Re: RICH when DECELERATING?

Car: 1991 Chevy Lumina 3.1MPFI
> > My mechanic tells me it's not a HUGE deal, but he noticed that my ECM > thinks I am running rich at idle, but even MORE when decelerating!? > For example, if I'm cruising at 40 MPH, and let completely off the > gas/throttle. I don't exactly know what he's talking about, nor did I > have time or money to get it all checked out now. > > I can't think of anything that has NOT been changed on this car in the > past year, EGR, plugs, wires, DIS, coils, O2, cat. converter, muffler, > PCV, throttle body cleaned completely etc..etc... > > So I can't think of anything that would cause this?? Especially since > he said it's fine when your accelerating or cruising?? > > I understand that the vacuum is at it's highest point when > decelerating, but I don't know if that's a clue or not? Perhaps the > Charcoal Canister?? > > Also, as I understand, richness can be caused by not only TOO MUCH > fuel, but also TOO LITTLE air, right?? > > Anyone have any ideas? > > THANKS!

When the ECM sees closed throttle, high vacuum (low MAP) and RPM somewhere (they never tell us exactly) above desired idle, it goes into what's called "deceleration enleanment" basically, it shuts the injectors off and doesn't pulse them at all. If the O2 sensor voltage is observed to be sufficiently high (rich mixture) as claimed by your mechanic, false fuel is being introduced into the engine from -somewhere-. That somewhere could be leaking injectors, ruptured fuel pressure regulator, the cannister system, via the PCV due to worn rings or valve guides, leaking intake gaskets, etc.

Charcoal Canisters usually produce rich symptoms, however if the canister has been purged enough and the purge continues at the wrong time (i.e., closed throttle decel from a leaking purge valve (or a plain old vacuum leak)) it can cause a lean condition.

Right.

If it is indeed fine at cruise and accel, I would look at something over fueling the engine. Like a faulty pressure regulator or injectors, Throttle Position Sensor fault, or the ECM seeing a lean exhaust.

Don't disagree with that. He just didn't mention the tech determined it was running rich. In fact, he said, 'he thinks it's running rich.'

I'd never tell a customer that I 'thought' they had a symptom. Maybe that they had a symptom and I 'thought' it might be XXX which could be determined by XXX1 test in YYY hours, but that's just me :)

True dat.

In any case, I would think that this would be a fairly simple diagnosis for a competent tech. I can't think of anything off hand that could cause this that would take more than 0.5 to 1.0 labor.