E250 Hesitation/Stumble off idle

I guess I should clarify... Van runs perfectly until you release the brake and let it start to roll, then it stumbles. If you hold the brake and load the engine, it will not stumble. We have replaced the speed sensor in the diff, due to bouncing speedo. Anyone had this one before?

Gale

"gale tyler" wrote

How's the idle in park with your foot off the brake? And if you stick your head under the dash, is there a hissing sound ONLY when your foot is off the brake? Sounds like it could be a leaking vacuum booster.

When you step on the brake, an internal valve seals off the front section of the booster (it now has vacuum in it), and opens the rear section to the outside air. The air pressure now in the rear is what helps you push against the master cylinder.

When you take your foot off the brake, the valve should seal off the outside vent, and open the inner valve, allowing the vacuum to equalize between the front and rear sections. If the outer vent isn't closing properly, you'll have unmetered air leaking into the intake manifold via the outside vent, the internal valve, and the brake booster's big vacuum hose.

"MasterBlaster" under the dash, is there a hissing sound ONLY when your foot is off the brake?

Blaster, I tried putting it in gear while holding the E brake to eliminate that... Still does it. No hissing either... This one is really challenging!!! The Ford garage said it may be the sensor on the speedo head??? The EEC is not getting the signal??? Ever heard of this one??? Thanks, Gale

brake?

New info... If speed sensor is disconnected at rear diff, symptom disappears... Dealer diagnosed torque converter, but this does not pass logic test. Engine is dropping cylinder. Anyone??? Thanks, Gale