so a few weeks ago i got the dreaded head crack in my engine. 01 sport w/122xxx miles. changed it this weekend and finished putting parts back in yesterday. I've noticed that it will heat up to 210 than drop down to about 200-195 for 10 minutes no matter the load on the engine. than it will gradually work its way up to 220 over time, about maybe 20-30 minutes and will cool back down to 210 than it will stay a 220 no matter what. This worries me as before i changed the head out it ran at a constant 210 no matter what. not a tick above, not a tick below. in the process i changed the t stat out since the housing had to come off anyways. i put in a stant 195. Could this be from not burping the system long/well enough, or a bum t stat?

 

 

It's really easy to test a stat in a pan of water on top of the stove using a cooking thermometer (takes 10 min). A 195 stat should appear closed at 195 or ever so slightly open at 195. As the temp comes up, the stat should open slowly/smoothly and be about 1/4" open at +/-210. About 1/4" will be wide open. Do not toss any stat into boiling water.

Have u removed the rad cap (motor COLD) to verify the rad is full to the top?

 

What kind of head did you put on it?

 

Brand new clearwater head, and new fel pro perma torque hd head gasket. I kinda also guessed when it came to putting the coolant back in when mixing the anti freeze w/ water. is it possible to much anti freeze can cause this?

 

That's a tough one. Water is better at transferring heat than Etheline glycol. I gather that is why we tun 50/50 instead of pure coolant. I can't say for sure one way or the other if that could be your issue. I went with a dealer T-stat after some issues with others Btw Also there is a chart in my sig with regular specific gravity readings for people with a "normal" hydrometer. (at 68* F, 1.082 would be 50/50)

The "burping" issues can be allot worse with an 87-90. (closed system). It doesn't seem to come up much for the newer.

 

May be a burp issue. Stant does not have the jiggle hole which makes it a little more difficult to burp the system. Stant is good and accurate but I always drill a 3/64 hole at 12:00 o'clock position. Without the vent hole air can stay trapped longer.

 

Negative.....there's a dime sized hole directly beside the stat that flows coolant/bleeds any air the moment the motor starts. Drilling holes in the XJ stat simply increases the time it takes for coolant to reach operating temp. The jiggle valve in the OE stat is closed (not bleeding) when the motor is running. It's only open when the motor is not running.

 

1. The dime size hole is for the heater circuit
2. The jiggle valve is CLOSED when motor is running

As the head/block cools the thermostat closes blocking coolant flow from radiator. As engine cools a vacuum forms inside the block/head the jiggle valve is really a one way check valve and its purpose is to break this vacuum and allow coolant to be pulled into the block. This is part of the burping process that eliminates air pockets in the head. After a coolant replacement and after few heat/cool cycles it is no longer needed.

 

Negative.... the dime sized hole beside the t-stat is the coolant bypass loop/circuit (with or without the heater loop/circuit)......and that's what I said, the jiggle valve is closed when the motor is running. No thermostat will seal completely air/water tight, therefore air will pass by the stat (even when it is closed) to the rad whether the motor is running or not and air will pass to the rad thru the jiggle valve when the motor is off.

The jiggle valve opens the moment the motor stops, not when a vacuum forms in the block/head. When the motor is turned off (hot), the cooling system will remain pressurized for quite some time but again the jiggle valve will fall open the moment coolant flow stops, as in motor turned off. When the motor cools enough while off (overnight for example), the valve in the radiator cap opens when the atmospheric pressure (outside the motor) and the pressure inside the radiator become equal thus allowing coolant to return to the radiator via the tube between the recovery bottle and the radiator. If an actual vacuum forms in the cooling system, that means the rad cap has failed. There should never be a vacuum inside the cooling system. If u see collapsed rad hoses on a cold motor, then a vacuum has formed, again due to a failed rad cap.

 

Well said. Too many people think the water pump creates pressure or vacuum.

All it does is circulate water in a loop.

 

Sounds like a burping issue or there's not enough water/coolant in the system. Tilt the front end up, take off the radiator cap, start it up and turn your heat up on high. This should burp it and then top it off with distilled water if the radiator is low. You should be holding 3 gallons IIFC.

 

Correct, simple fluid transfer. TEMPERATURE creates the pressure dynamics.
Temperature rise along with radiator cap creates positive pressure. Temperature drop creates negative pressure. This back and forth makes it hard to eliminate trapped air....usually a number of hot/cold cycles. Without a jiggle valve a pocket of air can form on the plug side of the thermostat causing wild fluctuations in temperature.
Vacuum was the wrong choice of words, I meant negative pressure flow. There has to be SOME vacuum. How to you think coolant returns from the recovery bottle.

 

Eliminating trapped air is not hard at all. Between refilling the cooling system properly, leaving the rad cap off while motor idles/warms up 'till coolant/air escapes at the neck of the rad and (as stated) a number of heat/cool cycles, the system is burped. U don't need to jack the front end up or remove the temp sending unit in the back of the head....those r all myths. Air naturally migrates to the highest point.....the top front of the motor and the top of the rad are the highest points.

Virtually impossible for a pocket of air to form at the back side of the stat when the motor is running. There is a large volume of water flowing thru the block/head the moment the motor starts and that large volume of water is flowing directly across the back side of the stat, again the moment the motor is started. Unless one does a really lousy job of filling the cooling system, the stat will constantly be immersed in rapidly flowing coolant. That's the purpose of the bypass loop......to keep the stat surrounded by rapid flowing coolant and allowing the block/head to warm up evenly from rapid flowing coolant.

Coolant returning to the rad from the bottle has nothing to do with vacuum inside the motor.......it has to do with atmospheric pressure (out side air pressure) becoming greater than pressure inside the motor, as in when the motor cools.

 

^I say point the front end up so people don't try and burp the system with the nose pointed down.