why is my 240 slow to return to idle?

Please help me with a seemingly not-serious, though puzzling issue.

I drive an '82 240 DL with CIS fuel injection, 159K miles, manual
transmission.

When the car is cold, it idles at around 600RPM like a clock, though
she shakes a bit due to, in my mind, vibrating resonances. No big
deal there. When the car is still cold and I go through the first few
upshifts and downshifts of my commute, the engine will return to
normal idle (600-900RPM) very quickly when I engage the clutch.
However, as the car gets warm, the return to idle takes longer and
longer. Sometimes it will only go down to about 1300RPM, other times
to 1000RPM. I cheat and engage the clutch a bit at stoplights to drag
the engine and artificially bring down the idle. When I do this it
usually stays below or around 1000RPM.

Here's what I've done so far to try and fix the problem.

Checked vacuum hoses not-so-thoroughly, though the car NEVER stalls,
and I understand this would happen if I had some vacuum leaks. Taken
out and lubricated with white lithium grease the accelerator cable
assembly. With the cable out, I started the engine and manually
turned the throttle assembly to see if there was any difference in
return to idle. Still slow return to idle, so the cable seems not to
be the problem. Sprayed throttle butterfly valve with appropriate
cleaner while working it open and closed.

To do this last bit I unclamped the hard, black plastic ducting from
the throttle valve assembly to the unit below (air filter tube?). As
I was pushing and pulling to get that free I noticed a section of the
plastic ducting was torn, right where the metal clamp holds it down
against the throttle valve assembly. I might be describing this
badly; sorry. But I was wondering, maybe this tear is causing my
problem? Not sure how that could fit in... Air leaking in causes
problems? Anyway, after I sprayed the cleaner up into the butterfly
valve and put everything back together, the engine still has the same
problem. Slow return to idle!

I don't know what to look for next. Any suggestions would be greatly
appreciated.

Matt

 

Does it have a constant idle system? If so check this valve, it's a
motorized thing that allows a controlled amount of air to bypass the
throttle plate. Also check the throttle plate to see what it's rest position
is, with the plate closed it should still be open a tiny amount, someone
else can fill you in on the adjustment procedure for that.