dtek001

Hello,

I need some help my problem it is twofold:
My 90 XJ 4.0 L Runs too cold. Very low heat output from heater. I installed a Mechanical Temp gauge connected to sender at thermostat housing. Warms up very slowly, barely reaches 160 degrees when warmed up. After engine shutoff, under heat soak conditions about 190 degrees and no more. Lousy heat, lukewarm air coming out of heater vents. Dash heater controls work fine. I've just replaced Thermostat, 195 degree unit, purged air out of system. I thought the thermostat would cure problem, it did not. Originally there was a 195 degree T-Stat also. There are no leaks. I plan to replace the heater control valve this weekend. Any Ideas Please?

98 JeepJeep

Did you bench test the thermostat? I have seen them out of the box new be faulty but have never replaced one on a motor due to failure......What is the condition/tension of the belt? try squeezing the lower radiator hose to "burp' the air out of the system...when i did my Freeze plugs on my 98 XJ This is how i purged the air to keep the water pump from cavatating. It sounds like you have low/no circulation either from a air pocket or a faulty WP.

finneys98xj

Flush out your heater core.

87 XJ

Yeah, sounds like heater core to me too.
My temp guage hardly moves unless I'm climbing a hill in the middle of the summer but the heater output is great.

The only other thing I can think of might be low vacume causing the heater to not work properly.

98 JeepJeep

when it warms up is the upper radiator hose hot??

dtek001

Thanks for the responses guys. I now have ideas about a few more things to check. The upper radiator hose does get hot once the thermostat opens but it could be opening too early? It was a cheap 195 degree unit. I will be backflushing the heater core to see what happens. Can I ask another question?? Where should the ECU Temp sensor be? There is a Sensor / Switch on the top of the block, next to the valve cover in the rear, driver side. This is the switch for the idiot light on the dashboard I pressume? Should the ECU temp sensor be on the thermostat Housing? I found a brass "T" with the bulb sensor for a mechanical gauge the PO installed. Is this where the Coolant Temp sensor for the ECU should be? Am I running in open loop the whole time. The gas mileage is not to bad 16 to 18 mpg
Thanks again

98 JeepJeep

Im not sure about the jeep but i know for a fact on my mustang one is for the guage and the other is montired by the ECU for timing adjustment. I would assume the Cherokee is the same.

robertj

Sounds to me like the thermostat is stuck open.

Jsny1010

yea i thought it was weird but mine stays under 160 all the time and the heater works fine.

dtek001

I replaced the vacuum actuated heater control valve and flushed the heater core. The heater core was clean inside. I also removed the idiot light switch to bleed the air out of the system. The sender/switch broke upon removal and fell into the bell housing through the hole next to the CPS. The engine was running and I could hear the flywheel buzz as the sender went by. I don't think it will hurt anything?? It should settle at the bottom and it is soft brass anyway?? I fashioned a plug out of an old oil pressure gauge adapter. It was getting dark so I don't think I got all the air out if the system. I loosened a plug on the thermostat housing and the idiot light pressure switch alternatively every 15 minutes. I jacked up the rear end but the temperature of the top heater hose it not hot evenly throughout the hose. The pressure bottle did show the level rising and both heater control hoses are pretty hot. The gauge came up to 190 degrees and upon acceleration there was one quick fluctuation of the needle from 160 to 180 so I think there is still air in there. I will try to bleed all the air out tomorrow and report the results. All of your input has been greatly appreciated and useful. I will be fashioning some sort of radiator, grilled cover to help the engine come up to temp faster in the cold weather.

98 JeepJeep

it still sounds like you have a circulation issue...either from a big air pocket or a poor flowing W/P.

chasemyjeep

hey just wondering if you ever found a fix, my jeep has been running a solid 160 with luke warm air, only did it after running out of water due to a cut hose. so just curious if it ended up being air in lines or in the heater core, mine is a 92 btw

djb383

Does it have a t-stat in it? Could it be be one of those so called "fail safe" stats and has failed open?

HopWorks

Old thread but did not see a solution here. Since my heat left, my Jeep runs cold according to the temp gauge, during the winter. When I first bought her new in 1997, she always went right up to 210 and stayed there. Now, in the winter, when it's colder than 50 degrees, the temp gauge hardly moves up from cold. Again I have no heat from the air handler. I suspect the heater core is clogged and going after that tomorrow. But would blocked water flow through the heater core hoses cause the temp sensor to show such a cold reading during winter?

Again, if the original poster found a solution and gets an email about this thread being reopened, PLEASE tell me what you did to fix it. We are heading to the mountains in late October an we need heat. I have to either solve the heater core issue, or jerry-rig a heating solution using an inverter, oil-filled heater, and a fan. lol

Thanks!

smooth move

i don't think that's a sending unit in the thermostat housing unless it's been changed out. on my 92, that's the elec fan switch.