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Oh definitely, the filthy throttle will do this. That's why it's always a service recommended by the people on here, who have often BT,DT.
Here's why:
Under normal operating conditions you regulate the throttle. You open and close it with your foot in response to vehicle speed, load, and acceleration required.
Idle is a special circumstance. The engine needs a signal to tell it "ok, it's time to idle, we're not going anywhere but we need to stay running."
This signal comes from a closed throttle plate. When the throttle is fully closed (in these cars it does go completely shut), idle speed is then controlled by a bypass air circuit- this is the IAC or Idle Air Controller. That's the silver canister under the throttle body, with 1" rubber hoses going to it. This thing is a variable valve that opens and closes to keep the engine speed more or less contant. It also responds to things like the air conditioning cycling on and off, changing engine loading.
When the throttle is all gummed up, the plate may not return fully to the closed position. This may allow extra air into the engine, possibly causing a fast idle- but in a lot of cases, what happens is the engine will go into a near-stall before it decides that RPM's are too low, and opens up the idle controller. In an automatic trans car, this will often result in a stall. With a manual trans, there's less load on the engine normally, and it may recover- or the driver may sense the stall and give it some gas.
A clean throttle working correctly is a big part of the basic maintenance of these cars, as you've found. My wife's 90 745 wouldn't idle well at all when I got it. I'd bet the throttle hadn't been cleaned in 50k to 100K miles. That car bounced between 600 and 1200 rpm about once per second...rrrmmm, rrrmmm, rrrmmm.... infuriating when you know that just a simple service will cure it.
Now go clean the flame trap!
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::: Rob Bareiss, New London CT ::: 92 244 M47 211K ::: 90 745GL M47 273K ::: 88 245DL AW70 190K ::: 84 242DL Project ::: 70 VW Bus ::: 70 VW Pickup Project ::: 71 VW Notchback :::
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