Engine running cooler than normal after water pump replacement

haulagebasher

Fiat Punto, model 1997-1999, 1.2 16V. The story is I did a timing belt kit and replaced the water pump along with it. The pump I took out was the original factory fitted unit. I've had this car over 5 years and I know it inside out and it has always run with the temp guage at exactly the 1/4 mark during normal driving. While in traffic it will rise almost to the 1/2 mark when the fan cuts in and keeps the temp at just over the 3/8 position before the temp drops back to 1/4 and stays there for normal driving. Now since the water pump change the temp is staying slightly cooler than normal and lags below the 1/4 mark during normal driving. I cannot figure it out. Everything was flushed refilled and bled according to the Haynes manual and I have checked the thermostat in boiling water and it opens and closed fine. Running cool would suggest a jammed open stat, which is not the case. The only thing I can think of is that the new pump was a QH one and had an impeller with longer curling vanes as opposed to the originals short straight vanes. Could an increased flow rate from the pump cause it to run cooler? Has anyone ever come across this sort of problem before? I don’t think it has anything to do with sludge being washed out of the rad and improving cooling as I have always changed the coolant periodically.

haulagebasher

Moderator: I think maybe the thread should be moved to DIY maintenance as it might be more suited to there. My bad.

Anan1

Might the thermostat be opening too early, ie at too low a temperature?

In the Brava I used to drive that had the same engine than your Punto the needle stayed bang in the middle once it had initially warmed up. Therefore I'm suggesting your thermostat needs replacing.

Why is it running cooler now? Maybe your new pump is more effective than the previous one or something could have gotten stuck in the thermostat (was there any crud in your cooling system when you drained it).

Edit: Too slow

haulagebasher

I don't think the stat is at fault because it was working perfectly before the pump was swapped and ran cooler immediately after. I feel it;s too much of a co-incidence for the stat to fail at axactly the time i changed the pump. Anywhay when I took the stat out there was nothing stuck in it. There is a tiny bypass valve in it which has a ball in it which seemed to be OK. OT: Damn glad I changed the TB too, it had two cracks in it!!

Stats don't suddenly fail but it is a gradual process. Brava was heating up sluggishly before I changed the stat and after that the temp was always bang in the middle.

Anan1

haulagebasher wrote: »

I don't think the stat is at fault because it was working perfectly before the pump was swapped and ran cooler immediately after. I feel it;s too much of a co-incidence for the stat to fail at axactly the time i changed the pump. Anywhay when I took the stat out there was nothing stuck in it. There is a tiny bypass valve in it which has a ball in it which seemed to be OK. OT: Damn glad I changed the TB too, it had two cracks in it!!

Might a failing thermostat have been compensating for a failing water pump? No matter how strong the pump, my understanding is that the stat should only open when the engine reaches normal operating temperature. If your engine never gets there then wouldn't it follow that the stat is opening too early?

firefly08

I would replace the thermostat, even if it appears to work. Because I had this problem once - removed the thermostat, fitted a new one, problem solved - but when I tested the old one, it tested fine. I don't know if the act of removing it unstuck it - maybe I'd have gotten away with putting it back in, but a new one is the only way to be 100% sure, and they are cheap.

Just to throw out a few more things that might cause cooler than normal temps:

Did you fill the system with the correct ratio of water/anti-freeze? Water cools better than anti-freeze, however you'd need a big difference in the ratio to see any difference on the gauge. Going from 50/50 to pure water might make a difference (but would be a bad idea obviously).

This one may be a stupid question, but did you remove any lower engine cover(s) and not put them back? That can make a big difference especially when driving.

Do you have an electric radiator fan? Can you tell if it's coming on/off at the right times? I always thought they fail by not coming on at all rather than coming on too soon, but it might be worth a look if you're stumped.

dr.fuzzenstein

Had bad overheating problem in my MX5 after changing the waterpump.
I flushed and bled and fiddled till i changed the rad, everything fine after that.
Sometimes changing the waterpump will merely highlight an existing problem.
Just throw in a stat, they're something like E10, that's cheap enough fault finding.

haulagebasher

I'll get a stat on the way home and fúck it into it. I'll let you know how it goes after that.