88 chevy 5.7 TBI CRAZY high idle

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...
Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Some thoughts...
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 don’t advise on doing Arc welding (12v) with those alternators.
Good luck
Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
In2hoppn wrote:

Bring it in. Disconnect the battery for about 10 minutes. Reconnect and see what it does. Sometimes the ECM just gets a hiccup and dropping all power will allow it to reset. Had it work many times.
--
Steve W.

Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

REALLY? THAT long? 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 at all... 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!
Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Try spraying some throttle body cleaner,on the tbi shafts each outer side,while it's running,if that slows the rpm's down you will know the shaft is leaking.Hope this helps. Bobby
Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
In2hoppn wrote:

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.
--
Steve W.
(\___/)
  Click to see the full signature.
Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Thanks Steve! 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... : )
Add pictures here
βœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.