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A/C blows warm under slight acceleration, then returns cold once cruising speed is reached

ZACH.SIKES

MEMBER
2003 FORD EXPLORER
4.0L
V6
4WD
AUTOMATIC
200 MILES

Hey all, I have the vehicle listed above that I’m having A/C problems with. The A/C didn’t work hardly at all when I got the car. I’ve gone through the entire system nearly, but this last part has me stumped. If I’m sitting in park in my driveway with the A/C on, it will work perfectly fine and be cold, even after I drive slowly it will be nice and cold as long as I’m super easy on it. When I got the car, it was overcharged, so I evacuated it and recharged with the correct amount of Freon. Now it works much better but under easy acceleration it will suddenly get warm until I cruise at a constant speed for a while. It isn’t the high- or low-pressure switch, it shouldn’t be the full throttle circuit, as I’m barely giving it gas, and I can floor the car if it’s in park, and it will never get warm. Only when in drive going down the road it will get warm. What could this be? The compressor works perfectly, all the vents/vacuum hoses are in working order. I removed the vacuum check valve and it tested good as well. It’s not vacuum related. I’ve also noticed even when the ac is working nice, and cold idling, the A/C lines are not “sweating” like they should in 100+ degree weather under the hood. Thanks in advance for any help/answers provided.

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Sunday, July 10th, 2022 AT 4:51 PM

23 Replies

JACOBANDNICKOLAS

MECHANIC
108,192 POSTS

Hi,

The HVAC on this vehicle is controlled by an engine vacuum. I suspect there is a leak in the system. When you place the engine under a load, the vacuum drops. When you let off the throttle, it increases. So, do me a favor. If none of the other functions are giving you trouble, try this. Listen closely for a hissing sound from under the dash. I suspect there is a slight leak between the controller and air temperature door.

I attached the vacuum schematic below and highlighted the air temperature door. Let me know if you hear anything and pay attention if any other functions act oddly when you accelerate or place the engine under a load.

Take care,

Joe

See pic below.

Image (Click to make bigger)

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Sunday, July 10th, 2022 AT 10:40 PM

ZACH.SIKES

MEMBER
16 POSTS

Hi Joe, thanks for the reply. Like I previously stated I have checked all of the vacuum lines under the hood and behind the dash that I can get to. There is no vacuum sound bleeding off. What I am wondering, would cause the pressure in the A/C system to drop rapidly under acceleration? It idles at about 50lbs of Freon, when I give it gas, the pressure drops to roughly 30lbs. I suspect this is the issue, as it’s probably dropping even more under strain while driving, causing the low-pressure switch to do its job. Thanks again!

Zach

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Monday, July 11th, 2022 AT 1:08 PM

ZACH.SIKES

MEMBER
16 POSTS

Again, I went and re-checked for vacuum leaks/noises behind the dash. Everything seems to work properly. The hot/cold air door seems to function perfectly, as when I’m in my driveway in park, I can rev it to the moon and have no problems.

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Monday, July 11th, 2022 AT 5:30 PM

JACOBANDNICKOLAS

MECHANIC
108,192 POSTS

Hi,

The pressures you indicated dropping, the low side will drop when you rev the engine and at the same time, the high side will increase. If you are getting a loss of cooling, there could be a vacuum leak anywhere that would cause the blend air door not to hold. If there is a leak under the hood, the vacuum in general will be lower.

That is what I suspect is happening.

Let me know your thoughts.

Joe

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Tuesday, July 12th, 2022 AT 2:32 PM

ZACH.SIKES

MEMBER
16 POSTS

Hey Joe, I’ve done some more inspecting i.E. Taking the glove box out and the intake tube and air box to inspect vacuum lines, I can’t find any fault in them whatsoever. Also, this car is not equipped with rear A/C, but it does have the typical two vents on the back of the center console. For some reason while driving it seems the rear two vents remain cold while all four front dash vents get warm/normal air temperature. Wouldn’t the air door cause these two vents to get warm as well? What would make the car only do this while in drive, going down the road? In park, I can rev to 3,000 to 4,000 RPMs and the air will remain cold, however the lines are not sweating and if at a standstill long enough, will start to get warm. I’m suspecting a hidden vacuum line I’m unaware of, or a bad mixture of Freon possibly. Let me know what you think, thanks for any more info, it’s super hot! (I will hook up a vacuum gauge tomorrow and let you know the readings.)

