So I tried searching this, there was definitely another guy with the same problem, but the thread died.

89 4runner 3vze 5spd 4x4

There is a very clear whistle while I'm stopped, or slowing to a stop, that only occurs while braking, or depressing the brake at a complete stop. The tone changes based on the pressure on the pedal, and if I'm at a dead stop, and I just stand on the pedal as hard as I can, the noise will go away.

The noise is definitely air, not mechanical. And it's very audible in the cabin, sounding like it comes from directly under the dash. I tried to listen to it from under the hood, but evidently it only does it after it's heated up a little bit, because it didn't do it on a cold start.

The vacuum line from the intake manifold to the brake booster looks fine. Is the booster leaking? I don't know much about the interface between the booster and the master cylinder, could it be an MC problem?

Do I need to replace the booster and the mc as a unit, or can I repair one part of the system without the other?

Thanks for reading guys!

 

You can replace 1 without replacing the other. Take the vac line off and plug it to the booster. See if noise is still there. NOTE: You will loose power brakes this way. If noise goes away, time for a booster change.

 

That's a good tip man, I'll give it a shot. And I guess if the noise is still present it has to be in the line, duh, good teching, thanks!

 

So, I still need to pull the sparkplugs and check for sure, but I have every reason to think the truck is running pretty rich, any chance this vacuum leak could be the cause of it?

 

IMO a vacuum leak typically causes an engine to run lean.

 

That's interesting. In my experience, any post MAF air leak will cause the engine to run rich. Fuel delivery is calculated based on the MAF sensor reading, so when the MAF reads "x-air" it delivers "y-fuel," but if air is lost pre-combustion chamber, the ECU has no way of knowing, so it still delivers the same amount of fuel that it calculated for "x-air" before any air was lost. After combustion, the O2 sensor should see that the mixture is rich, and tell the ECU to trim fuel out, but if the fuel trims are already max lean, it won't be enough... but I guess I should do little more research on this ECU and fuel injection. I'll start another thread on the mixture after I check the plugs to confirm.

 

If you let unmetered air into the engine the MAF thinks there is less air going into engine and will not call for more fuel for the extra air introduced. Your not loosing air with a leak between MAF and TB your gaining it.

 

Good point, I guess I need to re-adjust my thinking for naturally aspirated engines, lol! =) I'm used to intake leaks occuring under boost...