Luke warm heat

[Manche757]

89 MJ, 4.0, 93K miles.  Does anyone have a best guess why the heat is coming out luke warm?  The mechanic said it is coming out at 140F degrees even though the temperature gauge is showing 195.  A year and a half ago, I had ac installed (never had ac before) and had a new heater core replaced at that time. An electric fan was added for the ac but the mechanical fan is still there.  3 or 4 weeks ago, (long story short), after overheating, the mechanic replaced water pump, water neck (because it was pitted), and flushed the engine and radiator. He also replaced the clutch on the mechanical fan because it was not working.   The temperature gauge indicated there was still a problem.  It overheated again.  Then he replaced the radiator.  I asked him to replace the heater control valve even though there appeared to be no problem with it. I also asked him to look to see if it was fully opened when the temperature control on the dash was at max.  (I want the heater control valve to remain so the ac will be optimal). The mechanic thinks the heater core, even though only a year and half old, may be partially full of rust from the engine flush prior to the radiator being replaced.  Once the engine heats to 195F it maintains a steady temperature, but it does take 8 or 10 miles to reach 195.  I will likely leave well enough alone at this point.  If anyone has any thoughts about why the heat is luke warm, have at it.  FYI, Virginia Beach is right on the Atlantic Ocean and that keeps us a little warmer in the winter and a little cooler in the summer than 75 miles inland. Day time winter temperatures below 30F is cold for us and night time temperatures go below 20F only a few night a year here.

[jdog]

Either the blend door isn't opening all the way or there's a air bubble, you never told us if it was open or closed system

[Ωhm]

What's the mix set for? Too much antifreeze can act like a coolant.

[Strokermjcomanche]

If the heater core does have sediment\rust in it, I normally use a garden hose rinse it out with the hoses attached and then blow compressed air through one hose and it blows out the other side of the heater core  . If there is anything in the heater core you should be able to see what is coming out to confirm or deny that the heater core is plugged. 

 

 

If  the heater core isn't plugged,I would agree that it has something to do with your blend door/ heater valve or something in the HVAC box 

[coolwind57]

i might add my two cents.

 

This summer, I bought a 100K mile 94 Caprice wagon.  The heater sucked.  I drained and cleaned the whole system.  I spent a couple of hours flushing and reverse-flushing the heater core till I had clear water coming alternately out at both directions.  No real difference was made in heating the cab.  It would lose heating capability after a minute or two of having the blower on. 

 

I spend another half day cleaning the flushing and reverse-flushing a few weeks later.  I was surprised that I was still getting debris out of there when I started this 2nd time.  Part of this half day cleaning, I even used CLR inside the core, twice, allowing it to sit inside for an hour or so.  I worked my tail off trying not to have to pull that heater core.  After all of this, it still didn't perform up to par.  Very minimal results.  

 

So I pulled the heater core, and replaced with new.  Damn thing will run you out now.  So depending on what's going on inside your heater core, I now believe that in some circumstances, you'll never clear all debris or totally restore the surface areas in there to like-new or even feasible conditions.  Sometimes it just comes down to replacement.

 

I'd try isolating your heater core from the rest of the engine cooling system and vigorously flush in both directions with garden hose pressure or less.  Careful with pressure, regardless of using water or air.  Worse thing is that you breach with a crack or hole and you get coolant odor/wetness in your floorboard. 

 

Like JDog implies:  Purge the entire system well afterwards, especially important if you're still running with a closed system. 

Good luck man.    

[scguy]

In my experience, if you backflush a heater core and get heat, that's a diagnostic step, not a fix. Pretty much every time, it will quit again, and heater core replacement will be required. 

 

 

 

 

 

[coolwind57]

46 minutes ago, scguy said:

In my experience, if you backflush a heater core and get heat, that's a diagnostic step, not a fix. Pretty much every time, it will quit again, and heater core replacement will be required. 

 

 

 

 

 

A large number of the guys on the LT1 forums for these B-bodies usually flush their heater cores as an annual maintenance item in the Fall.  There's a restrictor piece feeding the heater core from the engine that I suppose is to restrict harsh pressure during heavy acceleration or something.  If that's indeed the case, then I suppose low pressure probably doesn't help longevity of these LT1 heater cores because they can't naturally move particulates as effectively.  Just my guess.  

 

So I can agree with what you're saying....probably at least on some period of borrowed time if you can get it to extract heat after flushing.

[scguy]

Now flushing regularly for maintenance isn't a bad idea at all, but flushing a plugged heater core usually means a comeback within 1-2 months. 

[gogmorgo]

Could be a vacuum leak, stopping the heater valve from opening correctly.

A couple years back while I was moving I drove a parts xj 1000 miles out here, and it wasn’t making heat which was less than optimal as it was -45 most of the trip. I had air flow out of all vents unless I really got on the throttle when it would switch up to defrost, just no heat. About 2/3 into the trip the alternator died, and while changing it I had the battery out, and noticed the line down to the vacuum reservoir in the bumper was broken. I put it back together with some heat-shrink tubing, and had excellent heat the rest of the trip.

