weird overheating problem??

[reluctant]

hello,
I have a 1981 rabbit with NA block, hydraulic head.  I've added a t3 garret turbo, some VDO gauges (EGT, boost, oil temp, water temp, volts, oil presssure) and last but not least a giles performance turbo IP. 

The problem is that the water temp gauge doesn't like to sit nice and steady at 90C.  We I cruise around 120km/h down the highway it will creep up between 110C and 120C (stock gauge is also going into the red) The car has a newer water pump that seems to flow good when I check the small return line in the reservoir?  I've kept putting new thermostats into the car thinking I had got a bad one like you always hear about, however I've never been smart enough to test the thermostats before hand

the rad fan does cycle on and off in short bursts, no leaks of coolant anywhere, no excessive pressure in the coolant system (like from a blown head gasket)

when the car does get that hot, if i keep running it it doesn't get any hotter and start overheating out of control....any ideas what could be happening??? a batch of bad t-stats that are sticking? both temp gauges reading high in error??

any help would be much appreciated...thanks

[TylerDurden]

You might get an IR thermometer and check various spots on the system:

Radiator at each corner (UL, UR, LR, LL)
Bypass  hose
Heater send
Heater return

See how they compare to the gauges.


Ensure the pump belt is not slipping

Ensure thermostat block-off is extending far enough. (stovetop test)

Ensure the pump impeller is not slipping

Ensure the heater valve is open (hot) when bleeding the air out.

[reluctant]

good ideas...

the sender for my vdo water temp gauge is located at the drivers side of the head, what temp should I be reading there?  i figure if the thermostat is supposed to open at 87C, then the coolant passes thru the block and head it would be hotter by the time is gets to the sender...

[745 turbogreasel]

Is there a rag stuck in one of the hoses?

[burn_your_money]

If you put the heat on max and the blower motor on high does the temp drop any?

If you are sure your thermostat and waterpump are good, I'd be looking at the rad. It could just need to be back flushed.