Sticky Thermostat?

My cooling system in my 90 4.0 started acting up when it snowed a few days ago. I thought it was just ice messing with it, but today when I went out, it started doing it agian. It over heats when I sit at a red light for a period of time, and the heater loses effectiveness. The engine cools back off if I increase throttle, or I start moving. On my way home today it started over cooling. It has a new radiator, thermostat, and fan clutch from the cooling system episode last summer. It seems to act up only when it gets really cold out side. I know that the coolant isnt freezing too, its a 50/50 mix thats not even 6 months old yet. Could this be a sticky thermostat?

Thanks.

Renix Cooling Systems....aaarg!

My last thermostat, stayed at about 170, except when I idled for long periods, then it would creep up to 210 or so, until the aux fan came on. Replaced the thermo, it held steady at 200 or so. Recently replaced the radiator (serious hole) and now I get swings, between 185 and 200.
The wifes old 87, seemed to need a thermo every couple of years, then I´d get a good one, that would last for five or more.
I believe, many of the thermo´s I changed out, were unnecessary. I usually found a very small leak (coolant out, air in),shortly before or after changeing out the thermo. Air in the system, will also mess with thermostat function, it often opens and closes, at odd times. Some very small leaks, evaporate almost instantly on a warm motor. The only way to see them is on a cold motor, warming up and then often, only from the stain where the leak is.
With the motor at operating temp., engine off, squeeze the top radiator hose vigoriously, until it makes a noise like a demented suction pump. Keep an eye on the surge tank, for air bubbles.
The fan clutch has a bi-metal that actuates the clutch. It´s conceivable, the bi-metal, isn´t getting warm enough, to activate the fan clutch, in very cold weather.

I do have a lower radiator hose leaking at the upper clamp. Could this be causing problems for this type of cooling system? I top the coolant off and after a normal day's commute, the overflow bottle is empty again. Its not burning any coolant, and the leak isnt that bad. Where's it all going?


Thanks

Sounds like persistant low coolant level...classic symptoms...and/or air entrapment..if it leaks coolant and gets low, it will trap air...get this fixed immediately...the areas that needs the cooling most (the head and upper cylinder) suffer most..unless you want to buy yourself a new engine for christmas.

Id been fighting a similar dillema with my 88 4.0 (temp gage spikes to the red as the heater output goes cold) it was an off & on thing, seemed to do it at the same spots -Ist at 5miles then at 25 miles into my 30 mile commute :dunno: Like the Jeep knew where it was Some road speed would bring the temp gage back to 170* or so and heat would blow hot again.

Anyway I'd burp the system and it'd help for a little bit, but always came back to pesk me. I even wired my Aux fan to a manual switch/relay so I could head off the heat problem, but it didn't help at all.

I finally bit the bullet a week ago Thurs. and bought a $10.00 Mopar 195* Tstat at the stealership. Got to work & popped the hood, to let the pig cool off quicker to work on it...My co worker was outside a few minutes later, and came back in saying: "Come listen to this!" Sure enough a high-pitched whistle coming from the heater hose at the firewall. AHA! I say, grabbed my screwdriver & tightened ALL the clamps (I had replaced the coolant, 180* tstat, hoses & clamps with worm-gear type when I bought the Jeep)

After the tightening, we backed the Jeep up a hill and I burped it once more...that was on 12-4 and I've driven the rig every day since, with NO cooling/heating issues to date.

I recommend finding & fixing any leaks you find. Trash those OEM squeeze-type clamps for some 'real' ones that you can tighten. Burp the system once or twice (don't let the coolant bottle run dry during...) With the 'closed' system of the early 4.0l, ANY leaks are bad (coolant>out = air<in; air<in = BAD) and will cause the problems you describe.

Would air in the system cause an over-pressurization? I was driving home today when my temp needle went into the red. I had lost my heat. I pulled over and openned the hood to see my anti-freeze reservior all blown up and about to burst. Luckily it did not and I made it home. I do not think its a head gasket or cracked head because the of the sudden lost or heat and no white smoke in the exhaust. Once the truck cools my heat returns. The truck has a new radiator and water pump. Any suggestions?

Keep the surge tank at least half full, make sure the bottom spout in the surge tank is covered with coolant on a cold motor.
Coolant loss (a leak) will cause it to overheat, the closed system will suck air into the same place it lost the coolant, when it cools down.
Once the system gets over 220-230 or so it will start to make steam, which will swell the hoses and surge tank. Fairly classic symptoms of air in the system.

Ecarlcl said:

I do have a lower radiator hose leaking at the upper clamp. Could this be causing problems for this type of cooling system? I top the coolant off and after a normal day's commute, the overflow bottle is empty again. Its not burning any coolant, and the leak isnt that bad. Where's it all going?


Thanks

Click to expand...

If your '90 has not been converted to an "open" cooling system, you do not have an overflow bottle. You have a surge tank that is part of the pressurized cooling system. The level of coolant in the system must be maintained or you get air pockets and uneven cooling.

Im taking it to the shop tommorow, I'd do it my self but its colder than sh** out there, no condition to be ripping my jeep apart in the apartment complex lot. I have not been allowing the temp guage to go over 210, so we should be dammage free. I will have the shop do my thermostat, and the lower radiator hose. I'm certain that my bottle isnt leaking. I got it off a totalled xj that had low mileage. Then we'll see how it cools.

Thanks,

Ecarlcl

Success!

I got it from the shop today with a fresh Chysler thermostat, and a new lower radiator hose, no leak, no over heating, no problem. Thanks for the support. I bet I saved some money by diagnosing it myself.

Thanks

Ecarlcl