Overheating problem

[Chris S]

While driving this afternoon my overheating problem again reared its head,
here is what happened,
Leave house, drive 5 miles (8kms) to motorway, temp gauge points vertically at 12 o’ clock dead normal, heater on defrost working ok blowing hot air,
Enter motorway speed up to 75mph (120kmph) then after about 7 or 8 miles,
although still set to max heat the heater is now blowing very cold air and the temp gauge is reading very hot at 2 o’ clock so I think the heater must be airlocked
the oil temp is up from a normal 96/ 98 deg to 114, the motor is now so hot you can smell it!
but if you slow down & drive further and over 20 miles or so everything gradually returns to normal,

The motor has only done 125,000 kilometer & the waterpump is less than 1 year old, as is the thermostat, the fan belt is new & is correctly adjusted, most of the hoses are new or have tested as good and I also have a brand new radiator, there are no water leaks & the heater has been properly bled, so why in buggery does it air lock & overheat?

btw, the outside weather was cold & wet today but it also happens on warm days as well!

I can understand overheating but I cant get my head round this intermittent air lock,

My check list for tomorrow is as follows

1) radiator cooling fans
2) rad temp sender & fuse box ( if fans dont come on)
3) water pump internals ( thermostat then impellor)
4) any more suggestions please (they will be gratefully received)

[no such user]

I had same problem before it was headgasket

My theory is air leaked from the headgasket tends to collect in places like in the waterpump and cases the impeller to cavitate hence the airlock cant move on and the water pump dosen’t pump well till drop revs and the airlock gets chance to rise out and then the system sorts itself. I spent a whole weekend swearing at a PB untill I realised that the top tank bubbled lots if I held the revs up for a few minutes then let it drop back down to idle

[Chris S]

Hopefully it is just a lazy thermostat as there are no bubbles in the header tank under normal round town running.

[no such user]

Have you run a proper cooling system flush through the system?

The motor might have only done that many miles, but how long has it been around? Possible corrosion in the waterways?

Is the stat a genuine VW one? Even if it is, I’d replace it for good measure.

[skattrd]

Seconded on the flush cooling system, also change the rad fan switch.

My Jetta was overheating when I got it and doing a flush and new switch sorted it out. The rad fan switch cost me £6, so you might as well just change it, it rules out one thing straightaway.

[ld50]

Chris i know it sounds obvious but does the water return to the header tank as you’d expect? (i.e. the rate at which the coolant returns goes up and down with the revs too).

I had soemthing similar on me syncro and after several weeks of hair-pulling-out it was just a small blockage on the rad return feed.

[Chris S]

Yes it does all that, its another new pipe as well

[holmes]

i would say a faulty thermostat also is the car actually overheating or does the dial just say it is, i only ask as weve had problems before where a non genuine thermostat and temp sender had been fitted and it was giving out the wrong signals thus the car thinking it was overheating when it wasn’t. my syncro at the moment does the same in which the temp dial shows going into the red, the warning light comes on at the same time as the fan cuts in, strange but nothing to worry about, ill just change it when i get a chance.btw my sender is non genuine.[]

[no such user]

Sounds like a clogged up, elderly radiator to me, Darren – exactly what my car was doing, too.

I replaced the stat and various other bits before giving in and replacing the rad – cured it immediately!

[Chris S]

Apart from the fact that I fitted a brand new rad in July I would agree with that diagnosis! []

http://www.vwsyncro.co.uk/data/forums/topic.asp?TOPIC_ID=459

Here is the uprated cooling.

The old & new fans.

The old and new radiators.

How it looks when fitted.

[Author]

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