1992 VW Jetta (base model) 1.8L engine, 5 speed, no A/C, 275,000km (171,000 miles), using 10W30 oil.

This problem began several years ago only in very hot weather, engine temp at the point the cooling fan would turn on and engine idling (ie. when stuck in traffic on a very hot/humid day). Increasing the RPM slightly would turn the light off. A nuisence but I have tolerated it as the car is not driven very much anymore.

Now I find the light flickers most of the time especially when cold, accelerating and engine RPM between 2000 and 3000. After engine warm up the problem seems to subside or even go away until the engine temp is very warm thus the original problem begins.

Is there any way of knowing by these symptoms if this could be the pump or a pressure switch? Any way of diagnosing which part is at fault. Any suggestions would be helpful. Any way to know for sure?

Thanks,

Rob