Overheating

Hi - I have a beautiful 1987 Pontiac Fiero GT (original owner) which (knock on wood) has rarely given me problems - but I a stumped now (probably because I know next to nothing about auto repairs).
The problem is overheating as follows:
1) I fill the place (where the thermostat is housed - on the rear engine) with a 50/50 mixture of coolant and water. This is done until it begins to flush out from the radiator (I had already taken the front radiator cap off).
2) I fill the Coolant overflow tank with coolant.
3) After about 5 minutes of driving with no problems - the heating gaige "redlines" - I pull over and it is hissing like it's read to explode. Please note: Nothing appears to be leaking because I always check - I know the original water and coolant is still in there - albeit probably turned to steam.
4) Since nothing was leaking - I assumed it was a bad thermostat so I purchased another one - yet the same problem.
5) I replaced the water pump in 2004 (8,000 miles ago - I drive the car very little) and replaced the radiator in 2001 (19,000 miles ago) - so I would assume it shouldn't be one of those (I hope)
Can anyone give me some guidance on what the problem might be. ?
Thanks so much in advance.
Rick
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Did this start after the new thermostat was installed, are you sure the new thermostat is installed properly and it's the correct one? I don't know anything about the Fiero as far as bleeding air from the cooling system but from reading your post I don't think that's your problem. If all is right with the thermostat check your hoses if the hose on the return side collapses it will restrict the flow. If your problem started after any work was done on the cooling system (I would guess that's why you were refilling it) I would look there first. If you don't find anything wrong, take it or tow it to a radiator shop and let them flow test it before the overheating does any damage.
Al

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If not leaking, but also not cooling, then I say maybe not circulating. If not circulating, but thermostat valve is good, then perhaps clogged radiator?
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On Wed, 07 Jun 2006 02:16:21 GMT, snipped-for-privacy@ihatespam.net (SgtSilicon) wrote:

Oops, or collapsed hose.
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