The 3900s' coolant system is unique and sensitive to small changes in coolant level in the Fiero arrangement. If my coolant level at the engine filler cap drops below about 10 ounces I might be headed for an overheat. The thermostat is located at the water pump housing and hot coolant scavenging from the upper return coolant crossover at the end of the cylinder heads is used to activate the thermostat. When the coolant level drops a little the hot coolant flow to the thermostat can be interrupted by an apparent air pocket which keeps the thermostat from openning and potentially causing an overheat because the air pocket prevents water from getting to the pump to keep the coolant circulating through the engine.

It turns out the heater core was the cause of the gradual coolant leak causing the intermittent problem.



The hoses were a little difficult to disconnect from it but for knowledge of needing to remove the speaker to get to upper screws in the housing made the removal fairly easy otherwise.

The new heater core is aluminum and I'm not impressed with it considering aluminum doesn't transfer heat as well as the brass unit and it's not as thick as the original either. Hopefully being new and free of 25 years of coolant scale net no perceptable difference in performance.



Since I'm also due to charge up the A/C system I decided to take a look at the evaporator core to make sure that it wasn't the cause of the leak discovered during the last recharge attempt. I was not prepared for what I found although I expected to see a little debris.




You do not want to find a sight like this in your car. It was very difficult to remove the smaller bits and pieces of leaf, stem and fiber from the condenser than I imagined. High pressure air will not do it alone. I had to use a combination of straight pin, wide strips of tape applied to pull some of it out, and a heavy duty shop vac with the core positioned between the vacuum and exhaust ends with tapping on the upper and lower flat aluminum plate borders and believe it or not the vibration caused by tapping on it with rubber insulated plier handles seemed to remove debris better than any of the other methods used. I also flushed it.

I'll report on how the heater core performs after it's installed.

+1 for the show-and-tell

My heater core started to leak about 10 years ago and had to be replaced. Replacement was not difficult. The original lasted just 13 years on a Fiero that had only 60,000 miles on the OD and was always garage kept. I would venture to say that the original heater cores don't last very long.
As for Joe's overheating problem; a 1/8" hole drilled in the thermostat may solve the problem. If not relocating the stat might be a consideration. Its so easy to lose 10 oz of coolant so why take a chance?

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Flotech Afterburner Exhaust, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

[This message has been edited by Dennis LaGrua (edited 12-28-2011).]

quote Originally posted by Dennis LaGrua: My heater core started to leak about 10 years ago and had to be replaced. Replacement was not difficult. The original lasted just 13 years on a Fiero that had only 60,000 miles on the OD and was always garage kept. I would venture to say that the original heater cores don't last very long. As for Joe's overheating problem; a 1/8" hole drilled in the thermostat may solve the problem. If not relocating the stat might be a consideration. Its so easy to lose 10 oz of coolant so why take a chance?

I believe heater core life expectancy may be tied into the average coolant temperature and pressure it is exposed to. I ran the car for years without a thermostat because it was turbocharged, but after getting a little more education and redirection back to the laws of thermodynamics I realized that the best mpg is achieved with coolant temps in the 220 deg range so I installed the stock 3900 stat which kept my coolant temps just above 196 deg minimum. The leak didn't start until some time after the 3900 swap and persistent higher average coolant temps. I purchased the car used about 10 yrs ago and there's nothing to indicate whether or not it has ever been changed.

Holes drilled in the thermostat face do not help (I've tried it). They may also complicate the matter by allowing just enough coolant over the stat element to make it open slower since the coolant flows in the opposite direction of traditional flow over a thermostat (pintle end first). Joshua riedl doesn't have over heat problems but he has complained of the constant higher coolant temps, even with the thermostat removed on an 85 deg day coolant temp will run about 188.

The coolant arrangement with the thermostat in the back side of the pump housing is just a bad design. If you look at the picture below you'll see there are two hoses feeding back to the pump housing. One is closed off when the thermostat opens, you can also see that one is connected to a metal line which serves as the return flow for the upper metal line connected parallel to it that comes up out of the top of the coolant crossover that is connected to the cylinder heads. It goes straight up about 3-4" above the filler cap on the crossover and is a potential air trap. It feeds coolant to the turbo which comes back around to the pump housing. I may eliminate that feed and tie the turbo and heater core in series between the lower return feed, seen to the left of the radiator return outlet, back to the pump housing.



If the pump housing gets an air bubble and stops pumping water, the stat will not open and the coolant will sit static. I experienced it yesterday when the heater started blowing cold air and the temp gauge started to climb as a confirmation. The stat should have been placed up high in the same manner that it was in the Fiero 2.8L, or at the back end of the intake manifold down low the way it is in the non VVT 3500 and below.

[This message has been edited by Joseph Upson (edited 12-28-2011).]