bcdd removed, now backfires at downshift

[freakshow]

I took off the bcdd valve a couple of days ago, the car runs much better now, no more bouncing idle, much better driveability, feels like it has more power(could be just me) but when i open it up, it has backfired a couple of times on shifting. usually going from 2nd to third from full throttle to no throttle, coming off of corners and such. i was wondering if my timing might be a little thrown off by a difference in timing. i also has a bad exhaust manifold gasket when i bought the car, and the po had been driving it like that for a while. maybe my backfire is caused by a burnt valve??? i could find that out by doing a compression test right?? what kind of reading would a burnt valve cause?? other than the backfire on shifting, the car runs awsome, lots of power, and no other issues at low speeds, part throttle, or other driving. ive replaced all my vacuum lines, all my fuel lines, and run 93 octane, with booster/cleaner most of the time.

p.s. i replaced the exhaust gasket too, and the car has a cold air intake, and a open/one glasspack 2.25 exhaust, no cat.

[PurePontiacKid]

I don't mean to go off topic, but what is a bcdd? Is that a turbo thing? lol, my car's N/A, so I don't know Is your car turbo or N/A?

 

But when my car is cold, and I hit the gas quicky (yea, you shouldn't do that, but oh well ) it has a small backfire...

 

And from what I know, a backfire is unburned fuel leaving the engine and getting burned in the hot exhaust, creating the "POP" and sometimes a flame... But that's just what I've heard...

[freakshow]

Boost controlled deceleration device. it is a throttle bypass valve that reduces intake vacuum when you let off the throttle. its what makes your idle stick at around 1500 when you are coming to a stop, it also helps keep your rpms up when you let off the throttle quickly, like when you come off a corner. I have actually kinda got used to driving it, and it doesnt backfire very often anymore. I just have to shift it quicker when i rag on it. my car is a 79 n/a with a very open exhaust, so i think that might contribute to the backfires too, almost no backpressure.

[Guest p_tree244]

from my past research , I believe that the BCDD has nothing to do with backfire in the exhaust, the popping backfire is more likely from an exhaust leak from the manifold or anywhere in front of the cat.

[freakshow]

the exhaust is solid, as I stated in my first post. the po had drove the car for a while with a leaking exhaust manifold gasket, caused by a broken stud. I replaced the gasket and had to have the manifold milled because the gasses had eaten the manifold at that port. i replaced both the front and rear stud with a larger one to hopefully prevent the same thing from happening again. I also replaced the manifold to downpipe gasket. my exhaust is sound, runs allt he way out the back, no leaks. and i dont have a cat. I believe my backfire is caused by excessive intake vacuum when i go from full throttle to no throttle. I also could possibly have a burnt valve from the previous exhaust leak, I havent done a compression test yet, but i believe i will find evidence that would support my theory.

[S130Z]

Boost controlled deceleration device. it is a throttle bypass valve that reduces intake vacuum when you let off the throttle. ........ my car is a 79 n/a with a very open exhaust, so i think that might contribute to the backfires too, almost no backpressure.

 

How do you have a Boost Controlled Decel. Device if it is a N/A? Boost usually comes with the turbo models, from what I'm told.

[S30TRBO]

nope they came on L28E too.

[silver280zx]

I was wondering just about the same thing, I have a 79 na, and it idles really rough now that i plugged the egr, and it back fires on deceleration, but the plugs are fine, nice light tan color. Just put a new manifold gasket on, and it has crane cams ignition. I have no cat, but i have a 6-2-1 header, 2.5 inch pipe all the way back. I havent removed the bccd, yet but im really thinking about it.

[BRAAP]

For you guys guessing about the BCDD, you do have a Haynes manual or other shop manual for your cars right? Being members of the internets highest performance Z car forum, we hope that you have been through it a time or six by now. This highly technical performance forum is not a replacement for a shop manual, so if you don’t hove one, you best ought to get one…

 

The Boost Controlled Deceleration Device, (B.C.D.D. for short) is covered in the Haynes manual!

 

 

I'm feeling generous today, so instead of reminding you guys about the search feature, (BCDD/exhaust popping has been covered ad nauseam), I'll give you a quick course on the BCDD. Come time, I expect you'll remember this gift and reciprocate...

 

In short, yes, removing the BCDD can and will often times lead to popping in the exhaust system during deceleration!

