Hello, everyone.

I've got a '76 280z that I've taken on as a second car. I bought it earlier in the year, and it ran like a champ. I drove it home from Temecula, CA to San Bernardino, CA. Same day, no problem. Problems started when I went to change the plugs, plug wires, fuel filter, and old vacuum and fuel lines before taking on the smog test. Turns out the BCDD was disconnected and lines running to and away from it had been blocked off. So of course, I reconnected them and since then, the car is not a fan of smooth idling. Idle starts out at about 1k, and it quickly drops down to 500 rpms and even lower at times. After I turn the car off, or if it dies. I'll hear a vacuum leak. I'm assuming I've got a bad BCDD? Thoughts?

I'm also considering a bad water temp sensor, and have purchased a new one. I'll have to install it in the following days and I'll report back if it's fixed the idle at all. Also have considered once again blocking off the bcdd, and fixing my problems that way. But that's a step backwards as I need to pass smog.

Thanks for reading, and I'd appreciate your knowledge. I'm a first time z owner, and I think it shows. haha 

-Oscar

p.s. - only even considering water temp sensor because I had to replace water pump because it began to leak coolant.

I wouldn’t be ready to condemn the BCDD quite yet, and I am confused by a couple things in your original post...

First thing is that there is only one externally visible connection to the BCDD, and that is an electrical one. There are a couple vacuum connections made to the BCDD, but none of them are externally visible. They are all made internally to the throttle body and are not visible without peering into the throat of the throttle body.

So when you say "the BCDD was disconnected and lines running to and away from it were blocked off", if you're talking about some vacuum lines being capped... Those vacuum lines originate from the throttle body, but despite how close in proximity they are to the BCDD, neither of them have anything to do with it. One of those vacuum lines should be connected to your distributor and carbon canister, and the other vacuum line is used for the EGR system. So if you're having troubles after messing with those vacuum lines, it's not the BCDD's problem.

If you are talking about the electrical connection to the BCDD, that connection is a kind of "dead man switch" such that when that wire supplies power to the BCDD, the BCDD is deactivated and should have no impact. But if that wire was disconnected, your BCDD was active at all times.

Second thing is about you hearing a vacuum leak after you turn the car off... As soon as the engine stops turning, the manifold vacuum goes away, and there are two places where that vacuum is "stored" while the engine isn't running. Those two places are in the HVAC system, and in the power brake booster. Both of those systems should have a check valve in series with them to prevent that vacuum from bleeding off, but I'd start looking there.

17 hours ago, redfairladyzee said:

 it ran like a champ.

 Problems started when I went to change the plugs, plug wires, fuel filter, and old vacuum and fuel lines before taking on the smog test.

 Idle starts out at about 1k, and it quickly drops down to 500 rpms and even lower at times. 

p.s. - only even considering water temp sensor because I had to replace water pump because it began to leak coolant.

CO is right, don't move too fast.  Collected the basics above.  If you mean "starts out' from a cold engine, then the idle behavior could be your AAR working correctly as the engine warms up, with a vacuum leak that causes the low idle after the engine warms up and the AAR closes.  Could also be your timing is retarded now and you didn't reset idle speed (idle speed drops as timing is retarded).

Find your service manual here, and use the Engine Tuneup, Engine Fuel, and Emissions chapters to get things right.

http://www.classiczcars.com/files/category/9-factory-service-manuals/

Forgot to say - the sensor will work fine even with a leak.  Just fix the leak, not the sensor.

Edited September 18, 20168 yr by Zed Head

I appreciate both of your inputs and advice on where to start troubleshooting! Exactly the type of responses I was hoping for. It must of slipped my mind, but I did also end up unplugging and reconnecting the I believe purge vacuum line to the carbon canister. So I'll have to go over the lines once again when I get home. Maybe a few photos will also help me explain which hoses I've since unplugged and reconnected. I have the efi bible, and I'll be sure to download the FSM tonight. Thanks again, both of you. 

- Oscar 

Edited September 19, 20168 yr by redfairladyzee

Cool. Good luck. Actually, my suggestion would be to put the car back to where it was when it ran good and then take some pics for us.

Then we can discuss emissions changes from that point?

Sorry for the delay, but I was just now able to pop open the hood and take some pictures. Unfortunately I didn't find the hardware/bolts that were used to cap the hoses by the previous owner, so the car is not back to where it ran good. Hopefully the pictures will still be helpful. Where the arrows are pointing to, there used to be a bolt, and that bolt blocked off the hose there. All I did was remove the bolt and connect the hoses. In the first photo, to the carbon canister. And in the second photo, I thought was to the BCDD, but now I don't know.

-Oscar

 

 

 

Edited September 21, 20168 yr by redfairladyzee

The red arrow points to the control signal line for the evap system. It should also be tied into the distributor vacuum line. It could be the source of a small vacuum leak.

The green arrow points to the vacuum supply line for the power brake booster, and it could be the source of a much larger vacuum leak. If your car runs fine with that brake booster line plugged but won't idle with that line connected, then your brake booster is probably leaking internally and needs attention. You also mentioned that you hear a vacuum leak when you first shut off the engine? I suspect your brake booster is toast.

Thank you CO for all of your help! I'll be sure to block off the vacuum at the green arrow and confirm, but I'm sure you've got it right. Any recommendation on rebuilt brake boosters? I tried doing a quick search for new ones, and at first glance, no luck. I'll keep looking in case replacing the unit as a whole ends up being my solution.

Captain O's most recent post on Brake Boosters.  Also lots more info if you do a search.

 

Thank you gwri8! 

Haha! Well don't be too sure I got it right! These cars have a way of teaching you a lesson.

You'll know more after you disconnect the brake booster and plug that line. Hopefully you'll get your smooth idle back.