Drove into work this morning just fine.  Got into the car for lunch and it starts up fine, but won't idle - rpm just drops down then it's off.  I was able with some tricky foot work to get it over to the repair shop, but that's not a good way to drive!  I'm hoping this is going to be an easy fix - just need a new idle air valve? - thoughts?

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This could be several different issues....you would need to start by checking for any codes in the system, to see if that leads to the culprit. Could be the IAC(idle air control) as you mention, but could also be a TPS(throttle position sensor), or something as simple as a vacuum leak. Lots of variables here, but I would see if it has any codes stored, and proceed from there.

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Joel Adams  
My Link

    
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

as joel said, there are too many variables in todays car to be able to pinpoint. good luck.

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the dynamic duo

c3vr life member #97

When my 85 did this, it was the fuel pump. I forget exactly how I determined it was the 21 year old original pump causing the no-idle, but it was. I distinctly remember driving it from where it first died (it was in the smog inspection bay getting its first "smog" in 12 years when I put it back on the road 12 years of storage) to the store to buy the replacement pump, and then home to install it. I'd push in the clutch to shift, and it would die. I'd shift, let the clutch out and it would fire right back up.

I know it doesn't make sense that it could be the pump, but it was for me. I had the problem fixed less than an hour after it first occured. That was 7 years ago, and has never died at idle since. Runs like an L98 should.

So the car is back from the shop and it was the idle air control.  Guess the pintle got stuck or something.  Dealership had to get the part from Michigan so it was in the shop until Saturday.  But it snowed a bit during the week so it wouldn’t have been driven anyway.  Snows all gone so it’s with me again today!

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Glad to hear you got it back, nothing worse than owning a vette thats "in the shop" ... trust me I know!

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Allan Brenneis Nashville Tn. 1990 Polo Green Coupe Saddle Interior Chrome ZR-1 Rims https://www.c4vetteregistry.com/member_uploads/14501_14600/14572/image2014329104347r.jpg

Ya...the pintles will get carboned up, and not move in/out properly. You can usually clean them, but it's better in the long run to replace.
Glad they were able to get it taken care of!


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Joel Adams  
My Link

    
(click for Texas-sized view!)             NCRS

"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

So I know the idle is controlled by the ECM, but why is there an adjustment screw at the throttle plates?  After the IAC repair it seems the car is running a little higher idle then before.  I would assume because it's not having to fight getting fuel in like before.  Right after I originally purchased the car I cleaned the throttle plates as they were wicked dirty and it ran a much higher idle then before so before I knew any better about ECM control I adjusted the screw out so to close the plates a bit, and that worked great.  Is there adjustment that works with the ECM?

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to adjust the screw, back it out and then turn it in untils it just touches the arm and then 1/4 turn is where it should be set to. if i remember correctly when i replaced my IAC, it said to drive the car at approx 38-39 mph for a couple of miles so it can adjust itself. the idea is not to let the car go into overdrive.

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the dynamic duo

c3vr life member #97

The screw keeps the throttle plates from jamming / sticking closed. The adjustment also affects the TPS closed-throttle voltage, and the IAC counts. TPS at closed throttle should be .5 Volt, and IAC counts should be 20-40. The screw affects more than just throttle plate position.

"Resetting the IAC" is an old wives tale. Urban legend, or BS, whatever you want to call it. There's no such thing.

When replacing the IAC, push or screw the pintle into the IAC far enough that it doesn't bottom in the housing when you install it. Because this will damage it if it bottoms while being installed.

When the ECM powers-up, it sends the IAC more than enough counts to fully seat it, then opens it a programmed amount of counts to allow starting. After engine is running, ECM adjusts as needed for desired idle speed. That's all there is to it. There is no "learning" by the ECM for IAC control. Anyone who thinks there is, doesn't know how the IAC control works, or has never monitored IAC on a scan tool during start-up.



|UPDATED|11/3/2012 12:51:31 PM (AZT)|/UPDATED|

Okay, so should I be checking that at closed throttle the voltage at the throttle position sensor is @ .5 and if not decrease the screw until it’s near to that?  Does the ECM take the lowest voltage it’s reading now and assume it’s at idle, or because the voltage is higher it think I’m giving slight throttle when I’m actually at idle? 