 

The BCDD is a valve that opens when you drop the throttle at higher than idle RPM, it allows air to bypass the throttle for emissions, leaning out the mixture. ALL EFI Z cars from ’75-’83 had this nifty little device. Early cars it is on the bottom of the Throttle body, later ZX’s it is on the bottom of the manifold itself.

 

With BCDD functioning, you will notice your RPMs are LAZY to fall when you let off the throttle. Sometimes will hang around 1300-2200 RPM or so then eventually fall to a normal idle. "Dat dar be da BCDD vahv axue-aden". With the BCDD removed AND the ports plugged, your RPMs will drop MUCH quicker. Down side to this, (some prefer it so it would be an up side for some, especially if you have a free flowing exhaust system), is popping in the exhaust while decelerating due the now richer mixture. If the popping is mild, it isn’t hurting anything. If the TPS is set per OE, the ECU acknowledging idle/closed throttle, then during decel from high RPM, you wont hear anything in the exhaust but then at exactly 2800 RPM, you will here a distinct pop or lots of popping as the RPMs continue to fall, or at least slight change in the exhaust note. This is because the ECU shuts off the injectors above 3200 RPM when the throttle is closed, then turns them back on at 2800 RPM. Some of you have found this out on your own if you have pressure washed your engine and inadvertently filled the TPS switch with water. This shorts the idle circuit in the TPS connector, and is a VIOLENT rev limiter, 3200-2800 RPM! This episode is actually quite embarrassing when played out as you leave the public pressure wash station, your fancy dancy eye catching sports car is bucking violently down the road. (Don't ask how learned about that 20+ years ago. OMG, it's been that long?!?! )

If you set your TPS so the idle switch does not activate, you might hear the popping at all RPMs while decelerating.

 

 

Hope that helps clear up the function of the BCDD.

Paul

[S130Z]

Much appreciated BRAAP. Clears things up for all of us. I don't have one of those and I guess thats why I like the sound of my exhaust.

[Hyuri]

This is because the ECU shuts off the injectors above 3200 RPM when the throttle is closed, then turns them back on at 2800 RPM.

Although, according to the FSM, the '83 model's magic numbers are variable and range as low as 1500 and 1300, respectively. With a head temperature of over 200F, but still.

 

But yes, very much appreciated. I never quite understood the point of the BCDD before now....

[S130Z]

Although, according to the FSM, the '83 model's magic numbers are variable and range as low as 1500 and 1300, respectively. With a head temperature of over 200F, but still.

 

But yes, very much appreciated. I never quite understood the point of the BCDD before now....

 

That clears some things up because I would always here mine pop at just about 1500 RPM.

[freakshow]

Thank you very much for the very informative post Braap. This clears up a lot for me. I have another question though. As I have removed my bcdd and plugged the ports, with my tps set as factory, is there any way to lean out the mixture with my factory efi? I dont have the money to pony up for a aftermarket efi system, although that is a future plan. Are there any adjustments i can do with the factory efi to make the mixture leaner?

[S130Z]

There should be an idle mixture screw on the side of the AFM. Just make shure you dont go too lean. A wide band o2 sensor is really helpfull in doing this.

[blueking]

it comes from having no cat

and i don't think its back fire that you all are talking about unless its coming directly from the intake which i have to admit sounds hard to diverge considering its right next to the dam exhaust haha

but my engine makes very discreet pops at the muffler and i have my cat so

i imaging having no cat will allow the after burn to sound a lot louder

 

try on a cat or a non glass pack muffler i mean its only a couple of bolts and if your certain its a back fire i recommend cleaning out your intake manifold and checking youd mast vac lines along with the hose on the bottom of the intake mani

the rubber one... what ever that one is called haha

[BRAAP]

it comes from having no cat

and i don't think its back fire that you all are talking about unless its coming directly from the intake which i have to admit sounds hard to diverge considering its right next to the dam exhaust haha

but my engine makes very discreet pops at the muffler and i have my cat so

i imaging having no cat will allow the after burn to sound a lot louder

 

try on a cat or a non glass pack muffler i mean its only a couple of bolts and if your certain its a back fire i recommend cleaning out your intake manifold and checking youd mast vac lines along with the hose on the bottom of the intake mani

the rubber one... what ever that one is called haha

 

 

Besides lacking any capitalization and punctuation whatsoever, your post is not really pertinent to this discussion of the BCDD on an EFI L-6, its removal and how that allows an overly rich mixture during decel/drop throttle causing the popping in the exhaust, which has already been explained, in detail.

