Strange idle issue...

[Ph34r]

When the car is in motion it idles at 2k, when the car hits 10mph or so, it returns to normal idle.

Every once and a while the brakes get squishy. I looked for the brake booster to check vac and nothing is wrong.

l28e, 1979 280zx, stock mostly... 5-speed.

Thanks in advance

[Jeff G 78]

BCDD is the cause of your idle troubles. Search for BCDD and you will find all the info you need. The brake issue could be many things. Old hoses, master cylinder going bad, air in the lines, etc.

[Ph34r]

Sweet, I think that I may just check and replace all the lines.

Thanks again!

Jason

[Ph34r]

Alright, I blocked off the BCDD like was stated in the writeup... and I still have the issue.

Between the AFM and the TB there is only one hose, on the top. It runs to something on top of the intake manifold. If someone has a picture of the BCDD, as I can not find it on my tech drawings of the motor, I would appreciate it.

Jason

[sblake01]

The BCDD is on the bottom of the throttle body. I don't know how you 'blocked it off' as it's an electronically controlled. The end of the hose you described connects to the air regulator.

[Ph34r]

If you decide to disable it, remove the rubber intake hose between the AFM and the throttle body. You will see two holes at the bottom of the throttle body. One hole goes to the BCDD and the other goes to the distributor if I remember correctly. If you plug the hole that goes to the BCDD, it will disable it and make your car run better (assuming you can't get the BCDD adjusted properly). In my case, I used a piece of QUALITY tape to cover the BCDD feed hole TEMPORARILY so I could determine for sure that the BCDD was the culprit. As a precaution, I cut the strip of tape long enough to extend upstream and wrap around the opening of the throttle body so that the rubber intake hose would help secure it in case it came unstuck. I didn't want to risk sucking it into the intake. Make sure the tape covers the BCDD hole only (the larger one) and does NOT obstruct the butterfly valve. After replacing the intake hose, take the car for a quick drive to see if the problem is gone. If so, the BCDD was the problem and you can either replace it or disable it by plugging the air hole with some RTV. On my car, the tape trick worked, but at the same time the idle was much lower and the car would stall when coming to a stop when the car was cold. This told me that the AAR was also not working. I replaced the AAR and the car now runs great cold and warm.

So... would unplugging it disable it?

I saw that and saw no adjustment screw or anything. Both VAC lines under the TB are blocked.

[sblake01]

You could also run a switched 12 volt to it's connection which would basically render it inoperative.

[Ph34r]

Ok, so ran a 12v switched to it and the car just idles at 2k all the time.

But... while my Ice cold A/C is blowing... the problem goes away.

Anyone got a Vacuum diagram for the 79ZX? Would like to go through and triple check everything.

[pbatura]

Here's a thought - Is it possible the fast idle actuator is somehow working in reverse? Meaning the dashpot advances the throttle until the A/C is turned on.

Perhaps you can observe the actuator operation with the aid of a friend.

Good luck.