checkd everything, still poor heat!! 1999 4.0

new here, but not new to jeeps or wrenching.
PROBLEM: everything worked great till the water pump puked. got a new one, a new thermostat, new cap, flushed the motor(with thermostat housing removed), the radiator, and heater core seperately till you could drink it, and flushed em all going both directions.
parked the drivers side tire on a big ramp(about 14 inches high)to elevate the cap side, cap off, brought motor up to 1,200 rmp and let it run for half hour to purge air bubbles. also bled thermostat housing buy cracking sending unit loose, replaced the coolant overflow hose, and added some coolant to the overflow tank.
when cold, idleing, temp hits 200 degrees, good airflow from vents, but heat is weak, warm at best. both hoses are very warm, and same temp(to the touch).
start driving, temp drops to 100 degrees, and heat drops to cold.
500 miles later, i got a new thermostat, same problem. removed new water pump to check for a reversed or loose impeller-all good. got a new pump anyway. also new stat.
same issues.
so i blocked the radiator with cardboard. no change. removed the clutch fan(and it is functioning correctly) from the water pump. now i get WARM air when driving(the same temp as when idling), temp is steady at 200-210, and electric fan keeps it steady at 200-210.
what gives here??!!?? i looked for the blend door to check its operation, but can't seem to find it.:rattle:floor, vent,defrost selector works good.
i'm ready to set the carpet on fire to keep warm driving to work-i work outside, and its been 20 degrees or less for the last 7 weeks and i drive 70 miles a day to work.:explosion
HELP?

i suppose i should try boiling the t-stat, and maybe drill a tiny hole in it at the 12 o'clock position, but thats it, unless it the blend door.

If your coolant temp is reading 200-210, the heat isn't coolant related. Either you aren't getting flow through your heater core or the air isn't flowing through your heater core.

Do the new style XJs have the vacuum valve in the heater core line like the older ones? If not then you either have a blocked heater core, which I doubt since you say you flushed it separately. So the blend door it is!

i doesn't have the valve. and i suspected the blend door, but i can't locate the actuator for it, or the door itself. and even if it was a partially blocked core or blend door issue, that still doesn't explain why the temp drops when driving down the road.
unless i have multiple problems, a blend door and or bad airflow thru the core, and a bad t-stat?

What year?

And dont set fire to the carpet, just take it out and you will never be cold again

its a 1999 with the 4.0. i put that info in the title, sorry.

The blend door on your 99 is controlled by an electric motor. The door itself isin the same place in the cab (under the passenger side dash) as your heater core. There's lots of walkthroughs on the Internet on how to replace your heater core; follow one of those walkthroughs to get at the blend door.

I think you might be able to get at the motor through the engine bay but I'm not 100% sure about that.

I'd probably verify the temperature of the heater core hoses in the engine bay before I started taking apart the dash to get at the blend door.

Possible that you've installed the wrong water pump? You need to make sure that the new one is a reverse rotation pump.

Definitly check the thermostat though, there's a history of problems with aftermarket thermostats in the 4.0. I went though 4 or 5 until I got one that worked properly.

ert01 said:

The blend door on your 99 is controlled by an electric motor

Click to expand...

The blend door on my '98 is operated by a cable, was it changed to an electric motor in '99?

I went through this whole thing about a year ago with a 99. It was definitely the blend door issue. This is actually pretty common since it's such a poor design. Here is a link to my info thread with pics of how and where you can access the door:

http://naxja.org/forum/showthread.php?t=1014264

If you do get into it and have any questions, feel free to pm me.

EDIT: and YES, definitely check both hoses to the heater core before going any further.

loosewrenchcowboy said:

flushed the motor(with thermostat housing removed), the radiator, and heater core seperately till you could drink it, and flushed em all going both directions.

Click to expand...

sorry i can't help you, and slightly off topic, but how do you flush the radiator and engine? I flushed mine at least 8-9 times with clean water (drain, fill, run engine for 10-15mins at normal temp, repeat), and the water still came out brown and gunky. i just gave up. I figure there must be a way to hook a hose up and let it run.

robhurlburt-i flushed the motor with the water pump off, and t-stat housing off, and compressed air. i used a long aluminum tube on the air nozzle to reach back into the motor and blow crap loose, then flush again. the radiator-with the hoses off, petcock open. then capped lower hos, fill rad, and drain again. to be honest i spent about 3 hours flushing the damn thing.

blend door-i checked it yesterday. i actually drilled a 3/4 inch hole in the heater box(behind the glove box, just above and to the passenger side of the blend door motor) so i could see it move thru its full travel. i wasn't convinced by just turning the blend door by hand(with the motor removed).

BUT-i did get another new stant super stat and new gasket, and a thermometer, boiled it just to check before installing-it was good, opened just as water started to boil. i also drilled an 1/8 inch hole in the t-stat(and i did install the hole at the 12 oclock position). when i went to install it, since the petcock is so tough to get to, i removed the inlet line from the heater core, stuck the end in a bucket, and fired up the engine to pump some of the antifreze out. over a 10 minute span, the engine hit 200 degrees, and i got about 20 ounces of antifreeze out of it. i revved the motor up, still virtually nothing came out. the t-stat housing has a coolant bypass port, and so does the gasket, and when the motor hit 200 it should have poured out, espically when i revved it up!

btw- i blew on the heater hose going to the core with my mouth, and antifreeze shot out the other side-so its not the core. and this problem started when i replaced the water pump and t-stat 2 years ago. until that point, the heat worked fine.
Also, when this problem started i took the pump back off the next day to check for a reverse flow impeller, or a smaller impeller, or maybe just a loose impeller-compared to the original one-nothing. it all looked the same. someone mentioned a few posts ago, a reverse flow pump? i suppose, since i don't seem to be getting any flow out of the heater hose, coming off the t-stat housing, i may have overlooked a reverse flow impeller?

I know this sounds weird, but there have been several threads about poor heater performance after a water pump swap. My brothers TJ had this same problem.
This sounds kind of crazy, but hear me out:

I think the Jeep heater systems are designed to work after the water pump has been "Broken in". As in it isnt pumping at its maximum. Im not sure how this would work, but it seems that water pumps with a few thousand miles on them give the best heater performance. I honestly dont know enough about automotive cooling systems, but from personal expirence, my brother's TJ had the best heat in any vehicle i have ever ridden in. Bar none. This fall he replaced a bunch of his cooling system at 150k. His water pump hardly had any blades on it at all. I dont see how that thing pumped any water. All the blades were eroded away. We put in a new water pump and his heat sucks!

I can't explain it. Some one more knowledgeable should tell me if this theory holds any water.

loosewrenchcowboy said:

robhurlburt-i flushed the motor with the water pump off, and t-stat housing off, and compressed air. i used a long aluminum tube on the air nozzle to reach back into the motor and blow crap loose, then flush again. the radiator-with the hoses off, petcock open. then capped lower hos, fill rad, and drain again. to be honest i spent about 3 hours flushing the damn thing.

Click to expand...

thanks. GL with the heat!