I've searched this forum for help in this matter but I haven't found anything to help diagnose my ongoing problem. I apologize for the length of the post up front, but I want to avoid creating a long thread of answers that may not apply.

I am constantly filling the radiator overflow container to maintain the coolant level. Overall, about half a gallon every 100 miles. There are occassional spots under the motor where I can see that there is a slow leak. I was worried that I was losing coolant because of a bad head gasket, but this problem has been around for over a year and I don't have the typical symptoms of a gasket problem. I think the leak is small, and it is probably hitting the hot engine / exhaust. I say this because I can smell the fluid cooking off on some days. I've climbed all over the engine bay and up under the truck but I can't find the source. I've replaced the radiator core, the water pump, the thermostat and both main coolant hoses. I've also replaced the small 'J' shaped heater hose that is closest to the t-stat. I don't have anything leaking into the passenger floorboard and no humidity is pumped into the cabin, so I think I can rule out the heater core. I've checked the main heater core hoses on both ends and cannot see any leakage.
One odd thing about this leak is that it doesn't seem to happen all the time. In other words, some days there will be a foot-wide puddle under the truck in the morning and most other days - nothing! I can't find any cooling schematics anywhere, so I'm not sure what cooling hoses / connections may be under the intake manifold or what could be leaky on the back (close to the firewall). I've pampered this old truck for over 150K and I keep the engine bay pretty clean, so I should be able to easily track down a leak, but I can only see the fluid that accumulates around the oil pan and front diff.
Knowing that there must be over a brazillion Ford 5.0 truck motors out there, I can't see why this problem is so unique and I haven't been able to fix it. I'm not ready to admit defeat and take it to the stealership for a pressure test. This should be an easy fix, but I just haven't gotten lucky yet. Any advice or new ideas would be greatly appreciated.

 

Does your year have the small coolant line that goes to the throttle body? I'm
pretty sure they did away with it but I can't remember. They have steel elbows
and they can rot little pinholes through the thin metal if the coolant lost it's ability
to protect and may be the source of the leak. Mine did it so I just disconnected it
and put jb weld on it then filled it with oil and routed a hose back into itself so it
will kind of act like a heatsink and keep it from rusting more.
I would also replace the drain petcock and if you wanted to do an upgrade, Summit
carries a drain petcock that is made out of zinc so it protects the cooling system
metals cathodically, Which adds a level of protection if you run 70%water to 30%
coolant and sometimes don't change out the coolant when you should. It is a
cheap insurance item.

Does the truck make much pressure? If it does I would try and leave the cap at the
half turn so it doesn't build up much pressure and see if you still lose coolant at the
same rate!! I run a 7lbs rad cap and 70/30 ratio and I don't have overheating
problems even in august!! I use the low pressure cap since it is an old truck and I
don't want to stress my plastic end tanks and the cooling system as a whole, I even
have a rad cap that doesn't have the pressure spring and it just lets the coolant go
into the overflow tank if it gets to that point just for diagnosing I got it from a Mack
granite cab truck.

 

Thanks for the post, Blue. Part of my delimma is that I'm not sure how the coolant is routed in/around the intake manifold ('94 does have coolant lines running to the TB). I have the factory chassis manual, but it doesn't contain much (if any) detail on the engine. I would like to find a schematic of the cooling system for the 5.0 truck motor, but I haven't had any luck. I'm hoping that a road map of the cooling lines would clue me in on places I need to examine to find the leak. I haven't checked my Haynes manual yet because I'm skeptical it has the type of detail I need.
It is difficult for this old man to climb around on top of my engine (especially when it's hot) and I'm sure the neighbors get a kick out of seeing the hood up with my legs dangling over the fenders. One last note - the leak seems to be towards the rear of the engine bay, not up front near the pump / radiator. Obviously some of the leakage is blow-back, but the drainage seems to be around the starter side / area.

