[DML] Re: Couple Questions- Coolant System & Idle

["endotex23" <endotex23@xxxxxxxxx>]

As Toby and David pointed out hunting of the idle is common but that
doesn't mean it should happen. It's "typical" only in the respect most
people end up accepting it. There are several different causes for the
problem, plus there are interactions between different systems that
can add to the frustration of solving it. This defeats many owners so
they just end up tolerating it for the few minutes it occurs. I found
it not only embarressing but a gauntlet thrown down by the car I just
had to pick up. In doing so I learned the multitude of reasons why it
happens. To add to the problem the service manual omits important
procedures published elsewhere for setting up the Bosch system used to
maintain idle.

There are distinct approaches for resolving the problem depending on
whether it occurs cold verus hot and with a little patience and a
methodical approach one can identify which of the issues is/are at
fault. It's not difficult but you need to understand the basic causes
and how each of the systems involved can interact to make the problem
tricky to figure out. Assuming you have no vacuum leaks and the engine
is in otherwise good condition the best way to solve this problem is
to start from scratch and set everything up independently, with each
system isolated and operating in open loop. The procedure is beyond
the scope of a single post but if you feel confident working on the
car contact me off list and I'll expand on it.

One thing you won't get rid of (without a mod) is the very minor idle
instability that occurs after the lambda loop becomes active. It's
caused by hysterisis in the fuel system as the FV corrects for mixture
deviations. Since the mixture is hydraulically controlled there will
always be a slight lag between the FV and injector pressure. There is
a silver lining in that cloud though: it confirm's your lambda loop is
alive. It verifys at a glance everything from the O2S to the FV is
operating.








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