AW: [DML] Re: End of life for idle ECU's

["Elvis Nocita" <elvisnocita@xxxxxx>]

And still - a good control loop can compensate any hunting of the idle,
especially the bad ones that let the idle vary with about 1Hz.

There's absolutely no reason or explanation behind an idle speed hunting 
between 900 and 1200rpm other than a badly designed control loop.
One cylinder firing better or worse won't affect the idle like that
at all, the other 5 will smooth it out !

And also there's a diference between cold and warm ! Sometimes when cold
the engine may hunt very bad but suddenly stopps to hunt when it gets warm.
And when I say suddenly I mean suddenly ! Not like it's getting slowly
better - no it's like blown away within less than a second. Like switched 
away !!!
This made me believe that it can't be just a vacuum leak or a bad
wire. And soon I'm gonna start to test it.

Of course everything needs to be in a good condition, a 23 year old 
O-ring or a totally broken O2 sensor need to be serviced anyway.

Elvis & 6548


>From my experience what I have found as a cause for a hunting idle is
vacuum leaks for sure but mostly 1 or more cylinders are not firing
evenly. A dirty fuel injector, a bad ignition wire, low compression, a
valve out-of-adjustment, is the kind of thing to look for. MY theory
is when the motor fires on the low cylinder the engine slows down. The
idle ECU tries to compensate by increasing the idle but by time the
correction kicks in the engine is already past that cylinder and is
now firing on a better running cylinder so the correction causes an
overshoot so now the motor is running too fast so now it slows the
motor down. Now you can see why the motor seems to "hunt". There is no
way to adjust any dampening into the control loop so the "fix" is to
get all the cylinders firing as evenly as you can. Do a compression
test. Forget the actual pressure #. Just compare them to each other
and if they are not within 5% you will have trouble getting the engine
to idle smooth. Next, clean and regap the spark plugs very carefully
so they are all the same. Clean the fuel injectors and replace the
seals. If it won't idle any better look for a vacuum leak, like the
"O" rings on the intake manifold to the head or the "O" ring under the
mixture unit to the cold start valve. It is also possible the idle
motor "sticks" and is not responding fast enough to the ECU. In this
case trying someone else's might prove that out. I know it is possible
for the idle ECU to go bad but it is a low failure rate item. A lot
more likely to find a bad ignition wire, a miss-gapped sprk plug, or
dirty fuel injectors.
David Teitelbaum
vin 10757






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