 

Don't forget that just like a roof leak water can cling to surfaces and then at the lowest point is where it will drip far from the source. The underside of the intake
can be carrying the water (since the motor is tilted to the back) to the rear of the
motor and you won't see the source. If you have a piece of copper pipe just route
the tb line into itself so it takes that potential leak out of the equation.
A black light will help you find the leak since coolant glows bright green in uv light
Wally world has a small handheld flourescent black light that you can use for this
and for finding leaks in your ac and motor oil with the dyes. I bought one of the
halogen black light type leak detector kits and wish I had just went this route since
you can't leave the light on for more than 5 minutes and you have to let it cool down
and of course it is glass right there and if you get water on the hot glass (POP!!)
I would try parking so the rear end is higher and see if the leak spot moves you
may find it that way if it is one of those migrating leaks.
You have a freeze plug near that area though and is a likely leak spot
since they are just zinc plated sheet metal plugs and they can rust and
develop leaks or even get pushed out a bit and leak. buying a new set of
plugs and popping them in is pretty easy. You will see a round plug about
an inch and a half or so in diameter in the sides of your block.
Autozone.com has wiring and vacuum diagrams online for free and might
have something that would help ypou out, At least you could always look
for websites with pics of your motor and then enlarge the pics.

 

Okay, here's the latest: I managed to find the source of the leak. It would appear that there is a very small crack in the engine area behind the right side of the water pump. I managed to trace the leak up to this area and if I wipe it dry, I can see the fluid begin to reappear and seep out. Obviously the area is difficult to see, but the location is also a little confusing (as a source of the leak). First off, this motor has never overheated. How would a crack develop if there hasn't been any overheating? Also, I tried tightening the water pump attachment bolts on that side, but this didn't help. Initially I thought the leaking area a little strange because it seems that the water pump attaches to the crank / cam chain cover. There shouldn't be any water in there. However, the crack may actually be where the main water port is located (on the driver's side of the front of the motor). It would seem that if this is actually a crack, one of my only solutions would be to use some type of internal sealant, or more likely I'll try to use some JB weld. Looking back, it was odd that when the motor was warm there was no leakage. I think this is because the engine would expand when warm and help seal the crack. When it cooled the crack would reappear - and leak faster as long as the cooling system was still pressurized. Now that I don't keep the radiator cap cranked down, it has slowed the leakage. Next step is to get out the JB weld unless anyone has any better suggestions.

 

jb weld will work just fine. ive used that on a cracked timing cover as well. I had a cover crack due to a motor being in a truck that had a front impact at one time. shoved the water pump into the cover. mine never leaked coolant though so i would suggest pulling the pump off and pulling the cover off as well to inspect further. you dont want that coolant to get into your oil pan. have you checked your oil for coolant?

 

No coolant in the oil and no white smoke. Those are a couple of the reasons I was so frustrated with this leak. I couldn't find the source of the leak because the hot engine would normally cause enough evaporation that I could only see the stuff that made it down to the oil pan / crossmember. It just seemed I was constantly filling the overflow tank. Like a leaky roof, the coolant went down the front of the engine, onto the oil pan rim and would work its way back towards the tranny. Was also weird because I would have no leakage when the truck was parked after work. Go out in the morning and there was the re-occuring puddle.

I guess I'm disappointed in the old girl because she's been pampered for so long, only to develop this crack. I'm not sure why the crack formed in the first place, other than some form of 5-liter osteoporosis. I love the truck - like a pair of old blue-jeans. Unfortunately I don't know how much longer she'll hold together. I guess the same could be said about me.

Anyway, is there anything JB weld can't fix?

 

Since yours is a keeper look at this as a golden opportunity. Timing chain sets are cheap and make a huge difference in performance and 150K is at about the right time to start thinking about it. I would suggest the timing set, water pump, fan clutch, FelPro gaskets and Permatex Tack and Seal. Search replacing timing chain for details. There are gaskets between the TC cover and the block and between the TC cover and water pump. Water flows through two ports in the TC cover to the water pump.