 

Alternatively you could swap it out with a ball valve, and connect the other two lines together. Open it when you want heat, shut it off when you need your a/c to be optimal.

[rokinn]

24 minutes ago, gogmorgo said:

Could be a vacuum leak, stopping the heater valve from opening correctly.

A couple years back while I was moving I drove a parts xj 1000 miles out here, and it wasn’t making heat which was less than optimal as it was -45 most of the trip. I had air flow out of all vents unless I really got on the throttle when it would switch up to defrost, just no heat. About 2/3 into the trip the alternator died, and while changing it I had the battery out, and noticed the line down to the vacuum reservoir in the bumper was broken. I put it back together with some heat-shrink tubing, and had excellent heat the rest of the trip.

 

Alternatively you could swap it out with a ball valve, and connect the other two lines together. Open it when you want heat, shut it off when you need your a/c to be optimal.

This was my situation as well, except for the minus 45.  Had only defrost and not particularly hot.  Repaired the vacuum line in same spot with shrink tubing und voila...heat in all the right places.  My fan resistor needed cleaning for the high speed to work.  Also put in ball valve to heater core.  Closed summer, open winter.

[cruiser54]

Keeping the heater control valve does NOT optimize the AC output. 

All they do is restrict flow, and then explode sometime in the future. 

[Manche757]

7 hours ago, cruiser54 said:

Keeping the heater control valve does NOT optimize the AC output. 

All they do is restrict flow, and then explode sometime in the future. 

I understand that the heater control valve can restrict flow of hot water and reduce the heat at the core.    If there is no valve, hot water flows in the heater core all the time and gives off heat in the plenum assembly at the same time that the ac evaporator is absorbing heat.  What am I missing?  Elaborate if you would.

[cruiser54]

There is a door in the plenum that controls the temp of the air. Air goes by the heater core or the evaporator.

 

Believe the guy from Arizona. No worries eliminating that piece of crap. 

[gogmorgo]

The proximity of the heat to the cold air does increase its temperature somewhat. Not a lot, but a little. Minuit did a small study on it.

The a/c will still work great with no heater valve, but it works just a bit better with the coolant flow shut off and less heat going into the cab.

[Manche757]

1 hour ago, cruiser54 said:

Believe the guy from Arizona.  

 

Who is this person?  Does he happen to reside in Yavapai county?  Thanks for his reply.

[cruiser54]

Yup. 90* day. AC vent temp at 42*.........

[Minuit]

The link in his post is a good read if I do say so myself

 

We've been over this a few times, but I'll continue to defend the heater control valve for optimum A/C. Because I tried it both ways on my truck and I saw a measurable difference with my eyes. That difference was mostly 3-5, but sometimes as much as 7 degrees Fahrenheit at the vents! And no, my blend door isn't stuck partially open. That's the first thing I thought too, but it hits its hard stops on both sides. In motion, bypassing the heater core (which is all the heater control valve is - a selectable heater core bypass) dropped my vent temps from the low to mid 40s down into the high 30s on two consecutive 85 degree days with summer in TN humidity.

 

Just because there's a plenum in the heater box doesn't mean the scalding hot coolant doesn't heat the ducting in the heater box. Not only that, but the heater core is right next to the A/C evaporator, separated by a thin plastic divider. That makes enough of a difference to matter. Jeep installed that control valve for twelve model years and completely redesigned it at least once in that time. They had a reason for including it. The air passes through the A/C evaporator first before going through anything else, even the mode doors. Once that's done, that air is as cold as it's ever going to be. Passing right next to the heat soaked plastic around the heater core isn't going to help.

 

I'm a guy from swamp-@$$ Tennessee. Not only does it get hot, but that hot comes with a lot of wet too. Colder A/C is a hell of a lot more important to me than slightly less warm heat. In fact, I went back and looked at historical weather data for the two days I performed this test, June 3 and 4 of 2019. Not only was it hotter in Nashville, TN than Prescott, AZ on those two days, it was significantly more humid on average through the day as well. And for what it's worth, it was a couple of degrees hotter on the "heater core bypassed" day - 87 versus 85.

 

You can choose to believe me or not. You're free to disagree with me - you're not gonna hurt my feelings. But I have a serious problem when you go telling everyone "it doesn't make a difference" in every thread when I have data showing that it most certainly does for at least some people's trucks, mine included.

 

Unfortunately, I don't have any data on heater performance with and without the control valve, but I wouldn't expect a massive difference. It's something I'd eventually like to do, but not a lot of time for that right now sadly.

 

Just, for the love of (insert your choice of supernatural being here) don't buy an aftermarket heater control valve. Get one from Mopar directly.

[Manche757]

In our combined search for knowledge, please keep answers friendly and non-territorial to maintain the mission of the CC. No one party is singled out here. Thanks for all posts.

