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Discussion Starter · #1 ·
1989 C900 Turbo Vert, 5-speed, 140K. I had this problem about 4 or 5 years ago. When slowing down or taking my foot off the accelerator, the car would just randomly die. It would always immediately restart with no issues after coasting to a stop and run fine, and there was no rhyme or reason as to when it would happen. You could go months without it happening. Investigated every possible cause (vacuum leaks, idle air control valve, grounds, electrical connections, etc.) Completely went away for probably 4 years. Now all of a sudden the problem is back with a vengeance. It's pretty much every time I go out now at some point or multiple occurrences. Making me worried about driving the car long distances. I've changed out the IAC valve with a known good one (original Bosch of course). The original one was spotless clean to start with. That made no difference and I actually have worse idle than before. Can't find any vacuum issues and the intake grommets are all very tight and in good shape. It seems now that it's pretty much guaranteed to happen the first couple times I slow down after starting out, then seems to be okay, but again no rhyme or reason as to when it happens. From watching the tach, it doesn't seem to "catch" it soon enough once the idle starts falling to prevent it from stalling. Open to any suggestions or anything else to look for that I am possibly missing? I've seen mention of something with the turbo bypass valve and that is one thing I have not checked?
 

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Discussion Starter · #3 ·
Bypass valve and TPS both test fine. I can hear bypass valve moving; it holds vacuum indefinitely. TPS has continuity between pins 1 and 2 with throttle closed, but as soon as I move it an iota, it no longer has continuity, per Bentley manual.
 

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Discussion Starter · #7 ·
Yes, tested turbo bypass valve. It moves in its housing and stays in position as long as there is vacuum applied to it. Does not lose vacuum or move back until vacuum is released.
 

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Discussion Starter · #9 ·
UPDATE: I replaced the IAC with a remanufactured Bosch, cleaned the grounding points near the bottom of the radiator and up behind the IAC (they weren't terrible, but they weren't great either). Have driven the car about 5 different times approx. 300 miles (stop and go, highway, etc.) Hasn't even tried to stall not even once, and the idle is rock steady from cold start all the way through the day, never wavers an iota. Probably the best idle I've ever seen on a C900. Hopefully that solved the issue . . .
 
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