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Old 01-14-2021, 08:28 PM   #1
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Dash vents blow hot air under heavy engine load?

Any thoughts on what would cause the dash air vents to suddenly start blowing hot air when the engine is under heavy load/high RPM (such as climbing a hill)?

With the temp control knob set all the way to cool, the vents blow cool air as you would expect when driving on a flat highway at about 2000 RPM. Once the RPM drops back to 2000 again after the hill, the vents slowly start to blow cool air again. This is on a 2021 Winnebago Vista 35U with the new Ford 8 cylinder engine. It acts just like the old vacuum controlled dash heat/AC/vent/defrost systems when they had a vacuum leak. As far as I can tell, this version of the Vista uses all electric motors to control the blend/mix doors and doesn't use any vacuum actuators or motors. I think the supplier of the dash heat/AC/vent/defrost system is Victory.

Since it is too cold outside, I haven't done any poking around or looking at the system. I have just been doing a little internet searching and thinking about it. The internet searches came up with lots of complaints about the vacuum systems. In fact, I had the vacuum leaks that I fixed on my previous 2003 Winnebago Sunova. Thanks in advance for your ideas.
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Old 01-15-2021, 02:26 AM   #2
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Originally Posted by Bones2003 View Post
Any thoughts on what would cause the dash air vents to suddenly start blowing hot air when the engine is under heavy load/high RPM (such as climbing a hill)?

With the temp control knob set all the way to cool, the vents blow cool air as you would expect when driving on a flat highway at about 2000 RPM. Once the RPM drops back to 2000 again after the hill, the vents slowly start to blow cool air again. This is on a 2021 Winnebago Vista 35U with the new Ford 8 cylinder engine. It acts just like the old vacuum controlled dash heat/AC/vent/defrost systems when they had a vacuum leak. As far as I can tell, this version of the Vista uses all electric motors to control the blend/mix doors and doesn't use any vacuum actuators or motors. I think the supplier of the dash heat/AC/vent/defrost system is Victory.

Since it is too cold outside, I haven't done any poking around or looking at the system. I have just been doing a little internet searching and thinking about it. The internet searches came up with lots of complaints about the vacuum systems. In fact, I had the vacuum leaks that I fixed on my previous 2003 Winnebago Sunova. Thanks in advance for your ideas.
When on vent, or AC?
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Old 01-15-2021, 09:30 AM   #3
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Here is some additional information about my problem and an answer to Wyatt's question.
1. Dash AC was turned off.
2. Recirculate switch was turned off.
3. Temp control knob was all the way to the left in the blue area to set it to the lowest temperature.
4. Control knob was set to the vent position.
5. Changing the fan speed did increase or decrease the amount of air flow but did not make a difference as far as the temperature problem.
6. Outside air temperature was about 50 degrees F.
7. While parked with the engine running, all dash heat/AC/vent/defrost functions appeared to work as they should.

Thanks again for the help.
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Old 01-15-2021, 10:57 AM   #4
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I get hot air from the vents any time the AC is not running. Figured it was just catching the air right off the hot engine.
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Old 01-15-2021, 12:54 PM   #5
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I get hot air from the vents any time the AC is not running. Figured it was just catching the air right off the hot engine.
Ok, that’s what I was thinking, but I’d trace the vent tubes and see if any are loose or disconnected in the front engine compartment.

To the OP, I’m not even sure if the recirculate function works when not using AC—never tried that—but if it does, I’d try that and see if you get the same results when using VENT.
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Old 01-15-2021, 01:43 PM   #6
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Might just be a high load cutout. AC systems are often calibrated to turn off under certain high load/pedal situations. Might only turn it off for a calibrated time, might stay off until the load drops.
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Old 01-16-2021, 12:00 PM   #7
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tderonne - my dash A/C was off. Would the high load cutoff make a difference with the A/C off?

