Vaccuum leak? Weird idle if pressing brakes

[cstarbard]

On my 96 2.3, I noticed today that tapping the brake pedal once, or repeatedly, will cause my truck to drop rpms (sitting at idle) and start to idle badly.

The more you pump the pedal the worse it gets. The rpms drop off quickly and the truck wants to quit.

I'm curious if this is indicative of a vacuum leak or other vacuum issue since I assume when you are tapping the brake pedal the booster is drawing/affecting vacuum?

Am I totally off base in thinking that? The truck idles and runs pretty well and I've looked at all the vacuum lines and they all look fine. All connected and none are cracked split etc. Don't hear any obvious leaks

Chris

 

[Mark_88]

The brakes are augmented by a booster on the firewall (that round can behind the master cylinder) and there have been members who discovered a leak that allows vacuum pressure to drop and bleed out. this will result in stalling when you hit the brakes or other noticeable symptoms.

It may be time to replace the booster and they are not that hard to swap out. They do not require bleeding brakes or even disconnecting the master other than pulling it away from the booster to remove it.

If the brake lines are corroded with rust they may break so it is a good idea to use care when removing. Cleaning the brake lines and spraying them with oil might help to allow them to move without damage but be prepared to do a bit of brake line work if they are too far gone.

I used a PB Blaster on anything that was too rusted...whether that penetrates the lines and makes them a bit more flexible may be wishful thinking on my part but I never broke a line when I did that...could be just coincidence though.

 

[cstarbard]

Thanks so much!!

I'm glad my thinking was generally on the right track.

When the booster leaks, is it internal (i.e., booster is trash) or is it from the white plastic fitting and hose coming off the top corner of the booster?

A while ago I put a clamp on the line that runs from the plastic fitting on the booster, because the hose seemed a tad loose. It seems like the fitting sits in a rubber grommet on the booster. Are the fitting and grommet changeable?

I'll go take a picture in a moment to show what I mean

If I have to change the booster I'm certain I'll break at least one line on the master, its toast. I should probably replace that line anyway... I used to use PB and Kroil but the best penetrating fluid I have ever experienced is 50/50 mix of Acetone and synthetic ATF per the youtuber 'Chrisfix' - amazing stuff!

 

[adsm08]

Sounds like the booster is shot.

The booster has a big rubber diaphragm in the middle. When the engine is running it pulls a vacuum on both sides of the diaphragm, when you push the brakes the rod that runs through it to the master opens a valve that lets air come in and push on the back side of the diaphragm.

If hitting the brakes affects engine performance enough to make it try and stall you are getting way more air in it than you should, so booster is probably shot.

 

[cstarbard]

Hey,

That makes sense.

I just removed the booster check valve to make sure it worked, and it does (can blow through it one way, but cant blow through it if I reverse it). I also replaced the line from the intake runner to the booster check valve.

What does it tell you if when I first removed the booster check valve, it hissed once I pulled it out? That makes me think the booster is in fact holding vacuum. Also a booster that is not sealing well should affect braking substantially shouldn't it? I just drove it around and the brakes feel good. It stops fast and the pedal doesn't go the floor or anything like that.

Thanks for the help guys!

 

[Mark_88]

If the booster is not the problem then there could be a vacuum leak in other places. The EVAP system has a vacuum line running from the intake to the driver side fender that can also leak if the hose gets weak..especially on the connecting end. It is normally closed but when it opens and there is a leak beyond the purge valve then it becomes a vacuum leak that affects engine performance.

If you only notice it when you brake then it is most likely the booster. Check the end of the hose where the valve sits to see if it is loose or broken.

Could be anywhere really...so a good idea to look at all the obvious places where hoses attach.

 

[cstarbard]

I just tested to see if the booster was the main problem by undoing the hose between the booster and the intake and capping off the line from where it starts on the intake. The truck didn't run any better. If I let it idle eventually it would start to idle badly and sputter.

So this leads me to believe that the brake booster is amplifying a vacuum leak somewhere else. Does that sound plausible?

Interesting that you mention the EVAP, Mark_88. Last week I noticed that I had a code related to the Evap purge solenoid. I can't remember what the code was, but I cleaned the Evap purge solenoid (it was pretty gunky in there) and replaced the lines before and after it, and the code went away and hasn't come back. I noticed that before I cleaned the purge solenoid, I couldn't hear the valve inside it move, and after I cleaned it, I could hear it click around if I shook it (tested this before re-installing it).

What I have noticed is that the steel evaporative line that crosses over the driver side frame rail, behind the power steering pump, is rotted off there, and I assume the rest of the line is missing because I don't see it dangling anywhere.

Could that be my issue? I don't fully understand how the EVAP system works so I don't know what the purpose of this rotten line is. What I would find odd about this line being the problem is that the truck didn't idle poorly until recently, and the line has clearly been rotten and gone for a LONG time. Unless cleaning the purge solenoid (which cleared my CEL) is now allowing a leak because the valve can open properly?

One other observation I just made; when the truck starts to idle poorly, I applied the throttle very slowly at first and it hesitates, then smooths out. At higher rpms it seems to run better than at idle but it still isn't running 100% normal then either.

One last thing, it seems like all of this started to occur after I let my brother borrow my maf sensor to test if his was bad (his truck starts pretty inconsistently and we are trying to rule stuff out). When he put my maf back in my truck, he didn't clamp the airbox housing together right and the truck ran like crap until I noticed that the airbox wasn't sealed right and fixed that. Thats about when all this started. Is that coincidence?

The only check code I had as of yesterday was for an 02 sensor that I know is junk, but I'll recheck and see if there is anything new when my brother comes home with his scanner. I will replace the 02 sensor soon but that code has been there for months and the idle hasn't been crappy until recently so I'm thinking they probably aren't related?

