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Unwanted hot air into cab

Fu Manchu

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It sits on the top on the HVAC under the instrument pad. To access it, guess what.
The pad has to come off to get to it from what I can tell. If you are really lucky, you might get to it by removing the top trim and centre trim around the headunit and that cover bit on the top of the pad.
You won't have to do the rest, like degas etc.
Removing the pad isn't too bad. seems bigger than it really is. You would need two people to do it.

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Fu Manchu

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Fu Manchu

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To get to the two hoses on the firewall, you need to remove the intake manifold. Only takes 5mins. It’s easy.
 

Fu Manchu

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the the air mix door is down inside the HVAC box. The only way you might get to see in might be down the windscreen outlet, only to do that, you’d need to remove the whole dash pad.
 

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The temps are controlled with a mechanical rod and armature set up on manual HVAC models.
 

ChrisG777

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Could it be just a vacuum hose to the Hvac.
No, because wiring the heater tap closed still leaves the problem. (The vacuum tube normally closes the heater tap). The air mix door is not controlled by the vacuum as far as I can tell, but by a mechanical rod from the temperature dial. And I have exposed that and the rod is moving correctly.
 

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What if the door has come away from the arm inside the HVAC box? I think we are starting to look at internal HVAC issues.

You know the other thing we might be looking at? What if the evaporator is freezing up at a particular air temp and that in turn is changing the air flow and output temp by air not passing through the evaporator.

Is this happening even when the ac is disengaged?

Anyone good with ac can tell if that is a viable explanation?
 

ChrisG777

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What if the door has come away from the arm inside the HVAC box? I think we are starting to look at internal HVAC issues.

You know the other thing we might be looking at? What if the evaporator is freezing up at a particular air temp and that in turn is changing the air flow and output temp by air not passing through the evaporator.

Is this happening even when the ac is disengaged?

Anyone good with ac can tell if that is a viable explanation?

This happens without the A/C engaged. Air out of vents with no A/C on is 8 degrees warmer than outside temperature. The air-mix door might be obstructed by some debris, as someone suggested, but I don't think it has completely come off, as you can get even hotter air by turning up the temperature dial.
 

Fu Manchu

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This is good because we can eliminate more and more things.

The water flow through the heater core isn't going to be the issue here. Even if the tap closes off 98% of the flow when switched to cold, the series of door positions further controls the airflow exposed to the heater core.

We can rule out the AC because it happens without the AC on.

We can rule out the temp dial, its been checked and OP has personally checked it's operation.

Is the rod adjusted correctly? It has a thread on it, if the thread is pushed into the mechanism at the wrong position on the back of the dial, the door might not fully open and close as designed. Just a thought. So effectively, the door is out of sync (of sorts).

It all points to removing the whole dash pad to access the HVAC, and possibly removing the HVAC to overhaul it, which might be a good thing. It's a big job and you'd want to be fairly sure to go the next step, so before diving in, make sure that rod is positioned correctly, or even operate it without it being connected to the dial to feel what it is doing.
You have the HVAC file to work from now, so use it as a guide.

Once the instrument pad is out, you will get a clear view down inside it from the top where the window vent is. That might be enough to remove the obstruction or see the problem.



*In hindsight, with both the ones I've done, I should have got one from the wrecker and reconditioned that one, so I had a refreshed one ready to go straight in as soon as the old one is removed. Speed the job up no end and less pressure on you to hurry things up so the car can be used again. Also have spares for the future or recon the spare after its out and sell it as a complete unit with new evap and heater core fitted, cleaned out and new seals.
 
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