Checking the IAC for operation

[Cntrysthbst]

Looked in all the forums on IAC but nothing really helped answer my question.

I'm checking the operation of the IAC because my idles alittle rough when it's warmed up,

Ok, if you unplug the IAC with the motor running you should notice the engine run different, to about a 2 to 3 hundred rpm difference. In turn that means the IAC is operational. And if you only notice alittle difference in the engine idle its not doing to good, and if nothing happens when you unplug it that means the IAC is bad.

I just want to double check to make sure this is correct. Thanks!

[MT-2500]

The IAC controls the eng rpm.
Rough idle points to another problem.
Unpluging the iac will just make the engine idle at the rpm it was set at when it was unpluged.
MT

[Cntrysthbst]

Ok thanks, I must be thinking of something else, or a different car.

[HotZ28]

The idle speed is controlled by the (PCM) and calculates the proper idle speed for conditions. The PCM calculates proper idle speed using information from throttle position, vehicle speed, and coolant temperature sensors and brake, and neutral safety switches. It then sends a signal to the Idle Air Control (IAC) motor. The IAC motor controls air flow through a passage connected to the intake manifold plenum or throttle body. The PCM controls engine idle speed, by adjusting the IAC motor to compensate for engine load or ambient conditions. They can get gummed up and lose calibration. It can be tested with a scan tool to determine if it is operating correctly.


Other likely causes could be the coolant temperature sensor, poor quality gas, (winter fuel), injector spray pattern, arching wire, weak coil, bad plugs and the list goes on and on!

[Cntrysthbst]

Ok, that helps alot! Points me in a better direction!

Now if the IAC is gummed and has lost calibration will it always set off a code?

And last, if the IAC is needing a cleaning, and/or needing recalibrated, can I get away with doing that and not have problems with it in the near future or is it better to just replace it?

Thanks!

[MT-2500]

Someimes it will and sometimes it will not.
What Kind of car and mileage and eng/
Any codes set.
MT

[Cntrysthbst]

Sorry, 92 PA, no codes, 3800 egn., and about 125,000 miles. Just wondering if its worth cleaning or better to buy a new one. Can the start of the gunk build up cause alittle unsteady rpm at idle?

Hot Z, nice rims, glad to see you finally get them, does it handle better? and last where did you get your coil packs? Thanks!

[MT-2500]

You can always try cleaning.
But a rough idle points to something else.
Check everything over good. Look for a engine miss.
Check for vacume leaks
How is your tune up plugs and wires and coil spark output?
If the light comes on check for code.

[HotZ28]

Rob, I will agree with MT2500 that the idle control is not likely your problem. It could be a weak coil or some of the other things mentioned.


The wheels & tires do make the "HOG" handle better however, the ride is not as good. (Not enough sidewall) I may go back to the original. Everybody that sees them wants to buy them, so the first person that makes a good offer with cash, can have them.


I got the coils from a local MSD distributor called, Barnett Automotive. They are the largest high performance automotive dist in the south. Paid about 45 bucks ea. You can also order from MSD or Jegs.

Welcome to MSD Ignition


MSD list one distributor in your area;


H & H SPEED SHOP
1308 Clearlake Rd.
Cocoa, FL32922
(321) 632-7143

[HotZ28]

You might try using the idle set method used by GM techs.

1. Reset ECM, by pulling the ECM fuse for a few seconds.
2. With A/C turned off, start the car and immediately put it in Drive
   with your foot on the brake. If the car does not start initially,
   keep trying, WITHOUT putting your foot on the accelerator.
3. Let the car idle in gear for at least 5 min and no more than 7 min.
4. Shut off the car for 10 seconds.
5. Start engine, place in drive and let idle for 5 min.
6. Shut off engine.
7. Idle is learned.

[Cntrysthbst]

Alright, thanks to you both, great info, and the link too. I'm checking into it all. I'm just fearing the timing chain for some reason.

I have no problems on a fresh start up but after she's warmed up it goes in and out. Sometimes when it's idling rough I'll turn it off and restart in immed. and it goes away. New tune up, no vacuum leaks. Can I test spark output from the coil packs with a volt meter? If a coil pack is going bad can it put out weaker voltage and still spark the same color? Can injectors do this also? Keep in mind that it does not idle rough all time, it comes and goes. Lately the idle is not getting more rough, but more lengthy.

[Cntrysthbst]

Quick question, when the title of a thread gets off the subject, is it better to start a new thread? reply off a simular one? or just continue this one?

[HotZ28]

Your fear of a timing chain problem is justifiable. I had an erratic idle last year that would come and go the same way. After I replaced all the easy things that could cause the problem, it finally set a code. At first, I thought it was the cam sensor, but after pulling the sensor, I found the magnet in the cam gear was broken. It had some small pieces left, just enough to trigger the sensor sometimes. If that is your problem, the injectors will fire out of time and cause a rough idle. Mine had around 150K when I replaced everything on front of the motor. I.e. chains, sprockets, tensioner, water pump, belts, harmonic balancer, idle pulleys. The MSD coils were purchased as a performance upgrade.

It doesn’t hurt to sway from the topic too much on a thread, as long as it comes back to the subject. PM's and new post are good if the subject changes all together, or if the post gets too long to keep it interesting.

[Cntrysthbst]

Good deal. I'm going to go ahead and replace both those sensors down there. (crank/cam). The one has 130,000 miles on it anyway. I just want to rule them out. Do I need to look for anything specific when replacing these? Any problems I might have the Chilton doesn't tell me? Thanks.

Question - I was listening to my fuel injectors through a hose, I noticed differences in the pulse rate so I used two hoses on two injectors at the same time and listened to them, ones slower than the other, 6 was almost a 3rd slower than 5. Seemed like the right bank had slower pulse rates than the left. They all sound the same when I had the idle about 1000. Does this point to something more specific?

[MT-2500]

Originally Posted by Cntrysthbst

Good deal. I'm going to go ahead and replace both those sensors down there. (crank/cam). The one has 130,000 miles on it anyway. I just want to rule them out. Do I need to look for anything specific when replacing these? Any problems I might have the Chilton doesn't tell me? Thanks.

Question - I was listening to my fuel injectors through a hose, I noticed differences in the pulse rate so I used two hoses on two injectors at the same time and listened to them, ones slower than the other, 6 was almost a 3rd slower than 5. Seemed like the right bank had slower pulse rates than the left. They all sound the same when I had the idle about 1000. Does this point to something more specific?

Depending on your PCM. Some can adjust the fuel trim and injector pulse on each bank. No problem as long as the differance is not excessive.
You need a good scanner that will give you all data read out and watch the right and left long and short fuel trims.
MT