My temp gauge has been erratic lately - really going to extremes in the morning before settling out and acting normal. I read here that water pumps with the plastic impellers are bad. How can you tell which kind you have or if one is slipping??

Have you checked to see if your coolant is full (NOT on the radiator end)? That caused a number of problems for me when I was having cooling system problems. I pulled out a metal pump and put a new metal one in though - haven't yet seen a plastic one. Might want to check your sensor as well. Does it *feel* that hot? Do you have one of those IR temperature-reader things?

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'87 Fiero GT, Automatic, 153k miles, stock everything, just trying to make it all work again.
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Your thermostat might also be going bad.

Usually, when your pump starts to die, you can hear the noise of the bearing failing. You'll also usually see a slight coolant leak from the weep hole on the top of the pump. One way to check the bearing is to grab the front of the pulley and see if there is any play in it.

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2 Corinthians 5: 17
1987 Pontiac Fiero GT, 5 Speed
2005 Pontiac Grand Am SE, 3400 V6

[This message has been edited by 87_FieroGT (edited 12-10-2006).]

Take the thermostat out, that rules out one thing. If it stops overheating, then its the thermostat, if it continues, read on.

When my waterpump went bad...it went completely without warning. One day it was fine, temperatures were normal, it made a 300 mile trip just fine. The next day, it started started overheating. With mine, I had a plastic impeller, so I guess these are the symptoms of one of those failing. The car would be fine, until it warmed up. Once up to operating temp it'd just skyrocket to 240+. From what I've heard from others, the plastic impeller would expand as it would heat up, and it would come loose on its shaft. So the pulley would spin, but the impeller wouldn't, which means zero coolant flow.

Extremes in temperature sounds like a problem with air in the system. If there's air trapped inside the cooling system, the air pockets will move around and hit the temp sensor, making the readings skyrocket. Then when water comes back into that area temps go back down. Try the burping procedure in Ogre's Cave, see if the problem goes away. The car overheats at all after that, replace the pump. It really isn't that hard to do.

You can put a mechanics stethoscope or (long screwdriver to ear) on the pump and listen. If your gauge is bouncing or pegging try this http://www.fierosails.com/tempgage.html as mentioned above check the weep hole for any leakage/drops. If you see new gasket sealant around the pump area it may be a good indicator that its been replaced, if the car came from a used car lot, I`d bet on plastic.

thats funny tha tthis come up
look at the water pump is htere green around it?

mine is leaking like well- a chevy- in my 94 saffari i am going to replace it monday lol. funny this comes up like that- i have found as long as i keep the antifreeze full than i am fine for now- at least for these comple of days. i think it is probably from being old and from me beating the piss out of it lol.

Thanks for all the input!! I've only had this car 3 months but it looks like relatively new sealant oozing out of the water pump (hard to tell). No leaks, and I've already replaced the fan switch, thermostat (180 deg.), and gauge sending unit. Shouldn't be going to 240 like it does even for a little bit.
Guess I'll try burping again and see if that helps then my plan is to check the resistance of the wire to ground at the sending unit at different temps to see if it's really getting hot or an indication problem.
I borrowed a friends infrared temp gauge a while back (awesomely cool!!) and the difference of the cooling pipes on both sides of the car was 50 degrees so I know the radiator isn't plugged. If the impeller is slipping at times, that would explain things.

Didn't see that anyone mentioned checking the coolant tubes under the car to see if any are damaged from sone dummy jacking up the car on them.
And like said run the car without the stat and see if it still overheats.

You can sometimes run the engine with the stat out and see how much flow there is from the pump looking in the stat opening.

Yeah...once your engine starts to over heat and you've checked the thermostats and radiator caps...looked underneath the vehicle for traces of coolant leakage from the radiator and water pump...and listened for funny sounds coming from the engine compartment sort of like something is lightly grinding or it could be as loud as a belt squealing due to coolant dripping from the waterpump on to the engine belts...then you know it's your water pump. They can be a pain to take off and put on...but take your time and pllllllleease don't over torque the bolts on the new one or you will crack the pump...whats worst is you may not know that you cracked it until you've finished with everything and added coolant back into the thermostat housing to find out that you have another leak...I did this and found out after I went through the motions of removing the new water pump that pieces broke off because I over torqued the bolts...this voids the warranty on the new water pumps too (unless you run into a cool guy at the parts counter who helps you out with the return).

Raifalino

quote Originally posted by 87_FieroGT: ...Usually, when your pump starts to die, you can hear the noise of the bearing failing. ...

Three or four pumps (metal) over 18 years all the same. Grinding noise. Yours sounds like burping to me.