Coolant and Heating overheating causes and fixes
Split hose from header to top-up tank
Mocki ok, how it works:
Top up tank to fill mark, when engine cold.......
Engine gets hot, coolant expands, builds up pressure with temp, and opens header tank pressure cap (VW Blue, after market black, Dalek shape, left hand rear of engine bay) forcing coolant into the top up tank........
Engine cools down when parked, coolant contracts, pressure turns to vacuum, and coolant is sucked back from top up tank to header tank, returning top up tank to fill level from its over full state when hot.....
So, if the pipe between is split or leaking, it would be pushed out of the header tank and out of the split pipe onto the zorst, not into the top up tank, and when the engine cooled down, the header tank would be not full.......
However, if you overfill the top up tank ( fill it when cold with Coolant at least 50/50 mix with water) it will push excess out of the breather holes in top of the top up tank, onto the zorst, and find its correct level.....
the header tank is the one with the darlek cap on, inside the engine bay the top up tank is behind the (infront of) the number plate flap.
the header tank should always be brim full the top up tank should be between the marks depending on the temp when you check it.It has vent holes in the top of it, so if it gets over full it vents out the top
the coolant is pushed out of the headertank into the top up tank, and sucked back ( through the same pipe and cap) when the engine cools and the pressure turns to vacuum, in basic terms.
so if the top up tank is empty it has nothing to suck, but will still blow into there.
it is a one way valve as such, pressure forces the coolant past it , but it allows vacuum to draw it back.
California Dreamin You can't even start to look at overheating issues without first checking the radiator fan and thermo switch are working properly. Nearly all T25'a (apart from 1.6D CS) have a dual speed fan which comes on in two stages. 1st speed comes on at 87°C and goes off again at 76°C 2nd speed comes on at 93°C and that goes off at 82°C. Most uses never experience the second speed which generally only comes on under the most extreme conditions (engine being worked very hard for a long period with little air flow through the radiator) perhaps driving up a mountain pass or similar. Under normal circumstances it is recommended that you check the the fans operation in the spring time by allowing your engine to idle and slowly heat up. The first speed should click in when the needle creeps a little over half way, it should stay on for 10 - 30 seconds whilst the needle goes down again to the half way mark (this is a rough gestimate and will vary van to van) this should start a cycle where the fan clicks in every few minutes for 10 - 30 seconds. As I said you will never hear the second speed unless the engine is worked very hard (under load) at slow vehicle speeds. Check complete...so the first time you get caught in traffic you can be fairly confident that you engine won't overheat.
If for whatever reason the temperature rises higher than 2/3rds without the fan kicking in DO NOT ALLOW the engine to get any hotter! suspect thermo switch issues (about £13.50 from Brickwerks) easily changed (when cooled down) access from the front right side of the rad...switch out new one in and this can be done with minimal coolant loss (a couple of egg cup fulls of coolant) quick rad bleed until coolant comes out..top up. Re-run the above check to determine correct thermo fan operation.