Coolant leak, cold engine.

[Mike65]

I noticed over the past few days the smell of coolant during cold weather cold engine start up on my wife's 99 Explorer Sport 4.0 SOHC w/90K miles. Today there was a small wet spot on the ground under the front cross member where it appeared to be dripping from. I noticed once before a week or so ago & put a pressure tester on the cooling system the next day but no leaks showed up. I have read here about the upper thermostat housing leak on the 4.0 SOHC which is why I tried the cooling system pressure test. I guess I will have to look on top of the engine on the next cold start up for any leak, unless any one has any idea's.

 

[swshawaii]

Mike, it's usually the LOWER thermostat housing, not upper. Interesting a coolant pressure test didn't reveal anything. Any signs of coolant residue inside the intake valley? Small mirror and strong flashlight may be useful due to the restricted visibility. Also, earlier 4.0L SOHC's are notorious for leaks around the ECT sensor and gauge sender threads. If by chance it turns out to be the lower t-stat housing, consider upgrading to the newer RH148 lower housing. See link below. GL

4.0 SOHC plastic thermostat housing | Ford Explorer and Ford Ranger Forums - Serious Explorations

EDIT- Motorcraft RH-144 has TWO sensor ports, RH-148 only has one.

 

[XLTrunner]

As someone who's been thru this with my '98 SOHC, I would definitely be looking at a leaking thermostat housing. Evidence of leakage can be very difficult to see by looking at the housing. But, if you take a flashlight and look carefully under the intake manifold behind the t-stat housing, you'll probably see a puddle of green coolant in the valley on top of the engine. When that fills to a certain point, it will run down the back of the engine and onto the ground. My driveway is at an incline and I would see it at home...but, not when I parked it on level ground in a parking lot. If that is the culprit, do yourself a favor and DO NOT waste your money on the Dorman or Four Seasons (a Div. of Dorman) t-stat housings. Go with the Motorcraft or chances are you'll be replacing it again very soon. In my case, I naively went with the Four Seasons and it lasted all of about 2 wks. before leaking again.

 

[Tech By Trade]

The damn seam on the lower is the one that fails. In the beginning its intermittent too. As you leave it, the leak becomes more consistent. If you go in for a look, you may notice a build up of dried coolant around where its leaking from. It sits in the front of the valley and rolls back down the engine. I got 100000kms out of my Dorman, and at 40 bucks, I don't mind replacing it again, vs 200 for the ford one that lasts slightly longer. Mine is an 07 though, so its a 10 minute swap because the intake is farther back so you have full access to the housing without removing anything else.