I've got a 97 (last of the 96's) F350 with the 5.8. Auto. about a month ago I discovered that I was low on coolant and there was brown gunk almost like clay all over the rad cap. I ended up adding a gallon of coolant and I cleaned the cap. today 300 km later I'm down another litre of coolant and the cap is gunked up again. I checked the oil and it's not all frothy looking. There are no obvious visible leaks anywhere. No pools under the truck.

Any ideas on what the gunk is and where the coolant is vanishing too? My guess was it's getting burnt but the exhaust isn't overly sweet smelling.

 

Head gasket might be starting to go bye-bye. Compression gasses will transform coolant into weird things! Do a cooling system pressure test.

 

damn, Correct me if I'm wrong but head gasket is a cheap but time consuming fix?

 

If you DIY, Yes.

 

Or cracked head. I had that. Oil would get in the cooling system but the oil still in the motor was as clean as could be.

 

Don't just assume, I know napa has(may be discontinued) a coolant system tester for checking to see if you have exhaust gases in your cooling system. Unit goes on where the cap goes, add the chemical and wait for engine to warm up if the color changes you have exhaust gases in the system. But without the tool you should see bubbling in the system and be adding coolant often. Has your oil level gone up? look like a milkshake?
What is the condition of the coolant? how old is it? have you tested for electrolysis? All the cast and aluminum add in the copper heater core(if original) can get angry with each other.

Curtis