Vehicle belongs to my son but he left it in my possession when he moved to another state. So, I am not thoroughly familiar with its quirks.

I am located in Ohio. I have discovered that the heater does not put out heat when the engine is idling, either parked or stopped in traffic. When the engine rpm increases, such as leaving a stop in traffic, heat output is noticed. But, it stops when coming to the next traffic stop, and so on. My son had the vehicle in a warmer climate before leaving it here in Ohio and this issue would have been less noticeable there.

I've also noticed that the temp gauge shows the water temp reaching normal temp, even from a cold start in winter here, within a very short time.

I suspect 1 or 2 issues. One is that the thermostat might need replaced. The other is that cooling system is clogged and needs a vigorous flushing. I suspect it has not been flushed for a very long time. The vehicle has about 200K miles on it.

Seeking opinions.

 

I would check the water pump. It is possible that the impeller blades have corroded away and at idle there is no coolant moving around. Once you get some RPMs, some fluid gets pushed through.

 

I had the exact thing happen in my 2002 3.5L Montero Sport - there is air in the system. We would also notice a gurgle sound in the dash when the heat would resume with increasing RPM.

The attached picture shows a vent screw in the jacket water system. I parked the vehicle on the crest of a hill with the engine at the top side. Removed this screw and filled the radiator till coolant came out this port. Reinstalled screw, radiator cap, and problem solved. This is the high point air bleed and where air collects.

 

Thanks for the replies; two good suggestions!

How difficult is replacing the water pump on a Montero Sport? Does the water pump run off the timing belt? Now, I'm thinking a weak water pump could be the cause of the temp gauge showing normal temp so soon after a cold start.

larry4406, I have not noticed the gurgle sound you mentioned but this is an easy procedure and seems work checking. I've done this procedure with a Honda Accord. For the Montero, can this be done on level ground?

 

Larry4406 is right about air trapped in the system. It can also result in engine warming up faster - lack of proper coolant circulation. Try bleeding the system first.
Water pump runs off the timing belt, so it is a serious job to replace it. It is totally doable, if you are comfortable wrenching and read up on the procedures first (especially making sure timing is correct once you are done).

 

I found I needed to bleed it on an incline as described. This ensured that the radiator cap fill point was at a higher elevation than the bleed screw so that as coolant was added to the radiator it pushed the air out and when coolant finally came out the port, I then reinstalled the bolt and continued to fill the radiator.

I agree with Hunter - water pump is deep in and timing belt is involved.

Try the bleed first.