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Discussion Starter #1
So this winter I noticed my heating system wasn't working like it was last year. Temperatures in NYC have been very cold this winter and I always warm my car up about 5-10 min before I even get to it and even then with my settings on 78-86 degrees, my car blows out semi warm/cold air. I shut the A/C completely off and still the same. After about 30 minutes of driving it finally gets warmer but not really warm. Back then I used to have my car on 72 auto all year round and in the winter it would be so hot in my car I would have to turn the A/C on.

I took it to my mechanic who is a family friend and he flushed the heating core and it did get a little better, but still not as warm as I would like it. Before it was very very cold in my car, now its bearable, but like I said not the same and I would like it to be nice and toasty again. I brought it back to the mechanic and he said that they might have to replace the heating core. That's the next step. But that's a big job. He quoted me $750 (once again, family friend, said he normally charges around $1500) and I told him why didn't he just do that from the jump, cause I paid $225 to flush the heating core. So he said he would just charge me $550 to do the job then.

Problem is I don't want to gut my whole dash. I feel like it'll just cause another problem down the line. A buddy of mine who works on cars told me it could also be the thermostat. But when I first brought my car to the mechanic I think he said the thermostat was fine.

What do you guys think about the whole situation?
 

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Did you use water in the system? any reason why the heater core might clog??
I said this because the Air Mix motor is usually the first to be blamed, if the Mix Motor is bad you wont get full heat as the motor has to open the coolant line valve in order for hot coolant to heat up the heater core.
I would double check my Air Mix Motor and the Coolant Valve.
 

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Agreed that the air mix motors tend to be a much more likely failure than the heater core, particularly if you haven't been doing anything questionable (like running water only, or adding gloopy additives).

You could try cycling the heat all the way up and down to see if it gets better. I know on a recent trip, my MDX was blowing too-hot air through the vents (yes, that can be a problem in January in Arizona), and I "fixed" it by running the temperature setting from 60° to 90° (or whatever the max is) and back a couple times. I'm thinking I will have to pull the air mix motor out and clean / relube it (had to do that with one of the other air mix controllers that changes airflow to various vents).

Also, check that the heater valve (lives connected to a bunch of medium-size hoses near the firewall) is fully opening. You could try disconnecting the cable at the valve and manually slamming it fully open (hot) and see if that makes a difference. I believe that means moving the arm clockwise as viewed from the top. If this produces lots of heat, then you can try adjusting the valve (not difficult - basically just disconnect both ends of the cable and set the system to full cold with the ignition on position II, and then reconnect the cables so that there isn't any slack in the cable, and the arm is fully closed on the heater valve.

The other thing you could easily check is to run the rear heater system at full hot and see if it's getting a lot hotter than the front (put it on manual rear control first if yours isn't a Touring model - not sure how the controls work on that one). If the rear isn't getting too warm, check your thermostat. If it's stuck open, it's going to take a LONG time for your heater to get hot, and the symptoms would be just what you're seeing. One of the easiest ways to check this is to buy a cheap IR thermometer, and shoot the temperature coming into and out of the radiator. If the inlet to the radiator (after the engine is warmed up) is much less than the thermostat design temperature (starts to open around 170°, fully open by 195°) then you need to replace the thermostat. Not only would a stuck-open thermostat interfere with keeping you warm, it'll likely keep the engine in the over-rich condition that it uses to warm up the engine (less performance and worse mileage). Harbor Freight sells a pretty good one that goes on (big) sale occasionally, but here's an option that might work quicker and cheaper: https://www.amazon.com/Nubee-Temperature-Non-contact-Thermometer-Adjustable/dp/B00JCFPODM/ref=sr_1_2?ie=UTF8&qid=1516914462&sr=8-2&keywords=ir+thermometer
 

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Discussion Starter #5
Agreed that the air mix motors tend to be a much more likely failure than the heater core, particularly if you haven't been doing anything questionable (like running water only, or adding gloopy additives).

You could try cycling the heat all the way up and down to see if it gets better. I know on a recent trip, my MDX was blowing too-hot air through the vents (yes, that can be a problem in January in Arizona), and I "fixed" it by running the temperature setting from 60° to 90° (or whatever the max is) and back a couple times. I'm thinking I will have to pull the air mix motor out and clean / relube it (had to do that with one of the other air mix controllers that changes airflow to various vents).

Also, check that the heater valve (lives connected to a bunch of medium-size hoses near the firewall) is fully opening. You could try disconnecting the cable at the valve and manually slamming it fully open (hot) and see if that makes a difference. I believe that means moving the arm clockwise as viewed from the top. If this produces lots of heat, then you can try adjusting the valve (not difficult - basically just disconnect both ends of the cable and set the system to full cold with the ignition on position II, and then reconnect the cables so that there isn't any slack in the cable, and the arm is fully closed on the heater valve.

