Much ado about nothing; and some very real danger. If you guys are burning through these heater fittings on a frequent basis, you've got something wrong and you're
not fixing the right thing. Sure, the fitting corrodes, and you need to replace the fitting.
But WHY did it corrode?
1. Coolant used beyond it's service life becomes acidic and will then cause corrosion. It can also create "radiator voltage" due to the electrical action of dis-similar metals--the iron, aluminum, brass, and copper sharing the same electrolyte (coolant) become a giant low-power battery. The stray electricity can cause corrosion. Solution is to flush and replace the coolant as needed, and use a proper concentration of anti-freeze which contains corrosion inhibitors.
2. FAILURE TO PROVIDE PROPER ELECTRICAL GROUNDS can "electrify" the coolant, causing electrolysis corrosion.
Original fitting on my K1500 went about 200,000 miles. Replacement (Dorman, I think) fitting has about 100,000 miles, 18 years, and is leak-free and corrosion-free. I pulled it out of the original TBI intake manifold, and screwed it onto the replacement (ZZ4-style) manifold using a brass elbow and close nipple about four weeks ago.
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^^^ That "crappy design" fitting is not bad looking, considering it's old enough to vote.
For the record...with outside temperature at -1F, the cab gets so friggin' hot in-town I have to turn the heater fan speed or temperature down. On the highway, I have to turn the fan speed AND the temperature down.
DANGER:
The original fitting is HEAVILY RESTRICTED INTERNALLY, which your hardware-store barbed fittings are NOT.
There's a reason GM engineered a restriction into the fitting--high engine speed (fast water-pump speed) can over-pressurize the heater core, leading to leakage. This isn't new or TBI-specific. GM has been restricting the water supply to the heater core for decades, to reduce the likelyhood of popping the heater core.
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Note restriction in heater fitting, lower left corner of photo.