Re: Help! 87 GT idle is screwy... too low, stalls!

Well... I figured out what it was.

This whole ordeal started with an engine that would stall after "gunning"
the engine... the RPMs would fall so rapidly that it would zip right by the
900rpm level and go to zero.

About a week ago, I cleaned up the throttle body real good and replaced the
IAC valve while I was at it.

It was at that point that it developed a low idle that was 500rpm-ish, with
the occasional high idle.

While debugging the issue, I became suspicious that the problem was the IAC
valve sticking in the fully closed position (this is because, like I said
before in the thread, the normal base idle with the ECM is service mode is
500rpm... this is the base idle with the IAC fully extended (closed)... this
is the idle that is settable using the screw).

Because it was staying at 500rpm even when NOT in service mode, that was
very telling that it was sticking.

So, I took the IAC valve out, manually rotated it in about 1 turn,
reinstalled it, but left it disconnected, and started the engine.

The engine, of course, idled a little high, but it ran... and did so very
smoothly. This test told me that everything else should be working properly,
and the IAC indeed was the center of the problem.

So, I removed the new IAC, cleaned up the old one, and put it back in.

And, suddenly, everything was working perfectly.

Which got me wondering why the new one was sticking but the old one (once
cleaned up very well) was not. And that's when I realized that the two IACs
were not identical. The old one did not "rotate" (or screw) in and out... it
simply moved in and out to open and close the passage. The new one, however,
screwed in and out to open and close the passage.

I can only guess that it was this screwing action that was causing the
pintle to somehow get wedged in the passage in such a manner that it could
not unscrew itself back out.

Does that sound plausible? In any case, it is running perfectly now... a
solid 900rpm idle when warm (which is at spec) and no codes.


"John Craker" <watts@dccnet.com> wrote in message
news:10da6grrf7el37f@corp.supernews.com...
> I've nipped out what looks 'right'.
>
> Your base idle is too low IMHO. I'd expect to see 900.
>
> I think you have a vacuum leak on the MAP circuit (or other related issue)
> as well, since I'd expect to see about 1.2-1.4V at idle off the MAP. 1.8x
> would indicate somewhere about 14" of vacuum which is totally whacked at
> idle.
>
> [oh, and BTW - if EVERYONE asked questions, and posted details like this -
> we could solve most of the worlds problems within a week, and then we'd
all
> be able to just sit back and relax! ]
>
>
> "I,Rossbot" <null@null.null> wrote in message
> news:yL7Bc.3010$5W5.1231@bignews5.bellsouth.net...
> > My 87 GT is giving me fits!! This is the 2.8L engine.
> >
> > The engine acts as if the idle is totally uncontrolled. If cold, it will
> > start and idle at about 1000rpm . However, after it gets warm, it fails
to
> > maintain the usual 900rpm idle, and will either be too high (1100rpm or
> > so)
> > or way to low (500-700rpm). When it is too low, it will get worse and
> > worse
> > until it finally begins to stumble and flat out die.
> >
> > The Idle Air Control valve has been replaced. I have observed it extend
> > the
> > pintle properly by placing the ECM into service mode while the IAC was
> > out.
> > (I, of course, rotated it back in before installing it). With the IAC
> > installed, and while in service mode (which places the IAC into full
> > extend,
> > closing off any idle air), the idle is 500rpm (which is the normal base
> > idle
> > for this engine).
>
> > The MAP sensor has been replaced. The voltage at idle is 1.85 volts or
so,
> > except when rpm is getting very low (500rpm) and it is about to stall,
> > when
> > voltage starts getting higher.
>
>