Pressing brake drops Idle speed a lot

[missouritig]

Hello,

Working on my 2000 k3500 5.7 4x4

When I press my brake pedal firmly at stop, the idle speed drops a lot. Truck doesnt stall, but this seems to be an obvious issue I can address.

I do not have the hydroboost setup, its the JB8 vacuum boost style.

Is the only option here a vacuum leak? I tried searching old threads but its mainly hydroboost power steering leaks/etc

 

[454cid]

I do not have the hydroboost setup, its the JB8 vacuum boost style.

JB8 is hydroboost. Either you don't have the JB8 RPO, or you do have hydroboost.... or someone has cobbled parts together.

 

[missouritig]

JB8 are hydroboost. Either you don't have the JB8 RPO, or you do have hydroboost.... or someone has cobbled parts together.

I Better go look again !

 

[GoToGuy]

It's easy, first do you know what a hydro boost looks like.
Open hood what do you see?
Do you know how to decipher RPO codes on glove box?
I'm pretty sure there are no or not many one tons, or K or C 3500's that are NOT hydro boost.
Some days your the bat, and some days your the ball.

 

[missouritig]

Ok it IS hydroboost. I feel kind if sheepish.

Not leaking ant fluid that I can see.

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[missouritig]

Ok im leaning on this being IAC failure as on this engine this is what controls the idle speed based on ECM input of power steering/engine load. Since I have hydroboost, im putting a greater load on the engine when hitting the brakes.

Will do some more testing

 

[454cid]

I'm pretty sure there are no or not many one tons, or K or C 3500's that are NOT hydro boost.

Early trucks could be vacuum, but his is a 2000.

 

[HotWheelsBurban]

Sounds like you're on the right track. If the brakes start self applying though, your booster is going bad internally. Just went through this on my '97 C3500 CCLB. Had to have booster unit replaced twice, the first part installed in July went bad in a couple of weeks. Argued with the shop, drove the truck till we couldn't and had it towed 3 times, then finally got it to a better shop who properly diagnosed and fixed it. Like a new truck now!

 

[HotWheelsBurban]

Ok it IS hydroboost. I feel kind if sheepish.

Not leaking ant fluid that I can see.

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When the seals inside go bad, it'll leak from the bottom of the unit, at the joint between booster unit and master cylinder.

 

[missouritig]

I found a break in the evap line, i rigged a plastic hose over it and will need to get a better fuel resistant piece but this seemed to have an effect on the idle drop.

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