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Discussion Starter #1
So here’s the sequence of events, as much detail as I can remember, and what I’ve done...

Last Saturday, the X started blowing out steam from the radiator. Never had the problem before. Was driving home on the interstate, about 12-15minutes, and kept an eye on the temp gauge which only started to spike once, at a light close to the house. I turned off the car, restarted and made it home without incident.

I added water to the radiator to see how it would respond and how low it was. Put in about 1/2 gallon and drive to take my daughter to a bday party. Same type of thing—only would begin to spike when stopped, with some steam coming from the radiator. Car ran fine, and—this is the weird detail, compared with below—I drove with the windows open and the heat on full blast and it was HOT. I took that to mean that I have good circulation through the heater cores with the heat on full, which I also thought meant that the thermostat was opening as it should...

I only drove back and forth to work during the week, which is almost all on the highway, for around 15 minutes. I kept water in the car, and whenever I stopped—when I was going to leave for work or when leaving to come home, I’d check the radiator and top off the level. It would steam at stoplights when idling and I’d shut it off. No temp spikes and car was running fine. I anticipated having time this weekend to fix stuff.

So on Thursday it got cold and the heat wasn’t really working—it was warmer than recirculated air without heat, but only barely. Car ran the same.

The first thing I did yesterday was replace the thermostat, because I figured the hot heat last weekend compared to the no heat now, meant that it was probably sticking. However, after replacing it, the car runs the same, streams at the same times, and still doesn’t have cabin heat.

Now I’m confused. I know I have a small hole in the radiator. I have checked to be sure I don’t have air pockets by doing the crazy 15-18, repeat forever bleeding procedure. Any suggestions?
 

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First, replace that radiator - constantly running the engine with lower than designed coolant levels is going to complicate any troubleshooting process. Gotta do it anyway, so might as well get it out of the way.

It's always possible that there was still air in the cooling system when you first experienced the "cool heater". The chances that your thermostat chose that particular moment to fail are nearly zero. Just be doubly certain that you didn't mess anything up when you did swap that thermostat (it's not unusual for someone to put one in backwards, and cause the kind of problem you're seeing now). The fact you had good heat before didn't mean your T-stat was sticking, but that it was working correctly.

And I have to ask - did you by any chance add a cooling system sealer when you first discovered the leak? That can cause all sorts of other issues, including those you're having now, since it can clog up heater cores or the valve just in front of the firewall that directs the coolant to your under-dash heater box.

And just to be thorough, make sure your heater control valve (kind of obvious just ahead of the glovebox area on the engine side of the firewall) is cycling fully when someone runs the temperature control from 60° to 90°. The valve, the cable or the controller can all get flaky and cause a less-than-full engagement of the valve (my controller got "weak" and had to be replaced recently).
 

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Yeah, re: the good heat before—that’s why originally I ruled out the water pump and thermostat—I was thinking I had good circulation in the system and just the radiator problem. So, I thought that was it.

Then when the heat went cold, I thought the thermostat may have been sticking.

I’m embarrassed to admit that I did use a sealer solution. So my next move was to flush the radiator. I did that this morning and got relatively little fluid out of it. Is there anything to do other than open the drain cock at the bottom of the radiator?

Re: the valve for the A/C and heat, I believe I can hear it engaging. It sounds like a pronounced clunk that is vaguely like a lid on a bucket closing/opening, right? And the thermostat was installed with the spring side in and the cone-looking side out, with the little vent flapper hole pointing up.

The other thing I forgot to mention was, after everything that has happened and has been done, after I did the heat/cold air pocket cycle ten times, the top heater hose was very hot—like engine coolant hot—and the lower hose was warm. That is as it should be, right?

I plan to replace the radiator, I’m just trying to figure out everything that is happening so that it’s not a repair that takes weeks, trying this and that. I don’t understand why there is overheating and excessive pressure, if the coolant is cycling through the engine the way it should be. I understand why the radiator is steaming, I just want to try to do all the repairs at the same time. I also want some reassurance that it is not the water pump that’s gone bad.
 

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1. Check that your cooling fans are working. If the A/C is working OK (proving that you have a charge in the A/C system), just turn on the A/C and both cooling fans should come on. If not, normal troubleshooting for fuses, relays, rodent eaten wires.

2. It is normal for the heater to go cold when you are low on liquid in the cooling system.

3. Be sure to bleed all air from the cooling system any time the level gets low. Open the bleeder as you fill until only liquid comes out. Do this with the engine off. This video shows how, although he has the motor running.

https://youtu.be/wBWldTfBEMQ
 

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New and interesting details:

Ran a hose in the radiator for a little while with the drain cock open to flush out as much junk as possible. Then I did the heat/A/C air bleed sequence and squeezed the hoses going to and from the heater box. That was definitely gunked up because the hose going to it was hot and the one coming away was just outside temperature—not hot or even warm.

