Vacuum Loss In Gear

92SCtk

Registered User
So I finally got my brakes done on my 91 (AOD), and can drive fast enough on the couple hundred feet I have without being "public" roads. New IAC, new fuel pump, new cam position sensor. Car starts immediately, comes down to idle right away, and vacuum will sit in the 16-17 range (have a separate gauge than the dash). Brakes applied in idle is not a problem. Plugs and wires haven't been changed yet, but they're on the list. RPM at idle is around 900. No CEL.

Shift into any gear, idle goes to about 500, but the vacuum drops to 10 and the engine stumbles over itself. It does not require me to touch the gas to save it; stays running. Accelerate slow, not in boost, and there's a clicking sound from the trans, vacuum bounces with each click. Seems like it "catches", smooths out as RPM increases but seems low on power. If I accelerate under boost, same initial symptoms, but being in boost makes them less noticeable. Both scenarios, it shifts smoothly into second.

Just looking for ideas to look at. Don't see any noticeable vacuum leaks upon visual inspection. Could there be a blockage in my exhaust/cats? How would I know without taking out the exhaust? Could it be the valve body in the trans? Any input is appreciated.
 
Vacuum loss when put in gear is normal. Although it does sound like it is low for a non cammed car.
 
Vacuum loss is secondary to the normal RPM loss from the extra load on the motor.

I know some EEC strategies are supposed to increase the desired RPM when in-gear for the auto cars, although you don't usually notice...it's supposed to compensate for the drop..not give a net increase.

If it's not dying on you, I wouldn't worry about it. One of our autos would drop pretty low too.
 
All of that makes sense. Only ever had my 5 speed so the AOD, and how it works with the car, is new ground.
So what I'm gathering then is I should start with the ignition (plugs, wires, coil pack, etc), and go from there. From the little history I know of this car, those were getting replaced regardless.
 
90% of all SC engine running vacuum related problems are due to improperly sealed IC tubes and related plumbing. Your situation sounds just like it. Your idle is too fast for an auto, and the stumble, and almost dying, point to a vacc leak.
 
90% of all SC engine running vacuum related problems are due to improperly sealed IC tubes and related plumbing. Your situation sounds just like it. Your idle is too fast for an auto, and the stumble, and almost dying, point to a vacc leak.

This is a possibility as well. It does look like the intercooler tube was removed at some point, thinking for head gasket replacement, but the collar looks like it was beat on rather than the wrench being used. Based on that and other parts of the car, I doubt the seals were replaced properly, or at all.
 
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