[This message has been edited by Mike in Sydney (edited 11-04-2023).]

quote Originally posted by Cliff Pennock: So first my radiator was leaking until it failed. I replaced the radiator and two weeks later, my heater core failed. Replaced that, had no problems for two weeks and now I have a large leak around the thermostat housing. I'm not sure if it's actually the housing, but I suspect it is. How difficult is it to replace it? And after I have done so, what will the next weakest link be? I've replaced the water pump two years ago so I think that's still in good condition.

Unfortunately, that's the way these things go.

Heater core (in my opinion) has always been the first one to go, and then the radiator. But I'll spare you the... this car is 500 years old and blah blah.


But basically, with a new radiator, and a new heater core... OTHER THAN HOSES, there's only three locations left where it might fail...

1 - The filler neck (either it's rusted out, or the gasket has failed)
2 - The water pump... yeah...
3 - The heater core cooling tubes that run along the center of the car.


#1 is super easy to fix. If you can find a replacement, just replace it. But more than likely all you need is a new gasket and some finger-applied RTV... just have to check.

#2 is obviously... you have to replace the water pump. You'll see it leaking through the weep hole, which means the bearing seal (and soon, the bearing) is going out.

#3 is more... well... problematic. To be clear, this isn't the stainless tubes that run along the underside of the car's thresholds on either side, but in fact the insulated tubes that run through the inside of the fuel tank tunnel. Reposting this picture from an old Fiero I unfortunately cut up, but as someone else mentioned... it gives a really good look at how these tubes are run...



The top (drivers side) one that is insulated is ONE of the cooling tubes that goes TO the heater core, which is incidentally right next to the A/C pressure line, which is hot (and ideally should be cool... which is why it's not insulated).

The bottom (passenger side) one that is NOT insulated, is the cooling tube RETURN line from the heater core, which is not insulated because the goal is for it to cool off before it reconnects to the drivers side return threshold cooling tube just before it goes back to the engine. This sits right next to the low-side A/C compressor line which is insulated because it's supposed to be cold.


... with that, these lines can eventually corrode and fail. I've never actually seem them fail, to be honest, but I've heard of people having issues with them in high road-salt states. And also... yeah, the car is 30+ years old at this point and these are going to fail if your car has been on the road forever.


Now, all of that aside, there are several hoses that can fail... and the Fiero has like literally, twice as many hoses.

- FRONT - Radiator to return cooling pipe
- FRONT - outlet cooling pipe to radiator
- REAR - return cooling pipe to engine
- REAR - outlet cooling pipe to engine

.. and then also,

- FRONT - Heater Core to feed line
- FRONT - return line to Heater Core
- REAR - heater core feed line conduit to water pump
- REAR - drivers cooling pipe to heater core return line (which is usually a crimp line)

... and then also, small lines connecting the throttle body (heater) to the cooling system. These are the two stainless tubes coming off the throttle body, but after the pass under the plenum, they turn to small rubber lines which can wear out and leak (and this may actually be your problem, they connect to the filler neck, etc.).


... and then depending on whether it's a 4cyl of a V6, there may be an additional cooling line that goes around the firewall to the rear side of the engine bay to warm up the intake (on the 4cyl), etc...


... and as Mike says, any one of the coolant sensors (either for temperature readings, or switch). And worst of all... a head gasket leak.

[This message has been edited by 82-T/A [At Work] (edited 11-04-2023).]

[This message has been edited by lateFormula (edited 11-05-2023).]

Cliff- on my thermo-housing I never could get the gasket to seal properly....after a while I tried sanding the ends of the flange slightly where the bolts go in, which puts more pressure on the gasket- worked well (You don't need much- my drawing is exaggerated)

[This message has been edited by cvxjet (edited 11-04-2023).]

[This message has been edited by buddycraigg (edited 11-04-2023).]