want to be stung for a big bill. All help would be greatly appreciated.

**Crasher** · 30-11-2012,10:41 AM

Certainly sounds like thermostat to me. Don't trust the temp gauge, they are woefully inaccurate reading 90 when the coolant is actually 72 and still reading 90 when it is really 117! Does the fan run continuously?

**AJB75** · 30-11-2012,10:56 AM

Thanks for the reply. The fan only comes on when the engine has warmed up. As I said it was working fine until Service, water pump & timing belt change and the fan was reconnected. I bought it off a dealer so don't know why previous owner disconnected it?

**AJB75** · 06-12-2012,09:14 AM

UPDATE: Thermostat changed yesterday but still getting cold air on longer runs. Water pump checked and working fine and no airlocks in the system. Mechanic thinks the radiator is stone cold at one side, hose going into radiator is hot, one coming out is luke warm..next suggestion is to remove radiator and get it pressure tested for a block. Anyone else any suggestions?

**Crasher** · 06-12-2012,09:59 AM

It is beginning to sound like the heater matrix but I have yet to see that on a Golf 5.

**AJB75** · 06-12-2012,10:54 AM

Is this a big / expensive job? I'm running out of ideas and can't afford financially to test / replace parts as I go.

**Crasher** · 06-12-2012,12:27 PM

I have no idea as I have never done one but they are quite similar to the 8E A4 and these take me about 2 hours. VW quote the time as just under an hour which may be a little optimistic but as long as the clutch pedal does not get in the way I think it will be an easy job. Its interesting they have gone back to the Golf 1 method as changing the matrix on a Golf 2, 3 and 4 was a pain and on the 3 and 4 meant taking the complete dash out. The matrix is £155 from VW but it is available aftermarket from trusted manufacturers such as Valeo, Hella, Behr and Febi.

**niall campbell** · 11-12-2012,01:49 PM

Replace fan sensor first , then go to radiator. You will be able to feel yourself if indeed one half or radiator cold. This indicates blocked radiator core................. however the car isn't overheating !! I suspect its blocked and fan going overtime to cool the water down. Then go to the bulkhead and locate two pipes from engine to heater matrix. Is one hot from engine after a good run ?? If not then follow pipe back to engine. There may be a smaller thermostat here which supplys the heater matrix with hot water. It could also be a cable broken from the heater control inside the cabin.

**AJB75** · 13-12-2012,03:36 PM

Niall, thanks for this..I have an update on what's been done too.

Mechanic changed thermostat (didnt help) then thought it was the radiator as half of it was cold so that was replaced yesterday but I still have the problem. The 2 heater pipes (go through the bulkhead / firewall) have been checked and are hot too..Mechanic sought advice from an Audi mechanic who said he thinks its the head gasket but only way to check is by replacing it. It's all getting a bit out of hand (and expensive) thanksfully so far I haven't been charged any labour and got parts at cost price.