Hey all,

I've read a whole bunch of threads on cooling issues, I haven't found one quite like mine and I'm in the process of replacing a couple of parts (the thermostat and water pump) but thought I'd post and see if there's any good feedback, which is all much appreciated. I'm a novice and just doing my best. Truck is 1994 4.9l 4x4.

1.) I bought the truck and it had a gauge issue, as soon as you would turn on the truck the gauge would instantly go to "HOT"/H, which would be impossible. After driving for 20-30 mins the gauge would return to normal and sit around the N/O/R on Normal.

2.) After a few months I noticed coolant issues, I was refilling a lot, it looked like the water pump seal was leaking, but I wasn't sure. It has an almost a brand new radiator in it. I checked the oil, clear as day, no milky/coolant. The leak seemed to get worse. I decided (probably a bad idea) to put in some coolant sealer into the cooling system hoping to plug the gap. I was a bit nervous because I couldn't rely on the gauge to know if it was overheating unless I was driving for more than 20-30 mins, and I use the truck mostly for 5-10 min drives (there's only 8miles of road where I live).

3.) The coolant leak seemed to subside after putting in the sealer (K-Seal), but then the gauge started acting completely different. Now on start up it seemed to have an accurate reading, starting on C and then moving up slowly, but it would (over 5 minutes) move all the way up to H. I checked the top radiator hose and it would be warm, the bottom hose would be complete cool, the coolant in the radiator was cool. The system seemed to pressurize, pulling back the pressure lever on the radiator cap caused air/coolant to flow back into the reserve tank. I got nervous about the sealant in the system and flushed the whole coolant system out and put in new coolant. But nothing changed. I then noticed the reserve coolant tank filled up after a drive (a couple inches from the top) which I read is basically normal behavior but it didn't seem to go back down to the cool line).

4.) My novice brain (saving head gasket for last because it's probably out of my league) seems to think it has to be a) the coolant is not flowing through the radiator, b) or the gauge is wrong, but I'm not banking on "b". I've also thought maybe it needs to be "burped" but I don't even know, reading about air in system it's not clear how to diagnose that. But as I said, just a novice so I'm just doing one thing at a time and sometimes throwing parts at it

5.) I've picked up a new thermostat/thermostat gasket and some rtv sealant, I've got a new water pump coming. I've drained the cooling system, and taken down the front of the engine, tomorrow I'll be installing the new thermostat and water pump. I put a garden hose through the radiator and it seems to flow just fine from top to bottom. The waterpump actually looked okay (to my novice eyes), I do think thats where the original leak was coming from though, possibly around the thermostat housing as well, the seal was grimy and it just looked dirty and not in the best shape, there was grime and moisture coming from both.

One obvious question, I bought an RT1139 Motorcraft thermostat from Walmart, it looks different than the thermostat I removed, and is there a certain orientation for the thermostat when installing? It has the little brass plug/thing, is there a specific orientation for it?

Thermostat: https://www.walmart.com/ip/Motorcraf...t-Asy/47930002


Any feedback/input tips/tricks is appreciated. Thanks for reading the wall of text.

 

First off, that tank connected to your radiator is an Overflow Tank, not a reserve/expansion tank. Once the coolant passes through the spring-loaded radiator cap it cannot return to the system as the plunger in the cap will reseal the opening. And, no, it's not normal for coolant to flow that direction to the point of filling the tank. Now, if you have sufficient air in the system it can expand with heat and increase pressure to the point where some coolant escapes, but if it's just a small amount it should find its way back to the radiator and be burped out. Low coolant will cause weird things as hot water splashes over the thermostat and temperature sensors, followed by hot air.

You indicate that your radiator hoses are not getting hot. The bottom hose is the supply line (cooled water --> engine) and the top one is the return line (engine --> radiator). They should both be really warm/hot when the engine is warmed up. If not then you either have a defective thermostat (not opening properly or installed backwards), a lot of air in the system, or a blockage. Blockages are uncommon, but you could run water through the engine while you have the pump off to see if it flows well. You already said the radiator is good.

