So went outside today to discover a puddle of coolant under the Excursion. It has never leaked coolant before, but is 18 years old now and has almost 200k. Got home last night from a 500 mile trip and parked it, then discovered the coolant leak this morning. It's leaking from somewhere at the back of the motor I think. It's running down on top of the trans and then running along the transmission, dripping from the back of the pan. It's definitely coolant as it's bright green. I tried looking around back there, but I can't tell for certain where it's coming from as coolant has covered everything. It does appear to be coming mainly from left side of motor.

I'm thinking it is the intake gasket though, as I can see a small amount of coolant on the left side of it, but there is no coolant down in the V and the right side does not have any standing coolant.

 

I have now noticed that when it warms up, the temp needle just gets the bottom of the normal range, or I'm guessing when the thermostat will open, it will start running rough, it feels like it is misfiring, but runs like it always had at idle or under load, until it begins warming up. I'm guessing what is happening is coolant is getting on one of the coils and making it short, but I can't see where the coolant is coming from. Could the intake manifold gasket or the manifold itself leak at a rate that significant, or is this a heater hose? It has never leaked coolant nor have I ever replaced any lines so I'm not sure exactly where to look. I'll check in the morning how much coolant leaked overnight. I'm not seeing any bubbles in coolant and there is no smoke or smell coming out of exhaust so I'm pretty sure it's not the head gasket or a cylinder head, but I can smell faintly coolant burning when its warm. Thermostat housing is dry, and I still have heat.

 

 

Turns out it is the Manifold gaskets. Coolant is leaking at the rear, and has ruined the coils for Cylinders 6 and 7. Plugs are about due, and coils are original, so will take the opportunity to just change them all. I'll take some pictures detailing the process. It's the same for all 5.4s and I think the same on the 6.8 as well.