FINALLY!
First the symptoms:
Car would warm up then go up to about 225-230, cool down a little, then stabilize around 220. Didn't like that so I put in a 180 thermostat and a 195 fan switch. Car runs 225-230 while driving but is OK in the driveway so I borrow a Tech 1 and discover the gauge is off - the sensor at the front of the block for the ECM is reading 180 while my gauge inside is reading high. I buy a new gauge sensor for the back of the head and a new gauge from Ebay for the dash (old one pegged out a bunch before the electrical fix and was jittery). STILL shows hot........
I use an infrared thermometer and when the cooling fan turns on in the driveway, I get a nice temp drop. 145-150 right side pipe temp going back in to the engine and the gauge is normal. Get on the road and drive, it overheats. I turned on the A/C (and cooling fan) and the temp drops! I do a lot of thinking then it finally hits me. I pull over next to a dumpster and rip off a strip of cardboard and cover the bottom half of my radiator (actually condensor) and drive. Temps stay at 180!!

What is happening is the radiator is getting stopped up - of course the bottom section. The cooling air for the radiator is taking the path of least resistance and just going through the bottom section. The chin scoop doesn't pick up enough air to cool the entire radiator. I don't have any hood vents so the cooling air is stagnant up at the top. I got home, took the relay out for the cooling fan, reached between the blades and felt the bottom section of the radiator and it was cool!! Confimed. I'm guessing now that the lower flow through the system is why the back of the engine is hotter than the front (or I've got the dreaded spinning plastic WP impeller). I'll let everybody know the results after I get my radiator in. Had to order it.

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Mick
1986 GT

How long does it take to get to 230 degrees? Mine takes about 20 minutes and I have a large Nova 4-core radiator sticking well below where the stock one used to hang.

I wonder if you found MY problem, too!

I have an "SBC Overheats After 20 Minutes" thread up somewhere, but I can't give the link because I am writing from my PDA.

I have something to check in the garage tomorrow. Thanks for your post!

Just pop the hood, it will let air vent that way.

ummmm
unless you have some mods that you havent listed you still have a problem
my stock 84 cooling system more than keeps up with my current setup.

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87 Fiero GT 5sp with Vortec L35 4300 Turbocharged V6
Bully Stage 2 clutch
Syclone intake manifold and engine management with Moates adapter and chip burner
Air/water intercooler and Devil's Own progressive water/alky injection
50lb injectors, 3 bar map sensor, Walboro fuel pump and Jabasco Intercooler pump
LM1 wideband on custom manifolds and 3" stainless exhaust system
T31/T04B S4 turbo with a Super T61 in the box
S10 caliper conversion.
Murphy's Constant Matter will be damaged in direct proportion to its value
Murphy's Law of Thermodynamics Things get worse under pressure.
Arthur C. Clarke "Any significantly advanced technology is indistinguishable from magic"

It took about 20 min for mine too but I'm with vortecfiero, I think you might have another problem. By all means check the radiator especially if it came off another vehicle. Let us know what you find out.
I've got a pretty strong water pump here at the house. Before I put my new rad. in I think I'm going to disconnect my hoses from the long pipes around the back of the doors and pump water backwards through the old radiator and try to unplug it. If nothing else I can see what kind of flow I've got. I already dropped the pipes to check if some bonehead tried to jack the car up there and crush them but they were good.

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Mick
1986 GT

You also now know that your ECM temp sensor is reading low, which will be hurting your gas mileage.

quote Originally posted by cropduster: It took about 20 min for mine too but I'm with vortecfiero, I think you might have another problem. By all means check the radiator especially if it came off another vehicle. Let us know what you find out. I've got a pretty strong water pump here at the house. Before I put my new rad. in I think I'm going to disconnect my hoses from the long pipes around the back of the doors and pump water backwards through the old radiator and try to unplug it. If nothing else I can see what kind of flow I've got. I already dropped the pipes to check if some bonehead tried to jack the car up there and crush them but they were good.

Lots of mods. See https://www.fiero.nl/forum/Forum2/HTML/078605.html - I have an SBC in the back, so that's why the bigger 4-core went up front. Previous owner installed it.

Someone else mentioned that radiators that are too big can be a problem - and your solution explains the reason.

I wonder how hard it would be to have the ECM temp sender also do the temp gauge and leave the regular temp sender for the fan and idiot light.

Seems like the ECM temp sender is more acurate than the gauge sender.

I read that the temp sender in the head will tell you of an overheat condition before the ECM temp sensor in the t-stat housing detects it. It seems to be the gauge is the real problem. People with aftermarket gauges have posted coolant temps equal to the ECM coolant temp from ALDL. Also, the overheat light is separate from the gauge part so it will light up without the gauge working correctly.