Zach

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Tuesday, July 12th, 2022 AT 9:46 PM

JACOBANDNICKOLAS

MECHANIC
108,192 POSTS

Zach,

I think checking the vacuum with a gauge will help. As I mentioned, the leak doesn't even have to be in the vehicle, it could be under the hood.

As far as revving the engine, I don't think the vacuum drops long enough for a change. On the road, you will hold the throttle open longer which provides more time for a drop in vacuum.

I don't know if you need it, but here is a link you may find helpful. Take a look through it if you get a chance.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Let me know what you find. I'm really interested in knowing. Since it happens under a load, a leak is my primary suspect.

Take care,

Joe

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Tuesday, July 12th, 2022 AT 10:33 PM

ZACH.SIKES

MEMBER
16 POSTS

Hey Joe, I’m still somewhat stuck as this vehicle needs other things and the A/C seems to work much better when it’s cooled out? Which it has been cooler. The reason I mention the interior/dash vacuum lines is because I know for a 100% fact every vacuum line under the hood, going to the firewall, are in working order. The vents do not switch to the “defrost” position, they just seem to get warm. The rear two vents seem to remain cold but I’m not positive. I am going to re-evacuate the system and recharge with new 134a, as vacuum seems ok, I have sat in the driveway revving to 3-4K for over 30 seconds before, and it never shuts off. Could a faulty clutch fan cause this? When the ac is working properly, the lines under the hood aren’t even close to “sweating” they are almost warm, with cold air coming from the vents. Also, the Freon dropping in pressure so much under acceleration is alarming. I tested this on a friends Crown Vic from the same era and his system dropped from 45lbs-35lbs max. Mine is dropping nearly 25 pounds from a good 3k rpm rev. I will update you with more info when I figure out more, thank you for the reply!

Zach

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Wednesday, July 13th, 2022 AT 6:35 PM

ZACH.SIKES

MEMBER
16 POSTS

Hello again Joe, I worked on the car for an hour or so today. I found that the “heater control valve” vacuum line, was entirely disconnected. (The grey one coming out of the passenger side fire wall) it also didn’t seem to have any vacuum of any kind in any position on the climate control knob. I hooked it back up to the heater control valve, but nothing seemed to change. Still have no ac when accelerating. I also got to messing with the climate control positions, and found that all of the defrost/floor vent positions on the knob do the exact same thing. Some air from floor and defrost. If you try just floor or just defrost, it won’t do one alone it will just default to floor and defrost at the same time. Thanks again.

Zach

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Wednesday, July 13th, 2022 AT 8:49 PM

JACOBANDNICKOLAS

MECHANIC
108,192 POSTS

Zach,

The idea that it is not changing from floor/defrost leads me to believe there is a leak.

Try this. Go to the vacuum supply from the engine. Disconnect it. With the engine running, is there a vacuum supply? If there is, place a vacuum pump on the interior side. See if it holds vacuum. To me, I think the issue is vacuum overall that is causing the blend air door to move allowing the temperature to change. This is a theory, but that is what makes sense to me.

Let me know what you find. Also, confirm if this has manual climate control or automatic.

Take care,

Joe

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Wednesday, July 13th, 2022 AT 10:14 PM

ZACH.SIKES

MEMBER
16 POSTS

This car has 3 knobs (not the digital climate control). It almost certainly has to be a vacuum leak behind the dash, if it is one. I have gone through legitimately every single line under the hood and they all seem to be near perfect. They’re not even brittle and they still bend quite easily. The inside lines are very soft like brand new, which is why I have been questioning it, the vacuum system on this car is probably in the best shape I’ve ever seen one of these in, but clearly something is amiss with vacuum if the vents aren’t working properly. Could this be the actual switches themselves leaking? The only time I ever hear the vacuum escaping inside is when I turn the climate off, I can hear all the air bleed off behind the dash. Are you asking me to disconnect the vacuum from the port on the intake? Then pull a vacuum on the line I disconnected and see if it holds? I also wanted to mention the schematic you provided. That is the only one I can find online for these Explorers, and the temperature door you highlighted, has no vacuum hose/port connected to it? Every other door on the schematic has a vacuum line that I can trace, except the door you highlighted. How is this door controlled? What vacuum line controls this door? Thanks

