bcdd removed, now backfires at downshift

[silver280zx]

So, if the bcdd squeels and whines when you decel, is it broken you think? Mine pulls a ton of vaccum right now. I think its advancing my ignition timing more than it should. WOuld it be better to just plug it bcdd?

[BRAAP]

So, if the bcdd squeels and whines when you decel, is it broken you think? Mine pulls a ton of vaccum right now. I think its advancing my ignition timing more than it should. WOuld it be better to just plug it bcdd?

 

Squealing/whining BCDD. Could be broken, out of adjustment, or just carbon build up.

 

Regarding your ignition advance.

The BCDD wont advance ignition timing and should have NO affect whatsoever on your ignition timing during decel with the throttle closed, whether it is operating correctly, broken, or non existent. What you are describing tells me your vacuum advance is connected to the wrong vacuum port. In short, if you have more ignition advance during decel than you do at part throttle, your vacuum advance is hooked up to the wrong vacuum port. Vacuum advance should only be active during cruise conditions. Not at closed throttle at any RPM, not at WOT.

 

It should NOT be hooked to the intake manifold directly, (see that a lot), or it will advance your timing more during decel with the throttle closed.

 

Your vacuum advance should be hooked to one of the ports on the bottom of the throttle body. Technically, that is called a “ported” vacuum source. To know which port is the ported one, it will be the port that does NOT draw a vacuum at idle, but will draw a vacuum once the throttle is cracked past 2-10 degrees or so. It will be the vacuum source connected to the small slit or small hole just in front the throttle blade itself. You can use a vacuum gauge to verify which port that is or just use your finger over the port on the running engine, (beware of the spinning fan!). If it is sucking at idle, wrong port. If no suction at idle, then as you start to open the throttle it starts to suck, that is the correct port.

 

Hope that helps.

[JSM]

... later ZX’s it is on the bottom of the manifold itself.

 

Really, I don't recall seeing this on the ZX under the Manifold?? You have a pic by chance?

[BRAAP]

Really, I don't recall seeing this on the ZX under the Manifold?? You have a pic by chance?

 

Here ye be... As far as I know, regarding ALL Z cars in the US market, if the TB doesn't have the BCDD on it, then it was on the bottom of the intake manifold as pictured below. Started at some point during the 280-ZX run, possibly 81+?...

 

This particular one came off an '83 N/A ZX.

[Daeron]

Wow. I read and understood the function of the BCDD about five or six years ago, when I first read through the EFI bible.. but it never *really* clicked in my mind because that was before my 3 year long affair with my Subaru, The Car That Taught Me Fuel Injection And Auto Mechanics. Now I know all about the practical upshoots of things like drop-throttle fuel cut, because I have been running the ECU's algorithm in tandem in my own mind the whole time I've been driving. (Pizza guy.. 500 miles or more a week?)

 

This thread just tied up the mental loose end there. Keep in mind that I haven't rotated a single wheel in an FI Z since my fire on 04, and everything has really been learned in practice in the intervening time..

 

But my question is this. Why are we feeding fuel to our engines on decel at such a high RPM?? Is there a lower threshold where we can set it? What determines the lowest threshold you can run no fuel, no throttle while coasting in gear??

 

 

As a pizza driver, these issues burn in my mind and soul often. The more thorough and detailed an explanation you can point me towards, the better; I understand most of it, its just that 2800/3200 seems AWFUL high, and I want to know why they set it where they set it, and what needs to change in order to change that.

[JSM]

Thanks Paul! I can't honestly remember seeing one an a ZX? I knew they were on the early ones on the TB, but not the ZX.

 

Now I'm gonna have to pay attention for them. They must have been removed from a lot of cars.

[Tony D]

"What determines the lowest threshold you can run no fuel, no throttle while coasting in gear?? "

 

The buck on re-engagement, and the customer's tolerance for a surging or bucking ride.

 

A lot of this gets into flywheel inertial ring weight now, as you can move things around when you got a heavy flywheel...

 

Mileage Motors have HEAVY flywheels. Ones with THICK inertial rings. Then you can run real lean and not 'feel' the surge most people say the find intheir Z's with the aluminum flywheels well before others notice it.

