This question is more a theoretical one than anything that needs a correct answer, as I no longer own the car and it’s on the other side of the world. But it had an odd problem that I never figured out and I’m still curious about it.
About a year ago I spent about 2-3 months in New Zealand and figured I ought to just by myself a car and then sell it right before I leave. This plan worked beautifully, but the car I bought did not. It was a 1998 Ford Mondeo with an auto transmission (possibly the same as a Mercury Cougar?) that cost me the equivalent of about US$800. This car had the exciting habit of stalling any time I came to a stop before the engine was sufficiently warmed up. It felt as if I was driving a manual with a clutch that only partially disengages if it’s cold. The engine RPMs would also surge up and down while stopped (sometimes stalling if it was cold) and it would get pretty hot, though it never quite overheated. It was certainly not a safe car for many other reasons, and I have no idea how it passed their strict inspections.
Well, I’ve been learning about cars a lot in the last few months and I just read Tom and Ray’s recent column on what happens within the torque converter if you step on the gas while holding down the brake pedal, which led me to an explanation for why this car was doing this. I’m wondering if anyone can tell me if I’m on the right track. My hypothesis is: the transmission fluid in the torque converter was so old and dirty that it was far too viscous to work properly when cold. Until it was warmed up, the wheels and engine were coupled together via the transmission and torque converter. So when the wheels stopped, the engine would stop as well.
Does this seem reasonable? And would new transmission fluid be likely to solve the problem?
I doubt that scenario. A stuck torque converter clutch can lead to stalling at idle, true.
But what you describe just sounds like a sticky idle air control valve or perhaps even a leaking brake booster - i.e. it sounds like a “vacuum leak” type of problem.