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Vacuum leak somewhere in the dash?

tmmby119

tmmby119

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I've been having this problem for a while but it gets progressively worse.My '90 Burb runs fine up until it gets all warmed up, then it has this nasty stumble in idle. It gets even worse when I drive on highway for longer period of time......the other day I got off highway down the off ramp, stopped on red light and the truck almost died. Went home and after hour or so it started up and idle just fine (-ish, it still have a bit of stumble ). I can hear hissing sound from somewhere in the dashboard, right behind the steering wheel.Is there more vacuum lines than the one from the brake switch? I can pump the brake pedal but it goes always to the floor. Never gets hard like all my other cars after few pumps......:dunno:
 
There should be only one vacuum line going into the dash from the engine compartment. It will go in just to the right of the "black AC box". It comes from the "black ball". Brakes are a different issue. You either have a bad booster or air in the lines. Most likely air if they go to the floor.
 
also there was a cancle switch for cruise control on the 80's trucks that went to the brake pedal along with the brake switch.

but being 91 and electric speedo and tbi not sure if there on that setup for yours.

and as said check the brake booster .

easy way to check fire up run for around 1 min. turn off. go back few min later and pop the 90* adaptor/check valve from the booster under the hood and it should suck in air. if not its bad check valve or boster dia and time for new booster.
 
Yeah....there is vac line running to brake pedal switch but that one looks like the PO put new one in. Imma gonna pull the booster out and pop new in.....those studs are going from the cab out, right? No need to take the dash out?
 
why not check booster function first ?

then if bad swap it.

and dont need to pull dash. but do need to get under it and in engine bay area also.
 
sweetk30 said:
also there was a cancle switch for cruise control on the 80's trucks that went to the brake pedal along with the brake switch.

but being 91 and electric speedo and tbi not sure if there on that setup for yours.

and as said check the brake booster .

easy way to check fire up run for around 1 min. turn off. go back few min later and pop the 90* adaptor/check valve from the booster under the hood and it should suck in air. if not its bad check valve or boster dia and time for new booster.
Click to expand...

how much of air sucking is normal? i ran the test and it did have some vacuum but not whole a lot....
 
Check the check valve on the booster first. I have seen many of these fail first. Take it off the booster (and vacuum line) and check to make sure you can only suck on one side of it.
 
do you hear a lot of suction noise when you press down on the brake pedal ?

It may make noise when you first apply the brakes,, but it should not be a continuous sucking sound....if it does...the booster is leaking...
 
Its common for a faulty brake booster not to leak vacuum unless your depressing the brake pedal---then it will start leaking,and you'll hear the suction noise..its very unlikely any of the smaller vacuum hoses under the dash for the heater controls or cruise control would affect the idle enough to cause it to stall or stumble,so I would say its more likely your brake booster thats got a ruptured diaphram in it...The only other vacuum hose large enough to stall the engine out would be the one for the PCV valve...and they dont get damaged all that often..
 
Isn't there a single master vacuum feed for all the dash controls at the firewall?
He could take it loose and plug it to see if the noise and stumble stopped.

That would narrow down the places to look.
As far as that goes, he could plug the line to the brake booster too. As long as he can hear the noise, he should be able to just plug things until it stops and then look at what that feeds.

Of course, he would not want to drive it like that.
 
Fordum said:
Isn't there a single master vacuum feed for all the dash controls at the firewall?
He could take it loose and plug it to see if the noise and stumble stopped.

That would narrow down the places to look.
As far as that goes, he could plug the line to the brake booster too. As long as he can hear the noise, he should be able to just plug things until it stops and then look at what that feeds.

Of course, he would not want to drive it like that.
Click to expand...
:waytogo:
 
It gets worse, the more I drive.....I will take it for a spin after work and will see ...thanks guy for your input!
 
I experienced brake booster failure last summer. Press the pedal and hear "whooosh". It was like that for a while before it started messing with the engines idle and overall run quality. It got to a point where you could step on the brake once, let up and immediately step on it again (like a stop and go traffic situation) and there would be zero boost assist. Full on manual brakes. That will catch you off guard.

But my point is, it was messing with the idle. Even a small vacuum leak will mess with the TBI system. So start following all the vacuum lines that run under the dash. I've found some of those heater control lines disconnected before.
 
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