79 280zx: Fast Idle 2200 RPM

[Dave WM]

so disconnecting the wire simulates above 10 mph, could I test the function but idling (maybe not high enough vacuum to test) and then there should be an increase in RPM due to bypass?

 

 

[Zed Head]

1 hour ago, Captain Obvious said:

So some points of interest...
The further run in the adjustment screw on the bottom, the easier it is to lift #9 off it's seat. That's why the BCDD will operate at a lower vacuum.
When the solenoid is energized (and the pin is retracted) this defeats build up of vacuum in chamber #17. This is because vacuum pulled into chamber #17 from the intake manifold side is just bled off by pulling in air from the upstream side of the throttle body. In order to enable the BCDD, they block the bleed passage on the upstream side of #17 and allow the vacuum from the intake manifold to build up in chamber #17. If that vacuum gets high enough, it will lift the plunger off it's seat.

Keep in mind that I didn't design the thing, but that's my read. 

So if the pin is retracted, in it's normal position with the key On (the click you hear when you turn the key On) and at a stop, you would expect a small amount of air to bypass the blade.  But that extra air is normally present and baked in to idle speed adjustment.  If the wire is disconnected the pin blocks the passage and idle speed should drop a small amount since that extra air is gone.  Does that follow?  I should be able to go out and disconnect the power wire and see a drop in idle speed.

If you have an idle, at a stop, that's stuck high, it seems to also follow that it's probably the main air bypass, designated by the arrows in the picture, that is stuck open allowing air through.  Or a hole in a diaphragm, or the main valve n the BCDD is stuck.

Seems like there are two normal ways for air to get through the BCDD, the little control passage and the main bypass passage, and one abnormal way, a hole.  I've never thought the idle hanging problem through completely, I just refer to what other people said.

[JSM]

3 hours ago, Captain Obvious said:

To test if your problem is the BCDD, you could probably take off the rubber boot on the front of the throttle body and put a small piece of tape over the BCDD holes. Put the boot back on and start the engine. I wouldn't rev the engine for fear of sucking the tape off and down into the engine, but as long as the throttle butterfly stays closed, you would be OK for just a test.

We'll figure this out! 

Thanks.  I was thinking as well.  I'll give it a shot hopefully later today.  

My 79 has the wire, My 80 does not.  If it does test bad,  I do have an extra from the 80 I could try. I need to figure out if the wire is needed if I use the 80.  

Edited January 26, 2017 by JSM
Bad Grammar

[Captain Obvious]

Zed, I dont know how large the passageways involved are, but yes... If my understanding is correct, it could/should result in a small decrease in idle speed if you were to disconnect the wire at idle. And yes, it would just be baked into the idle screw position.

JSM, good luck with the investigation!

[JSM]

If the 79 turns out to be bad I'm going to try and use the 80. Ideally, I'd like to leave the BCCD functioning and since it works internally I'm going to cap 4 of the 5 vacuum sources as I'm not going to be running any of it (except the Distributor ported vacuum) . 

 

My goal is to steal the 79 EFI and distributor and run it on my 71.  Please no bashing.   This is more of an exercise of:

1. Can I get the Parts car to run. Done

2. Can I get the EFI in the Car.  From all of my reading thus far, I'm pretty sure I can.

3. I like carbs, but I like FI better.  Plus it's such a simple system.

4. I have it why not. 

5. I've been able to test a bunch of other parts that I've had for ZX cars.

This is from the 80 Emissions section. Going to cap all the hoses but the blue circle:

 

 

 

 

Edited January 26, 2017 by JSM
Bad Grammar

[Zed Head]

That's one of those interesting things.  They stopped using the solenoid in 1980.   Looks like they only referenced one side of the blade also.  A new puzzle.

In 1981 they put the BCDD on the bottom of the intake manifold.  Seems like they never quite got it where they liked it.  Some designs last for years,like the AAR.

This is the problem with the BCDD - you can get drawn in and spend a lot of time here, for little value.  Like eating celery.

 

[JSM]

12 minutes ago, Zed Head said:

This is the problem with the BCDD - you can get drawn in and spend a lot of time here, for little value.  Like eating celery.

I know, I know.  I enjoy the learning if nothing else.  I only eat celery w/ Peanut butter or ranch dressing!

I was reading my directions last night for my weber 60mm TB.  I can simply plug the hole for the BCDD on it!

[Dave WM]

some notes:

BCDD coil resistance 30ohms tested out of circuit

power at wire disconnected KOEO approx. 11v

meter used simpson 260 analog

 

[Dave WM]

Re read the FSM for 75 and 76. going to try this

start car get up to operating temp (so the AAR is no longer bleeding air)

with BCDD wire attached rev engine up to 3k and then quickly shut throttle. Since the car is no moving the solenoid should be on and the BCDD not active allowing RPM to quickly drop.

Next with the BCDD wire off (simulating above 10mph) do the same, I am expecting the quick shut off of the throttle to generate a high vacuum, activating the BCDD and bypassing some air, there by keeping the RPM up at least a bit (with the car not moving the vacuum will not sustain long anyway so I don't think it would hold it up very long).

 

[JSM]

Success. I really believe it was a bad BCDD valve. I could actually see more light through my clean 80 TB then the carbonized 79. 

 

Also here is an example of a bad fusible link though it looks good:

 

[Zed Head]

Now you can take the old one apart and see if a diaphragm is blown or the vale is just clogged or stuck.  So, to summarize - you swapped an 80 BCDD,with no solenoid, in place of a 79 BCDD that had one.  So now you're using a BCDD with no speedometer control.  It's parts car anyway, but curious...

Don't forget to inspect the 79 distributor breaker plate and bearings.  If you get there soon enough you might be able to save the bearing cage before it breaks.

[JSM]

27 minutes ago, Zed Head said:

Don't forget to inspect the 79 distributor breaker plate and bearings.  If you get there soon enough you might be able to save the bearing cage before it breaks.

Correct  I'm trying to steal the EFI for my 71.

Is there link for this (bearing cage)? My 80 only has 90K on it. The amount of rust is bit unbelievable. The 79 has at least 155k. Is there a mileage or time this usually goes out?

Edited January 27, 2017 by JSM