Wyatt - according to the ducting diagrams, the recirculate function doesn't look like it would make a difference but I can try it again. Unfortunately, it might be awhile before I am able to drive it again under those load situations.
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Old 01-16-2021, 07:09 PM   #8
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Here is some additional information that is interesting about this problem. The A/C evaporator and the heater coil share the same assembly (sometimes referred to as a combo coil). There is no temperature mixing door. There is a 3-port water valve that routes hot coolant from the engine through the heater coil to produce heat. Otherwise, that valve shuts off the water flow for A/C or regular unheated vent. There is a door that is controlled by the "fresh air" or recirculate button but it has nothing to do with temperature. The temperature knob on the dash heat/AC/vent/defrost control is a potentiometer that sends a voltage to the water valve to control the amount of hot water going to the heater coil to raise or lower the heater output temperature. What is not clear is why putting the engine under a high RPM load would cause the 3-port water valve to send hot water to the heater coils?
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Old 01-17-2021, 06:31 AM   #9
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We have the same problem. Don't know if yours is the same, but on ours, the fresh air intake for the system is located in the engine compartment on the left side looking in from the front. The intake is a rectangular opening located on the bottom of the air box which is drawing in warm air from the engine compartment, radiator, etc. I read a solution posted somewhere that a duct was attached to the intake opening and run down below the source of the warm air where it could draw in cooler air. The person claimed it cured most of the problem, but I’ve not tried it myself yet. It's on my to do list
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Old 01-17-2021, 09:36 AM   #10
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We have the same problem. Don't know if yours is the same, but on ours, the fresh air intake for the system is located in the engine compartment on the left side looking in from the front. The intake is a rectangular opening located on the bottom of the air box which is drawing in warm air from the engine compartment, radiator, etc. I read a solution posted somewhere that a duct was attached to the intake opening and run down below the source of the warm air where it could draw in cooler air. The person claimed it cured most of the problem, but I’ve not tried it myself yet. It's on my to do list
thompwill, most all Winnebago Class A's on the Ford chassis have had that configuration for many years. You should only be drawing engine compartment heat when you are stopped for awhile like waiting for a red light (and then only if the the fresh air damper door is open). Otherwise, with the vehicle moving the engine heat flows mostly to the rear of the coach and doesn't impact the air intake too much. To me, adding that duct would not be worth it and may actually be masking another problem. In my experience, if everything is working correctly the Winnebago dash HVAC system works really well. However, there are a lot of different parts to the system that could have a problem.

I appreciate your input and comments though. I'm still looking for ideas on my original problem so hopefully someone on the forum will come through.
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Old 01-17-2021, 09:43 AM   #11
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I get hot air from the vents any time the AC is not running. Figured it was just catching the air right off the hot engine.
theSane, that is not normal and its hard to catch engine air when you are moving. Sounds like you have a problem with the water valve that runs hot engine coolant through the heater core to provide dash heat. Could be the valve or something that controls that valve.

Thanks for your comments though. I think my original problem might also be related to that water valve but I'm having a hard time seeing how engine load/RPM would make a difference.
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Old 01-17-2021, 11:46 AM   #12
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We just swapped out a 2015 Ford Focus which had a problem from day one of giving warm to hot air if we had it on vent and did not have AC on. It was drawing air which was way warmer than wanted, just off the engine, even though it was a very small engine!
If we were in slow traffic like normal we almost always had to have the AC on to overcome the "too warm" outside vent air.

I might be able to say the larger engine working harder might give off far more heat to make your problem. I might want to look at how air might flow through the radiator and then be picked up for inside.

But that would be a guess which needed lots more checking!!!
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Old 01-17-2021, 12:19 PM   #13
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I might be able to say the larger engine working harder might give off far more heat to make your problem. I might want to look at how air might flow through the radiator and then be picked up for inside.

