Strange Overheating???

Post by **Jamie25** » 15 Dec 2007, 20:33

Sorry this post is so long, but can anyone help? My '85 1.9dg (running on gas) has been behaving in a strange way lately.

Last winter it ran fine for ages, then suddenly the hot air in the cab would turn cold, followed shortly rapidly raising water temp needle and then a flashing red light. I would promtly pull over and keep the engine revs high, opening the bleed valve on the rad. Some air would come out, then water, I'd re-tighten the valve and all would run well for a few more weeks.

It did this several times and then the problem seemed to stop.

Last year the temp needle used to sit right in the middle, lately it's barley moved out of the white block on the left. It overheated last month - did the same as above and it was fine, although I never got any heat out of the exchanger! A couple of weeks ago it overheated again, this time with lots of steam from the back. After unloading the 'boot' onto the side of the road, I decided that I could undo the expansion cap just 'one more turn'and was rewarded with coolant all over my face, clothes, ceiling and a blister the size of a half tennis ball on the inside of my wrist

. After lots of swearing I managed to limp home.

I've replaced the thermostat (and the top housing with bleed screw) and all went well for a couple of days. The needle now sat about a third from left. I then braved a trip to Devon and after about 40miles i had lots more steam. After letting it cool a little and (very carefully) removing the cap I noticed that the pipe forward of the thermostat (to the rad?) was not pressurised at all, but the other pipes were rock solid. This was with the cap off. Let it cool some more, topped up, limped to a garage and bled system as best as possible. It made it to Devon and back without missing a beat.

Finally, to my question.... WHAT IS GOING ON? Short of any advice on here, my ideas are, in no particular order, flush/reverse flush radiator, replace expansion cap, attempt to test thermostat is working properly.

I don't know if I'm missing something obvious that someone can point out??? Could it be as simple as I'm not FULLY bleeding the system?

Post by **kevtherev** » 15 Dec 2007, 20:40

does the top up tank level ever move?

I ask as the expansion tank caps are prone to failure.

Post by **Jamie25** » 15 Dec 2007, 20:44

Last time it overheated it was actually overflowing out of the top up tank. i don't if this is good or bad?

Post by **kevtherev** » 15 Dec 2007, 20:53

That is bad
and maybe not
The cap allows flow both ways...so check that the cap does that

or...you have either an air lock or a blockage

I suspect an airlock

Have you followed any of the bleeding advice in the wiki?

Post by **andysimpson** » 15 Dec 2007, 21:01

was the thermostat geniune?

Post by **Mocki** » 15 Dec 2007, 21:01

sounding like a head gasket job here or cracked head........ combustion gasses in the coolant by any chance?

Post by **Jamie25** » 15 Dec 2007, 21:02

I have followed the advice, although probably not every time.

I think I shall probably get a new cap, flush the system and make sure I bleed the system properly. Is the usual advice to only use a genuine VW cap, or are there acceptable cheaper options?

Sorry, should have said, earlier, thanks very much for the advice and for reading the entire thing in the first place.

Post by **Jamie25** » 15 Dec 2007, 21:05

Mocki - your scaring me now! How do i tell if there are combustion gasses in the coolant?

Andy- no, unfortunately not

Post by **andysimpson** » 15 Dec 2007, 21:10

Jamie25 wrote:Mocki - your scaring me now! How do i tell if there are combustion gasses in the coolant?

Andy- no, unfortunately not

The chances of getting an aftermarket thermostat for any vw that actually works as it should are very slim, geniune is a must.

Post by **Mocki** » 15 Dec 2007, 21:11

if you bled it properly, and you are sure, unless it is the cap( replace) and it keeps persistantly getting air in the system, chances are its a head problem.....

however, it maybe just a leek you cant see, letting air in as the engine cools down.

How much coolant are you loosing over a month?

Post by **Jamie25** » 15 Dec 2007, 21:19

How much coolant are you loosing over a month?

Bizarre as this sounds for a T25, apart from when it actually over heats I don't think I'm loosing any!

I think it's a new genuine cap and thermostat and a few hours bleeding then.

Post by **Mocki** » 15 Dec 2007, 21:23

yep bleed it properly, then when you think you have got all the air out, do it again!!

Post by **Jamie25** » 15 Dec 2007, 21:34

Thanks for all the advice everyone. I'll get the new bits, flush the system and bleed it properly. I shall let you know how it goes.
That's after a careful trip to Devon tomorrow - cooling problems or not, that swell can't be missed!

Post by **kevtherev** » 15 Dec 2007, 21:50

have a safe day dude....

Post by **Jamie25** » 16 Jan 2008, 11:01

Right - having bled it properly the van ran fine for two fourty mile journeys and then all of a sudden it stopped blowing hot air from the heat exchanger and overheated.
I've since bled and re-bled, but it keeps on happening. I've replaced the thermostat and expansion tank cap with genuine vw items. I can't see any obvious/serious leeks. I'm loosing quite a lot of water even though I've only been using it for short journeys, about five/six miles at most.

Mocki - you've suggested
........it keeps persistantly getting air in the system, chances are its a head problem..... however, it maybe just a leek you cant see, letting air in as the engine cools down.

How can i tell the difference - will a compression test give me any clues?

I think I best start saving for a replacement engine?????!!!!!