I've had no heat in my 991.1 turbo since I bought it from the selling dealer (I found out later it was purposely not disclosed). I recently brought it into a shop to have the heat fixed, but they can't seem to figure it out. They initially changed the COV and changed the actual coolant bypass valve that the COV controls. Once this didn't work they realized the COV wasn't actuating. If you remove vacuum from the COV, the heat works fine, so the heater core and physical bypass valve are working fine. The mechanic also said that when they manually actuate the heater COV in software that heat still doesn't work. It seems like at this point for some reason the AC module isn't commanding the COV to actually open. He said he verified that the solenoid harness is receiving power and ground also.

Is there anything I can recommend for them to check? This just doesn't make any sense at this point.

 

Hi. Are you referring to the solenoid changeover valve? There are several on the 991 that haven’t been known to go bad. I believe they’re six solenoids and a couple of them have to do with heating.

 

Yes that is what I am referring to. According to a diagram there is one associated with heat and that is the one that is being looked at.

 

OK, yeah there’s a couple others. It’s been a while since I’ve dealt with this. I can’t recall, but I’m pretty sure it’s the same unit. Maybe they can change a couple of the others. There’s a lot of connection with all these lines back there. Do they have access to getting recall and repair bulletins and a PWIS?

 

Yeah they have access to PWIS and said when manually actuating that nothing is happening. I told them to dig a little deeper into the solenoid (and be 100% sure of the vacuum/fresh air lines).


I gave them these instructions (just based on my own logic):

 

After sending my troubleshooting instructions the mechanic was able to discover that the HVAC COV (single COV) was getting vacuum on both lines instead of vacuum on one and fresh air on the other. After fixing the line routing everything is working now.

 

Misconnection of the accessory vacuum system's lines will - *will* - become an increasingly common problem with 991s as more 'technicians' touch them.

This, because the lines form - effectively - two mesh networks: one for vacuum source and one for "vacuum release" (*) and it's all too easy to get a new COV connected "backwards" into the networks.

Any DIYers (and techs) should always spend the extra 5 minutes to inspect the line connections every time vacuum bits are touched. That, after making sure they know which COV connection is which.

(*) It's not "fresh air." It's filtered air actually.

 

Makes you wonder who did that to the HVAC and messed it up……

 

It saddens me that techs don't check the basics before starting with part replacement. How are we going from a COV replacement to a coolant bypass valve replacement and we haven't full tested COV actuation and vacuum application. I should have just done this repair myself, because I would have immediately done all that thorough testing before replacing anything. I would have discovered the line issue within a few hours.

Also, I'm aware it is actually filtered air, but the docs reference it as fresh air (as opposed to vacuum), so I was just using their language.

Exactly. In this case it had to be the people I bought the car from. The car was actually in a total loss event with the previous owner (only some front-end body damage) and the dealer/body shop I bought it from did all the repairs. I actually had a few conversations with the previous owner who told me THEY had reached out to him about the same issue (no heat) asking if he had that issue. He told them he never had an issue with the car. So somehow these lines got messed up (must have been them or someone they had work on the car) and they chose to hide and not disclose that in the C&B auction or to me directly. When I confronted them they played dumb saying they never noticed an issue blah blah.. which is impossible because you see this issue in hot or cold weather. Hot weather the car just keeps getting colder (doesn't blend hot and cold properly) and eventually overshoots the temperature target. In cold, obviously there's no heat at all.

 

Hmm got any pictures? My car seems to be doing the same thing right now after being put back together.

 

I'll see if they took any. I'm picking the car up tomorrow. From the diagrams I've looked up it's very straightforward and testing the lines is simple. The only thing you need the computer for is to manually actuate the COV.

 

Actually, you don't. Sure it's a tad easier, but the COVs are a simple 12v "switch." If you've dug down sufficiently to access vacuum lines, it's easy to unplug the two-wire connector and hit the COV with 12v.

 

I'm speaking more to having the actual computer actuate them. You can obviously apply 12v to test the actual solenoid and that's a good test of the part, but if you wanted a full test of the harness/computer, doing it via PIWIS is the way to go.