d00med

If the thermostat is failing on my '89 4L Pioneer, what symptoms should I expect? Overheating quickly, obviously. But I am losing fluid and while my radiator seems to have a slow leak, thus accounting for loss of some of the fluid, I don't know why I should be running hot within a few miles with just a leak being the problem -- a leak that doesn't leak at all sometimes, and other times dumps less than half a pint to the ground.

After many years of faithful service, and then many years of being neglected, the old truck is getting attention from me again, and I want to (re)learn all I can. Thanks.

Turbo X_J

A 1989 subjected to years of neglect? It's a safe bet to replace everything.

djb383

Almost sounds like one wants to neglect their poor old Jeep even longer because it's just a slow leak.

Gimmy

fix the leak...a leak means there is no pressure, no pressure will result in a lower boiling point of the coolant.

if you have an air bubble in the system it will over heat because the coolant isnt traveling through the full radiation system.

Does the heater work?

if not then thats a good indication the thermostat isnt opening to let coolant through. or a sign that there is not fluid in there at all.

FIX THE LEAK

and while youre at it replace the stock radiator with one that has an open system and get rid of the over flow tank at the firewall

djb383

If I'm reading between the lines of the OP correctly, it sounds like there may not be a good understanding, by the OP, of how a properly functioning cooling system works. If that is the case, then the OP needs to understand the differences between closed/open cooling systems. He needs to understand that there's more to it than just getting rid of the overflow tank at the firewall. Both systems require a tank, but they are plumbed differently and one is pressurized and one is open to the atmosphere. He needs to also understand that neither has an advantage over the other regarding cooling efficiency.

JWheeler

This. I had a customer argue with me for an hour and tell me I was trying to rip off his wife even after I explained this. Couldn't grasp the concept. Had a badly leaking thermostat housing and insisted that could not be the cause of overheating because he always kept the level up.

Gimmy

Yes that was worded bad and in a hurry. But by swapping to a different style radiator it, in some cases, makes the system easier to work with and keeping out air pockets

Firestorm500

Running hot after a few miles and consuming coolant is a couple of telltale signs of a blown head gasket.

If a thermostat fails, it tends to fail open, leading to slow or inadequate warm up.

OP said that radiator "seems" to have a slow leak. Where is it leaking coolant? The dripping on the ground could be a result of overheating. Has he looked for stains on the radiator itself?

Before throwing parts at it, someone needs to do a thorough diagnostic.

d00med

The OP agrees that "someone" does. Meantime, you are all right that I need to understand proper operation better.

Symptoms are not entirely consistent. Going a mile this way or that, it doesn't warm up too much, nor does it run too cold. But if I go far enough I start steaming. Leakage while parked is very slow, usually from zero to a few drops, but occasionally a cup's worth, from somewhere along the passenger side of the radiator and nowhere aft of that. I was going to replace the thermostat out of principle but haven't yet. Heater works fine.

Back story: Prior to this, I though my problem was just that the overflow bottle had cracked, but replacing that only shifted the problem.

Head gasket idea is scary but I've no indication at all (foam) on the dipstick.

Would love to fix this leak without replacing / modifying the system. Never done that, some research is in order, and tips are appreciated. Thanks all for the input.

Firestorm500

Head gasket possibly blown into the cooling system, causing quick overheating, cracking pressure bottles and radiators due to pressurization from combustion gasses.

It's not the thermostat.

Chronic overheating is ruining your engine.

d00med

That's worth having checked, and fixed if need be. Will do so asap. Thanks.

highmileage

Head gasket would be the easiest way, but if it keeps it up a crack between the valves on the #1 is a place to look( RENIX heads like this spot). Pull the plug and see if it wet or rusty, or all of the plugs.
Also you can try starting it with the cap off and look for coolant being readily pushed out or lots of bubbles and an exhaust smell.
Head gaskets do not always blow to the oil side, when they blow to the cylinder side you get lots of heat and a deteriorating cooling system.