My girlfriends 94 Grand Cherokee 4.0 was leaking coolant terribly. So after replacing the radiator and upper rad. hose, I start it up to check for leaks and let it warm up. Sitting there idling it gets up to 230 degrees in under 3 minutes. I find this weird... thinking it might be the thermostat. What do you guys think??

 

Thermostat sounds likely.
Fan could also be an issue...
Or the radiator cap.

 

I put a new rad cap so i doubt thats it. Although it is one of those safety caps with a red pressure lever. Ive never used one so I dont know how well they work. What about the fan do you think could be the issue?

 

if it heats up to 230f in 3 min it must be the water pump or air in the system, does it blow hot air out of the heater (inside the car). Is the upper radiator hose hot.
Have you bleed the system? I have found out that the best way to bleed the system is to remove the upper hose on the thermostate housing. Fill the radiator, let it stay, fill the radiator, let it stay and repeat until you see the cooling liquid in the thermostate house. This will only work if you have the correct thermostate WITH bleeding hole. When the thermostate house is full, attach the upper hose and it should be fine. remember to fill the reservoir too. To check if there is good pressure after you shut down the engine after it got good working temperature, the upper hose should be really hard to clench.

 

I think I bled it right. When I did the temp went back down to 220 with the heater on and stayed around there. Took it for a drive down the road and it went fron 210 to 215 and back and forth (with the heat on.) There might still be some air in the system

 

This may not have anything to do with you, but I've seen guys put the thermostat in backwards, that is with the copper slug towards the radiator. This makes to open very slowly and restricts flow. Dumb thing to do, but hey, it happens.

 

I just replaced the motor on my 1993 ZJ, and I had the same problem. Changed the thermostat, and I found out it had really bad air lock in the block from air that got in during swapping everything. Vaccum purged the system and that did the trick.

 

With the 4 Liters, after you replace the radiator, replace the cap. Don't use the one with the pressure relief valve, not worth a crap. Use a regular cap. Also, go with a new thermostat. Avoid the thermostats that are fail safes.. stick with a standard thermostat.

Also, when you fill the radiator, put the heat on the hottest part.. just put the vent on your face if you will, fan speed doesn't matter and let the motor run till it's at it's hottest point so that is can run through the heater core. Shut it off, let it cool on its own. Also, the fan relay might be bad as well. If the fan doesn't kick on by a certain temperature on it's own, relay is bad. That's underneath the facia on the passenger side headlight.

 

I always drill small 1/8th hole in the new thermostat before I install it, and I make sure the hole is at the top when I install it. The hole lets the air bleed out as you fill the system, but does not effect the engine temp at all.

 

does that trick work on the 4.7 as well?

 

Any engine.

 

I never honestly thought of that

 

Other than putting it on the top so it can bleed, how far from the top?

 

As close to the top of the housing without breaking into the sealing flange. Some thermos come that way. On the 4.0 I usually take the temp sensor out and fill it until it runs out, then put it back in and finish filling it. It's worked for me every time. If you don't have a coolant control valve in the heater hose, it's a full flow heater and it doesn't matter if you have the heater on or not, it will bleed itself.

After thinking about it a while, I remember seeing old Wranglers that had an air bleed petcock in the thermostat housing for this purpose.

 

I'll revive this old thread and add an opinion. I was searching for the trick on getting air out of the heater hoses. I drained my weak coolant solution and refilled w/ new, then started it to let it pump around so I could get it full. Pretty quickly it was showing well past 210 deg. I didn't think that was right because it had been sitting for 6 hours. I remembered reading about "burping" the system. I squeezed the lower hose as best I could while first filling. It was pretty stiff. At least I guess I know the anti-collapse spring is still good.

I drove up to the overpass and pulled up the concrete ramp underneath to get it at a steep angle, hoping it would bleed the air out. The temp had gone up to the mark below 260 as I was driving the short distance. After I backed down on the road and started driving, it swung really quickly the rest of the way to 260 and the Check Gauges dinger and light came on. I stopped and shut it off. I squirted water on the radiator and noticed that it didn't really boil off of there like it was real hot. Some splashed on the cast iron of the head too and it sputtered and boiled but not very violently. I put my hand down on the radiator (engine was off) and it wasn't very hot. I started it and let it run a few seconds, then squirted more water through the radiator just in case. I restarted and the gauge was ~halfway between 210 & 260. I eased the 1/2 mile back home, mostly downhill, and when I turned in the drive the gauge quickly swung back up to 260.

After letting it cool some, I released the remaining pressure and removed the temp sending unit so I could bleed it. I noticed some steam come out and saw it was dry in there. That makes me think that earlier the coolant temperature actually was not hot, but rather that the sensor gets surrounded by steam, which is hotter than the liquid down in the block, and that's what the gauge was showing.

I filled the rad until coolant came out the sensor hole, put it back in, then poured in what little more it would take. I started it, let it run, and the gauge acted normally, stopped when the stat opened at 195.

However, now that the weather has got cold, I found that I have no heat. (Well, it is slightly warm.) Going to squeeze the lower hose some more and try going up an incline, both forward & backward. What else is the trick to getting the air out of the heater hoses and/or heater core?