suction noise after turning truck off

Hello all,

just picked up a stock 01 with a 3.4L V6 TRD 4x4 ext cab with 217K. It is throwing code P0420 (which it seems can be cat) but I am trying to make certain it is before I cross that bridge. When I shut the truck off I hear a hissing sound as if it is sucking air in on the drivers side close to the firewall near the intake manifold. I can hear it if I stand by the driver down with the hood closed. I can also hear it while it is running with hood open and leaning in. Here is what I have done to troubleshoot.

1) inspect intake hoses for slits, breaks or damage
2) while running spray carb cleaner around all intake hose to see if idle changes
3) Used a smoke machine to fill intake and look for leaks. I tried the brake port, front and center port (on my truck it was blocked off, but I think it is power steering line on some) and coming in from the throttle body.
4) hooked up a vacuum gauge while running. I was pulling 17 at idle and passed all the test of blipping the throttle (would go/head toward zero) and slowly increasing the throttle (stayed at 17).
5) Then I got the air compressor out to double check and filled intake with air and looked for leaks with soap/water

Then I got to thinking maybe it is exhaust related...So i filled the exhaust with air and checked for leaks with soap/water. I found a two SMALL leaks on the passenger side before it turns down and one SMALL leak at a joint before the last O2 sensor (this was all when cold so not sure if they are there after truck is at operating temperature).

I am stumped, anyone been down this road? I need to replace my valve cover gaskets which requires intake to come off also, but I was hoping to fix this problem before I dig into the next project (helps eliminate variables when I put everything back together).

thanks in advance

 

Have you tried a fog machine? Fill the intake with fog smoke and this can tell you if you have any vac leaks hot or cold. Same with the exhaust though you can start from the tailpipe up... I've seen it done before with a simple dog machine and a small hose. You can do it yourself but having a second helper makes it easier.

 

Will a fog machine give me different results from the smoke machine that I used in my "step 3" above?

 

The charcoal canister by the brake booster will make these sounds. It's kind of normal as it may be venting or testing.

 

Did you separately smoke test the brake booster? I believe you have to separately test it because of the one way check valve.

 

I didnt smoke check the brake booster at all. Should I go in from the same line that goes to the intake manifold? Or is that where the one way check valve is?

 

The one way check valve is directly on the booster if I'm not mistaken. Not looking at my truck right now

 

My idle definately changes if i take off the brake booster line with engine running (can hear it sucking air also with line off). I cannot blow thru the brake booster line but i can suck air out of it (check valve working). Because of the one way check valve i cant smoke test the booster unless there is another port/line that i am missing. With engine running i sprayed the circumference of the brake booster with carb cleaner and heard no change in idle speed. Maybe my next step is a trip to a mechanic.

 

Mechanic thinks it is the brake booster
starting to fail, immediately after the the truck is off I have two "assisted brake pedal pushes." After restarting truck, letting it build the vacuum back up, turning the truck off and then waiting till the noise stops (vaccumm gone) then I have no "assisted brake pedal pushes.

Thanks for all the help! Hopefully this thread helps the next guy!

 

So I installed a new brake booster (Cardone 5C-34905 off Amazon), the hiss definitely went away (so the air leak was from the brake booster).

Now the brake pedal is hard to push.

I did the hold the "pump the brake with the engine off and hold it, then start engine test." When I started the truck the pedal went closer to the floor (booster is boosting). A bad booster can have a "hard pedal" symptom (although mine didn't).

I will check the vacuum hoses and the check valve.

Is it possible that I messed something up installing the new brake booster that would make the pedal hard to push?

 

I did an aftermarket booster a while back. Something inside is different. Its not as soft as the OEM. Might be normal.

 

Interesting, do you feel like it has becomes softer over time? Or not noticeable?

 

I changed a brake booster (reman'ed) on another make of truck once that had no 'softness' to it.

It was defective.

Had to change it again and all was well.

 

no change in feel or increase in assist. just learned to press a little harder...