As for the thermostat, yes, they are to be installed in a certain way. The spring side goes toward the engine. This being the side that will get hot first it will activate the spring and open the valve as needed. You also need to ensure you have one rated for the proper temperature. Speaking of the valve, I've heard that you can put an aspirin in the valve to hold it open, or two for balance. This will hold it open while you are filling the system and make it easier for trapped air to get out. Not something I've ever remembered to try, so I can't say if it really works or not. I've also drilled a small hole in the thermostat flange to serve the same purpose, if there wasn't already one there. Parking on an incline while filling the radiator with coolant can also help get the air out. Let the engine run as you top it off and that should (almost) guarantee that there will be no air in the system.

In your case there's a good possibility that the thermostat is the wrong temperature, or installed backwards. If your hoses are cool then you have no flow. No flow means the coolant in the engine is just getting hotter and hotter. This, combined with any air in the system, will create a back-pressure that can force coolant out into the overflow tank. This is just a theory, of course. Anyway, you're probably on the right track to replace the thermostat and water pump. If the problems remain after that, well, just cross that bridge when you get there.

 

Thanks for the help @seschenburg!

UPDATE: I took down the front of the engine, the gaskets on the water pump and thermostat housing were rough shape and looked like definite signs of leaking from one or both. I replaced the thermostat and installed it correctly (the old one was the right way as well). The water pump still seemed in good shape to me and I cleaned it up and put it back on, new gaskets on both. After starting up, it didn't look like any leaks around pump or thermostat housing, seal looks good now.

Was waiting on sender to arrive and a few days later started back up the truck and the temperature gauge seemed to be working, it would hover around halfway on the NORMAL and drop down to about the OR, never seen a gauge move like that so didn't trust it - took a couple short test drives. But on third short drive, gauge jacked up to Hot and steam/smoke came from under hood with a hissing sound, lifted the hood and it appears there was a big leak around where the Air Pump is, hissing steam coming from that area, signs of lots of coolant around that area. Gauge had gone up to Hot just before it started hissing. It was late at night and so couldn't see much with all the steam/smoke, this morning I took a look and there's coolant all around that area. I had noticed some white smoke from exhaust when truck comes to a stop and is sitting idling but quickly goes away when driving. Engine oil is clear as day.

Before I dig in there, just trying to get a sense of what it actually means. Would it be a hose in that area? The air pump itself? What would coolant be leaking from in that area? Any help here is appreciated.

 

I'd agree with what seschenburg said, except for one thing. The motor can pull fluid back through the rad cap.
There is a large spring which stops fluid from going out until it reaches 12 or 15 psi {whatever the cap is rated at}. This pressurizes the coolant and raises the boiling point.
When the pressure exceeds that fluid is pushed out to the overflow tank.
In the center of the rad cap is a small brass button which is spring loaded (very light spring) in the opposite direction. This allows the engine to pull coolant back from the overflow tank as the coolant in the engine shrinks while cooling.
That's what the little button in the center is there for.
Parking your truck on a slight uphill incline so the rad cap is the highest part of the system and then running your engine through several heat up and cool right down cycles will usually purge any air in the system.

 

Wish I had more info for you. I'm not all that familiar with the straight 6, though I had one in my first car many years ago. But, if it was hissing then I would fill it back up and then let it idle until it warmed up. Then maybe get someone to run it a little fast for a while until it starts hissing again and you may be able to see where it's coming from. Could be a freeze plug or maybe the head gasket. Just guessing here. I would suspect the head gasket if your overflow tank keeps filling up.

Of course, if there are other things that could cause high pressure in the cooling system, maybe someone else on here could share them.

 

Did you have this large leak before you pulled the water pump? I would pressure test the system and see where the coolant is coming from. There are several bolts on a 4.9 water pump which penetrate the water jacket, did you clean up the bolts and apply some thread locker or sealant on the threads? If not, they are likely leaking. I would also look at the lower hose connection to the water pump.

 

Update:

Put in a new sender, the gauge send to function properly now. Found a leak in the lower radiator hose so I replaced the hose and new clamps. Added coolant, started it up. Topped up the coolant.... Now I'm pretty sure I got a head/head gasket problem that has become apparent now that everything is sealed up tight. I started to get smoke out the exhaust, smoke (looks like burning coolant, will post a picture below) from the engine and it pushed coolant into my coolant reservoir.

Here's a picture of where I see coolant on the engine... Can someone help identify... I'm guessing it's the worst case



Looking in - for reference...

Zoomed in, you'll see the coolant there...