Losing heat in the Matrix
 

Well, I have what I think is probably a water pump going bad. It just started this weekend when I was comming home from a trip to LaCrosse (across the state). The car ran great on the way out, but on the way home we didn't have any heat for the first few minutes of the ride despite the coolant temp being high enough to normally get good heat (130F+). It seemed as soon as we got on higher speed roads the problem aleviated itself. Again this morning it did the same thing. On the low speed roads we didn't have much heat, but once we hit the highway it warmed up more. As soon as we got back to low speed roads it cooled off again. The curious thing about it is, when I slowed back down I started watching the coolant temp and it actually went up. So, my guess is that the pump is going bad and the lower RPMs aren't flowing enough coolant to keep the heater core and engine cooled down. Any other thoughts?

I gotta say, if it is the water pump I'm a going to be disappointed with Toyota. My Tercel has over 200k on the clock and is still running the original pump, same with my wife's old Tercel with 180k+ miles. The Matrix only has just over 100k.

First things first, check your coolant when the engine is cold. It could've gotten hot enough to boil over (grille block) leaving you a bit short. It gets hotter at low rpm because there isn't enough liquid for decent circulation at the lower pressure, but if you bump the impeller speed up there's enough pressure to push whatever is left through the cooling system.

in some cases the fins on the water pump actually just go. so while the pump is turning its not really moving the coolant. another guess is you have a leak and lost some coolant or your burning some coolant. if your burning some coolant through a head gasket or if you have a leak there is a chance you could have gotten air into the system. allowing air bubbles to come in. to getride of the air bubbles start your car and remove the radiator cap add coolant to system and watch for the bubbles to keep coming when it stops bubbling put the cap back on and shut off the car.

or it could be what roflwaffle posted

I did some searching on matrixowners and genvibe. Apparently some people have had problems with coolant weeping out from around the water pump area. Over 10k miles the weeping makes a difference in coolant level. I topped off the coolant and I think the problems are solved.

Problem solved... except for the weeping!

Poor, sad weeping car.

My wife took the car on a trip this weekend and it ran beautifully again. Same with this morning, ran great.

1ZZ owners check your coolant levels though!