Zach

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Thursday, July 14th, 2022 AT 8:18 AM

ZACH.SIKES

MEMBER
16 POSTS

Could a PCV valve leak cause this? Update: the PCV system has a very strong vacuum, and seems to work properly. I’ve tried using carburetor cleaner all over the engine bay and there is no change in idle anywhere I spray.

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Thursday, July 14th, 2022 AT 8:50 AM

ZACH.SIKES

MEMBER
16 POSTS

Just hooked the grey vacuum line into the black vacuum line with a vacuum “T” and still no change. I can sit in one spot & rev it as much as I want for as long as I want, and it will remain cold. If I’m driving, slight acceleration over 2,500 RPMs will cause the air to get warm.

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Thursday, July 14th, 2022 AT 11:38 AM

ZACH.SIKES

MEMBER
16 POSTS

Okay, I completely evacuated the system again, and charged with new freon again. The problem persists. No sweating A/C lines, lose cool air under any acceleration. I have a buddy sending a new compressor, he believes the pistons inside have worn out from being overcharged and it isn’t pushing Freon like it should.

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Thursday, July 14th, 2022 AT 3:52 PM

ZACH.SIKES

MEMBER
16 POSTS

Okay, the heat/cold control knob is electronic, so vacuum wouldn’t affect it. The only thing that isn’t electronic is the position of the vents, and that functions properly. Enough.

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Thursday, July 14th, 2022 AT 7:25 PM

JACOBANDNICKOLAS

MECHANIC
108,192 POSTS

Okay, remove the blend air door and inspect the actuator and the door to see if it could be moving when accelerating,

Also, since the pressure drops (low side) I suspect the high side increases. That shouldn't be the compressor. However, we don't know the high side.

I just sent a message to the site owner. He is excellent with A/C. Let's see what he suggests.

Joe

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Thursday, July 14th, 2022 AT 8:08 PM

ZACH.SIKES

MEMBER
16 POSTS

Isn’t that a dash out process? Also, the heat works perfectly, and switches over without issue. If that matters. Again, the pressure is dropping far too much. The problem definitely lies around the fact that I have proper refrigerant, good clean condenser, a running compressor, and still don’t have cold sweating lines or even cool lines on the low side.

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Friday, July 15th, 2022 AT 11:51 AM

ZACH.SIKES

MEMBER
16 POSTS

I have a new compressor and manifold set on the way.

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Friday, July 15th, 2022 AT 2:30 PM

JACOBANDNICKOLAS

MECHANIC
108,192 POSTS

Hi,

Since everything is the way, it should be, the last thing I can think of is the compressor.

If possible, could you let me know if that takes care of the issue? Also, my theory behind the blend door was the direction it moved may have changed under acceleration. I try to think of things that are less expensive to fix, and I didn't know how the high pressure was changing when the low dropped. The compressor, which at this point I do agree must be the issue, is the last thing I want to recommend. I hope you understand.

Regardless, if you could let me know, I would be interested in knowing.

Take care,

Joe

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Friday, July 15th, 2022 AT 9:48 PM

ZACH.SIKES

MEMBER
16 POSTS

I will let you know if it fixes the issue for sure. I don’t want someone else to waste as much time as I have trying to diagnose theirs. It’s definitely not a common issue/symptom that I’m having. If the compressor doesn’t work, it would 100% have to be the electronic temperature control door that’s broken. Even though it seems to function perfectly, who knows. Will be updating you in due time.

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Saturday, July 16th, 2022 AT 9:41 AM

JACOBANDNICKOLAS

MECHANIC
108,192 POSTS

Hi,

I suspect the compressor is simply worn out. I hope it does take care of it for you. I will watch for your reply.

Take care and have a good weekend.

Joe

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Saturday, July 16th, 2022 AT 5:04 PM