'93 Explorer XLT overheating-pressure-cooling system problems

[rstumpy]

Picked up a '93 explorer XLT, auto, 4.0L v-6 for my wife the other day. It only has 92,000 miles and appeared to be extremely well-kept. Runs great, by the way.

After driving it a few hundred miles the cooling system began acting up. It would peg the temp guage now and then, but only for a second or two. When that happened the heater would only produce cold air, and it would go hot again when the temp dropped. I replaced the thermostat, temp sending unit, lower hose, and completely flushed the system, and carefully "burped" any air out of it. At that time I noticed that the freeze plug at the back of the passenger-side cylinder head and been popped-out and a rubber compression/expansion plug is now in it's place (holding fine and not leaking at all).

Sitting in front of the shop I ran it to operating temp tonight...everything was great...drove two miles down the road and the heater started producing cold air, the temp guage was at "M" in the "normal" range, and about that time, POOF-- the freeze plug on the block near the starter blew out. Obviously my cooling system is building pressure in the extreme?
There is NO oil in the water/coolant, and NO water/coolant in the oil. On the other hand, when the engine was warming up there seemed to be a lot of water vapor in the exhaust (the usual white dissipating clouds of exhaust, but more than I might expect on some rigs).

I'm fearing a cracked head or blown head gasket. BUT again, the fluids are not cross-contaminated and the thing runs absolutely great. None of the earlier symptoms where consistant either.

One additional tidbit. While I was flushing the other day (three times now), I had an adapter and garden hose stuffed into the radiator neck. After freshwater flushing for 5-10 minutes with the engine running (via a flush T in the heater core hose), I observed the outflow from the garden hose (I'd put the end in a 5 gallon bucket). The water looked clear and clean by then, BUT I did notice an occasional bubble, or even a stream of small, fizzy bubbles like from a scuba regulator....not constant, mind you, but every ten seconds a few would appear.

So again, considering the facts that the rad cap and fan are functional, I have repeated backflushed (with good flow results), properly replaced the thermostat (after testing it, by the way), temp sender unit, a couple of hoses, etc., I'm wondering if there might be a bad spot in a head gasket that is letting a small amount of compression-pressure into the cooling system without either the coolant or crankcase oil being cross-contaminated? I doubt the cast block is compromised, but a head could be cracked, eh? ....as opposed to a bad head gasket.

Any other possibilities anyone can think of or suggestions? Again, the actually cause of the issue is NOT related to a lack of coolant, poor quality coolant, a non-functional thermostat or fan.

I'd sure like some outside input before I start tearing things apart. We need the rig and can't afford to go looking for another just now.

Thanks much,
Rob

 

[NHale623]

Is your water pump moving any coolant? Does your heater work?

If you're certain the cooling system is working, you should perform a compression check.

Pull the spark plugs, see if there is any evidence of water.

 

[rstumpy]

The water pump is apparently functioning. The heater works until the (I'm guessing) pressure builds up and (guessing again) an air bubble is formed at that point in the system.

Basically, the pressure build-up, when it happens, has caused something else (blow the freeze-plug out, burst the lower hose, and sending about a gallon of coolant through the recovery tank and onto the ground are the three happenings to date) seems to happen BEFORE the engine overheats.

Night before last it sat for 15 minutes idling away just fine and dandy in front of my shop....warmed up, coolant was circulating just fine, heater was working. Then two miles down the road with the gauge in the mid-range of normal the freeze plug behind the starter blew out.

Like I previously mentioned, the frustrating thing is that there's no apparent cross-contamination of the fluids and the rig runs A-1-A. If there is no evidence of water in the crankcase oil, do you think it will show on the sparkplugs? I am going to hunt up a compression gauge and check it tonight or tomorrow though, before starting to wrench on things.

Thanks for the input,
Rob

 

[NHale623]

If there is no evidence of water in the crankcase oil, do you think it will show on the sparkplugs?

It is possible. If the gap where the coolant is entering the combustion chamber (either head gasket or a cracked head) is small enough, the coolant that is drawn into the cylinder would be burned off when the spark plug fires. There may not be enough coolant sitting in the cylinder to make it past the rings into the oil pan.

Dealerships should also have the equipment to detect combustion gasses in the coolant, if you want to go that route.