But that would be a guess which needed lots more checking!!!
Hi Morich, thanks for your idea but I'm pretty sure that is not the situation. I was driving 55 MPH at 2000 RPM with AC off and cool outside air blowing through the vents. For every hill I encountered, the transmission would downshift, the RPM would jump up to 3500 or so, the speed would maintain at 55 MPH but then hot air would start to blow out of the vents. Once I topped the hill, RPM drops back to 2000, transmission upshifts, and cool air starts to blow out of the vents again. It happens consistently and fairly quickly (although not instantaneously). These were not huge mountains but the hills around Springfield, MO so climbing them would only take a few minutes usually. Hard to envision air flow off the engine doing that. Like I said, it blows cool (as expected) except under load.
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Old 01-17-2021, 12:43 PM   #14
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Hi Morich, thanks for your idea but I'm pretty sure that is not the situation. I was driving 55 MPH at 2000 RPM with AC off and cool outside air blowing through the vents. For every hill I encountered, the transmission would downshift, the RPM would jump up to 3500 or so, the speed would maintain at 55 MPH but then hot air would start to blow out of the vents. Once I topped the hill, RPM drops back to 2000, transmission upshifts, and cool air starts to blow out of the vents again. It happens consistently and fairly quickly (although not instantaneously). These were not huge mountains but the hills around Springfield, MO so climbing them would only take a few minutes usually. Hard to envision air flow off the engine doing that. Like I said, it blows cool (as expected) except under load.
Oh, yeah! Know those hills well as home town and they are not the long hard pulls I had in mind that bring the temp up to almost crisis levels!
When they were doing the blasting for Hwy 65 down to Branson, my wife and I once dropped off to sleep while waiting for the road to clear and woke up with all the traffic just cruising by and looking at us setting there in the road!

I've almost totally lost track of the controls on new engines but it sounds like one of a dozen things is not sensing it right? UGH!
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Old 01-17-2021, 08:17 PM   #15
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Is it possible the the mixer valve is opened during high rpm/load demand to help in cooling the motor?
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Old 01-17-2021, 08:59 PM   #16
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if the blend door is operated by vacuum as on many vehicles the pull up a hill will deplete the vacuum storage capacity. If this is the case check your vacuum hoses for leaks.
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Old 01-17-2021, 10:54 PM   #17
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I had that issue in my 06 outlook. Turned out to be the vacuum reservoir was leaking. Replaced it and never another issue.
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Old 01-20-2021, 05:29 PM   #18
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When on vent, or AC?
You probably need a bigger vacume tank. Sounds like you run out of vacume due to the engine not producing under that condition. Make sure the check valve is operational.
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Old 01-20-2021, 06:54 PM   #19
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Thanks jonmurray, jeepster05, and mdka99 but as far as I can tell there is no vacuum involved in this 2021 unit. Everything uses electric stepper motors instead of the older vacuum motors/actuators. Your comments are exactly the problem path I was looking at - acts like vacuum so it must be vacuum (based on my experience with my 2003 Winnebago) until I found out everything is supposed to be electric. I haven't given up on that somehow being involved but it is hard to see at this point. I'm trying to find a vacuum diagram for the new Ford V-8 but haven't had any luck yet.
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Old 01-20-2021, 07:15 PM   #20
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Here is some additional information that is interesting about this problem. The A/C evaporator and the heater coil share the same assembly (sometimes referred to as a combo coil). There is no temperature mixing door. There is a 3-port water valve that routes hot coolant from the engine through the heater coil to produce heat. Otherwise, that valve shuts off the water flow for A/C or regular unheated vent. There is a door that is controlled by the "fresh air" or recirculate button but it has nothing to do with temperature. The temperature knob on the dash heat/AC/vent/defrost control is a potentiometer that sends a voltage to the water valve to control the amount of hot water going to the heater coil to raise or lower the heater output temperature. What is not clear is why putting the engine under a high RPM load would cause the 3-port water valve to send hot water to the heater coils?
You could be getting somewhere with the 3-port valve. Here are my thoughts, no science behind it:
1) The higher load, higher engine temp, and higher pressure on the engine coolant system. The engine water pump is pushing full flow through the engine thermostat. With all of this pressure, it could be either pushing on the 3-port valve shuttle forcing water through the closed port or actually backflowing in through the return side.
2) Curious if the 3-port heater valve is vacuum actuated or electric (solenoid or motor). If vacuum-actuated, the hard work on the motor is impacting the pressure in the crankcase, this could affect low vacuum actuators.
If electric or solenoid, it could be engine coolant pressure as mentioned in "1" forcing through the closed port of the 3-port valve.
3) Even though the recirculation damper is closed there are some style dampers that still let fresh air in. This is by design. Not a lot of air, but there is some. Although, this is not likely to cause a significant measurable temperature increase; unless, the outside air intake damper is not properly sealing when closed.

It seems like you've done your research and have a good handle on the knowledge of your coach dash ducting system. Therefore, I would look at the 3-port valve either leaking or being forced open.

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