I just bought a 1998 Expedition with the 4.6L V8. Vehicle has 149,000 miles on it. It is your typical "only had the bare minimum maintenance performed" vehicle. Pretty sure the shocks are factory original and the diff oils have never been changed. Has two exhaust leaks where the pipes join and the clamps rusted away. The plug wires say AC Delco on them. Engine has a minor oil leak at the oil hoses going from the filter base to the block. The coolant is green (not sure what the proper coolant is for this engine/year). I noticed in my inspection that the coolant seemed new, as did the upper radiator hose. Then noticed coolant running down the driver side of the bellhousing. Looked up along, and saw it was running down a harness or hose in split loom. After the test drive, I noticed a strong anti-freeze smell, and when I opened the hood, I could see "smoke/steam" coming from the valley. I hunted and looked, and finally found the source. At the thermostat housing, just below the flange on the intake side, there is a pinhole that was peeing coolant out. The valley has filled up with coolant.

The good news is, I was able to drive it home 60 miles without it overheating. The body and interior are in pretty good shape. A little rust at the inside bottom of both front doors. No bad stains/ smells or anything inside. Compared to what they go for around here, I did pretty well. Especially if I re-sell it.

I priced a new lower intake and gaskets, but I think I am just going to try JB Weld first. My question is, why did this pinhole occur? Was it as a result of some sort of corrosion in the cooling system? Is this a common occurance? Does it need special coolant to prevent this sort of thing from happening?

I have searched a little bit, and am getting ready to go back to it. If I decide to remove the intake system and replace the lower intake, I plan to thoroughly clean everything I can get to, replace the plugs and wires, and any hoses that are easier to get to without the intake installed.

 

I'm pretty sure that pinhole is supposed to be there it could be one of 2 things

1) The "jiggle valve": allows air to escape and not become trapped in the cooling system and create a vacuum. In other words it is there to help the head fill after coolant has been drained.

2) The hole in the casting to allow vapor (steam) to vent. If you have a lot of fluid coming out of it, it may be a sign to check your water pump.

Regarding your special coolant question: Yes, there are different grades of coolant required, just MAKE SURE it is aluminum safe

 

I'm absolutely certain you are dead wrong. Unless of course these engines are designed to leak coolant into the valley and drain the cooling system completely overnight. Get real dude. Pin hole is supposed to be there. Seriously?

It is definitely as a result of either corrosion or damage from a botched repair. Either way, I am fixing it and probably selling it.

 

Here is a picture of the pinhole. The first picture is me pointing at it with a panel clip tool. The second pic highlights the pinhole, which is is just below the thermostat neck.

 

Now that I see the photos I understand what you are talking about, and you are right It doesn't belong there. The thermostat assembly does have a "relief valve" like device which can be classified as a pinhole.

 

This hole was due to corrosion. I prepped the surface with a well worn flap wheel, red scotchbrite, emery cloth, and lint free paper shop towels. I cleaned it using acetone based brake clean (CRC green can), and straight acetone (Dupli-Color brand) to ensure the area was free of any and all debris, grease, corrosion, etc. Repaired it using standard JB Weld, and it never leaked a drop after that. I saved myself a few days of work and a few thousand dollars in parts by doing this. The hole was actually in the head. I would have had to pull everything including the head to replace it due to this one tiny pin hole.

I traded the vehicle in for an Excursion.