78 280z vacuum system problem affecting idle and heater control

[Bighare56]

the engine runs anywhere from 1500 to 2000 rpm idling depending on the ambient air temp. When I first got it, it used to run right about 1000. I originally had a problem with the defroster not working properly. It wouldn't always close the valve to divert the air from the rest of the system to the window. I replaced the heater core and reconnected everything. It worked long enough to pass inspection, then the same problem started. I hand over handed the vacuum system and found the boots into the vacuum cylinder in the engine compartment had dry rotted. I replaced those with aftermarket off the shelf connections and again it worked for a little while. I suspect it to be a vacuum problem causing both problems, but am at my wits end trying to figure this out. Any ideas?

[Willoughby Z]

Unless the mode door "dash pot" is failing intermittently, you still have a leak in a vacuum hose. I had the same problem with the defrost setting (air flow from the center vent only). I found a tiny split at the magnetic solenoid valves- underneath the bracket. Then the defrost would quit again and I would find another hose with a split, trim the end, mode door worked. Then it would stop working and I'd find another split. Again and again. Rubber seemed supple but obviously was drying out and failing. Replaced everything vac line under the hood. There is a plastic "T" between the valves that is fragile and may have a crack. I have also had a check valve fail. The leak may be up under the dash, especially since you were in there replacing the core. It's possible that the selector valve (behind the center vent) is leaking- there's a write-up for refurb/repair.

The small leaks weren't enough to affect the idle and a large leak would cause the idle to drop and roughen, I would think.

BTW, welcome to the best car club on the internet!

Check my signature for xenon's site to download the FSM, very useful diagrams for the HVAC system. I nearly had them memorized while chasing my AC/ heater/ vent issues.

[boomer3]

I'm having a similar problem that I'm trying to diagnose. Thanks for the info Willoughby, huge help with the FSM etc! I think the thing is to just get in there and replace every single thing that is rubber.

[dankfrye@charter.net]

I have a '78 also and had the same problem. After replacing all my hoses and such, I still had a high idle so I removed my BCDD valve at the bottom of my throttle body and capped it. This solved my problems. You should be able to turn your idle screw all the way and kill your engine. If you can't then you have a vacuum leak. The seals in the BCDD valve get old with age and leak air, bypassing the throttle body. It is just an emissions device on deaccelation anyways.

[cygnusx1]

First thing to check is the fast idle control device. It's right on top of the intake manifold and looks like a little sideways UFO. Car must be fully warmed up to do this. With the engine idling, tap it with the handle of a screwdriver and see if the idle changes. It may be hanging up. Pinch off the hose connected to it (on either side) using a channel lock. If the idle drops, that's your problem. Car must be fully warmed up to do this. As a second possibility, the BCDD is adjustable, FYI...

Fast Idle Device:

http://media.photobucket.com/image/datsun%20fast%20idle%20device/datzenmike/L%2520Z%2520Heads%2520and%2520Motors/200sxcoldstsrtvalve002Large.jpg

Cracks in the rubber boots are also a major culprit, but you already ruled those out.

Edited March 29, 2012 by cygnusx1