Slow return to idle on ‘81 308 GTSi — Tech Help Needed

Try it in the engine bay by disconnecting the throttle cable and manually opening and releasing the throttle valve directly on the throttle body (TB). This will eliminate all of the linkage from the loop, and you will be able to observe how fast the butterfly plate snaps back on its own ---- if it's slow, it could be a weak TB spring or sticky / binding valve mechanism or an obstruction.

If it snaps back quickly (which it should), I'd start looking for vacuum leaks in everything that communicates --- downstream of the butterfly valve --- with the air intake system & the throttle / idle air bypass circuit.

Also be sure to check the small diameter vacuum hose that runs from the intake plenum to the DigiPlex boxes (ECUs) in the trunk ---- for leaks / cracks. This hose commonly goes bad and not only bleeds in un-metered air, but also screws with the ignition advance curves ---- both will make the engine run faster than it should at various RPMs.

 

Last but not least, if all the above checks out OK, there is a "Deceleration Cut-off (Overrun) Air Valve" on the the TB that connects both upstream and downstream (to by-pass) the throttle valve. It is a simple vacuum diaphragm / spring valve in a small metal canister. Check its hoses and connections thoroughly for leaks. The diaphragm inside can also fail, of course. It's rare, but it happens ---- you can check it by disconnecting both ends and doing a standard "push / pull" forced air test on it.

But, again, do all the other stuff listed above first ---- 99 / 100 times you will find the problem before you get to having to check the Cut-off Valve.

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Good luck and have fun !

BTW, if you don't already have one, go to the parts store and buy a hand-held vacuum test pump kit........ it will save you a helluva lot of time on this !

 

Make sure the clamp holding the cable housing is tight.

 

Floor mat moved against gas pedal. I know how ridiculous this sounds.

 

So, sounds like she needed to be driven. I'm certain my cars function better after a good drive, and better still if I get the oppy to drive a couple of days in a row.