Hi all.

My son has a problem with his Mk4 Golf 1.8t GTi. It is not reaching the normal operating temperature (which by all accounts should be bang on 90deg). He has had the car for several weeks and recently had the timing belt and tensioner changed. At the same time, he had the thermostat replaced with a VW one as apparently a dodgy thermostat can cause the cool running issue.

Unfortunately, the issue still exists, so he changed the sender in case that was the problem, but it did not cure it. The car is really drinking more fuel than it should be but I guess that is because the computer thinks it is running cool so must need more fuel (i.e. choke scenario)? I tried clamping off the feed from the thermostat housing to the radiator to make it run hotter, but it did not make any difference.

The other thing I noticed is that there does not seem to be any movement of coolant through the expansion bottle. After about 20 mins of running I carefully removed the cap and there was a very slight 'hiss' but nothing like I would have expected after 20 mins of running.

Any advice/help would be much appreciated as I am stumped!

Cheers guys and galls

Edit: He has just checked after leaving the car running for a while again, and it seems that there is a very slight trickle of water coming into the expansion bottle now, but no pressure to speak of when he blocks the neck off with his palm..

[Edited on 28/3/13 by Daddylonglegs]

Sighh.. it's a VW what did you honestly expect?

VW's are infamous for water pump failures (impeler comes adrift) Possibly it's only a stuck open thermostat?
Either way it's time to get in there and look..

A friend of mine had the problem as well, thermostat replacement cured his.

That engine shares a few components with the one in my little polo... the temp sender and thermostat are well known for failing...

I have got through 3 genuine temp senders in 5 years.... Gives low gauge readings but also seriously messes up the fueling....

He bought the replacement water pump with the metal impeller, a good quality thermostat and sender. Not sure where to go next then

You're right too tegwin, screws up the economy something rotten!

You have to work logically changing a water pump for over cooling isn't logical.


The first thing to check is the engine actually running cool or is the gauge reading cold.

If the engine is running actually too cool then it is the thermostat.

Clamping off the top hose at idle won't tell you much in current weather as modern engines don't need much cooling at idle.

If the gauge is showing the wrong temperature and if as on most modern cars the gauge is driven by the ECU then it could be either the sensor or sensor voltage connection/refernce voltage/earthing fault.
A lot of ecus are programed to show "N" on the gauge if the sender is disconnected, most will also show "N" on the dashboard at any temperature between 75c and 115c

I would measure the real temperture at the cylinder head and compare this with OBD II live data.

I had the 1.8 turbo and it was remapped an absolutely brilliant car to drive, the water pump is a known failure and usually gives way at the 60 to 70k mark, as said prviously this will cause overheating and cause the car to go into limp mode. I know as this happened to me at 65k. You mention you bought a good quality part, it may be good quality but may not be the correct one. If this is the case it will send the wrong information to the ECU causing a fuelling issue. Personall I would buy this part from VW they will match it to your registration and year rather than one size fits all. I is also worth asking VW about the warranty on the coil pack as they are prone to failure and about 30 per chuck. Mine was a 2001 MK 4 and 2 years ago they where still classed as a warranty item VW had to replace them though at no cost as they claimed the labour back from VW Germany. (I found this out from a friend who worked in the dealership).

Another part that causes all sorts of problems is the footbrake switch this can also affect the mixture as it is connected to the ECU to alter fueling on braking.

I would also recommend that you search flea bay for vagcom and lead available for less than a tenner you can then read fault codes and reset them

IF the engine isn't reaching normal operating temperature it can only be a faulty/incorrect thermostat.Some VWs will self destruct the thermostat if they overheat which they can do if they have an air lock on refilling the system. If , as already suggested it is reaching normal but not showing as such it is a faulty gauge or more likely sensor (very common problem on VWs)

Change the thermostat again (I've used halfords QH ones in VAG engines and they work fine) - all the symptoms point to a thermostat failure.

New parts are defective sometimes.


Certainly a thermostat if it gets up to 90'c if you sit with the car parked and just hold the revs to about 2-2.5k rpm.


Both my Passat (1.9tdi) and Audi (1.8t) needed thermostats changing. The Audi one twice after a genuine replacement kept getting stuck open.