Lots of "no heat" threads appearing here... so I wanted to offer a little general advice, since I see this at work almost every day.

Here are some ways to diagnose the cause of your problem that almost anyone can do.

Things that can cause little/no heat:
1. Blocked/Plugged Heater Core
2. Low Coolant
3. Bad Thermostat
4. Faulty blend door or actuator

Blocked/Plugged Heater Core
If coolant can't flow through the heater core, well, then you're not going to get sufficient heating.

Here's how to test it.

Observe this photo.


See the two black rubber hoses that run right by the left side of the valve cover? Those are your heater hoses.

Get your engine up to operating temperature, leave it running but MIND the cooling fan and hot metal parts. Take your hand and touch each hose. Don't worry, it won't burn you. They might be hot, but not enough to burn you.

Is one hot, but the other cold/warm? They should both be almost exactly the same temperature. If they're not, that means there might be a blockage in the heater core. Disconnecting the hoses from the heater core (where they plug into the firewall) and using a garden hose to flush out the heater core will likely fix this problem, if it's the cause of your issue.


Low Coolant
If your coolant level is low, it's not flowing through the heater core sufficiently, and there is likely an air pocket in the system.

Your temp gauge SHOULD change dramatically, constantly while driving... but that's not always the case.

Simply check the coolant level in both your overflow tank (the big plastic jug) and the radiator itself. If it's low, adjust the level to where it needs to be.

Check it while the engine is COLD (so the radiator cap doesn't blast coolant out when you open it and burn you).

If your coolant IS low, simply remove the radiator cap (again, while the engine is cold) and start the engine. You might spill a little - that's ok, just clean it up or have a can underneath to catch it. Add coolant until the radiator is full, and let it run until the bubbles stop coming out.

No, you WON'T overheat it by doing this, not unless you let it run for hours and hours. This shouldn't take more than 5 minutes.

You can also do the heater hose test I described above to make sure the coolant is flowing through the system properly.

If you have an air pocket in the system, it might be because of a previous cooling-system related repair, or you might have another problem. Leaking water pumps, bad head/intake gaskets or other cooling system failures can allow air into the system. Inspect for leaks.


Bad Thermostat
Your thermostat is a little device in the cooling system that actually RESTRICTS coolant flow - slows it down. It does those so the the coolant takes longer to circulate through the system, allowing it to heat up faster. Once the coolant reaches a certain temperature, the thermostat opens and allows faster flow of the coolant.

Most thermostats are designed so that if/when they fail, they fail in the OPEN position, therefore NOT restricting coolant flow.

If coolant flow is not restricted, the coolant will take longer to heat up. On really cold days, it might not reach operating temperature at all.

One way to test the thermostat is to simply start your COLD engine and pay close attention to your temp gauge (driving it or holding it at 2,000 RPM will speed up this process). Your temperature should rise to a certain point, just above operating temperature, and you might head a "thunk" noise, then your temperature gauge should fall down a ways.

If it does this, your thermostat is working correctly. This is NOT a fool proof method of diagnosing a thermostat.

If your engine takes a very long time to reach operating temperature, or never does at all, you can bet that your thermostat is stuck open, and it needs to be replaced.

MAKE SURE you've checked your coolant level before testing.


Bad Blend Door/Actuator

The blend door is just like it sounds - a little door in your vent system that opens and closes allowing more or less heat into the air. Think of it like turning your hot water **** in your shower.

Sometimes these doors can break. Since these are run off of vacuum (they should make an air hissing noise when you change from cold to hot), sometimes the vacuum actuator can fail, too.

Read this for more information on testing/diagnosis/repair.

http://www.heatertreater.net/jeep%20...e%2097-04.html

 

Nice!

 

Thanks for this, it might save a lot of copy/paste and link referrals to many of the current heat threads.
Of note: the 99-01 have the servo motor operated blend door, previous years have the cable operated.

Nice work!

 

It's easier to disconnect your heater hoses for flushing towards the front of the engine, rather than at the firewall.

Also, I don't think your temperature gauge should have a constant, wide variation once everything is at operating temperature.

Otherwise, nice article.

 

Absolutely.

 

This would be a great write up. But, no one seams to know how to do a search. Now if we could get people to do a search before posting hundreds of post about the same problem it would work out perfectly.

 

So true. More than half of the threads would not be there if an attempt to do a search was done...but thats how a forum sometimes works...

 

This helped a ton.. Going to try all of this tonight.

 

One question. I have an 85xl 2.5L and under the hood looks nothing like the photo above. Mine has that funky looking valve with the vacuum connector at the firewall where the heater hoses connect to the heater core. Can you flush the heater core the same way with that setup, or is there something special you need to do with that vacuum valve to make the water flow through the heater core?

Thanks for the great writeup, just couldn't quite figure out that one thing

 

You'll have to wait for a RENIX pro to drop by and tell you how to get rid of that heat control valve...permanently.

 

Just flush it on the other side of the heater control valve closer to the fire wall. But I would do like CCKen said and just get rid of the valve al together. If you have never flushed the heater core before I bet that the heater control valve will end up breaking on you anyways, Perfect time to trash it.

 

Disconnect your heater hoses downstream of the valve, then flush and reverse flush until only clear water comes out.

Leave the valve system intact like factory as it works just fine.

 

It was easier to remove than I thought, it is now gone. The core was not clogged but the valve would not move at all. This thing had set parked for about 6-7 years before I bought it last spring, so I guess that long without moving took its toll on it.

Difficult to tell just how much difference it made since it is 60F outside today instead of 5F like it was on Monday lol. But it did seem to blow hot even before the engine heated up.

Pretty simple to do since the hoses were long enough to reach the heater core fittings after the short pieces holding the valve to the core were removed. The tubing on the valve were beginning to rust quite a bit where the tubing was attached, so it probably would have begun leaking pretty soon.

Thanks for all the info everyone.

 

I just came across this issue for the first time and I've been working on my own cars for 20 years. This will be an awesome guide for me to follow. I already replaced the thermostat but I've also noticed a leak on the passenger side that I think is coming from the radiator, so I'm going to put a new one in and try your steps from there if needed.
Thanks

 

I have an 88 XJ Pioneer. The vacuum actuated heater control valve can cause intermittent loss of Heat due to changes in vacuum. I disconnected the vacuum from the control and plugged off the vacuum line. I then disconnected the arm from the vacuum plunger. I can now manually open the valve in the winter to have heat and close it in the summer so coolant doesn't flow through the heater core.