[S130Z]

Well I just got my exhaust completely redone. I got long tube headers, running 2.5" piping, no cat with a single glass pack and I have yet to get back fire. I also have removed my BCDD. I only get a little rumble at around 1800rpm and I have oversized injectors.

[Niku-Sama]

BCDD is adjustable, it shows how it is in hayes manuals but i would trust the FSM over the hayes. i had to adjust mine, the hanging at 2000 rpm was just not right. i think some one had adjusted it to lessen the vaccum so the idle would stay up and not die when it was cold. i think its lessen i cant remember its been a while since i have done this. either way when i adjusted it just enough so that no matter the speed i drive its drops down to idle the car ran alot better and my milage went up a bit. i just thought i would toss that out there, i am running no cat and no EGR (its there its not hooked up) and i have always gotten popping out of my exhaust.

 

the dieing when cold was fixed with a valve cover gasket (sizable leak) other vaccum leaks fixed and a new coil. after 30 years the coil couldnt hand out the juice when it was cold

[freakshow]

The reason I originally removed my bcdd was because it was making a horrible racket at idle, sounded like someone was trying to grind up a bunch of cats. I suppose I could have rebuilt the unit, but I dont have emissions, and i plan on upgrading my system later, and I didnt want to waste money on something I would be removing later. I got increased vacuum at idle, and also got better oil pressure. The car idles smoother now, and doesnt creep up when I sit at stoplights. I assume the original cause of the noise was a wore out baffle in the bcdd.

The car does not backfire out of the intake, and I cleaned everything out when i replaced the intake manifold gasket. On the other side, before I removed the bcdd, on startup, if I tried to rev the motor before it warmed up, it would backfire out the intake. Now it only backfires at the 2800 rpm mark.

After I read that, I payed more attention to the spoke of rpm, and that is what I have seen from my car.

The car has never had a cat, the whole time I have owned it. From the looks of things, it hasnt had a cat for a very long time.

[Tony D]

Dead on, the BCDD admits air to the plenum to allow a combustible mix in the exhaust to continue burning.

 

Without the BCDD, on drop-throttle there isn't enough oxygen to support combustion, and you flood the hot exhaust manifold with unburned HC (gasoline)...what happens then is it keeps building and at some point down the tube when there is enough oxygen (free flowing exhausts it happens sooner) you get a reburn phenomenon, nad can get a violent backfire out the exhaust.

 

My brandy-new 1980 Chevy LUV would do this when I downshifted at 45mph into third. The exhaust would go completely silent as the carb pumped gas into the exhaust, and the AIR pump pumped oxygen in at the same time... You could count: One....two...threeeeee...BOOOOOM!.

 

The truck was absolutely silent for those three seconds and then a HUGE violent backfire in the muffler occurred. No catalyst on the truck, just that AIR pump working to get the entrained HC in the exhaust up to the LEL or combustion limit, and then relight.

 

One day I was demonstrating this technique to a friend, after getting VERY adept at it, and after the boom (which turned heads in the coffe shop I did it in front of...) there came a rougha-rumbla-vroom vroom sound of an unmuffled truck.

 

Seems the explosion in the exhaust was so violent I split the seam on the 3" long Stock Muffler and blew it WIIIIDE open!

 

I replaced it with a Thrush...

 

My brother, once moving to the U.P. near Marquette, eventually removed the AIR pump and the 'popping' went away completely...we just spewed unburned hydrocarbons into the atmosphere unabated and unburned from that point forward. Till it rusted back to the earth from which it came.

 

The issue isn't making the overall mixture leaner---it's just that on drop throttle in almost anything you go pig rich in the exhaust. The BCDD in conjunction with injector cut lesens this phenomenon.

 

Hell, VW Beetles had an External BDCC on them in the late 60's for emissions purposes. Everyone trashed them when they put on QP Mufflers anway, but on theirs it was an arm that held the throttle open, and they rarely gave any problems. Adjustment was easy because the screw faced you when looking at the engine. Didn't stop them from being thrown out. They have been around for a while, and the physics/engineering on WHY they are there, and how they work/what they do hasn't changed one whit since then (working to re-earn wiki since being dubbed horny...)