Idle drop while braking,backing out of driveway

[1994rcsbLS]

Did you vise-grip the brake booster hose closed and try pushing the brakes to see if the idle dip goes away when pressing the brake pedal?
Note: Do this in your driveway, not on the road!

Also, with testing it this Way, unsafe to drive with no booster vac. How do you safely test this with foot on brake and in gear. With no vac to booster couldn’t I risk damaging diaphragm?

 

[Scooterwrench]

Increase the hold position by 10% and set the start ramp to about 50 rpm over your curb idle. If it feels like it's holding too long you can decease that to 1sec. Decay time looks good,3sec should ease it down nice and gentle like.

 

[1994rcsbLS]

Increase the hold position by 10% and set the start ramp to about 50 rpm over your curb idle. If it feels like it's holding too long you can decease that to 1sec. Decay time looks good,3sec should ease it down nice and gentle like.

I’ll give that a shot, I just looked at both my data logs from both trips. Everything looks fine But you can tell exactly where my foots on the break.

 

[Schurkey]

During braking( usually coming to a stop light/sign/ traffic ) the trucks idle dips and almost wants to stall. my normal idle is 9xx-1050,

WHY is your idle speed so high? Seems totally unreasonable.

while braking and coming to the stop it drops to about roughly around 700rpm. once stopped the idle levels out and is fine.

Seems like a booster problem.

While braking coming to a light. Pedal felt fine before stall. Went to stall. Pedal got hard, saved it. Pedal normal. Got to stop.

Again, seems like a booster problem. No vacuum reserve. One pedal-push exhausts the vacuum supply.

The lack of vacuum reserve causes the engine to lean-drop when the brake pedal is pushed, leading to driveability issues.

Also, with testing it this Way, unsafe to drive with no booster vac.

Unsafe? No, not really. Just that you'd have no power assist on the brakes, so you'd need a heap of "foot-pounds" on the pedal to get the car to stop.

How do you safely test this with foot on brake and in gear. With no vac to booster couldn’t I risk damaging diaphragm?

The diaphragm isn't damaged with lack of vacuum. Although, you may have that--or other--faults in the booster already.

Maybe I'm wrong. I don't know dickety-doo about Holley injection systems. But to me, this is a mechanical problem, not a "computer tuning" problem.

 

[Scooterwrench]

WHY is your idle speed so high? Seems totally unreasonable.


Seems like a booster problem.


Again, seems like a booster problem. No vacuum reserve. One pedal-push exhausts the vacuum supply.

The lack of vacuum reserve causes the engine to lean-drop when the brake pedal is pushed, leading to driveability issues.


Unsafe? No, not really. Just that you'd have no power assist on the brakes, so you'd need a heap of "foot-pounds" on the pedal to get the car to stop.


The diaphragm isn't damaged with lack of vacuum. Although, you may have that--or other--faults in the booster already.

Maybe I'm wrong. I don't know dickety-doo about Holley injection systems. But to me, this is a mechanical problem, not a "computer tuning" problem.

He's got a big cam. 900-1000 normal,low vacuum normal.

 

[1994rcsbLS]

WHY is your idle speed so high? Seems totally unreasonable.


Seems like a booster problem.


Again, seems like a booster problem. No vacuum reserve. One pedal-push exhausts the vacuum supply.

The lack of vacuum reserve causes the engine to lean-drop when the brake pedal is pushed, leading to driveability issues.


Unsafe? No, not really. Just that you'd have no power assist on the brakes, so you'd need a heap of "foot-pounds" on the pedal to get the car to stop.


The diaphragm isn't damaged with lack of vacuum. Although, you may have that--or other--faults in the booster already.

Maybe I'm wrong. I don't know dickety-doo about Holley injection systems. But to me, this is a mechanical problem, not a "computer tuning" problem.

Idle is set higher due to large cam, I’m gonna look into the booster some more. Try a few things out. I’ve been getting a lot of it’s the booster failing comments from other places as well. Just wanna be sure

 

[Schurkey]

He's got a big cam. 900-1000 normal,low vacuum normal.

Idle is set higher due to large cam

238/242, .600'/.600', 112 LSA, 110 ICL is not "that" big. AND you've got fuel injection that doesn't rely on venturi signal/vacuum to draw fuel.

Was that cam degreed when installed, or just shoved-in "dot-to-dot"? What is the cranking compression pressure?

 

[Scooterwrench]

I agree it's not that big but I'm running a cam with the same overlap and my motor doesn't like to idle under 900. Some people like that slow lump-t-lump but not me. Too many idle issues!

 

[L31MaxExpress]

My best guess is the converter is too tight and possibly have a leaking booster and thus it leans out. What is your idle vacuum in gear? 6 in/hg? Are you using idle spark timing variation to help control the idle speed?

 

[Scooterwrench]

My best guess is the converter is too tight and possibly have a leaking booster and thus it leans out. What is your idle vacuum in gear? 6 in/hg? Are you using idle spark timing variation to help control the idle speed?

Hmm,
Good test for that would be block booster,run engine up to about 3000 in park and hold it there a few seconds and chop the throttle and see if it drops down below your set curb idle or stalls all together. If all goes good with that then set the parking brake HARD and using the foot brake do a stall test and see if it falters. If it passes that test it's pretty much a given the booster is shot.