 

This can go off on a BIG tangent, a thread all it's own.

 

I had a 69 Ghia that would get 55+ mpg back in 1979...but it was a pain to try and cruise with with the original 12# flywheel. We ended up putting a stocker back on, along with a Berg Equalizer (think 8# Thick-A$$ Harmonic Pulley on the other end...) to keep it smooth while cruising during mileage marathon competitions.

 

Feeding fuel on the top end after drop-throttle is merely a function to allow combustion to complete and not dump unburned HC into the Catalyst and overwhelm it. I have said it many times before: our engines are calibrated to run too rich for best fuel economy. They set 14.7 because it's optimal for catalyst function... you can run a LOT leaner and drop your emissions to near catalyzed levels on HC and CO, but your NOx number skyrocket. So then you pump a lot of EGR into the chamber to cool the combustion process down (or spray anti detonant like water---yes in an N/A you spray water into the intake...) and your NOx is reduced. I ran my 73 on a set of SU's clean to 83 Catalyzed Standards with nothing more than one header tube having an AIR injection into it (#1). Got 27mpg on the freeway at 65-70mph.

 

I was visually failed at SMOG.

 

I was forced to reinstall the EGR and Flat Tops.

 

I dropped to 19-22 mpg. I had (no exaggeration) 10X the HC and CO#'s that I had with the 71 Setup on tuned as I did...

 

At that time they didn't test for NOx.

 

My guess is it would have failed NOx if they did, given the way it ran.

 

Don't get me started down this path! Don't, dammit!

[silver280zx]

Squealing/whining BCDD. Could be broken, out of adjustment, or just carbon build up.

 

Regarding your ignition advance.

The BCDD wont advance ignition timing and should have NO affect whatsoever on your ignition timing during decel with the throttle closed, whether it is operating correctly, broken, or non existent. What you are describing tells me your vacuum advance is connected to the wrong vacuum port. In short, if you have more ignition advance during decel than you do at part throttle, your vacuum advance is hooked up to the wrong vacuum port. Vacuum advance should only be active during cruise conditions. Not at closed throttle at any RPM, not at WOT.

 

It should NOT be hooked to the intake manifold directly, (see that a lot), or it will advance your timing more during decel with the throttle closed.

 

Your vacuum advance should be hooked to one of the ports on the bottom of the throttle body. Technically, that is called a “ported†vacuum source. To know which port is the ported one, it will be the port that does NOT draw a vacuum at idle, but will draw a vacuum once the throttle is cracked past 2-10 degrees or so. It will be the vacuum source connected to the small slit or small hole just in front the throttle blade itself. You can use a vacuum gauge to verify which port that is or just use your finger over the port on the running engine, (beware of the spinning fan!). If it is sucking at idle, wrong port. If no suction at idle, then as you start to open the throttle it starts to suck, that is the correct port.

 

Hope that helps.

 

Thanks, that helps a ton. Ya its hooked up right. The guy that had the car before me had the vacuum advance T'd into the brake booster, has anyone ever heard of this?

[tech9]

I took the BCDD off my 77 280z engine, and it has a backfire when shifting at high rpms also, its just because its slowing down so fast.

[280zx 2by2]

In all my 1 1/2 years of experiance with my NA 81 280zx i have never heard or read of this BCDD(except for the altitude application), nor have i ever felt this rpm stall in deceleration(my rpms fall straight to 800 or 1000 depending on engine temp)

 

i do get an ever so slight backfire or surge when coasting to a stop in gear(but i belive its a leak in my mannifold)

 

to the FSM i go for the rest of the night...

 

EDIT: how long does this rpm dec. stall last? 1/2 a second or more?

[Niku-Sama]

i depends, mine would stay at about 1500-1700 rpm untill i came to a complete stop. after asjusting it it would come down normally but before if you were coasting down a hill it wouldnt come down for any thing.

 

it was something i noticed before i bought the car but having a similar problem with the BCDD on a maxima i knew how to fix it. was a good haggling point