Diagnosing vacuum leak (brake booster?)

[zak]

Per my post about a year ago, my car has thrown consistent KOER code 41. With gas at $3.35 a gallon some attention was clearly needed.

I picked up a Silverline mightyvac unit and have been starting to check out the vacuum system for leaks. The car has good vacuum at idle (85 5.0 HO CFI, stock 86+ H-pipe) of around 20 mm Hg, with enough fluctuation at idle to indicate that valve guides and/or seals are on the way out (2+ mm rapid swing). To be expected with a little over 150 K miles and 22 years old.

Anyhow, I did some static (engine off) vacuum tests at the main vacuum distribution log, blocking off the large port to the intake manifold. A fair bit of pumping got 10 mm Hg vacuum, which decayed to only 9 mm Hg after one minute. I isolated everything except the brake booster and got only 0.5 mm loss in one minute (10 -> 9.5).

Here is where things got strange. Depressing and holding the brake pedal down meant that no amount of vacuum pumping (engine off) would get a vacuum to build. However, with no brake pedal pressure, at times vacuum would pump right up, other times it took some time.

Is this a sign of a leaky brake booster and/or brake booster check valve? The book that came with the tester does not discuss brake boosters at all.

Any suggestions appreciated. I think V8only tracked down an issue with an Explorer V8 swap to a brake booster, but not sure what steps to take to do this.

zak

[Whtstang]

Well, checking the check valve is easy. You should be able to blow air into the booster hose but it shouldn't allow it to escape. To check the booster assist turn the engine off. Pump the brakes repeatedly untill all the vacuum has bled off. Then, while holding the brake pedal down, start the engine. With a properly working booster you should feel the pedal go down a little. Try these tests & post back what you find.

[zak]

To check the booster assist turn the engine off. Pump the brakes repeatedly until all the vacuum has bled off. Then, while holding the brake pedal down, start the engine. With a properly working booster you should feel the pedal go down a little. Try these tests & post back what you find.

Whtstang, thanks for the reply. Finally had a chance to poke around on this. It passes the test above easily. With engine running, I can rapidly pump the brakes and get it to drop from say 20 mm Hg to 17 mm Hg, but it stabilizes and returns right away when I stop pumping the brake pedal. I think I'm concluding that the power brake booster is not at fault.

I checked on the other two vacuum circuits that draw from the rear of the manifold. The red stripped circuit that goes to the BMAP sensor checked out fine. The other one, that seems to service many different actuators, would not hold static vacuum for even a few seconds. I traced the worst of them down to the air cleaner, where I found that one of the vacuum duct motors (that direct hot air from the exhaust manifold to the air cleaner with engine cold) was not working and had the diaphragm leaking. Also found split in one of the very thin plastic vacuum lines, and made a repair there.

I also found that the two vacuum "switches" that supply air to the warm up duct motors would not hold vacuum, even tested individually. Ambient was right around 50 F though. One was "inboard" of the air filter and appeared to have a reference tube coming off of it to sample vacuum just inside the filter - did not try capping that off. The other was a grey vacuum switch on the OD of the air cleaner housing, from which the lines to the vacuum dampers for warm up emanated. I ended up just capping the vacuum supply line to the whole air cleaner (yellow stripe).

Could the leaks I've found thus far accrue for the KOER Code 41, and very limited rich/lean switching (only during the middle of the warm up cycle) I've seen in the recent past on this car? I have not had a chance to instrument the O2 sensor again (or run KOER).

Appreciate any advice anyone has - 13 mpg is just not cutting it, and my other Ford is in many pieces for a clutch R&R and complete suspension upgrade.

[zak]

Mods , this is really engine related and not brake related, not sure why it got moved here - zak

[JACook]

The 'brake booster test', as described, really does not mean the booster diaphragm isn't
leaking, only that the engine vacuum is sufficient to pull brake assist even if there is a leak.

KOER code 41 usually means either you have unexpected air getting into the intake. Or
in the case of a mass air system, the MAF isn't properly measuring all the air that goes
through it.

Your brake booster should hold vacuum pretty much indefinitely when the pedal is at
rest, and should not fully deplete it's reserve of vacuum when you press the brake pedal
down and hold it. If a single press of the brake pedal is enough to empty the booster
of all it's vacuum reserve, that's not normal.

[zak]

After 20 minutes engine off, I still had vacuum assist left on the brakes. That plus Whtstang's test and the other tests I've run leads me to believe the brake booster is not the culprit. On the 8 mile drive to work this AM I had the usual full rich runing, with some rich/lean switching in the middle of the drive (just for a minute or 2). Would snap to full lean (*from the air injection I assume) in either neutral (stopped) or closed throttle coast down from speed.

I am going to check fuel pressures. Still wondering whether I still have a vacuum leak somewhere, and also what do folks know about the vacuum damper motor and the vacuum switches that activiate them, on the air cleaner (see post above).

[Whtstang]

Looking at this post & the previous threads about the code 41, I'm wondering if there is something wrong with the HEGO circuit. I gather that the car is not stock?