Cooling Puzzle

[Exgrayxj]

On my '90. 4.0 it suddenly started to overheat today.

I was playing around in some wet stuff, revved it up maybe 35-4000, nothing special about that.

Got home, resevoir is leaking at the seam. Pissing out.

 

OK, no biggie.  But then I realized, it seems to be making water! Reservoir gets fuller and fuller, temps are climbing.

 

Let it cool, opened my bleeder, nothing.  Filled things up, ran it, same story, making water and getting hotter.

 

So I pulled the thermostat, it was still open,  wide.  Placed it in some tepid water, it closed.

 

What happened?

 

Water pump was new when I got the truck, and I have only put on about 3k miles.Up until it did this I have been having no problems (with the cooling, at least).

 

I am thinking that revving it up, perhaps the impeller has broken or come loose, not spinning.

 

Is this possible?

 

I have been thinking of selling the truck (financial problems) and maybe this it the truck putting a stop to that idea.

 

Any suggestions?

[Zeke&Me]

Turn your heater on.. Do you have any heat? Sounds like your coolent is boiling over. Which means head gasket. Does your car emit white smoke when you first start it after you let it cool down?

[Exgrayxj]

No smoke, no coolant loss until this.

 

The thing has been running good for a while now, I had been more worried about the failing rear end.

[nitroxsteve]

If tank is cracked it can't hold any pressure and the water will boil. Pressurizing the water raises the boiling point. I would fix the tank first and then see what happens it may be thst the taink just failed. It happens.

[cruiser54]

Either convert to an open system now, or get a new bottle and use a 703-1396 cap on it from Napa.

[Lee21490]

If tank is cracked it can't hold any pressure and the water will boil. Pressurizing the water raises the boiling point. I would fix the tank first and then see what happens it may be thst the taink just failed. It happens.

Correct me if i'm wrong, But the res tank is never sealed? So having a leak in that wouldnt affect the water pressure?

 

Sounds more like a radiator cap to me.

[nitroxsteve]

I think 90 4.0 still used a closed cooling system . On the closed system the tank is under pressure.

[Lee21490]

I think 90 4.0 still used a closed cooling system . On the closed system the tank is under pressure.

Ahh. Wasn't aware.

 

Never had to deal with an older 4.0

[Eagle]

 

If tank is cracked it can't hold any pressure and the water will boil. Pressurizing the water raises the boiling point. I would fix the tank first and then see what happens it may be thst the taink just failed. It happens.

Correct me if i'm wrong, But the res tank is never sealed? So having a leak in that wouldnt affect the water pressure?

 

Sounds more like a radiator cap to me.

 

Since you asked -- you're wrong.

 

The Renix-era 4.0L XJs and MJs used what we call a "closed" cooling system. That plastic bottle at the firewall is not an overflow recovery bottle -- it's a surge tank that is part of the pressurized cooling system. The cap is a pressure cap. As has been noted above, if the bottle (tank) is split and can't hold pressure, then your cooling system has no pressure and will boil the coolant. As coolant gets hotter, it expands.

 

That bottle is not supposed to be full. When the system is cold, the bottle should be filled to the halfway level -- there's a post inside with a notch in it, and you fill to the notch -- no higher.