I went back and forth between the heat and A/c a bunch of times always adding a little flush/water closing it up for a few minutes and then switching to the other. Eventually the heater core got a little cleaned out, at least enough to make the cabin air hot. It also got warmer when I revved the engine, but I figured that was because of higher pressure through the heater core. That eventually stopped again....

However, one thing that was interesting was the radiator only fumed/steamed when the cabin air was on heat; not on the A/C setting. I got out and looked in the engine compartment and noticed the fans only come on when it’s on A/C. In fact, I can turn the heat all the way up and leave the A/C on and the engine fans stay on, with no steam at idle, for an extended time. In the heat setting, the fans would stay off, even at idle, and it would eventually steam.

Throughout all of this, the engine temperature stayed constant at about 1/3 up from the bottom.

Does the fan circuit have different sensors to tell it to come on? It seems like they may be part of the problem. One or more not working correctly...
 

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The fans should come on only when the coolant temperature gets over normal operating temperature. This time of year, that can take a while - the turn-on temp is really pretty high. The fans should only come on after idling for a long time - and if it's cool out it might be a REALLY long time.

Yes, the sealant could be one of your problems - it could be clogging up the "pinch points" in your system (the valve and heater core, for example).

To make sure that the under-hood heater valve is working, I'd recommend having a friend (or some passing vagrant) run your heat controls all the way up and down - you'll see the lever moving, and when the vagrant (heh) moves it to full hot, leave it there and see if you can manually reach behind the (hopefully not hot) engine and push it further. The controller can get wimpy, or the system can get out of alignment (not hard to fix, but you have to know).

I'd say that you should probably use whatever kind of cooling system flush you can find to try to dislodge the gunk in the system - getting it out is pretty necessary to restoring order in the universe. I guess I'd suggest running a lot more fresh (tap) water through the system, then put in the flush agent and follow the directions (which I'd guess include running the temperature up to normal operating temperature, and opening the heater valve fully to flush the heater core).

Oh, and THEN (and only then) replace your radiator - you really don't want to have the gunk clogging up a shiny new radiator.
 

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Discussion Starter #7
The fans should come on only when the coolant temperature gets over normal operating temperature...

To make sure that the under-hood heater valve is working, I'd recommend having a friend (or some passing vagrant) run your heat controls all the way up and down - you'll see the lever moving....

I'd say that you should probably use whatever kind of cooling system flush you can find to try to dislodge the gunk in the system - getting it out is pretty necessary to restoring order in the universe. I guess I'd suggest running a lot more fresh (tap) water through the system, then put in the flush agent and follow the directions...
Ok, so, in order from above...

The fans ONLY come on if the AC is engaged. I tried this many times. If the AC switch on the dash is on, the fans come on; if it is not, they don’t. It doesn’t matter what the temperature setting on the cabin controls are. It also does not seem to matter if the engine is hot. The fans also do not (should not) come on, as you explained, unless the coolant temperature is high enough. But, when going through the alternating air bleed steps of idling the car at 1500 rpm for 5 minutes with the inside cabin controls set at 60 with AC engaged, and then shutting off the engine and topping coolant, resetting the cabin controls for 90 degrees and holding at 1500 RPM for 5 minutes, the fans come on ONLY during the AC phase. If I have the car in the AC time, and I switch to 90 degrees from 60, and keep the AC engaged, the fans stay on. If I do the same thing and disengage the AC switch on the dash, the fans shut off. When doing that, the coolant temperature must be the same, so there must be two switches: one that goes on temperature and one that goes on whether the AC is engaged.

I can certainly have someone move the heater valve while I try to push it manually, but I think it has been open for me to get any higher cabin air temperature. I opened it using the temp controls on the dash and was able to get it to be hot, after fiddling with the hoses—squeezing them, etc.—so it can’t be the valve that’s causing that problem. I think I have gunk in there and there must be enough in the system that that area, where the coolant has to slow down because of the angles and turns, is just the sticking point for the gunk. I have not tried to manually move the valve, but I have gotten varying degrees of hot air—some very hot. Unless you’re saying that it could be intermittent—that it might fully open sometimes and not others. But I figured that it was either weak or not. The cable seems to be straight and not gunked up at all—very shiny on the part that moves back and forth into the guide cover.