Thanks

 

[Mark_88]

A rotted line can cause a vacuum leak. Try plugging it up and check performance is all I an suggest. The EVAP system needs to be sealed and anything from an improper fitting gas cap to breaks in the lines can cause it to detect a leak and throw a code. I just finished dealing with one that was caused by a new cap that was leaking. I didn't suspect that at all because it was new...

To be honest, it could be any number of things and leaks, dirty components, poorly fitted hoses, moisture on wires, burned sheathing on wire harnesses, improper components can all be at fault.

Could also be that your MAF was mixed up somehow and is not the right one or maybe your brother mixed it up with an old part and gave you the wrong one. Weirder things have happened...not implying any wrongdoing...stuff happens without us plotting against each other...lol.

I've had it all happen at one time or another so nothing surprises me...check and move on is all we can do...or backtrack on something you've done recently that might have knocked a line off or it wasn't sitting properly.

 

[cstarbard]

Update: I smoked out my vacuum lines with a cigar and I found a pretty good leak. It was really hard to see the line itself, right under the throttle body.

I replaced that line and tested with smoke again, no leaks, but no change in the idle.

A bit stumped at this point

What else could be contributing to this new found crappy idle?

Coil packs, plugs and wires are all new
Idle air valve got cleaned
Maf sensor got cleaned
Evap purge solenoid and thermistor cleaned

Have a code p0135 for upstream oxygen sensor, but that's it. Could the 02 sensor failing be affecting the idle? It sounds like a misfire at idle

 

[adsm08]

Hey,

That makes sense.

I just removed the booster check valve to make sure it worked, and it does (can blow through it one way, but cant blow through it if I reverse it). I also replaced the line from the intake runner to the booster check valve.

What does it tell you if when I first removed the booster check valve, it hissed once I pulled it out? That makes me think the booster is in fact holding vacuum. Also a booster that is not sealing well should affect braking substantially shouldn't it? I just drove it around and the brakes feel good. It stops fast and the pedal doesn't go the floor or anything like that.

Thanks for the help guys!

Ok, so when the brake pedal is released the booster is a sealed unit with the valve between the two chambers being open. When the brake pedal is applied the valve between the chambers closes, sealing off the forward one where you vacuum line connect, and opening up the rear one to atmosphere.

So on a properly working booster the engine should not get any extra air until the pedal is released.

If the diaphragm is ruptured somewhere it can allow enough air in to the system to cause problems, but the hole may not be big enough to effect braking yet. It will probably grow.

 

[cstarbard]

Thanks I'll play with it more tonight

Ill replace rotten steel line and see if that helps at all

 

[cstarbard]

Also thanks a lot for explaining the booster to me. That's really helpful.

I have a feeling that the booster probably is on its way out because the truck definitely wants to choke up more at idle when you're playing with the brakes

But it also seems to just idle badly in general as well now too.

It's weird because the truck actually idled pretty good two weeks or so ago

 

[tomw]

If you diddle on the pedal, you are not doing 'normal' braking. It will affect idle, as you are allowing in more air as you cycle the pedal, and thus the valve. When you quit playing with the brake pedal, does the idle improve? Have you checked the vacuum level using a gauge to see if it is steady or fluttering?
The upstream O2 is used by the computer to fiddle with injection pulse width. If yours is not working, the computer will increase the injection time as much as it can, and add extra fuel. Adding fuel to an engine at idle will not make it idle better, and perhaps can make it idle worse.
Fix the bleeping O2 sensor and quit chasing problems that may be caused by something that is shouting out that it is broken and is being ignored. Better to try to fix something with parts already installed you know are good, and no stored codes to mess up the trouble shooting.
tom

 

[cstarbard]

If you diddle on the pedal, you are not doing 'normal' braking. It will affect idle, as you are allowing in more air as you cycle the pedal, and thus the valve. When you quit playing with the brake pedal, does the idle improve? Have you checked the vacuum level using a gauge to see if it is steady or fluttering?
The upstream O2 is used by the computer to fiddle with injection pulse width. If yours is not working, the computer will increase the injection time as much as it can, and add extra fuel. Adding fuel to an engine at idle will not make it idle better, and perhaps can make it idle worse.
Fix the bleeping O2 sensor and quit chasing problems that may be caused by something that is shouting out that it is broken and is being ignored. Better to try to fix something with parts already installed you know are good, and no stored codes to mess up the trouble shooting.
tom

That's a very good point, I understand "diddling" with the pedal isn't normal. As you can probably tell I'm no professional mechanic by any means lol and am trying to learn everything I can as I go When I stop "diddling" with the pedal the idle gets a hair better but is still crappy. And it happens from more normal brake pedal use, like pressing it once, or pressing and holding it for a second. I just started diddling with it to see if it changed anything. Curiosity killed the cat kind of thing.

I don't have any kind of vacuum tester.

The 02 sensor is at the top of my list but I'm out of work for a few more weeks so I'm just trying to gather information until I can spend $ on parts. A quality one is like $50 from what I found or I would have just done it right away. Just trying stuff that is easy and free for now is all. Like vacuum leaks, since I have a lot of spare hoses etc.

I completely agree that playing with things while I know that the 02 sensor is junk is somewhat frivolous, I'm just confused as to why if it has been junk for a while and only now the idle is changing. What you said makes a lot of sense, thank you for sharing that.

Thank you for all your patience in teaching this "shade tree mechanic" kid

 

[cstarbard]

Also I replaced the EVAP line that was rotten from the canister to the tank and replaced a shot vacuum line under the throttle body and it made no difference. Just to update.