GetCaughtDead

Hi, guys. My '06 tC has about 121k miles now, and I just had to replace my water pump (and thermostat, for good measure) yesterday. Not the easiest job to do in a parking lot, in February, with temps in the 35-40 deg range. But everything seemed to work out just fine, I flushed the system with Peak Super Cleaner and Flush, and refilled with Toyota SLLC.

I first noticed while doing the multiple flushes (water, cleaner, water, water) that it was idling rather low, like around ~350 RPM or so. I thought that maybe it was a side effect of having the battery disconnected all day and that it would correct itself when the ECU had some time to relearn. But after driving 16 miles to work this morning, it really didn't get any better and I even stalled it at a red light.

What's more, my gas milage was terrible! My SGII showed that I got 18.5 mpg, when I normally get 30 on the same drive. I'm not getting any codes, don't seem to have any leaks, and the temp gauge indicates that it's heating normally. What could be causing this??

GetCaughtDead

Update: So, I spent several hours of company time researching this yesterday and it doesn't seem to be an uncommon problem, and not just for the tC. I actually found several other queries around the web by owners of other makes and models who experienced this after replacing their water pumps. Suggestions ranged from disconnected or cracked vacuum lines, crankshaft position sensor, alternator/regulator wires, and some other, vehicle-specific suggestions that didn't apply to the tC. Alas, nobody seemed to have a verified, definitive answer, and I spent over an hour checking all of these things, to no avail.

Lo and behold, as I was driving to an appointment after work yesterday (17 miles) I actually averaged 29 mpg, though my idle was still troublingly low and it managed to stall at 3 different redlights. But on my way home from there (22 miles), I averaged 31 mpg and the idle was solid around 800 RPM.

The only thing I can really chalk it up to is the ECU taking way longer to relearn than I expected. Apparently, 16 miles and 30 minutes just isn't long enough. Altogether, it took around 50 miles and over an hour of actual driving (on top of the combined 45 min or so that it ran during the coolant flushes) before everything was back to normal. I had no idea it could take so long, considering nothing changed.

I think the actual water pump repair was purely coincidence, as may well be the case for the other vehicle owners experiencing this. One might find similar stories of people replacing alternators, stereos, or anything that involves an ECU reset. So, although you might call it somewhat of a false alarm, I figured I'll leave the post here for posterity, rather than delete it.

Of course, if anyone has something to add, feel free.