heater suddenly blows cold air at running temp

hi guys already posted this on vwvortex but no solution given wondering if you might be able to

i've got an '88 Golf 1.8l GL with no air con

basically i can watch the temp gauge rising and from the end of the thick 'cold' marker it starts to give out warm air through the vents gradually getting hotter and hotter as the engine temp increases

however once it reaches normal running temperature just after the led in the middle it suddenly blows cold air it doesn't matter whether i'm driving or sat still cold air is all that comes out of the vents

if i'm sat still and allow it to keep running the engine carries on heating up to approx 3/4 to 4/5 of the gauge's travel and i hear the radiator fan kicking in and watch the temp gauge fall back down exactly as it should yet nothing but cold air comes from the vents

in a bid to sort this out i've changed the matrix and thermostat, replaced the foam on the blender doors and bled the system thoroughly to ensure no air locks yet to no avail the coolant mixture is approx 50/50

has anyone got any suggestions to explain this wierd behaviour?

i'm at a total loss now and ready to pull my hair out

cheers dave

I assume coolant level is full. If you feel the two heater hoses at the firewall & they're hot then you know it's a problem in the air box. Did you repair the foam on both of the metal flaps in the air box, with the heater core removed? I can't remember the configurationof a non-A/C air box. The heat flap is cable controlled, correct? Is the cable fully opening the flap? Is there a vacuum operated flap for recirculate vs fresh air, & is it working?

checked, checked and triple checked everything you've suggested

after replacing the foam on the flaps i made sure the cables operated them perfectly and with the dash put back together i can hear them moving and closing into place

both the inlet and outlet hoses to the core are red hot suggesting there's no air lock in there

i'm not aware of a vacuum operated flap but if you tell me what i should be looking for i can double check

where would this be around the blower fan somewhere?

cheers dave

As I said I haven't seen a non-A/C box in a long time... it may or may not have vacuum operated flaps for the recirculate vs. fresh air function as the A/C boxes have. You should have vacuum lines coming off the control switch, follow them to see what you have.

well as far as i can tell all that comes off the heater controls are the 3 cables to the flaps in the air box and there's no other flap vacuum operated or otherwise that i can see

spoke to a mechanic today and when i listed what i'd done he looked at me gone out and said he hadn't got a clue!

i'm clutching at straws here but it's not likely to be one of the sensors/switches messing things up with the water temp is it?

i'm not much of a mechanic myself but i was under the impression it's only the opening and closing of the theremostat and the radiator fan that actually determines the coolant temp

from what i can tell the fan works at the right temp and the replacement thermostat was from a vw dealer (i wont go into the hassle i had to get one of those) so it should be ok

cheers dave

Well, I don't suspect it's an engine or cooling system problem as you stated your temp gauge seems to read correctly & the heater hoses are hot. I finally found a diagram of that system (99.9% of US Golfs have A/C) & noticed it referenced a heater control valve, which our NA models don't have. I know the Cabrios used this valve. This is where I would start, it should be on the engine side of the firewall. I cannot tell if it's vacuum or cable operated. Check to ensure it opens/closes fully & doesn't seem stiff. If it's vacuum operated it may be closing back when hot. If cable operated it may be defective internally & restricting flow.

that sounds interesting there's nothing to suggest such in the manual i have and maybe a place to look

there is certainly something on the inlet pipe but whether or not it's a valve i don't know i just thought it was a junction as it's not cable operated for sure

do you have a link for that diagram?

It wasn't shown in a diagram, just mentioned in a description of how the non-A/C system works. All it said was the heat control lever moves the "heater control valve lever", it may just be refering to the air box flap. A valve would be obvious in the heater hose from the engine.