Thread: evap
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Old 05-29-2012, 03:56 AM   #64 (permalink)
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clank - '99 jeep tj sport
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Originally Posted by some_other_dave View Post
A vacuum leak can cause a lean condition, depending on where in the system the leak is. Leaks right by the intake valve do not have as much of a chance to propagate back up to the manifold, and can cause a lean cylinder.

However, most vacuum lines will be hooking up to the manifold itself, so your point is right on the money.

O2 sensor feedback has a limited amount of "control authority". In general, it cannot change the base mixture so much as to prevent the engine from running, at the very least. So a very very lean condition will make the O2 circuit go to full enrichment, but it might not be enough.

Again, though, this is a "corner case" and is not that likely to be what is actually happening when a vacuum hose is disconnected. But it is possible.
it wouldn't make a lean condition in a speed density system; that's fuel is controlled by map, intake temp, water temp, and o2 sensors(most systems are 2 or 4; unless your talking obd1)

map= manifold air pressure
vacuum leak= lower vacuum pressure
most engines run around 20 inches if you have a vacuum leak(by the way he said 15 inches) it would cause the engine to run rich, no matter what the intake or water temp was, and the 02 is for trim(long term/short term) or where in the system the leak is unless it's behind a valve the way i was running around with a vacuum leak(and no decent amount of heat for march) for a month; high idle and pig ass rich.

2. plastic vacuum lines are crap, they break or wear through faster, and melt easier. than rubber

a maf(mass air flow) system on the other hand, a vacuum leak can cause a lean condition.

some_other_dave; do you drive a chrysler product?

Last edited by baldlobo; 05-29-2012 at 04:06 AM..
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