Hi all, I just bought a 91 Explorer, 4X4, 4.0 V6. Got it for dirt cheap cause it is overheating, sorta. Here's the scoop. Fellow that had it thought head gasket was blown. I checked, no water in oil, no oil in water, and let it run for about 10 minutes and did not overheat. Figured can't be blown.

When I went to look at it there was straight water in coolant system, and I noticed that Fan clutch was bad. I drove it for about 3-4 miles and temp stayed normal, about 1/2 way up guage. Drove it another 5 miles to get money from ATM and still seemed fine, until I shut it down to do the paperwork. Then, about 2-3 minutes after I shut it off I could hear it boiling over at the resevoir. Started it up and checked guage, was only 2/3 way up guage, still well within the 'Normal' brackets. Hmmmmm.... figured it was due to just water in system since water boils at 212 deg. So, I bought it anyways. Hey, 125K miles and got it for $400, figured can't go wrong.

So, I drove it about 10 miles to a friends house and drained radiator and added 1 1/2 gallons antifreeze and topped off with water. Then, drove it about 25 miles to my house. About 1/2 way still was staying less than 1/2 way up guage, so turned on A/C, still never got much over 1/2 way up guage all the way home. Got home, parked it and was not boiling over, so shut it off. Well, within a minute or two it started boiling over. Started it and temp guage was still just over 1/2 way or so. What gives?

Today I got a Block Tester. Checked it quite a bit and blue liquid never turned yellow. Used an infrared thermometer to check temp at the sending unit located where the upper hose goes into the engine, was reading just under 200 deg, guage was reading just under 1/2 way. Seems about right, no?

FWIW, I do have a freeze plug, located just above LH motor mount near front of engine, that is leaking. Any relevence?

This thing has me stumped. Any ideas? I havent' replaced thermostat or fan clutch yet cause I don't think those are the problem and don't wanna put any extra money in it unless I can figure this out first.

 

it could also be a radiator cap. some cars do that. and for around 5 bucks its worth a shot LOL

 

The radiator cap is a good guess and the reason for boiling could be because the coolant/water is standing still when the engine is off and as it absorbs the heat stored in the block it will expand. Where exactly is the coolant leaking from when it overflows?
There could also be an air pocket in the back of the block and the system may just need to be bled.

 

It's boiling over at the Reservoir.

Makes sense about the air pocket. There is a freeze plug, left side of block right above the motor mount, that is leaking. Could be that the coolant leaked down enough to get an air pocket, then when the previous owner noticed it they just topped it off without bleeding the air out, resulting in the overheating problem we're dealing with. That would really explain it all.

So, how do I go about bleeding the system on this vehicle?

 

First you have to replace the core plug that is leaking so air cannot get into the system. It sounds like you got a good deal in that truck once the plug is replaced. Don't waste your time or money with any of the many "sealers in a bottle", They will let you down at the worst possible time. Do the job right and replace the plug.

 

I've never replaced a freeze plug. Do I just pry it out and then hammer a new one in? Is it that simple?

Sorry to sound such a newbie. I've done plenty of repairs on vehicles. Just a couple weeks ago I replaced both head gaskets on a 2001 Windstar. Even rebuilt my 2.0L engine in my 1988 Ford Ranger (twice actually, since the first time I didn't do it right it went out again a couple years later). But I've never replaced a freeze plug. Are they in there pretty tight? Or kinda like a bearing race on a brake rotor, just hammer it in using the old race to guide it?

 

You will have to move anything that will be in the way so you don't damage it with the hammer you will have to use. Then on the solid inside part of the plug, using a punch on the inside edge of the plug, hit it until the plug turns in the hole to nearly 90* degrees, then using pliers or vise grips pull the old plug out being careful not to damage the bore of the hole. Then put the new plug into the hole and use a socket that is the same or larger diameter than the plug hammer the new plug into the hole until it is flush with the top like the original was. DO NOT USE ANYTHING THAT GOES INTO THE CENTER OF THE PLUG FOR INSTALLATION! If you do you will distort the plug and it will not properly seal and you will do the job over. Good luck with it and let us know how it turns out for you.

 

Did you replace the rad cap yet? It is not over heating, the cap is blowing off pressure that it can no long maintain.

Also, change the thermostat just for the hell of it. No telling the condition and a new one will at least give you a solid temperature basline to then troubleshoot tep sensors etc...

Nice $400 steal by the way.

 

Sorry for taking so long to get back. Yes, I replaced the rad cap, and 3 freeze plugs, and it is no longer boiling over. Even with straight water.

So, for all who are interested, my problem was definitely a bad radiator cap.

Thanks to all for the help. I never would've guessed that to be the culprit.

I haven't changed the thermostat. May do that some time soon, just don't have time right now.

 

I just replaced the rad cap in my 1993 Ex a couple months ago - original one. It was boiling over like that way too early in the hot season. That fixed it then. But now at 112 F, it does boil over after I shut it off. Even with the electric fan left on...nothing I can do about that. Coolant tank seems to hold it so no mess on the floor. glad it worked for you.

 

A bad 'new' thermostat was my problem.

Tested my thermostat and it never opened even after the water boiled- 212+ degrees. I found out that it takes a bit of heat for coolant to boil- maybe 340+ degrees and this is what my truck was doing- boil after parking and then leaky freeze plug.

Since I replaced my thermostat (actually went with a cooler unit 160) my freeze plug no longer leaks so my freeze plug was just doing that which it was designed to do, keep my engine block from blowing up or cracking.

My friend told me that usually a bad freeze plug will continue to leak even after the engine reaches a cold temperature after being shut off. However, it is my experience that freeze plugs are easy to replace anyway. The earlier mentioned method using a socket works the best.

 

Quote:

Originally Posted by sfjose

A bad 'new' thermostat was my problem.

A leaking core plug should always be replaced once it starts to leak as they do not heal them selves. A bad core plug only gets worse and if not replaced when it is found to leak can cause major overheating and the loss of the engine.

 

Thanks Moto Mel for the corrections. Every bit helps.