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