OK. Now it's my turn. I have a '91 4.0 Ranger with 168K on it. It just started losing coolant and I can't find the source. More likely, I'm just in denial. In 100 miles or so, coolant level is down about 24 oz. No external leaks, no steamy windows. I even tried using the UV dye to help locate external leaks. Hose connections, water pump, thermostat housing, freeze plugs, radiator (new), no water in oil or vice versa. All the easy stuff checks good.

I wonder if I'm overlooking something? Is there a definitive way to isolate the leak to the head gasket? I've had the heads off on my 4.0 Aerostar, but I can't remember if the lower intake has water passages in it. If so, any clever tips on isolating the leak to the intake or head gasket?

 

Pressurize the system.

 

^ + 1 on the coolant system pressure test.

To check for a head gasket problem, remove the rasdiator cap, start the engine & look for exhaust bubble signs in the coolant. A cyl blow down test will help narrow down the faulty cyl bank.

Lower intake manafold gaskets are a known source of leaks on that era 4.0L.

You might try retorquing the fastners to spec, to see if consumption changes.

Good idea on the coolant dye test, if you still have the dye in the coolant, pull a plug or two & see if there are any signs of dye on them with the UV light.

Any signs of coolant in the oil????

 

pull your plugs especially the end ones and look for a plug that is pure white. If it leaks into the intake runner you won't see any external leaks, nor water in the oil but it will steam clean the spark plugs.

 

Thanks for the suggestions. There's no oil in the coolant and no coolant in the oil. I did the pressure test, but since the leak isn't external, couldn't tell how the water was getting into the cylinder. No signs of bubbles in the radiator. Since both the oil pressure and cylinder pressure are higher than the cooling system pressure, I'm hoping it's nothing more than the intake gasket. I'm just trying to isolate the problem correctly so that I can fix it the first time.

 

You could rig an unlit propane torch, with a piece of tubing attached, a sports needle on the loose end as a probe & with the engine at idle, pass it around the intake manafold to see if the idle will increase when it gets a sniff of propane. It can get in where a liquid wont & its sensor safe. Of course if its an intake leak that isn't open to the outside, it won't work, but if the engine is ingesting coolant with dye, it'll show with the UV light on the plug internal insulator.

Keep us posted on your trouble shoot.

 

Man, do I feel stupid. And relieved. But, I'm not relieved that I'm stupid.

I thought I had done a good job checking for external leaks. After reading the suggestions above, I decided to check the lower intake manifold bolts to see if they were loose. When I got to the front passenger side bolt, I couldn't see it. It's near where the thermostat housing connects to the upper radiator hose. There's a big wire bundle in front of it and a wire bundle in back of it. I moved the wire bundle that was behind it up out of the way of the bolt so I could tighten it. Then, I noticed a tiny drop of green fluid on the bottom of the wire bundle. So, I moved the wire bundle that's in front of the housing up out of the way and grabbed the black light. The bottom of the hose lit up pretty good. There was a 60 - 90 degree arc that looked like it was leaking.

I drained the radiator enough so that I could remove the thermostat housing. I noticed that there was a lot of rust on the location where the hose joined the hosing. I scraped off the housing and wire brushed it to get it nice and clean.

It's back together again and I took it for a short spin. I'll drive it to work this week and see if I continue to lose coolant. I'm hoping that the source of the leak was the upper hose/thermostat housing joint.

 

Good find, fix & feedback. Hope this was your only leak spot, at least you found & fixed it though. Keep us posted.