wedgeheaded

I bought this car last month in NC and drove it home 6hrs. It ran great but idles a bit fast. I can't tell how much as the tac reads high but it's noticably faster than my buddy's 85. (Tac is on my list of things to fix) I understand the idle is controled by the ECU and IAC. I broke the TB down and cleaned everything including the IAC and it's pocket, put it all back together and it still runs fast. It's OK, it's just a bit fast and sounds wrong. I unplugged the IAC while running and it didn't change anything. I also unhooked the TPS, nothing. I pulled the PVC hose and still nothing. I thought that would change things. I plugged the hose port with my finger, no change.
As I understand the IAC from reading the FSM it's basicly 2 coils in oposition to each other. One pulls the valve to let air in, the other pulls the other way to restrict it, depending on what the ECU is calling for. Do I need to reset my ECU to get everything back to square one? Is my IAC bad? (I just realized while writing this post I may well have ruined my IAC by unhooking it while energized. I know that will fry an AC coil and I'd guess a DC coil is the same. I can read it with my VOM to see if it's OK. Any suggestions from those in the know would be welcome.

Corvette40

To test your IAC, remove the IAC from the throttle body but leave the electrical connector on. While holding the IAC have someone turn the ignition key on but don't start. The IAC pintle should retract all the way then go back out to its set point. If it doesn't its bad. You should feel the IAC vibrate a little if it has power. The pintle should be set 1-1/8" from the tip to the gasket surface.

You should make sure everything else is set up correctly also. If someone has opened your throttle plates too much, idle screw, then the IAC will make no difference. Go through the "set minimum idle" procedure after testing your IAC.

C409

......... The most common cause of a high idle in our aged C4's is a vacuum leak ... check ALL the hoses on that little diagram on the radiator shroud and check the brake booster ... there's a vac connection behind the distributor for the HVAC system and cruise control too ! and do the timing , TPS and minimum idle settings as mentioned earlier .........

wedgeheaded

Good stuff, thanks. I need to make getting the tac fixed a priority. I wanted to do that when I fix the sloppy tilt wheel and replace the ign sw. As I mentioned I have the FSM for a 90, it's a great tool but I keep getting lost and haven't quite got the hang of looking in the right place yet. I read the IAC with my VOM and it read 49.2 ohms, both coils. At least I didn't open the coils when I pulled the connection. I'll try operating it out of the TB to see if it strokes. I meant to check the length of the pental but forgot. I'll check all my vac hoses too, just clipping off 1/2" and remaking the ends might do it. It idles and runs very smoothly though. In my experience a vac leak is a bit inconsistant. Still, I'm big on checking the free and easy stuff first. Good call. OK, lots to do and read up on. Thanks.

aklim

Before we do anything, lets establish that you are really faster than normal. For that trick, use a scantool. Why? Because you don't know what the RPM really is and you cannot see what the ECM is seeing. IF your TB is clean, and you took off the IAC and the housing and cleaned that too, did you reset the base idle? What is the base idle set at in the chip? That too can be seen on a scantool.

To do it right, you would need to establish that the timing is at 6 degrees. Once that is right, we can proceed. Take a propane torch WITHOUT THE FLAME ON to all hoses and joints when the engine is running. IF the RPM goes up, we know where to start fixing.

Just chopping off the ends doesn't mean that much. How do you know if the hose has a leak somewhere else? Sure, ends are the most likely problem but we can't rule out a crack somewhere.

Finally, if you want to see what is going on, you need a scanner. Check the IAC counts with everything off but the engine. It should be around 20 to 30 counts. With that, we can see if there might be a leak or not. If your IAC counts are 0, we know that there is more air than needed, be it a leak or that the throttle blades are too far open and the IAC cannot adjust any more.

wedgeheaded

I've looked in my FSM for the procedure to check base idle. I've seen it there, or read it here, but can't find it now. Yes the best fix for the hoses is just get new. I'm off mon and I want to get some vac and vent hose material to replace my hoses then. I was just hoping to get lucky before then with clipping the ends. I've heard of the propane trick but never tried it, now might be the time. OK, more good stuff. Thanks

aklim

I set my idle using a scanner. I adjust it up or down based off iac counts

wedgeheaded

I just learned my buddy has a scan tool, if he works this week end he'd going to bring it to work. That will get me started straight.

aklim

Buy your own if you can. They aren't that expensive and will save you a lot of guesswork.

