Water light flashing randomly and high temp quickly when idling? Why?

[ScottMk2Gti16v]

Hi all, I've got a problem with the water light flashing randomly. The car isn't losing water, I mean I top it up now and again but nothing unusual and not often, but the water lights started flashing for random periods, usually 3 times in a row then it'll stop for a few mins then do it again. It also seems to come on when I put my foot down but goes out when I come off the gas and just cruise along. Sometimes it'll keep flashing for 5 mins, and other times it wont come on for 20 mins.

Also if you sit with the car just idling over the temp gauge rises right up to max - it'll take 5 mins but will go right up, but when you start driving it goes back to normal and stays there (between 1/4 and 1/2 way usually). The temp NEVER rises stupidly while actually driving.

A mechanic (who I don't rate greatly tbh) had a quick look and couldn't see anything obviously wrong and suggested maybe a faulty sensor in the water tank? Or maybe a blockage in the system?
Do you agree? Any ideas on what to check/replace?

Cheers

[Leon R]

That sounds very strange.

I think i'm right in saying that there is a coolant level sensor built into the expansion tank so you will need to buy the whole unit if the problem is due to this. However a faulty coolant level sensor won't explain why your temp guage goes to the max as this works from a reading from one of three sensors on the right hand side of the head near the dizzy.

I'd check for:
Coolant level sensor plug/wiring ok
Pressure test cap if you can or look for signs of boiling over
Check the wiring to the 3 sensors on the side of the head and the connections it makes
Then test each sensor using a multimeter and haynes manual. Should be about 22ohms resistance
Check your water pump is circulating or leaking from under the car

Changing the tank will be a good start tho as this should fix the random light. I'm sure ive read other people having the same symtoms. However the high temp problem is something new to me.

[filthy]

Also, try the thermostat, and the thermo switch for the fan 

[ScottMk2Gti16v]

Cheers for the pointers guys, I've got a new expansion tank with the level sensor built in, and I'm sticking it to a garage tomorrow to get that changed (amonst some other bits - wheel bearing and gear box mount) and will get them to have a proper look at what it may be. It's actually got a little worse in the last 2 days, temp gauge goes right up very quickly sometimes when your driving along, just for 20 seconds or so, then goes back down as rapidly as it went up. Just does this randomly every so often, usually when at slower speeds (30-40mph).

Is this likely to be a sympton of something getting worse? Or is it so bizarre that it's more likely to be something like a faulty thermostat?

[russ-vdub]

what condition is your rad in? when was it last changed. a new one will slove so many over heatin prblems

[ScottMk2Gti16v]

Not sure to be honest, I'll get that checked to. I've not changed it in the 2 years ive had the car and dont think it was done any time soon before that - I'll dig out the old receipts and check but dont remember anything about it on them.

[Conker]

As filthy said, check your fan is operating normally. My 16v regularly needs its fan in prolonged traffic. If your not losing water, then it must be electrical.

It could also be a weak water pump perhaps? To check, look inside the water tank, you should be able to see water coming out of the thin feeder pipe, and the flow should increase when the engine is revved.

[ScottMk2Gti16v]

It could also be a weak water pump perhaps? To check, look inside the water tank, you should be able to see water coming out of the thin feeder pipe, and the flow should increase when the engine is revved.

Looks like you've got it bang on with that one. Car went to garage and I got the expansion tank changed, which hasn't solved the flashing water light problem - but I'm hoping it's all related to the fluctuating temp gauge which is still happening. Left the lights on for ages today and killed the battery so had to get the AA out, I told the guy about the problems and described the symptoms and he's certain it's the water pump. He checked as you described above and told me about the flow when revving (no life at all in my expansion tank when revving!). Quite handy to as I've just moved to a new town and wasn't sure where to put the car to next - he pointed me to a good small garage where the guy's a VW enthusiast. The AA had a VR6 himself he said.

How hard is it to change the water pump? AA guy thinks you have to take off the fan belt and replace that as part of the job? Would be handy knowing how many man hours I'm maybe looking at?

[cняis]

shouldnt cost a great deal in labour as the alternator belt is a smooth one (ie no timing issues) and the water pump is easily accessible. new belts are about £5, id say job done in 1 - 1.5 hours at most!

unsure on water pump costs.

[Conker]

The only major issues with changing the pump would be...

1) Are the hex bolts on the two big sprockets seized? These can be a pain to get off. Perhaps start spraying them occasionally with WD40 now.

2) My lower cam cover was a right pain to get off too.