[BeBe]

I had noticed that on a cool day with the heater on and the top down that the heat seems to cycle on and off. When it is on, it is almost sickeningly hot. Is the operation of the temperature control similar to adjustable interval intermittent wipers where what you control is how long the system is off between cycles?



I noticed this morning (top up) that even with the temperature control all the way down that periodically there is heat coming from the system. Does this sound like something is broken or just an adjustment?

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[DaveN]

Maybe you have a sticky <acronym title='heater control valve'>HCV</acronym>. It is known to age poorly as it's next to the exhaust header and the plastic doesn't like all that heat. It may be time to check it's operation and possibly change it out. If it goes you are stranded and have a big mess to clean up.

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[Lear35A]

Dave probably nailed it. This valve is positioned by vacuum regulated by the temperature knob in the cabin. You can check the movement by observing the arm on the valve when someone in the car dials the knob full scale one way and then the other with the motor running and warmed up. It should be smooth the full range. You can also check it (motor off) by pulling the vacuum hose off the top of the valve diaphram controller and, well, sucking on it (piece of hose to extend is really needed). The vacuum you can generate by mouth is more than enough to close the valve (it is normally open and closes when the motor's running and the temp knob is set low).

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[BeBe]

If the knob setting is calling for heat and not moved, should the temperature of the air fluctuate?

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[DaveN]

The knob may be calling for heat but the little circle grid on the dash actually controls the <acronym title='heater control valve'>HCV</acronym>. It monitors the ambient air temp in the cabin and dynamically turns on/off the HVC depending on what temp you have set.

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[BeBe]

Thanks Dave. That explains the fluctuating air temp from the vents. Now I just need to determine if the <acronym title='heater control valve'>HCV</acronym> is working properly. Or, more likely, I'll just replace it to be safe.

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[lbpesq]

The <acronym title='heater control valve'>HCV</acronym> is one of those parts that regularly fails, will strand you if it goes (coolant leak), and is cheap. Mine went earlier this year, requiring a tow. When I replaced it, I bought an extra which now lives in my glovebox next to my extra DME relay.



Bill

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[BeBe]

Is there anything complicated about replacing the <acronym title='heater control valve'>HCV</acronym>? Aside from disconnecting the old one and putting in the replacement, does anything need bled or something like that? Any pitfall to watch out for?

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[DaveN]

I haven't done it yet but it looks straight forward. I assume one would have to pull the cam cover which means having some hi temp RTV on hand when reinstalling as they leak in the corners. That means removing the petrol feed lines at the rear as well. You will have to bleed the system as well but there are plenty of how-to's here to help with that tricky maneuver <img src="/forum/images/smilies/968/smile.png" class="smilie" alt="" />

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[Lear35A]

It's a tight squeeze in there, but I don't see why the valve cover would have to come off. Might help to have the oil filter off, so combine with an oil change. There's coolant in there. Be prepared to catch it somehow.

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[BeBe]

I was actually looking around in there this evening. It will definitely be tight getting to the clamps. The <acronym title='heater control valve'>HCV</acronym> and hoses/clamps actually look pretty new. I was surprised to see a heat shield between it and the exhaust manifold. It was not a D1R design, but it looks like it should protect the <acronym title='heater control valve'>HCV</acronym> anyway. Haven't conducted the test suggested by Lance yet. Even if it looks like it is working smoothly, I suspect that it may not be closing all the way -- which is why I'm getting some heat when the control is rotated off.

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[flash]

sorry i just saw this. likely is at the HCV. it could easily be a vacuum leak at the hose there.



bebe - the heat shield you have is likely the OEM one, as i'm guessing your car is a tiptronic.

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[BeBe]

No, it's a 6spd. It may not actually be a heat shield, but it is between the <acronym title='heater control valve'>HCV</acronym> and the manifold and looks like it would shield the <acronym title='heater control valve'>HCV</acronym> from some heat. Doesn't look like it would shield much else, as it actually goes to the firewall side of the oil filter and under the manifold, not on top and to the side like the D1R. Don't know my way around under the hood yet.

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[Darryl]

I replaced my <acronym title='heater control valve'>HCV</acronym> when I first got my car with the help of my trusty german auto specialist buddy....I was learning my way around at the time and for that matter, still am!! I know for a fact we didn't remove the cam cover but we did bleed the coolant system afterward. I remember breaking the plastic drain plug, so be careful with that. I also replaced both of the small hoses on each side of the HVC as the larger of the two had a nice bubble starting to build.



- Darryl

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[flash]

ah - yes, there is a small heat shield there. it doesn't do much of a job, but it is something anyway.

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[BeBe]

I was planning to buy the D1R heat shield, and it sounds like I still should.

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[Lear35A]

It's also possible to check for a vacuum leak. The 4mm hard plastic vacuum tubing runs directly from the firewall to the 90 degree rubber piece on the <acronym title='heater control valve'>HCV</acronym>. It starts as the blue line that also comes out of a 90 deg rubber hose piece on the firewall. Like everything else it is tight in there, but pull the blue tubing off at the firewall and the black tubing off at the <acronym title='heater control valve'>HCV</acronym>. Jury rig something to block off one end and suck on the other. It should hold the vacuum.

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