First I though I had a leak. Would see the overheat on temp gauge and the idiot light, and when I opened it up, it was low on coolant. But I could not find any leak, it did not drip, nada.

During overheat, I would pull over to add water, and found the coolant reservoir flooded full. WOuld slowly release pressure, and was getting bubbles from the engine into the reservoir. Once all the bubbles were out, suddenly the reservoir would be empty or 1/4 full.

Then I finally caught it when I jumped out while it was in o/h and saw coolant boiling through the cap on the plastic reservoir. Again with the bubbles, and low after releasing all the pressure. Still can't find any leak.

About this same time, saw the gauge go from mid-normal to full overheat peg instantly, and as soon as I vented it, was back to normal.

Thought maybe the thermostat was sticking closed.
So I decided to thoroughly flush it and change tstat. Drove for one day without tstat, ran cool, no overheat, no fluid loss.

Put new tstat, overheated to peg needle once it reached operating temp while I was driving. I coasted, turned engine off coasting, turned ignition back on, and temp was normal. It did this several times.

Then I stopped, vented, and left the cap off the reservoir to see if I could see anything. Like maybe small foam or bubbles if there was a head gasket leak.
There was a cyclical surge (about every 20 seconds) of coolant into and out of the bottom port - the level would go from normal, down to about 1/4, back to normal, sit for 10 seconds, then back down to 1/4. I've never seen a surge like this before... hope someone else has!

ANy ideas?

I will take it to a shop to test for exhaust gas in the coolant
I need to look at the oil for water
Other posts here make me suspect slight head warpage that does not leak until it gets fully warm.

 

Intake manifold....known for coolant leaks....

 

Need to use a pressure tester and see where it is leaking. Either a head gasket or intake as mentioned above.

 

I had a similar thing and it turned out to be the heater core that was plugged up....only problem I ever had with the mod motor...

 

Found it for sure.
My mechanic taught me one way to look for head gasket leak into the cooling system.

Start with Cold engine. fill the system COMPLETELY with water. Lipping tipping full. Run it for 3 or 4 minutes - not enough to warm up, but gives time for a hg leak to pressure up the system.
Then, check for pressure. Feel the hose. Nice and tight? And when you loosen the cap, you get a good sized splash of cold coolant? Congrats. YOu have a headgasket leak.

My mechanic (McAllen Auto in McAllen TX) who is both GOOD and CHEAP quotes $1400 M&L (I think that includes 14 hrs labor), and will check the cam chains tensioners and sprockets while he's in there.

 

Post mortem:

I think this is all self inflicted.

During the past month of chasing it down, I changed out the water pump. The impeller was rusted away down to a shuriken, no vanes at all ($40 and an hour of beating the stuck pig out of the block, plus $100 in upper and lower hoses, and another hour of struggling with the Ford Special lower hose clamp at the radiator. SHEESH!). The bad pump may have been the initial cause of an overheat and warped head, resulting in head gasket fail. It was the original 1998 pump and was not leaking.

New water pump overcame the rust plugging my rear heater core. ($60 and 4 hours, including sawing off the crimp clamps to replace the rear hoses under the spare tire.)

Plus thermostat and reservoir cap, and flush and several gallons of coolant during the course, about $60. So, overall, about 8 hours labor, and 250-300$ -before- fixing the head gasket.

Moral: change the blasted lifetime coolant every 3 years anyway!!!

 

A 98 would have green coolant which is supposed to be changed every 2 yrs/24k miles. In 02 they switched to gold which can go for 5yrs/100k miles. There is no lifetime coolant.

 

OK longbed.
I was exaggerating - point is I shot myself in the wallet by not changing the stuff out on a firm schedule. Done it a couple times, but obviously not enough. I've been using the gold replacement since 02 when I bought it.


And I understand the wierd overheat now:
Leaking headgasket allows compression vapor into the water jacket. Cold thermostat does not pass the vapor, (The vent in the ts is not big enough to pass all the vapor) so the block fills with vapor and pushes coolant into the (pressurized) reservoir. (So if I run without TS, I get no overheats or spew)

If there is enough coolant, it will push coolant out of the reservior, until the bubble is big enough that it starts bubbling into the bottom of the reservoir. The reservoir is connected directly to the lower radiator hose. Thermostat still closed.

Engine still warming up. Eventually, the hot vapor, or conduction, overheats the temp sender and the head overheat sensor, showing me overheat on the gauge and an alarm light. Coolant is still only warm, but spewing out reservoir, or venting vapor out reservoir.

If I turn the engine off and wait a couple minutes, the ts vent passes enough vapor to flood the jacket with coolant, and the temp indicator instantly shows normal, and it quits spewing, although still plenty of pressure.

Finally, the ts warms up enough to open, so the bubble goes through the ts to the top of the radiator, stops spewing coolant, and the temperature remains normal.

Continued driving continues to vent vapor through the reservoir cap.

 

Was this on all modulars starting in 02 including the aluminum 4.6L in the Explorer's?

Why did they change to that golden garbage? Is it because of the materials used for the radiator to keep from rusting like the GMs? Is it safe to run green coolant in these later modulars?

 

well, I got Big Red back from the mechanic. $2020 for the whole job.

That was:
remove and install
flatten the heads
head gasket kit
intake gasket (separate)
thermostat, antifreeze, oil change

extras:
timing chain kit (I had two cracked guides ate 180 miles)
belt idler and tensioner
spark plugs

All the parts were NAPA or Ford, so none were bargains.

all this was %800 in labor. So now I just have to drive it a couple more years to get my value back out of it. He was impressed about how smooth it ran after this job. It's always ran well, just gotta stay on top of maintenance.

He did tell me that the 4.6 bottom end is very robust, he's never had to fix one. Just head gaskets and timing chains - and if real unlucky from a broke chain, you get valves and pistons broke too.