Any help would be appreciated. I have A 1997 Sidekick Sport 1.8L. It overheats when driven hard. I've changed the radiator, thermostat, and upper and lower hoses. Nothing has helped so far. I've replaced the water and antifreeze every time. It will not get hot just sitting at idle but when ran at about 3500 rpm for a short time the temp climbs. Funny thing is just before it starts to get hot the heater will start blowing cooler air, not cold just cooler and then the temp quickly goes up. I took both heater core lines off and blew through them thinking it was clogged, it was not. I do not know what to try next. SOMEBODY please help!!!

are you loseing fluid?  That sounds like the symtoms of a head gasket.

That's what someone else told me, but I'm not losing water and the oil is not "foamy" or milky. I really do not know what to try next. The heater thing is what I do not understand.

Sounds like the water pump is the only thing left to swap out?

Before you do that, have you tried making sure your new thermostat is opening at the proper temp? I hear the best way to do this is with a thermomitor, and a pot of watter on the stove.

I've replaced the water pump and ran it without the thermostat. I've also boil checked the new stat and it opens at the right temp. That is why this is so puzzling to me!!!

how's the fan and shroud??

Fan is good, clutch has some tension when not running. Shroud is all in tact and in place.

Are you just going by the temp gauge, or does the over flow bottle start to boil?

It could just be a screwed up sensor.  You've replaced the entire system pretty much, so there's not much else it could be.

man that would suck if its the sensor

I'm just going by the temp gauge. The water in the overflow has never boiled that I know of. If it was the sensor why would that suck???

after spending that much on everything else.  at east you now have a new cooling system
the sending unit is easy to change. right on the front of the intke manifold

The 2 times mine has over heated, the overflow tank boiled and coolant was flowing out of it's release pipe (or whatever you call it). 

I have a strong feeling that maybe your gauge and temp sensor are giving you false readings.  I drove a car yesterday that showed the temp dropping as you drove it, instead of it rising.

Temp sensors are $10-20 i think, and easy to replace.  That should be your next move in my opinion.

The sending unit is at the back of the engine on a 1.8L.  "Coolant Temperature Sensor" is the correct name for it. Don't go asking the guys at AutoZone for a temp sending unit, 'cause they'll say they don't have one. They will, however, have a "coolant temperature sensor"! Mine was about  $40 from them.
   It is a three wire setup, and sends signals to both the computer and the temp gauge in the dash. I had the reverse problem on my Sport, the temp gauge would stay right in the middle, but the engine would boil fluid out. I had put an electric fan in place of the missing(when I got it) mechanical fan, and the aftermarket fan sending unit went bad. Anyway, I replaced the sensor, and now the gauge moves at least. I'm putting in a set of digital gauges to keep a closer eye on things though. As a plus, my mileage went up about 1 to 1 1/2 MPG!

Thanks for all the help guys!! I hope I can get this fixed. I love my Zuki but not when it gives me this kind of trouble. By the way I have a 1.5" coil spacer lift, 3" Masterkit body lift and 30.5" MT tires. I also have a "homemade" cold air kit and Glasspack muffler with a 16" long x 3" stainless tip. It sounds great!! I also sell the 1.5" coil spacer lifts on E-bay if anyone's interested. Look under the seller name: hoepkers. Thanks again!!