Hello Everyone,

I'm new to the forum but read through some of the other threads to get ideas to tackle my issue - so far no luck. I recently started getting a 2004 WJ Laredo 4.0 sorted out with my son for him to start driving. We're having an issue with no heat and I was hoping someone would have an idea we have not tried yet. The radiator leaked badly when we go it, so far we have tried/tested the following:

New radiator cap
New Thermostat (at idle temp is now 210 but drops to 175 ish when driving)
New Water pump
New Electric Fan
New hoses (radiator and heater)
Flushed heater core three times
*First flush garden hose - got a little gunk out. Flushed both directions with water flowing freely. Inlet hose very hot, outlet ice cold.
*Second flush put water in heater hose trigger 20-25 psi to blast out water. Got tons of gunk out of the core on this one. Flushed both directions with water flowing freely. Inlet hose very hot, outlet ice cold.
*Third flush power washer both direction - no gunk. Water seems to go in and out of both hoses easily. Inlet hose very hot, outlet ice cold.

After new components and flushings used bleeder funnel for 20 minutes adding coolant to get air pockets out of system.

Blend doors (not a dual climate WJ) when going from hot to cold or vice versa, goes from freezing to just cold and you can hear the what sounds like a change in the air blowing so I think the actuator is working. Is there a way to verify this? I focused mainly on the heater core flushing since the outlet heater hose is cold - would this hose remain cold if the blend door wasn't calling for heat?

Aside from a better way to diagnose the blend door, is there something else I'm missing?

 

Welcome to CF!

The heater is a full flow design and water flows thru it at all times. Just for grins, check the steel pipe that goes into the water pump for being blocked with crap. If both hoses aren't hot, you've got a blockage somewhere and it isn't the core. Also, change that new thermostat. The engine temp should not drop below it's set temp at any time, that being 195*F. I have a Stant Super Stat in mine and it stays between 195* and 210* all the time.

One thing I've noticed now that the air temps are near the single digits, the incoming air is too cold to transfer heat from the core on high blower. Turn the mode switch to recirculate and it should get very hot! But not if you don't have coolant flow. The only problem with doing that is the windshield doesn't get heat on recirc. Or your feet!

 

Thanks for the idea on the tube. Makes sense that if the whole heat loop can't complete it wouldn't heat up the hose out of the core. I'll check that and the t-stat (also a stant 195) tomorrow.

 

I blew compressed air through the tube @ 25 psi until it bubbled out of the top of the radiator. I then setup the bleeder funnel to eliminate the air I put in the system and now I have full bore heat. It looks like once the weather is a little nicer I will have to do a top to bottom cooling system flush because whatever I dislodged is now someplace in the cooling system. Thanks for your help dave1123!

 

Good to know! I may even check mine! I'm having basically the same problem with mine when the air temp gets below 20*F but is fine above that.

A common newbie mistake, not saying YOU did it, is to put the thermostat in backwards. AND even the best brand of part can be defective right off the shelf. I've noticed some of the major brands I use don't deal with China, but use manufacturers in central Europe or South America instead. Hell, even Mopar has parts "assembled" in Mexico, which gives me the whillies!

When you get around to your "complete system flush", pull the block drain plug and flush out the block itself. Any heavy crap will settle there and remain forever. You may have to probe the hole with a screwdriver to get it clear, but you'd be amazed at what comes out! I used to do it regularly on Chevy small blocks, but haven't done it on a 4.0 yet. It is usually a 1/4" pipe plug just above the oil pan gasket surface on the driver's side of the block and there may be 2 on that long block, IDK.

Okay, doing a little research, I found it's a 3/8 flush NPT pipe plug with a 8mm square socket hole in it. 8mm is .314" and 5/16 is .312". Same'o same'o. Lots of people claim that it doesn't matter if you clean it out, but in my experience, it DOES! There is NO other way to clean out the water jacket around the cylinder bores of heavy debris and core sand that pulling that plug. My mechanic tells me of a block he had come in for a rebuild that had plastic parts of a broken water pump impeller in it. The ONLY way to get that stuff out is to invert the block, bang on it, and hope it comes out thru the water pump hole. OR boil it in a lye tank.