Churchkey

May have seen it here - post #2



http://forums.corvetteforum.com/c4-g...vent-hose.html

aklim

I really don't like the FSM method. That only will work if you are perfectly stock. Once you go beyond, all bets are off especially if they change the idle speed via the cam. I prefer to set it using the IAC counts. Target is 20-30 counts. So rev it up and let it settle down with ALL systems except engine off. If it is higher. Screw the adjustment screw in and rev and check. If it is lower, screw the adjustment screw out, rev and recheck. That will let me know if there is wacky hunting by the IAC

Cliff Harris

The definitive article on setting the base idle was written by Lars Grimsrud. Here's a copy (Lars' site has been dead for many years):

http://www.batee.com/corvette/dcrg/r...scuss_sim5.htm

The IAC is a stepper motor. The ECM sends pulses to it that are 90° out of phase. If phase A leads phase B then it turns one way and if phase B leads phase A it turns the opposite way. That's why the tuning folks talk about "steps" -- those are steps that extend or retract the pintle.

The IAC is used by the ECM to fine tune the idle speed, based on the O2 sensor reading. It is also used in "throttle follower" mode to keep the engine from stalling when you let off the gas suddenly. It opens as you mash on the gas pedal and then (relatively) slowly closes as the car stops, lowering the idle back to normal. It is also used for a "fast idle" mode when the engine is cold to keep it from stalling, just like the old carburetor days.

I wouldn't run the IAC when it's not mounted on the throttle body. When you turn on the ignition the ECM steps the IAC closed (extended) and then steps it open (retracted) to 144 steps. If the IAC is not mounted on the throttle body when the ignition is turned on the pintle will shoot out of the IAC and fly into an unknown location, never to be seen again...

aklim

But if you look at that article, your idle speed disconnected has to be about 450. That is too low for many modified vehicles if you are doing head, intake or cam changes where the ECM requires a faster idle speed. If you tried that on my car, it would probably only work under very narrow circumstances since the command idle is much higher. That is why I prefer to set it using IAC counts.

wedgeheaded

I know this is an old topic but I think I fixed my problem. This spring I bought new injectors and put them in. My car ran faster than ever, but smoother. I discovered I forgot to hook up the IAC wires. This slowed it down but now it cycled up and down. I found a place in my FSM where it said that running the motor with the IAC unhooked, then hooking it back up, would cause it to cycle this way. It also described a reset procedure for the IAC. I did it and it worked. My car now idles at a respectable idle and outside of a off idle stumble, runs great. I suspect that my IAC my be on it's way out and need replacement.
Another discovery I made is that the IAC is a small DC motor, turning one way to drive in the pintle and the other way to pull it out. I thought it was a coil. Cool beans!

MrWillys

How about telling us what injector you removed, and what you replaced them with?

wedgeheaded

I bought 8 new injectors from the Injector connection.com. 8 balanced, remanufactured Bosh injectors, with a five year warranty, all the gaskets and O rings needed, and a new diaphram for the FP reg, for $250.00. Mine were yellow. (stock L98 injectors.) Sweet deal I say. Great guy to work with too.

Cliff Harris

It's not a DC motor. It's a STEPPER motor. Not at all the same thing. See my post #12 in the thread.

wedgeheaded

OK, so it pulses the rotor in steps. It's still based on a DC motor only not operated as one. I'm learning. It is a cool way of doing it. The FSM talks about steps and now it becomes clearer to me what they mean.

Cliff Harris

The ECM recalibrates the IAC every time you turn on the ignition. You don't need to do anything (assuming it's working OK). Given that, the FSM procedure of setting the pintle to some specific length is stupid.

...Except if the pintle sticks out too far. In that case it's possible to break it when it bottoms out in the passageway and you keep on screwing in the IAC. Been there, done that -- broke a brand new IAC. Consider the FSM pintle length (28mm in my FSM) to be a MAXIMUM to avoid breaking it. I just push it all the way in, install the IAC, turn on ignition, turn off ignition, start engine.

wedgeheaded

OK, so now with new injectors and my idle where it should be, my only issue with drivability is a slight hesitation at "just off idle" acceleration. Like edging up in traffic. Keeping in mind this car has 192,000 miles on it, where should I look for my hesitation? (The motor was rebuilt and runs great but I just changed out the OE injectors so I doubt anything else was replaced.) It runs glassy smooth on the road.