problem with cab heat & fluctuating temp gauge

Hi,

I have an 89 Nissan Pickup. 416,000 km and counting (that's nearly 250,000 miles for all you imperialists). This truck is very reliable and generally runs great.

However, just recently, an intermittent problem has come up - the heater in the cab will blow cold air while the temp gauge moves into the danger zone. As well, the truck engine idles high. But then the temp gauge will slowly go back to a normal operating range, the idle will go down to normal, and the heat will start working again.

The water pump and radiator were replaced less than 18 months ago.

what could be causing this? faulty thermostat?

Most likely the thermostat

I've replaced the thermostat, but the problem persists...

The new thermostat could be faulty but it really sounds like a blown head gasket.

yes, I'm starting to wonder about that.

It turns out the water level was pretty low, so, as the truck was idling, I slowly filled up the rad and let the air work itself out of the cooling system (I was really stupid not to check in the first place!!). Anyway, this seemed to work - the temperature finally stabilzed... for a while. It started fluctuating again - though it isn't nearly as bad this time.

So the problem still exists. And today I noticed a bit of what might be an oily spray (coolant and oil?) on some of the engine components, which possibly suggests what you said (?).

I'm gonna take it in to my mechanic to have a look at it. Hopefully it's just a blown head gasket and not something more serious...