Bosch (Volvo) Constant Idle System Instead of AAV & B.C.D.D

[Captain Obvious]

Yeah, sorry. For some reason I thought the modifications you made under the throttle body took that into account.

I've just been so awestruck with your productiveness and the scale of the prolific improvements that you've been making. I must have been blinded by the bling somewhere else.  

On edit... You said "I'll remove the plate I made & plug the front side port." 

Make sure you plug the correct side. If I understand your idle air controller plumbing*, you want to plug the high vacuum side of the throttle plate and use the low vacuum side for your idle air supply. So I'm not sure what you meant by "front side port", but I'm just bringing it up to make sure you think about it. 

 

* (which is not guaranteed) 

Edited September 23, 2023 by Captain Obvious

[HusseinHolland]

10 hours ago, Captain Obvious said:

Yeah, sorry. For some reason I thought the modifications you made under the throttle body took that into account.

I've just been so awestruck with your productiveness and the scale of the prolific improvements that you've been making. I must have been blinded by the bling somewhere else.  

On edit... You said "I'll remove the plate I made & plug the front side port." 

Make sure you plug the correct side. If I understand your idle air controller plumbing*, you want to plug the high vacuum side of the throttle plate and use the low vacuum side for your idle air supply. So I'm not sure what you meant by "front side port", but I'm just bringing it up to make sure you think about it. 

 

* (which is not guaranteed) 

Thanks CO 🤪

By front side I meant the high side vacuum port that faces into the plenum at the flange - should be easier to plug that that the underside, as I'm using the now larger low side for the vacuum draw on the valve cover, having enlarged that orifice, I'm not sure plugging the now smaller inner/central orifice will be so easy from the underside. 

 

EDIT - I did also check the ignition timing, as I think it was mentioned that doing the HEI mod can alter the base timing - it was still around 12º-14º, as far as I could tell, not 15º-20º, for instance. Given the current high idle rpm, I'm assuming it may be slightly over normal base setting.

Edited September 23, 2023 by HusseinHolland

[HusseinHolland]

OK - so I had a break in the rain this morning, so I was able to remove the TB during one clear patch, address the ports I hadn't closed off, then get it back on during another break in the rain

Larger port I tapped with 1/8" NPT

thread sealant on plug

tapped smaller port with M5x1, used the snapped off stud from the ignition ballast to make a plug

done

Back in. Ran the motor - don't have the tach installed, however the rpm's felt much more in line. A little low perhaps, so I ran it to normal op so I could check the base setting. With CIS, the easy way to do that is close off the low vacuum tube from TB to the IACV. It stalled out, so base was too low. Opened the idle thumbscrew a couple turns (I had completely bottomed it out) & got the idle what felt around 750. Releasing the tube, the idle motor cycles & the idle rises to what feels like 850-900 range. I will set it more accurately once the tach is in, but it feels good. Motor revs freely & returns to idle.

 

 

[Captain Obvious]

1 hour ago, HusseinHolland said:

 it feels good. Motor revs freely & returns to idle.

Great.

So you said this device replaces the BCDD. Does it mimic the same kind of behavior? Limits the intake manifold vacuum from going too high?

[HusseinHolland]

40 minutes ago, Captain Obvious said:

Great.

So you said this device replaces the BCDD. Does it mimic the same kind of behavior? Limits the intake manifold vacuum from going too high?

The main objective is to maintain a stable idle and base rpm, regardless of varying load conditions (AC cycling, etc.,). It provides low flow mode in decel, so that would be like the BCDD in that it prevents excessive manifold vacuum, and then has a high flow mode for any throttle position under acceleration. So, 3 modes.

[Captain Obvious]

That sounds like what I did with my throttle body work. I switched over to a TB from a Sentra and it has a built in idle air controller. I'm not using it to actively control the idle speed, but I am opening it up once I'm off idle to act like the BCDD.

I don't think it does as effective of a job of limiting the manifold vacuum as the original BCDD did, but it's better than nothing.

[HusseinHolland]

39 minutes ago, Captain Obvious said:

That sounds like what I did with my throttle body work. I switched over to a TB from a Sentra and it has a built in idle air controller. I'm not using it to actively control the idle speed, but I am opening it up once I'm off idle to act like the BCDD.

I don't think it does as effective of a job of limiting the manifold vacuum as the original BCDD did, but it's better than nothing.

So on yours, you use a TPS wired into the stock EFI to set the integral IACV high flow mode off idle?

I haven't seen anything like the BCDD on any early EFI Volvo - I'm curious why they went to such elaborate lengths to limit high vacuum on decel - don't they have oil control piston rings that would limit oil and unburnt fuel being sucked past the pistons on decel? I can't imagine it's there for any other reason than for emissions. I don't recall there being an issue with that on the 70's L6 Volvo EFI (D-Jet) motors, or on the Fiats that used the same L-Jet flap meter setup.

[Captain Obvious]

Actually I didn't do anything quite so glamorous. I've got my electronic idle air control valve wired "backwards" and I'm using the stock BCDD cut control signal to open it once the car starts moving. So when the car is below 10 MPH and the BCDD disable signal is high, my idle air valve closes. And when the car is above 10 MPH, the idle air valve opens up a little to reduce manifold vacuum when coasting.

Now because of the way I'm doing it and the simplicity of the way I'm using the existing circuitry, I didn't have a lot of adjustability. I have the idle air valve "almost as closed as I could get it" when powered, and that also dictates how much the valve will open when it IS unpowered. So I get an effective 3-400 RPM boost off idle once the car is moving.

At idle, I add the leakage past the idle air valve plus the air bypassing the idle speed control screw, and I get my total idle air.

If I coast to a stop in neutral, I can see the switch close and the idle air valve close and the RPM's will drop from 1200 down to 700-ish where I want my idle.

It's not glamorous and it's relatively unadjustable, but it was simple.

Edited September 24, 2023 by Captain Obvious
Old brain cells

[Captain Obvious]

19 minutes ago, HusseinHolland said:

I haven't seen anything like the BCDD on any early EFI Volvo

Datsun actually had some sort of manifold vacuum limit on deceleration even with the early carbs. There was a vacuum actuated diaphragm pull device that would yank on the throttle linkage if the manifold went too high. They kept refining that system through the carb evolution and eventually ported the same concept to the EFI systems.

I don't know much about Volvo or Fiat though. I know what they are, but that's about it.