Overheating, mystery coolant disappearance
 

Hi all,

Having some trouble searching for answers on this specific question and thought I'd throw up a thread. Here's the scoop:

Bought 1994 Ranger XLT 4x4 with 4.0L OHV V6, noticed coolant was a little low, added some coolant to the overflow tank until level was above COLD FULL.

Took the truck on a long highway drive followed by a steep, twisty mountain road. On the highway, temp gauge was in the upper quarter or so but stayed steady. On the mountain road, pushing over the highest pass, it was pinned to the top of the dial--I didn't see any "check engine/check gauges" lights but the needle on the temp gauge was as high as it would go. I hadn't checked it in a while so I'm not sure how long it was overheating, but not more than 20 minutes at the outside and possibly considerably less.

Pulled over immediately, let it cool off for ~20min, heard it gurgling and saw it steam when I dripped some water on the outside of the radiator. Coasted/limped the rest of the way (it was mostly downhill.). Needle crept alarmingly high when I wasn't coasting but I kept it out of the red.

Diagnostic tests: coolant level in overflow tank had dropped below "cold full". Coolant in radiator is full and very green. No visible exterior leaks. No white smoke on startup, no sweet smell. Tried the glove test (got that off these forums btw, thanks guys) and no movement/inflation/"dance". Oil on dipstick looks clear and brown (it was changed recently, though before the overheating episode), not "milky". My next thought is thermostat and I'll probably try replacing that and/or bypassing the heater core in the next couple days. The upper radiator hose seems to warm up but maybe not as fast as the engine--so I'm thinking maybe the thermostat isn't all the way stuck, or is sticking sometimes but not always.

Of course, I'm interested in any general advice you may have, but I'm also concerned about the coolant level in the overflow tank. Could it just be that the coolant I added had to circulate throughout the cooling system? Or is there somewhere else it could be leaking to that my tests wouldn't have detected?

Thanks all,
psmith

Welcome to the forum

Just FYI, there is no Temp warning light in a 1994, just have to watch the gauge, no low fuel light either

Reads like clogged radiator, may be thermostat, but more likely radiator
Clogged heater core will cause temp gauge to go up and down randomly, not just go up and stay there, but easy to change on a 1994 Ranger

Easy test for radiator
Warm up the engine, thermostat open, upper hose hot
Have a friend hold engine at say 2,000rpm
Squeeze upper rad hose then squeeze lower rad hose
If if upper is hard as a rock and lower rad hose is easier to squeeze then rad is not flowing well
Have friend REV engine repeatedly while you squeeze lower hose, if its easier to squeeze as RPMs go up then water pump is sucking in coolant faster than it can flow thru the radiator.

Rad hose at the top and bottom are the same size so there should be no restriction on flow thru the rad, so lower hose should have exactly the same amount of coolant flow out as is coming in on the top hose

As the engine gets overheated the coolant expands and exits via the overflow hose to the overflow tank and tank will over flow so some coolant will be lost
As engine cools down any coolant left in the tank will be sucked back into radiator, to keep it full
So your coolant was lost in steam and overflow because engine was running too hot

1994 4.0l should run with temp gauge just below 1/2 way, up a long grade or pulling a trailer it will go above 1/2 but never to 3/4, that's over heated

Thanks much RonD, super helpful. Do you think this could also be a water pump issue?

I ask because prior owner says he recently replaced the radiator, so I don't think this one would have clogged just by virtue of getting old. But there's an audible whine at idle that might be water pump bearings, and a water pump on its last legs might cause circulation issues + clog the radiator with metal shavings.