Rapid Temp Rising

[87MJTIM]

87 Renix 4.0, closed system.

 

My engine was rebuilt last year and has only a few thousand mile so far.  I believe all small parts were replaced.

 

On Friday, I loaded up the MJ for a weekend camp trip.  Nothing heavy, just tent, sleeping bags, cooler, etc. and a passenger.  This is probably the most weight it has carried since the rebuild.  The outside temp was mild - 70s.

 

I started out and looked down at the cluster.  The temp gauge usually stops at the first line.  Very seldom does it rise above this.  Well, I was watching it continue past that and go above 210.  I had only driven 3 miles.  Then the temp dropped down to "normal," just above the first line.

 

I stopped at a McD's for coffee.  I looked under the hood and noticed antifreeze around the reservoir bottle.  The cap was loose and would no longer screw down tight.

 

i turned back and tranferred all my gear into a Suburban.

 

I have ordered a new cap from NAPA.

 

For the big question:  Should I replace the T-stat also?

 

What may have caused the sudden, rapid rise, and rapid drop?

[Eagle]

First, just about every 4.0L Jeep I have ever seen typically runs with the gauge showing around 210. The thermostat is supposed to be a 195-degree, but the rear corner of the head (where the sender is located) is a slightly hot spot. So 210 isn't anything to be alarmed about. The temperature would then go down because the thermostat opened.

 

How high do you fill your coolant reservoir? It's a pressure bottle, and also an expansion tank. It should be filled the the HALF level with the engine cold -- no more. Overfill it, and as the coolant expands it has nowhere to go. Air can be compressed. Liquids do not compress, so it would have to force its way out at the weakest point, which is typically the cap on the plastic bottle.

[87MJTIM]

I had not added to the reservoir since I got it back on the road last fall.  I had not checked the level, either.  I have not seen any leaks and no smoke from the exhaust.

 

I do remember the temp gauge reading close to 210 when it was "so much younger than today."  But for years it has been reading lower.  The low reading continued even after t-stat changes (195)  and a gauge cluster swap. 

 

It was unusual for it to rise that fast, then drop back down to the lower temp.

 

I will just change the cap and see what happens.

[nitroxsteve]

If the cap is leaking it will boil fairly fast. Replace that first then see what happens.

[87MJTIM]

Thanks.  i was thinking that the cap may have popped earlier and last Friday was just the result of that.

[onlyinajeep726]

If you plan to retain the closed system, get the Volvo pressure bottle cap.  The stock one and any stock-like replacement is junk.

 

 

http://www.jeepforum.com/forum/f11/volvo-coolant-tank-cap-xj-613128/

[ftpiercecracker1]

If you plan to retain the closed system, get the Volvo pressure bottle cap.  The stock one and any stock-like replacement is junk.

 

 

http://www.jeepforum.com/forum/f11/volvo-coolant-tank-cap-xj-613128/

 

 

 

Wanted to quote this again for emphasis. Probably the best and simplest thing you can do to improve your aging cooling system.

[cruiser54]

Napa # 703-1396.

[87MJTIM]

I found that part no. after ordering 703-2396, the OEM cap, from NAPA.

 

I will order the above and use it.

 

Thanks

[cruiser54]

I found that part no. after ordering 703-2396, the OEM cap, from NAPA.

 

I will order the above and use it.

 

Thanks

You bet. Your MJ will be delighted, as will you. 

[metrictonner]

Yea, Yea, Hot Renix.  Mine ran hot too until I replaced the radiator.  Now it runs cool like newl.  Before the radiator I replaced the fan (reverse rotation), thermostat, electric fan relay and sensor, serpentine, the hoses, the computer sensor, the gauge sensor, flush and fill...what a waste.  Yep...try getting rid of your 25 year old radiator and save some money.

[nitroxsteve]

Yea, Yea, Hot Renix.  Mine ran hot too until I replaced the radiator.  Now it runs cool like newl.  Before the radiator I replaced the fan (reverse rotation), thermostat, electric fan relay and sensor, serpentine, the hoses, the computer sensor, the gauge sensor, flush and fill...what a waste.  Yep...try getting rid of your 25 year old radiator and save some money.

so replacing the radiator will fix the issue of the coolant cap not sealing?

[87MJTIM]

I replaced the cap.  It seems to be operating at the "new" normal temp.

 

i had the rad replaced about 10 yrs ago.

[cruiser54]

I replaced the cap.  It seems to be operating at the "new" normal temp.

 

i had the rad replaced about 10 yrs ago.

The green cap? 

[cruiser54]

 

Yea, Yea, Hot Renix.  Mine ran hot too until I replaced the radiator.  Now it runs cool like newl.  Before the radiator I replaced the fan (reverse rotation), thermostat, electric fan relay and sensor, serpentine, the hoses, the computer sensor, the gauge sensor, flush and fill...what a waste.  Yep...try getting rid of your 25 year old radiator and save some money.

so replacing the radiator will fix the issue of the coolant cap not sealing?

 

Good point.

[87MJTIM]

 

I replaced the cap.  It seems to be operating at the "new" normal temp.

 

i had the rad replaced about 10 yrs ago.

The green cap? 

 

Not at first.  I got the OEM cap on it.

 

I drove it NAPA to get the green cap.  I will swap it later when the engine cools off.  I will keep the old "new" cap in my parts/junk bin.

[cruiser54]

 

 

I replaced the cap.  It seems to be operating at the "new" normal temp.

 

i had the rad replaced about 10 yrs ago.

The green cap? 

 

Not at first.  I got the OEM cap on it.

 

I drove it NAPA to get the green cap.  I will swap it later when the engine cools off.  I will keep the old "new" cap in my parts/junk bin.

 

Excellent plan.....