As I was saying above, I’ve run a hose into the radiator with the draincock open, while the engine was running over the course of 15 minutes or so. I’ve also run the flush through the system for a while—I think that must’ve been what dislodged some more gunk that got stuck again in the heater core and clogged it again. I’m leaving the flush in there and running it over the next couple of days, so I’ll be able to try the manual movement of the heater cable and also fiddle with the hoses a little more.

I keep referencing the steps to bleeding the system from the Acura service manual. I am following the repeat procedures in step 17 (repeat steps 15-18, until there are no more bubbles). I thought I saw the .pdf of the manual on this forum but couldn’t find it to attach, however it was reproduced here:

Despite what the dealer may tell you, you should change your coolant every 2 years (3 years max). It's pretty easy to do, you only need a few simple tools.

Time Required: 2-3 Hours (if your first time)

Difficulty: Easy/Moderate Difficulty

Parts Needed:
-1 or 2 different size flathead screwdrivers
-Needle nose pliers
-12mm socket (preferably 1/2 inch drive)
-Socket wrench and various extension or two
-Drain bucket that can fit underneath the standard (no jacks) clearance
-Plastic drain hose, preferably 4+ feet long and flexible
-2 gallons of coolant (Acura Long Life Antifreeze/Coolant Type 2 (P/N OL999-9001) (Acura Long Life Antifreeze/Coolant Type 2 is a mixture of 50 % antifreeze and 50 % water. Do not add water.)
-Funnel (optional)
-Various rags/towels for spills and cleanup

Engine Coolant Capacities:
(Including the reserve tank capacity of 0.6 L (0.16 US gal)):
After Coolant Change:
7.1 L (1.88 US gal)
After Engine Overhaul:
9.0 L (2.38 US gal)

Step 0: Make sure the engine and radiator/coolant temp isn't hot. Some people like to drain it warm, others wait until completely cool. Ultimately it won't matter much either way, just don't burn yourself!

Step 1: Remove the splash guard (see picture). There are approximately 6-7 (I think) plastic rivets you need to remove. Use your flathead screwdriver to pry them up, then use the needle nose pliers to pull out/down the inside and the rivet will pop out. This step may not be entirely necessary, but I found it useful to remove it in order to access the radiator drain plug.

Step 2: Turn the ignition switch ON (II). Set the climate control system to 90 of (32°C) or heater controls to full hot, then turn the ignition switch OFF. Make sure the engine and radiator are cool to the touch.

Step 3: Position your bucket and loosen the radiator drain plug (front passenger side). Turn it left to loosen it. Should take about 10 minutes to fully drain. (See pictures below for the location of the radiator drain plug.)

Step 4: Turn the wheels as far as you can to the left so you can access the engine block drain plug (see picture). It is behind the front passenger wheel on the rear of the engine block. Access the drain plug by unclipping the wire/sensor (see picture) and prepare your bucket and rubber hose. Lay on your back and crawl under the plug and use a 12mm socket and break the bolt/the copper head of the drain bolt to the left. Once your break it, position your hose, then use your fingers to loosen it/turn it to the left. You may need to experiment with a 1/4 drive socket and an extension or two to get a good position on it. Allow the coolant to fully drain until you get a very slow drip. Should take about 10 minutes. Re-tighten the drain plug snugly (or 9.8nm if you have a torque wrench).

Step 5: Empty the coolant reservoir. This is the white tank on the passenger front side near the windshield washer fluid tank. I used a MityVac to empty mine, you can also use a long turkey baster and do a few pulls, or you can remove the tank to empty it. Get creative if you have to! If you can't do any of these, really you're only talking about a very small amount of leftover coolant and with replacing everything else it won't be a big deal (you'll have replaced 90% of it anyway).

Step 6: Pour Acura Long Life Antifreeze/Coolant Type 2 (P/N OL999-9001) into the radiator up to the base of the filler neck. Do this SLOWLY, you don't want to create a bunch of air/bubbles which are the enemy of a cooling system. If you use a funnel, pour slowly down the side of the funnel, like pouring a draft beer.

Step 7: Leave the radiator cap off (this is so air can escape, i.e. the bubbles you tried not to create it Step 6). Start the engine. Hold the engine speed at 1,500 rpm until it warms up (the radiator fan comes on at least twice). Make sure the thermostat is open, i.e get things warm. You may start to see steam rise from the radiator filler neck that you left open. You may also get some initial spillover during the first start. Grab a helper or some towels to clean it up or catch some around the next when it spills out. Check for leaks/drips from your radiator drain plug and engine block drain plug (don't mistake some spillover from the radiator for leaks).

Step 8: Turn off the engine. Check the level in the radiator, and add Acura Long Life Antifreeze/Coolant Type 2, if needed.

