eta brake booster leak

[LateFan]

How likely is a random vacuum leak from the brake booster? Wouldn't that show up in the feel of the pedal? It brakes really well.

We've got an '86 es with a completely random vac leak (we think). Everything has been changed, two smoke tests done, but both used the brake booster port on the intake.

Idles perfectly most of the time, then will barely start - black smoke, sputtering, sooty plugs. ICV, TPS, o2, valve adjust, AFM - all new.

This is why I wonder about a big leak - when it runs well, pulling the dipstick or oil cap will of course make it stumble or quit. When it's doing the rich running / sputtering, pulling either of those has no effect on the idle. I assume it's leaking somewhere else, and the booster was out of the loop for the smoke tests we did. I did put a new brake vac line, 90 degree check valve, and grommet at the booster.

[Gregs///M]

Its possible, there is a o-rings between the cylinder and the booster. Try plugging a vacuum pump to the booster and see if it holds a vacuum.

[LateFan]

Thank you. Is there a vac number I should look for, or just see if it drops off?

[jlevie]

When the smoke test was done was the AFM replaced with a plug (preferably with a nipple for smoke injection), the exhaust plugged, and smoke pressure of 2-4psi held for over 5 minutes?

[LateFan]

Yes, we made a plug for the end of the intake elbow, and clamped it (roll of electrical tape peeled until it was just the right diameter, with a sheet of heavy visqueen over the back side).

For the smoke tester, I used the big fitting on the intake where the brake booster hose attaches. So I've removed a whole system from the test, right?

I wasn't sure how to plug the exhaust - literally plug up the tailpipe?

We had sustained pressure until it blew our intake plug out. Our egg carton fire in the can may have fizzled before we had air flowing into the engine(?) I never saw smoke, but heard the rocker plugs on the back of the head hissing. All gaskets and vac hoses have been replaced.

What other system could produce a serious vac leak in such a random way?

[jlevie]

You could make a plug for the intake out of 3/4 MFD, though aluminum is better. It could be cut with a saber saw or band saw and should be turned to diameter on a lathe, but careful work with a scribe to a scribed line is almost as good. Drill a hole in the plug, tap it, and thread in a nipple. For the exhaust you need expanding plugs. You will will also need a source of long persistence oil smoke and a pressure regulator and 0-5psi gauge.