Cooling system - overheating

[Siegfried]

Hi,
I have read a few posts about coolant overheating etc. but I have a problem, I cannot get to the bottom of... Please can anyone suggest anything else?   The camper overheats randomly - I don't believe it is the electrics - having those checked anyway - get a small spray - escape from bottom water bottle when heating - over back window.  No evident coolant leaks - no loss. However, when the engine is cold and I remove the dalek cap - at times I don't get the short hiss of pressure release - assume therefore a leak somewhere?

I can stop, let it cool and then drive another 90 miles home, when it will randomly overheat again!  Interestingly, when it overheats, the heating in the cab goes off - absolutely linked every time - so I know when I have a problem coming!!

I have a 1990 T25 1.9 DG wbx (Autohomes conversion), petrol, RHD (LPG too). I have very heavily invested in the camper.  Last November it randomly started overheating. This is my current position:

• A fully reconditioned engine fitted (June 2022) (VWEngineCompany - Kent) - done 7.3k miles so far.
  
• Stainless water pipes - front to back
  
• New water pump fitted
  
• New thermostat fitted
  
• New radiator fitted
  
• Flushed through
  
• Bled, topped up etc.
  
• Pressurised
  
• New dalek cap
  
• Chemical test of coolant - nothing noted
  
• Thermal imaging - apparently nothing noted
  
• Coolant levels correct
  

What am I missing - or need to test again?

Much appreciated - Cheers Duncan

[Mocki]

Ok, if you dont already know , The system is very simple....
The headertank ( the one in the engine bay ) should always be full to the brim, and the top up tank ( behind the number plate flap) should be full to the max mark when cold. When the engine is up to normal running temp the coolant expands and is pushed into the top up tank, when the engine cools down the coolant is sucked back into the header tank, so removing the header tank cap before its cooled down prevents it from sucking the coolant back that has expanded , causing airlocks . Removing the header tank cap after it has cooled and sucked coolant back will not produce the “hiss” anyway , the vacuum has gone .

When you say over heating, is this really over heating, or the gauge telling you it’s too hot ??
Normal is anywhere from 1/3 to 3/4




The flashing red light in the temp gauge has two functions ....
1. If the gauge is above 3/4 and it's flashing it means over heating
2. If the gauge is below 3/4 and it's flashing , it means low coolant in the header tank.

[Siegfried]

Hi.
Thanks.  I am not a mechanic, but as one does learn and remember - although disaster if I do anything!!  I generally understand the cooling system is supposed to be quite simple.

I am sure it overheats, as I get a small spray of coolant across the tailgate.  However, it was slightly over filled.  The gauge normally sits just on the right edge of the small red light. When "overheating" the arm moves right across to the furthest point right and the red warning light starts flashing.  Fan does work too - although I need that checking as at times I thought it wasn't on...

I don't believe that the sensors and electrics have been checked yet - which disappoints me as I have been to two "VW" garages.  I am trying a new one in 10 days time and got this listed as a job.

Cheers Duncan

[mioba]

f you have cold air when heating is on then you have air in the matrix.

Check the circuit for leaks, water leaking out - air getting in
Air in system explains sporadic temp spikes.

[Mocki]

The spray accross the tailgate says a leaky pipe , the top up tank isnt at anytime under pressure , indeed it has three breather holes in the very top where it fits into the bracket, but the most it would do is flow out and drop onto the exhaust guard then onto the floor , more of a dribble than a spurt . If that was the case it says darlek cap or header tank fault .

I cant even picture how you would get any coolant spraying onto the tailgate , even with a spot pipe it would go on the floor undertook he engine bay

[Siegfried]

Thanks guys.
The spray is not just water, it has a slightly slippery texture, and washes away easily - not greasy - so can only assume it is coolant.  The spray is also random!  I have had some come out of the three holes - when slightly over filled.  Coolant levels generally keeping to the proper levels.  Fitted a new dalek cap two weeks ago.  Will consider new bottles. Also getting the heat exchangers checked.  
Beginning to sound it is likely that air is getting in. Other than the random spray - there are coolant leaks.

Every thought and suggestion - really helpful - thanks.  Making lots of notes to discuss with garage.

V frustrating - this is the first time in 17 years that we have had a longish term problem stopping us using the camper - being going on three months now.

Cheers D

[mioba]

Not sure how handy you are. If you have the means, go for a drive, get the bus warmed up so the stat opens. Keep it running and inspect everywhere for leaks. A pinhole on the rad can be hard to find as the cooland may evaprate fast.

If water is leaking out the tank at the number plate, then you have overpressure in the main tank forcing coolant into other tank.

But you say coolant keeping levels (strange).

[ajsimmo]

Well, that is a strange but familiar set of symptoms. The thing that caught my eye was the phrase
"at times I don't get the short hiss of pressure release". This implies that at other times you do get a hiss of pressure release when cold, which is a bad thing. It may mean that your cooling system is still under positive pressure once cooled. The usual cause of this is a failed compression seal between cylinder and head. Sometimes it can only leak under full load/high speed driving (eg motorway journey), but be ok under light load (eg town driving). It can be down to poor assembly procedure (eg sealant used under seal), or lack of clamping force due to failed headstud or insufficient torque on head nuts.