Step 9: Set the climate control or heater control panel to maximum cool. Start the engine. Hold the engine speed at 1,500 rpm for 5 minutes, then turn off the engine.

Step 10: Check the level in the radiator, and add Acura Long Life Antifreeze/Coolant Type 2, if needed.

Step 11: Set the climate control or heater control panel to maximum heat. Start the engine. Hold the engine speed at 1,500 rpm for 5 minutes"then turn off the engine.

Step 12: Check the level in the radiator, and add Acura Long Life Antifreeze/Coolant Type 2, if needed.

Step 13: Set the climate control or heater control panel to maximum cool. Start the engine. Hold the engine speed at 1,500 rpm for 3 minutes, then turn off the engine.

Step 14: Check the level in the radiator, and add Acura Long Life Antifreeze/Coolant Type 2, if needed.

Step 15: Set the climate control or heater control panel to maximum heat. Start the engine. Hold the engine speed at 1,500 rpm for 3 minutes, then turn off the engine.

Step 16: Check the level in the radiator, and add Acura Long Life Antifreeze/Coolant Type 2, if needed.

Step 17: Repeat steps 15 through 18 until the coolant level does not change in the radiator, then install the radiator cap loosely (half type of turn where you can feel it secure, but also loose so that air can escape).

Step 18: Set the climate control or heater control panel to maximum cool. Start the engine. Hold the engine speed at 2,500 rpm for 1 minute.

Step 19: Fill up the coolant reservoir to the MIN mark. I prefer to just fill it up to MIN as the cooling system does its thing over the next few drives. I then check it after each drive and eventually after a few drives I'll fill it up halfway between MIN/MAX.

Step 20: Re install splash guard.

Step 21: Have a cold one!
So, I guess I think I have a problem with the fan control switches or their circuits. They don’t come on when the car is not in AC setting. This is probably why I haven’t had the overheating problem until now—I’m in Memphis and it’s really only gotten cool enough to have the heat on just recently. The AC has been the only conditioned air used for months and with the fans working there is no steam from the radiator, so as long as we were using the AC there was no apparent problem.
 

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Discussion Starter #8
That being said, I don’t know anything about how the fans operate...where the switches and sensors are, etc...
 

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Discussion Starter #9 (Edited)
1. Check that your cooling fans are working. If the A/C is working OK (proving that you have a charge in the A/C system), just turn on the A/C and both cooling fans should come on. If not, normal troubleshooting for fuses, relays, rodent eaten wires...

3. Be sure to bleed all air from the cooling system any time the level gets low. Open the bleeder as you fill until only liquid comes out. Do this with the engine off. This video shows how, although he has the motor running.

https://youtu.be/wBWldTfBEMQ
I watched the video and couldn’t really orient myself to tell where he was. It looked like it might have been around the thermostat, but my vehicle is not that open there, unless he has stuff taken off the engine, like the intake...Are you sure there’s a bleeder valve on the 1G MDX? If so, why on earth would you need to do the elaborate changing from heat to AC steps in the coolant refill procedure?

If there is a bleeder valve, that would be great. That was what I was originally looking for when I ran across the service manual information. Every car I’ve ever changed coolant on has had a bleeder of some kind.

Re: the fans, you’re right—see my crazy-long post above—but they only come on with the AC. Do you know of another sensor or switch that would make them come on with high coolant temperature and the AC off?
 

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The good news is it sounds like the thermostat is installed correctly. For future reference:


I didn't find the temperature that the fan comes on for low speed but the high-speed circuit doesn't happen until 206° F (pretty darned hot). The MDX isn't like our old V8 lumps - it's a very efficient, modern motor that generates a relatively small amount of heat, especially when idling. The thermostat doesn't even fully open until 194°, and once that happens it's going to start circulating coolant through a whole lot more mass. If it's a cool day (almost a certainty) even without air being pulled through the radiator, it's going to take a long time to heat up the entire system enough to reach the turn-on point for the fans (I presume it's between 194° and 206°F - reading between the lines in the manual, I believe the low-speed fan should come on at 204°F).

Our MDXs don't have the old-school fan thermo-switch that could easily be tested, but the sensor feeds the PCM which controls the fan relays (and thus, the fan motors). Here's a circuit diagram:


I'd suggest that you not get too excited about the fans not coming on until you can measure temperatures at or very near the boiling point (say 210°F) on the upper radiator hose (an IR thermometer would be just the ticket for this test, though it wouldn't hurt to point it at a pot of boiling water to make sure the calibration is good - if it was off 10°, it could send you on a wild goose chase).

Oh, I should mention that the condenser fan motor is mislabeled in the diagram (as a "relay", though it's clearly the fan motor).
 
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