Temp running up and fluctuating

[xirdneh]

My friends tercel is running hot between 5/8 and 3/4 when it usually ran just under 1/2 way on the gage.
It fluctuates between 1/2 and 3/4
turning on heater and its fan to high speed brings it down fast to 1/2
turn heater and fan off and it shoots right back up

we installed a new radiator last fall (aluminum core)
today I installed a new radiator cap and thermostat
no change so i swapped temp sender and still no change
waterpump belt is tight
timing is fine (slightly less advanced than my two tercels that run fine)
he says it has been like this for two weeks and has not had to add water (i want to believe its a head gasket)

what do i have left to check but the waterpump?
the impeller ?
i have seen waterpumps that leaked when seal and/or bearing went bad but this one is not leaking and even if it did leak that would not cause this problem
has anyone had the impeller go bad?
is there something i am missing?

[dlb]

i'm leaning to head gasket but if he hasn't had to add water, that's odd. i would do a compression test and see if anything shows up.

[Petros]

It is acting like it has limited cooling capacity, when you add the heater temp comes down, and than goes back up when you turn it off.

Make sure all of the trapped air is out of the system. Also, verify the lower radiator hose is firm and not collapsing when the engine is reved (when the pump suck water from the radiator). Not likely a water pump issue would cause it, if there is not leak at the seal, the pump is likely good.

All of the behavior indicates a bad thermostat. A head gasket will not cause that behavior unless it is also consuming coolant, which means you will see steam vapor out the exhaust. If you are not loosing coolant, than it is not likely the head gasket. If the cooling system is full, even a bad head gasket will not cause it to over heat until you consume enough coolant to make it run hot. Than it will not fluctuate, but just get hotter and hotter as the coolant is consumed.

Sometimes even "new" thermostats are bad right out of the box, that is why I only buy the premium quality ones, and even than do not trust them until they have proven they behave properly. Also, you might verify the radiator is not blocked up. The only two causes of this behavior is plugged up radiator, hose, or t-stat not opening all the way.

[aoeu]

It sounds like a clog or a weak pump. Since it's not overheating, you might check the EGR and VSV since those lower combustion temperatures (make sure you aren't running high temp plugs as well). It also might be worth temporarily setting the timing at 5 deg BTDC just to make sure that's not the cause.

[marlinh]

Has anyone confirmed that the cooling fan is coming on? When does the temperature go up. Sitting idling? Climbing a steep hill? Cruise? Stop and go traffic?

[xirdneh]

back at it again today
installed new lower hose (wasn't that)
removed and ran without thermostat (wasn't that)
removed water pump, it was pristine so (wasn't that)
replaced upper hose (wasn't that)
Cooling fan was working properly

replaced new (six month old) doublerow aluminum core radiator with old single row radiator

that fixed it

I am assuming the radiator got messed up somehow when he blew a head gasket last february and we installed a different engine. i will be taking it to a radiator shop for testing

[Petros]

might just need a back flush. The rad shop can remove the top and clean out the tubes with long pipe cleaners if necessary, thought costs about have as much as a new rad, it is still cheaper than buying a new radiator.

I had to have that done with a six month old very costly Mazda MPV raditor. Crap "OEM" (lie) remanufactured water pump shredded it self into metal shavings after only 13 months, and plugged up the near new radiator.