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So I wasn't able to figure out which of the 3 wires I needed to check as any combination I tried didn't give me any voltage reading.  My meter may be faulty too.  Anyway I went ahead and backed out the throttle adjustment screw all the way out, and then back in until touching and a 'tad' more.  Seems to be working great, and the idle is where I believe it should be.

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first off you never adjust the throttlebody screw for idle problems,the tps has screws on slots to adjust the voltage,not the throttle plate.by doing this you will not have full throttle when you floor the acelerator,also this can cause a stumble.mid america sells a cable to just that for $14.99 with instructions on how to adjust the tps,all you need is a digatal meter.the part# 601-092.you now will have to put the throttle plate back to where it was  make further adjustments.tlavac

I was having the stumbling, and I knew that I didn't have full throttle options before as the screw was pushing the throttle plate more open.  I've since backed out the throttle plate completely and noticed the idle is where it is supposed to be, and I'm not having a stumble from a stop.  So I didn't adjust the throttle screw for my idle issue as I know that the ECM controls that, but having the plates partially open caused me to have a higher idle.  Definitely frustrating trying to figure out what previous owners have done or tried to do incorrectly.


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that's the most painful thing is what previous owners have done. best to get things back to the proper position and go from there. the adjustment for the screw is from the factory manual. great to hear it is running better.

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the dynamic duo

c3vr life member #97

i'm glad its running better.get some factory manuals and it will help you adj the plate to factory clearance and then the tps will need to adj accordingly.get that harness and manuals and you will be all set.tlavac

Okay not to shoot a dead horse - but I went to the hardware store yesterday.  I wasn't in there for a horrible long time, and it was a hot day yesterday.  Finished my shopping, I got in the car and she started up a little hard, the idle was really weird - then she died.  So restarted her and she wouldn't idle.  After she would die she wouldn't start right back up but had to crank her for a bit.  I got her home with some fancy stickshift feetwork, she didn't idle at all on the way home so I had to keep restarting her.  At home I turned her off and went inside to tell the wife we need to take her in to the shop, and not a few minutes later went back out and restarted her and now the idle is running fine.  Had to make couple of more runs to hardware store later on - needed to get my home a/c working.  Corvette didn't repeat any of the idle issue and is now running fine.  Drove her into work today and no problems.  Nothing worse then phantom issues - anyone have any ideas what this is about?

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Might be time to check fuel pressure, both when cold(first start), and after it gets hot. The fuel pump could be dragging when it heats up after runing a while, and not provide enough pressure or volume. If your pressure drops when hot, it's prolly time for a new pump. Could also be time for  a new fuel filter. 
An ignition module can also give these same symptoms.

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Joel Adams  
My Link

    
(click for Texas-sized view!)             NCRS

"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

Adam you hit it on the head.  So this week Tuesday got into the car for work and she started up, but no idle.  Started to drive down the street hoping it'd 'snap' outta it but still alas no idle.  Drove home to grab the wife's car and decided going to already be late for work, so we drove over to the shop (and of course now it idles fine.)  So days later (with no call back from the shop) I just got off the phone with the service writer and they have found that the fuel pump has indeed gone south.  She isn't keeping up pressure and showing low pressure on one bank.  I had to double question him as at first glance a fuel pump and lack of idle don't make any sense.  So what I was hoping was just a clean and check out the idle air control turned into an $750 job.  Blah .  Super glad we have this site though as I didn't realize my problem happened so recently (June) that she did this before.  So not that I have much choice but we're going to get a new fuel pump and filter put in.  They said I should have it back by next week... there goes my weekend of fun.

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So did I. Back in October:


I didn't think it was the IAC valve.  It never is. Glad you have it fixed. The shop should refund yer money for the IAC. Because that wasn't it.

I apologize to all previous posters as yes when I re-read this there were multiple mentioning about the fuel pump.  Adam was just the most recent.  I really appreciate all of your help and your wisdom. IAC is a much easier part to think about ($) versus a new fuel pump ($$$).  

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Got her back from the shop all fixed up with new fuel pump / filter.  She seems to be running great!  Thankfully had a 5% off coupon as out the door $800 with oil change.  Drove by a woman today on the way into work in a Gray C4 with a vanity license plate "her half" ...good stuff.

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That was a lot to deal with.  Glad you got the fuel pump issue sorted, but sorry it was so costly a repair.  Considering the location of the pump, it seems as if some of these places sure charge a lot   Well, at least you're back on the road.

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