The other thing you could easily check is to run the rear heater system at full hot and see if it's getting a lot hotter than the front (put it on manual rear control first if yours isn't a Touring model - not sure how the controls work on that one). If the rear isn't getting too warm, check your thermostat. If it's stuck open, it's going to take a LONG time for your heater to get hot, and the symptoms would be just what you're seeing. One of the easiest ways to check this is to buy a cheap IR thermometer, and shoot the temperature coming into and out of the radiator. If the inlet to the radiator (after the engine is warmed up) is much less than the thermostat design temperature (starts to open around 170°, fully open by 195°) then you need to replace the thermostat. Not only would a stuck-open thermostat interfere with keeping you warm, it'll likely keep the engine in the over-rich condition that it uses to warm up the engine (less performance and worse mileage). Harbor Freight sells a pretty good one that goes on (big) sale occasionally, but here's an option that might work quicker and cheaper: https://www.amazon.com/Nubee-Temperature-Non-contact-Thermometer-Adjustable/dp/B00JCFPODM/ref=sr_1_2?ie=UTF8&qid=1516914462&sr=8-2&keywords=ir+thermometer
I’ve never touched my heater core or added any water or additives. I tried putting my rear ac on manual and cranked the heat up on high and did the same with the front and the rear is blowing hot air. Way hotter than the front. I’m assuming that means my heater core is fine?
 

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It looks like you know two things now...

1) Your thermostat is probably OK (or your rear heater wouldn't be blowing hot)
2) Your rear heater core is fine.

The question now is whether the problem with the front is a semi-plugged heater core, or if it's a problem with the heater valve, or the mix motor. I'd recommend manually adjusting that heater valve, since it's the easiest thing to check. I'd also cycle the temperature up and down fully a few times to see if you get a different result.

Oh, and FWIW, you can keep the rear heat on high and warm your car up a whole lot quicker while you troubleshoot the problem. ;-)
 

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Assuming your coolant level is correct, here’s a guess: There is a diverter valve that routes coolant to the heater core. It’s on the passenger side of firewall inside the engine compartment, behind the engine. With you engine on, heater running, if you have a helper shift the heater temp from cold to hot and back while you look in the engine compartment (be careful of the moving pulleys/belt), you should see a wire on the diverter valve turning about 90 to 180 degrees. If not working, might be stuck or wire broken. If not turning, with engine off, you could try turning it by hand or pair of pliers.

The link below was searching “image of Acura MDX heater diverter valve.” Looks like the page has some discussion forum links too.

https://www.google.com/search?client=safari&channel=iphone_bm&source=hp&ei=qp1qWuKzDcu6jwPWz7rADw&q=image+of+acura+mdx+heater+diverter+valve&oq=&gs_l=mobile-gws-hp.1.0.41l3.0.0.0.3069.1.1.0.1.1.0.0.0..1.0....0...1..64.mobile-gws-hp..0.1.33.3...221.dUydfU-351o
 

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i agree with leftoverparts,my 03 mdx did the same thing.i removed the screw that goes into the heater valve because it was stripped.found one slightly bigger,gooped it up with loktite and wound it back in.is an easy fix and cost nothing.
 

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Discussion Starter #12
Assuming your coolant level is correct, here’s a guess: There is a diverter valve that routes coolant to the heater core. It’s on the passenger side of firewall inside the engine compartment, behind the engine. With you engine on, heater running, if you have a helper shift the heater temp from cold to hot and back while you look in the engine compartment (be careful of the moving pulleys/belt), you should see a wire on the diverter valve turning about 90 to 180 degrees. If not working, might be stuck or wire broken. If not turning, with engine off, you could try turning it by hand or pair of pliers.

The link below was searching “image of Acura MDX heater diverter valve.” Looks like the page has some discussion forum links too.

https://www.google.com/search?client=safari&channel=iphone_bm&source=hp&ei=qp1qWuKzDcu6jwPWz7rADw&q=image+of+acura+mdx+heater+diverter+valve&oq=&gs_l=mobile-gws-hp.1.0.41l3.0.0.0.3069.1.1.0.1.1.0.0.0..1.0....0...1..64.mobile-gws-hp..0.1.33.3...221.dUydfU-351o
Checked and the wire turns when cycling from cold to hot.
 

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I have an 06 MDX with similar symtoms. Rear was plenty hot, front was warm at best. The heater cable was moving when adjusting heat but the valve was stripped where the lever connected, so that valve was not opening/closing.
Replaced heater valve and all is better now:)
 
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