your thoughts on this?
A friends truck... and he tells me, "problem all began when / after trying
to jump start another vehicle"...
What I found is: this truck wants to idle fast. It's not the idle air
valve. It already got a new one, and I've even tested the old one. Any
vacuum leaks I believe are minimal. The computer is just trying to maintain
idle at 1000+ rpms. There is some wear in the throttle shaft. I took
advantage of the throttle stop screw to try to lower the idle, and left the
idle air valve fully closed and unplugged. It's at least drivable now.
With the IAC connected, the computer keeps the idle too high. If I force
the throttle to close further to drop idle, the iac data starts climbing up
to around 10 and idle is back to too high. If I put the truck into drive,
rpms drop to about 650. Then IAC number climbs on up to 80 to 90%, tryiing
to reach this high idle while in gear. That's a lot of added throttle, and
not a good thing here in NH with snow on the roads. Either the rear wheels
are a spinning, or the front wheels are a skiing! With IAC number that high
in drive, when you put it back into drive the rpms go up to like about 3K
before the number comes back down. My conclusion is the computer got
damaged?? I know that's not typical, but this thing is crazy! All data,
including coolant temp looks fine. What else would make this thing try to
idle so high when in gear? Truck has A/C but it doesn't work. Data shows
A/C OFF. AFAIK, there is no power steering switch to up the idle...
Leaky regulator diaphram? ( just a guess, maybe the IAC is compensating
for the extra fuel )
Leaky Injector? ( same reasoning. Check with a timing light, you will
see the fuel pulse out of the injectors )
Vacuume leak? (usual suspects )
I'm wondering if you can disconnect an injector at a time to see if
there is a difference, easy to do, worth a try.
Jump starting , as far as I know, will only put the Alternator at risk.
Since they use Avalanche Diodes, the arc may take them out.
They also dont advise on doing Arc welding (12v) with those
Actually, one injector is getting wet on top where it plugs in. Otherwise
spray pattern looks good. Block learn / int. look OK, and my thinking there
is that extra fuel would cause it to try to lean out the fuel, not up the
idle rpms. But then, if a little extra fuel from injectors is combined with
minor vacuum leaks... that would certainly increase idle. But still, why
would the computer keep jacking up the iac when I do things to lower the
idle. Especially when it's in gear. I goofed in my orig post... it should
have read that the IAC value climbs to 80 -90% in drive, and then when you
put it back into PARK, it revs like crazy with until the IAC comes back
down. It appears to me that the computer program itself may have become
corrupted? Repair data (mitchell) says idle is computer controlled and not
adjustable... but it doesn't say what the computer is SUPPOSED to do, or
under what conditions. I know from past experience that typically on a cold
start the computer will normally jack the idle up at first. Perhaps it is
stuck in this mode? Data shows engine comes up to temp (185F at least) at
idle. It's like the system is working perfectly except the idle target has
been raised. When he comes back with the truck next time, I think I will
check the alternator for any AC output. Otherwise, it's charging fine.
I had it disconnected for about 30 seconds (and it cleared codes).
It has memory that clears after longer (as in up near 10 minutes)??
Like clear codes amount of time vs. cause a full reboot sort of thing??
I'll certainly try that... but it's not with me any more. And to do that
and see if it helps, I'll need to get the IAC valve working again. I did a
little something there to absolutely prevent if from working it's way open
You say "Had it work many times." Can I ask exactly what problem(s) you had
it work for? This exact or very similar problem?
I will definitely get him to bring it back over sometime in the near future.
Truck is old but not overly rusted (this is a NH salted truck). I certainly
wouldn't mind digging into gaskets and even doing injectors (as one is
leaking slightly on the top) if I can resolve what the ecm is doing. I
always prefer to get to the root of a problem and fix it right... but with
his funds low, I didn't want to do a bunch of work (even though justifiable)
to have it end up with the ecm still jacking up the idle. So I went with
the "cheat it" into submission for now.
In the past, I've heard of ecms being damaged by disconnecting / connecting
battery terminals with the key on. I imagined that, given how side mount
terminals work, one can end up pulsing the power on and off rapidly and
effecting the programming somehow. So when he tells me it all started when
he was trying to jump start his other car (didn't sound like he got it
running either) I can only imagine what he may have done with it running or
KOEO or what...
Thanks to you for replying, and any who can provide more insight!
The longer time allows ALL of the caps and any power systems to
completely drain. Plus it also allows you time to run a test of the
battery, and clean and check the terminals while you have them off.
I have had it clear various "problems" , high idle, odd behavior of ECM
controlled items (like IAC motors that wouldn't reset the pintle
position, TPS that read WOT when at idle) Electronic 4X4 that seemed to
forget what it was doing. Plus other odd behaviors, like door locks that
would cycle on their own.
Most of the problems seemed to happen after a jump start or other power
surge type event.
On newer vehicles you can hook up a good scan tool and reset most of
these items but on some of the early OBD systems pulling power from the
ECM is faster and easier.
Just the sort of thing I was hoping to get from coming here!
I don't know when (if ever ; ) ) I'll get this truck back here again, but
when I do I will definitely follow up on this post! I know it should idle
down lower with IAC completely sealed off, so there is likely some vacuum
leakage involved. The other factor is ignition timing, and it wouldn't
surprise me if ecm is getting that wrong.... I did check that distributor
is tight and no-one has messed with it at all for the couple few years he's
had this truck so assumed the base timing is correct. I know "assume" is a
dangerous word! But I've never found timing wrong on anything in my life
except when someone else has been messing with it. I will put a light on it
and see what it's doing when (if) this truck comes back. I intend to get
him to return with it. But since I did get it "fixed" compared to how it
was, he may not want to come back for a while... : )
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