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[Siegfried]

Hi
Thanks again. Unfortunately my knowledge is very limited - have relied on the same VW garage for 40 years and now it is shut - now very lost as to who really knows...

This implies that at other times you do get a hiss of pressure release when cold, which is a bad thing. It may mean that your cooling system is still under positive pressure once cooled. The usual cause of this is a failed compression seal between cylinder and head. Sometimes it can only leak under full load/high speed driving (eg motorway journey), but be ok under light load (eg town driving). It can be down to poor assembly procedure (eg sealant used under seal), or lack of clamping force due to failed head stud or insufficient torque on head nuts.

The bit about hiss/pressure really worries me, as I was advised that it was ok and good!  Also the the overheating is generally when under load. Is there any way the sealant you refer to in the assemble process can get into the cooling system?   

Cheers D

[Siegfried]

Hi Ajsimmo

Just had a thought - if there is a leak from the cylinder head - is this where you can get the coolant tested for any chemicals etc. which shouldn't be there?  Thanks D

[ajsimmo]

What I meant was that some engine builders use an inappropriate sealant under the metal sealing rings (commonly called the fire-rings). This then sets or hardens, breaks down under repeated exposure to excessive heat, and leaves a tiny crack/void that combustion gas under pressure can force its way past, leading to an increase in pressure in the cooling system and subsequent displacement of coolant into the expansion tank. This often only happens when cylinder temperature and pressure is at its absolute maximum (ie loaded at revs, eg motorway speeds). Just bimbling around town gives the engine a much easier time and cylinder pressures are much lower, so no leakage of gases into the coolant.
No setting sealants should ever be used on these rings. I use a tiny smear of grease around them just to hold them in place during assembly. This is forced out upon tightening the head down, leaving a perfect face to face seal between barrel, seal and head.
Chemical tests often do not indicate any trace of dissolved gases. This could be because of the dilution rate, as the cooling system volume is much greater than a car, and the gas leak is tiny.

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[R0B]

Funny you should mention that Andrew. I had the same problem with my engine.(Well, one of the problems)

" some engine builders use an inappropriate sealant under the metal sealing rings"

[Siegfried]

Thanks Ajsimmo.

The key issues that remain:

1. Can any sealant/excess sealant get into the coolant/system? We found quite a bit clogged in the water pump - which has been replaced. Could sealant become stuck or circulating?
   
2. The overheating has happened immediately on start up - i.e. engine gets up to normal temperature and then thermostat keeps going over to right and warning light comes on - this is after 2-3 miles only, with speed not exceeding 30-40 mph.
   
3. We did 800 miles after we replaced the dalek cap etc, with no overheating - driving sometimes 50-60 mph. Then suddenly it happens again. If it was the rings/head/cylinder how could we have done the 800 miles?
   

Thanks for the contributions - trying to think of where to focus on tests and diagnosis - already cost me a lot of money and still not resolved. Going to a garage next Monday...
 

[Michael4]

I'm new to this but is the water actually circulation? Sounds like maybe not.  Is the thermostat sticking?

I'd take the thermostat out and see what happens without it.

I may well be barking up the wrong tree, if so ignore me.

[ajsimmo]

Thanks Ajsimmo.

The key issues that remain:

1. Can any sealant/excess sealant get into the coolant/system? We found quite a bit clogged in the water pump - which has been replaced. Could sealant become stuck or circulating?
   
2. The overheating has happened immediately on start up - i.e. engine gets up to normal temperature and then thermostat keeps going over to right and warning light comes on - this is after 2-3 miles only, with speed not exceeding 30-40 mph.
   
3. We did 800 miles after we replaced the dalek cap etc, with no overheating - driving sometimes 50-60 mph. Then suddenly it happens again. If it was the rings/head/cylinder how could we have done the 800 miles?
   

Thanks for the contributions - trying to think of where to focus on tests and diagnosis - already cost me a lot of money and still not resolved. Going to a garage next Monday...
 

The short answer is yes, but not from the fire rings. The builders who use too much sealant tend to use it everywhere. If you find it in the water pump, I'd guess it probably came from the bottom of the barrels that sit in the water jacket. There shouldn't be any sealant used there, but some "technicians" put copious amounts of silicone there in a false belief that it's a good "belt n braces" approach, or it may just be a way of avoiding using better (costlier) parts. Who knows?
I won't list the ones I know to do it, but they're mostly based in the south east.

The overheating straight from startup might be because you already have an airlock. Were your heaters hot? Then when you replaced the dalek cap, did you bleed it again, thus expelling the airlock and starting with a full complement of coolant? If so it would take a while to manifest itself as overheating, especially at a gentle 50-60 mph. Some vehicles only do it above 60, ie fine on single carriageway, but boil on motorways.

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