HVAC problem - no air from any vent after 20 miles

Shropshirewrangler

Shropshirewrangler

TJ Enthusiast
Joined
Jul 9, 2019
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264
Location
Shropshire United Kingdom
When I start the TJ from cold I can adjust the air flow into the cabin through the demister vent, face level vents and lower vents as expected, everything works properly. As the vehicle warms up the blend control works properly and so does the blower speed, the air con works well.

Roughly 20 miles into the journey, whatever the position of the vent/air con selector switch no air comes through any vent, the blower runs at all speeds and the air con clutch works.

After stopping for an hour or so air flow is restored and HVAC works normaly for 5 or so miles, then no air from any vent whatever the switch position.

I suspect some sort of vacume problem but have no idea where to start looking or how to test anything.

Guidance and suggestions for solutions greatly appreciated.
 
The default operation of the HVAC system when there's a vacuum problem is for it to blow out of the defrost. I think it must require vacuum to close that vent, where as the others require vacuum to open (but Im only speculating). If you have NO airflow, that might mean you have something else going on rather than (or possibly in addition to) a vacuum problem.
 
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Vacuum Leak Potential Sites.JPG
vacuum line firewall (2017_11_20 00_38_12 UTC).jpg
 
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I think this one comes up often enough that it would be good to have a resource thread about it. Especially if we could list any most-common locations for vacuum leaks. I'll soon be chasing down one of my own, so will see what I learn in the process.
 
Those check valves go bad, and I’ve had the under hood vacuum lines crumble at the lightest touch.
 
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Thank you for your replies.

I've given the vacume lines in the engine bay a good coat of looking at and nothing is obviously wrong, so my next step is to check the non return valves and reservoir.

Thinking about it, rather than pull the dash apart to look for a "ghost" problem, it might be more sensible to wait until it stops working and go from there.
 
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