[Manche757]

The heater core was replaced a year and a half ago when new a/c was installed. The engine and radiator were flushed after that and were badly filled with rust.  Then the new radiator was installed.  Has anyone come up with a non-metal flexible tool to attach to a drill that would flex its way through the core and scrape the inside like cleaning a sewer pipe?

[cruiser54]

I can't imagine the heater core is that dirty and it would happen that fast. 

Hold your hand on each of the heater hoses at the firewall. Same temp?

either side of the HCV. How do they compare?

 

If you have an IR gun, use that. But, your hands will likely tell the story. 

[Manche757]

39 minutes ago, cruiser54 said:

I can't imagine the heater core is that dirty and it would happen that fast. 

Hold your hand on each of the heater hoses at the firewall. Same temp?

either side of the HCV. How do they compare?

 

If you have an IR gun, use that. But, your hands will likely tell the story. 

Good idea.  Your theory is that if there is little rust, the intake will be hotter than the discharge hose, indicating good heat transfer?   Glad to know that there should not be much accumulation in the year and half even though the old radiator was pretty nasty.

[coolwind57]

On 12/6/2020 at 3:48 PM, Manche757 said:

The mechanic thinks the heater core, even though only a year and half old, may be partially full of rust from the engine flush prior to the radiator being replaced.

 

A couple of years ago, I flushed out my engine....and I mean FLUSHED OUT MY ENGINE.  I did a freeze plug replacement, so I pulled all freeze plugs, including those behind the engine/bellhousing.  I flushed out that bad boy for HOURS, even squeezing fingers into the freeze plug holes and pulling out tons of what appeared to be rust scale and sand-like stuff.  Sand--from casting process?  Maybe. 

 

I removed hoses and and ran my garden hose in every orifice associated with the cooling system, including those freezer plug hoses.   I remember having a pile of sand like stuff--especially at the rear freezeplug. 

 

So I have a theory here:  If your mechanic's flush process was really kick-@ss, then maybe you did clog that heater core up pretty good.  Like I said, I was astonished at what I had in my engine prior to my freeze plug work.

 

So here's my take on the heater control valve:  I removed mine because i was reading a lot about sudden failure.  They do seem to be built a bit weak-- and on a vehicle that's 30-years old.    I junked mine and I didn't really notice a difference in AC...but I don't live in Arizona or Tennessee.  We hit 100-F here on occasion, but AC has never been a big deal to me.  Perhaps its up to individual toleration.  

 

I had been bouncing around the idea of ball-valving the heater core and maybe draining it and capping it during the Summer to maybe help preserve it and perhaps even netting a few degrees on my Summer AC.  

 

Just some of my thoughts, Manche757.  Anxious to hear how you resolve your issue.  Do share.

 

[cruiser54]

If there is no obstruction in the heater core, the temps of each hose should be rather close when checking near the firewall.

 

Likewise on both sides of the HCV.  Let's get this info before proceeding any further. 

[Manche757]

    It will be the end of the day.  

[Minuit]

53 minutes ago, coolwind57 said:

 

 sudden failure.  They do seem to be built a bit weak-- and on a vehicle that's 30-years old.

 

Don't think that I'm recommending that people leave a 30 year old piece of plastic in their cooling system. 30 year old plastic pieces are time bombs and they need to die before they strand you. Also keep in mind that I'm a bit of an A/C junkie - I'm willing to trade 4 extra hose connections in an easy to reach area for extra A/C efficiency.

 

@Manche757 If there is a blockage in the heater core (and thus low flow of coolant from one side to the other), the input hose will be significantly warmer than the output hose, especially with no load on the heating system. Do this test with the mode set to "heat" but with the temperature control set most of the way (not all of the way) to the cool side. This is to make sure that coolant is flowing through the core (or at least trying to), but isn't being cooled down by the air blowing over it. There are quite a few redundant passages in the heater core, so I'd be a little surprised if they were plugged enough to present a serious restriction to the coolant flow, but it's a possibility.

 

I think this is probably the right time to start putting some numbers to your problem. If you can get a thermometer and stick it up the vents and see what it says, that would help us. "I have 95 degrees at the center vent at operating temperature on fan speed two" is much more informative than "my heat is lukewarm" but for what it's worth, I'd say to manage your expectations as far as vent temperature goes. The heater core in the MJ is fairly small and the air does not spend a whole lot of time in it especially on a high fan speed. Don't expect the air temp at the vents to match the coolant temp.

 

I went through the FSM quickly to see if I could find an acceptable spec for vent temperatures and came up short, but if there was a spec I would expect 140 to be within that spec, especially on a very cold day. In my mind, 140 is roughly where "good heat" starts, and I personally wouldn't call 140 "lukewarm" either, so there may very well be something else going on here. Like I said, it'd be best if you could collect your own measurements of the vent temps and report back.

 

Fun fact: setting the climate control to "MAX A/C" might help warm the interior up faster. That will pull in air from inside the truck rather than outside. I would expect this to start making a difference when it's below freezing outside. The A/C won't kick on if the evaporator is below about 34 degrees.