Use a 1992 Jeep 4.0L thermostat housing to 'burp' air from early / closed cooling systems.

Trapped air can cause overheating and crack your engine's head... so make like a pinko hippy and "Purge the system, man!"

NOTE!: NEVER open or vent your cooling system on a HOT ENGINE! This is intended as a guide for venting the air on a COLD / COOL engine!

This is the common engine coolant 'burp' point. By removing this Coolant Temperature Sending Unit, located at the rear of your head on driver's side and parking with the nose downhill you can bleed trapped air from the coolant system. It's a minor inconvenience but one you don't want to try if you have the engine running... the pressure will shoot this sucker to the moon and spray anti-freeze everywhere.


NOTE!: This is NOT the Coolant Temperature Sensor!
This is the temperature sending unit for the temperature guage or idiot light. It is often removed to 'burp' air from the engine coolant.

I had already purchased a 1992 thermostat housing and expected to be using it for my converted open system's electric fan thermostat. Unfortunately, at that time I was unable to find a sensor that would work with the older Renix computer and ended up using an aftermarket thermostat control. I went through every *@^#%$@$#!!! sensor that two auto parts stores had and couldn't fine one that would accomplish a simple 'on/off' function at the correct temperature so (at that time) I chucked that idea right out the window. Since then, other people have found sensors that will work but in it was much simpler just to hook the electric fan control into the factory harness exactly where the old fan sensor plugged in without running any new wires. That meant that I didn't really need the new coolant thermostat housing neck now ....or did I?

(UPDATE: - There IS a sensor that will work to control the electric cooling fan that will fit the HO thermostat housing and function just like it was made to be there. See the update section of Aftermarket Fan Control.
Parts Needed:
1992 thermostat housing - dealer part #53006192
195* thermostat & gasket. NO - you do NOT need a 180* thermostat!
The Jeep 4.0L likes to run at 210*. If it won't run properly at 210*, then you have OTHER cooling, sensor or engine problems that you need to FIX! A 180* thermostat will NOT fix your motor's other problems! Diagnose cooling problems here
Teflon Tape
Brass radiator petcock
Brass bushing with outside threads that fit into 1992 thermostat housing threaded sensor port (see photo below) and inside threads to accept brass radiator petcock.
50 / 50 mix of fresh anti-freeze & water.

Burping air from a closed cooling system.

THE HARD WAY: Let's face it, burping trapped air from the head of a 4.0L in-line six cylinder is an easy way to burn your hands or get scalded when ya remove the temperature gauge sender on the head of a hot engine. If you're into pain, or just absolutely just hafta vent trapped air from the motor this way, PLEASE LET THE ENGINE COOL DOWN FIRST!!!

Please be assured that if ya drain out, or lose coolant below the head that air has gotten trapped in the system. Closed systems are especially sensitive to this and seem to have a hard time purging trapped air. Here's how you get rid of that air without doing this mod:

  1. Park the nose of your rig downhill so that the engine is visibly tilted nose down so any air pockets will flow to the back of the head where the computer's temp sensor is. (This is NOT the sender for the electric fan. On factory closed cooling systems, that is in the radiator tank on the driver side below/beside air box.)
  2. DO NOT DO THIS ON A HOT ENGINE! Pressure in hot/warm engine can blow out scalding hot coolant!
  3. With engine off, remove the temperature sender's lead and loosen sender until coolant begins to flow out and no more air comes out. Keep an eye on the coolant bottle level.
  4. Tighten sender back up and reattach the leads.
  5. Top off coolant bottle with 50/50 antifreeze/water mix.
  6. Remember! Do this with the engine COOLED DOWN!

Upgrade: A better way to burp...

THE EASY WAY: For closed systems, or when upgrading from the closed to open cooling system you can use a 1992 housing and it's built in sensor port to give you an easier way to burp air from your cooling system! Using an adapter bushing, install a radiator petcock where the temperature sensor would normally mount in a 1992 thermostat housing.

To do this mod, take the thermostat housing to the local parts house or hardware store and get a brass radiator petcock as well as a brass bushing that will both thread INTO the thermostat housing's sensor port and allow the petcock to thread into the bushing. Grab some teflon tape while you're there. (Shouldn't set ya back more than a few dollars for the hardware store trip).

  1. Now wrap the threads of the bushing with teflon tape and install the brass bushing into thermostat housing.
  2. Wrap threads of brass petcock and install petcock inside previously installed brass bushing.
  3. Set those aside and drill a tiny hole in the flange of your NEW 195* thermostat to help bleed out air from the head. NOTE! DO NOT USE A NAIL OR PUNCH TO DO THIS! Punching a hole risks deforming the thermostat and impairing it's ability to function. Be aware that some thermostats will already have this hole predrilled and you may be able to skip this step.
  4. Install thermostat with correct side facing out from engine and with HOLE ORIENTED UPWARDS!
  5. Install gasket, thermostat housing and reconnect hoses.
  6. Park on a flat surface, or better yet, park facing up a slight incline and (on a COOL engine) open the petcock and add 50/50 antifreeze & water to the radiator or coolant bottle until it starts flowing out the petcock.
  7. Shut the petcock.
  8. You are likely to have some air trapped in the heater & heater hoses. Start engine & turn on heater to open heater valve. Let engine cool. Repeat burping and top off coolant.
  9. That's it.

Note! - Hose off the area to wash away any puddles of antifreeze. Pets will drink antifreeze and can die from drinking it!

1992 Thermostat housing with petcock inside bushing with both mounted in sensor port.

Picture above showing 1992 thermostat housing with brass petcock and brass bushing in temperature sensor port.

Note! Quick & Cheap! - If you don't have a thermostat housing laying around and cash is an issue, you can buy a 'Prestone Flush & Fill' kit instead. It will include a heater hose connector 'tee' which you can insert into the uppermost heater hose (as shown below) and accomplish the same thing. I have both on my system. This also is great if you have added the thermostat control for the auxilliary fan to the thermostat neck or have an HO engine.

Prestone 'tee' installed on upper heater hose.


Mine failed after approximately 1 1/2 years from heat / age!

The dang cap on it cracked & began spewing antifreeze everywhere! Nothing quite like hot anti-freeze to make you appreciate the sheer genius of engineered obsolesence and planned failure. No sense in this crap happening on a part doing exactly what it was "designed" to do. Failure required removal and insertion of a heater hose repair pipe to get on the road. The Prestone 'tee' itself appears undamaged but I won't reuse it, or buy another. Instead, I will replace it with one made from brass fittings & will update this page accordingly.

Electric Fan Control "How To".

Overheating Diagnosis Tips.

If you have pics of your own repairs or can suggest other methods - please contribute your ideas (and pictures) to this article!

Revised on: November 11, 2006

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