Mk 2 Golf Overheating

[grace_golf91]

My mk2 1991 runs at idle quite happily, temp guage at middle and radiator warm. Fan goes on and off as normal but still once moving the car overheats quickly. Gauge flies over the red mark and that dreaded flashing light comes on. Already tried replacing thermostat but still the overheating continues. Have noticed that when overheated, the system hisses alot and suspect that it may be the expansion cap on the reservoir, but also perhaps a leak somewhere? When it overheats also the pipe from the bottom of the radiator is cold, and when I take the pressure cap off, this pipe then becomes instantly hot and the cooling fan kicks in. Anyone experienced similar or know the way to fight the problem?!Thanks! Would be great to know.

[Simpson]

i am not too clued up on this as never had the problem, best thing to start is to run it from cold with the expansion cap off whilst squeezing the bottom and top pipes to gte rid of any air locks, also is there a lot of preesure in the system? i.e do the coolant pipes swell and are they rock hard when warm? also check wether there is oil in the water and vice versa to start. Hope this helps 

[rubjonny]

replace the tank cap 1st, see where that gets ytou.  It may be your waterpump has about had it!
Check my FAQ post on how to bleed & check your coolant system as well

[grace_golf91]

Many thanks for the tips. Don't think the pipes are swelling, felt normal when left running. Will try when giving it a rev. No water in the oil. About the water pump - it looked ok but how do you tell? The propeller looked fine and it turned smoothly, how do you tell if it's had it?   

[rubjonny]

without having the lower pipe off and checking its not spinning on the shaft there isn't really any way to tell!
If you rev the engine check for a nice strong spray of coolant from the thin hose to the top of the header tank.  If its weak or not spraying at all either the hose is clogged or perhaps the water pump has gone