After overheating a few times (5.0), I swapped out the 30-year-old stocker (which had really been crappy) for a nice aluminum performance radiator, and all was well until the winter hit. Then it started not consistently holding normal temp after initial thermostat opening. I had replaced the thermostat along with the radiator, with a "fail-safe" model designed to basically destroy itself fully open if the engine overheats. Before long, I had a stuck-open thermostat. It had destroyed itself, even though I hadn't overheated. So that was enough of that, and I put in a quality regular thermostat. But now it's acting like the thermostat's letting too much through again, though I never had any heater deficiency. Then I noticed the cooling system was consistently not building pressure, like I had a leak or bad cap. Further, the cap and outlet side of the radiator weren't getting hot, just lukewarm.

Then it came to me- is my new radiator "too much"? It's a 2-row aluminum, with 1" tubes, which could put it at the equivalent of a standard 4-row. But isn't the thermostat supposed to regulate regardless of radiator size? A quick internet search seemed to indicate a common belief it could not be possible to over-radiate, because the thermostat regulates, but then something was mentioned I hadn't thought of- the heater circuit. It runs a constant flow of coolant from the intake manifold through the heater core to the pump, which BYPASSES the thermostat into the radiator. This now represents a baseline systemic coolant circulation untouched by the thermostat. If the radiator itself never warms up enough to send fairly warm coolant back into the engine, the thermostat is going to have to react by closing back down, and given the unregulated heater-circuit flow, it might well have to close down quite far into its operating range, where it just doesn't modulate engine temp well.

So I took the very easy step of blocking half the radiator with cardboard, and my temp and system pressure have bounced back. I'd guess you just can't send overcooled coolant back into the engine. So now i'll just have to keep observation on it when the weather warms back up, as i've obviously also blocked half of the a/c condenser.

 

I had similar problems. The first thing to do is look for the old style Robert/Shaw 333-192 thermo with the foot on it. If affects flow. TMI has new ones at 195 degrees.

 

No such thing as a radiator that's too big. You did right though in blocking the airflow with cardboard. The real test is in summertime not winter. You are correct in that the thermostat is the regulator of the temps. What temp t-stat are you running ?

 

I suspect that with a big enough radiator the little bypass hole in the thermostat may come into play.

 

This is definitely a t-stat issue. The heater core is not large enough to cool the engine. Plus, if it were the heater core flow, turning the heat off would cause the engine temperature to rise up.

 

Yes it is possible to put to much rad on some of these motors, the early 5.0 and 4.9 for example. These low output motors really don't need more than a 1 row rad, yes the system will "work" with a bigger rad... the thermostat will attempt to do it's job but at really cold temps the system may never stabilize cycling up and down in temp as the stat opens and closes and you will see over cooling at sustained highway speeds where engine temps never reach optimum levels unless much of the rad is covered. When I had the stock 5.0 in my '90 I created this exact problem when I replaced the stock rad with a 2 core, had to install a rad cover for winter driving or the engine would never reach full operating temp towing sleds up north in the winter. Never had that problem after I put the 5.8 in the truck or with the cammed 5.0.. both of which made a lot more power than the stock motor and as a result generated more heat.

 

 

Good point, if it's a 180-185 instead of a 192-195 then the engine will run cold and get less MPG.

 

If it is the correct thermostat, it will throttle the flow of coolant until the engine heat rises to the point that the thermostat begins to open. They are very responsive, so if if the coolant coming up is cooler the thermostat will close it off until the engine heats it up, again opening the thermostat. Not much can go wrong other than the thermostat getting stuck open, (over cooling) or stuck closed. (over heating.)

 

That depends on the rest of the system. If the system is less than optimum, it will run hotter than the rating of the T-Stat.