heater stops blowing hot air when I stop

93 Cherokee Sport 4.0 liter

While I am driving down the road my heater works great. However, when I stop for a long stoplight or at a drive-thru, my heater starts blowing cold air. When I start driving again, the heat soon returns.

Could this be linked to my overheating problem? It seems that everytime my temp guage goes over the 210 mark is when my heater stops working.

My Jeep has had a recurring overheating problem ever since I bought it. When I stop moving for more than a minute, my temp guage goes up to around 220. If I let it sit for 30 minutes, it will eventually reach just below redline, but never beyond that point. When it overheats, I do not hear any boiling and the Jeep runs just fine. I assume the problem is simply the guage, but I could be wrong.

I have replaced many things trying to solve the overheating issue including multiple radiator and engine flushes, many new thermostats, new water pump, new heater control valve, and new hoses. My Jeep is showing no signs of a blown head gasket and I have took it into several mechanics trying to fix the overheating problem, with no luck.

Could these problems be linked?



I have searched the forum several times trying to find a similar problem but have been unsuccessful.

Have you tried bleeding the cooling system really well? Set the front on jack stands so the radiator cap is the highest point in the system. Let it run with the cap off and give the upper hose a small squeeze every now and then. Once upto temp, it should push any air pockets up and out

Thanks Hippie, I have not bled the system because I have paid mechanics to do the work. I assume they bled it but I am not sure, ill give it a try this weekend.

The only 2 things I have not did after extensive searches on this site are:
- replaced the heater core
- used a 70/30 mixture (whatever that is). the mixture in mine is 50/50.

Sounds like a vacumn leak to me.

Sounds like two different problems. The overheating sounds like fan clutch is bad, and or the e-fan is not turning on soon enough, and that is controlled by the computer based on the temp sensor data from the sensor in the thermostat housing. IF it is not one of those, look for an exhaust leak blowing hot exhaust on the block, or all three!!!!

The heater core is good if it works part time. The heater core issue sounds like a vacuum leak near the heater core vacuum operated valve near the fire wall and heater core hose connections.

Look under the dash while you move the temperature selector control between hot and cold. Is the cable connected to the blend door?

May also have air in the lines, but I would start with simplest explanation first

A 1993 non Renix Xj should not require bleeding. Simply fill the radiator and the over flow/coolant recovery bottle and run engine until it reach operating temperature. Let the engine cool and refil the recovery bottle as needed. To get the air out of the heater system, turn the heater control valve (and I am assuming the valve is in place and working properly) on fully while the engine is running. Add coolant to the recovery bottle as needed. If every thing is setup correctly, you should have plenty of heat.

Now, the over heating is a different matter. I am certain you will receive plenty of help with that here on the forum.

I just halfway got my heat working as well, I think I have a bad climate control assembly but thats another issue. My heat wont work until I get to full operating temp or pretty close to it. Anyone know of an hvac vacuum diagram? There's alot of parts to it or so it seems, a vacuum diagram would be hella sweet to find/sticky for any others that run into hvac issues including myself.

Mike

Wow! Thank you for all the suggestions, ill try some this weekend.

For the record, I do have an exhaust leak and old dryrotted vaccuum lines. My electric fan does not turn on until 220 degrees, so I would also assume i have a bad temp sensor. I'll tinker with these and bleed my system then get back to you.

Thanks again for all the information.

Is your coolant level ok?

+1 on checking the coolant level, my jetta had the same problem with blowing hot air and it was because it was the coolant was low

Problem Solved.

Thank you all again for nailing this problem right away.

Solution:
Coolant level was low by 1 gallon. Apparently when the mechanic flushed the system, he forgot to turn the heater on when filling it back up. Because of the mistake, he fixed it for free. Now my Jeep does not overheat and the heater works perfectly.

he didn't fix it free, you paid once and he did the work twice

Nick_XJ said:

Problem Solved.

Thank you all again for nailing this problem right away.

Solution:
Coolant level was low by 1 gallon. Apparently when the mechanic flushed the system, he forgot to turn the heater on when filling it back up. Because of the mistake, he fixed it for free. Now my Jeep does not overheat and the heater works perfectly.

Click to expand...

You should find a competent mechanic

Glad it was something easy.

I have the closed system renix and I had to put the rear tires on ramps and take out the temp sensor and fill up my recovery tank til it bled out the sensor hole... I changed a bunch of parts chasing an over heating issue just to find out it was lack of coolant. I also read to drill 2 holes in the thermostat at 12 and 6 to help bleed the system. Now I'm running at like 180 driving down the road (barely above the 2nd line) and I'm going to a trail today to see if it will see 210

The Renix OEM design has a plumbing problem in the design. The convoluted upper hose acts like a plumbing P-Trap, and the flow is going in the wrong direction to let the air out of the top of radiator. Air is lighter than water and wants to rise, not drop, so it tends to trap air in the upper half of the radiator during a normal refill process, and the upper hose tends to trap in the radiator even with one end of the hose pulled off at the T-stat. I got tired of it, and added an inline hose radiator filler neck that uses the standard size radiator cap, used a 16 cap, and kept the plastic turtle bottle and used a Volvo cap on the plastic turtle bottle (the new Renix bottle caps are junk). Now I can fill it to the top, with out standing on my head, in just 2-3 minutes.

Jegs sells the inline filler necks on line. The hose is 1-1/4" IIRC. Look for the old threads on this here. On mine the radiator cap has never opened in 24 months now, as the Renix bottle (with air in the bottle) keeps the operating pressure down around 9-10 psi with peak temps at 200 F in the summer, at 105 F ambient running the A/C!!!!

I do not need to elevate the rear, fiddle with the temp sensor, or drill the thermostat out now or pray for divine guidance or help to get the air out now!!! LOL