Idle Dips When Brake Pressed

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Jun 13, 2006
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'85 4R newly rebuilt top end. When engine is warmed up the idle will dip down every 4-5 seconds when I brake. I'm sure it is a vacuum issue but I don't know how to solve it. All my vacuum lines are new. Where do I start?
 
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Common "adjustment needed" :D (You think you're soooo great...... boy if I met you in a dark alley......)

no dark alleys in the country :D

but i might poke you with my poking stick :D

stickpoke.gif
 
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A dip at idle can also be a problem with the brake booster iirc while pressing on the brakes.

I will be chasing down the same issue this weekend. All my research is directing me towards vaccum leak or break booster.

To check break booster do the following:

On vehicle inspection: (before you rip the booster out and yell and scream
at it)

1 check air tightness
- - start engine
- - Stop engine after running for 1-2 minutes

2 pump brake pedal several times.
- -If pedal goes down deep the first time but gradually rises after the 2nd
and 3rd times, it is in good condition. :)

3 If there is no change in the pedal height when depressed the 2nd and
succesive time it is defective. :-(

Check the operation:

1 With the engine stopped, pump the brake pedal several times with the same
pressure. Ensure that the pedal height does not change.

2 Start the engine while the brake pedal is depressed, place one tablet of
prozacs into the brake fluid reservoir and see if the brakes recover from
their depression. No really, If the pedal goes down slightly at this time,
it is in good condition. If there is no change in pedal height, it is
defective. :-(

Check air tightness under load:

1 With the engine running, depress the brake pedal.

Then stop the engine while keeping the brake pedal depressed.

2 Hold the pedal for 30 seconds. If the pedal height does not change, it is
in good condition. :)

If the pedal rises, it is defective. :-(
 
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I just got one of these trucks about a week ago and I'm having the same issue. After it's warm the idle dips upon braking. I've all ready tried adjusting the idle screw and it's turned all the way in with no more ability to turn further.

any other suggestions?
 
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I just got one of these trucks about a week ago and I'm having the same issue. After it's warm the idle dips upon braking. I've all ready tried adjusting the idle screw and it's turned all the way in with no more ability to turn further.

any other suggestions?
When you turn the screw in all the way, does the engine stall? Yes? Good. No? The idle air valve is clogged up and isn't working. Probably corroded.
 
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So I finally was able to take the truck off the road yesterday to work on it. Solved my problem after some reading, doing a vacuum test and thinking long and hard about the symptoms. It was in fact the Air Idle Control Valve, not the brake booster. It had a nasty brown log running all the way through the AICV upper and lower valve stems and hoses, completely clogging flow. Replaced with a Napa new one and new Napa hoses. Also Flushed the coolant system thoroughly with distilled water and added fresh Napa 50/50 coolant. Truck is running great now and is idling perfect. Still chasing an exhaust leak but that’s another story...
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