Problem with IAC control from ECM.... - Team Camaro Tech
Troubleshooting Diagnosing problems done here.

 
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post #1 of 15 (permalink) Old Jan 28th, 04, 12:38 AM Thread Starter
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Just bought an 85 Z/28 from son-in-law. It has one problem, so far.

When cold, I have to open throttle alittle for it to start. Once started, I release the throttle, it high idles, then falls into place as it warms up for the next 90 seconds and continues to idle correctly.

It almost seems the IAC is not set for fast idle. ECM coolant sensor and its connector are new, I replaced it last week along with rebuilt ECM controller and transferred the CalMap & eprom. Runs much better after this change, but, only problem is no fast idle.

If I don't give it any throttle, it will slowly start in running mode, and then gradually work its way up to fast idle rpm.

Hot start rpm is the same, slowly works up to idle rpm from start rpm.

Thanks guys, all I have to troubleshoot with is DVM & Mac Tool Scanner #ECM-1.

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post #2 of 15 (permalink) Old Jan 28th, 04, 02:51 AM
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Everett
Sounds like the IAC air passage is blocked or restricted. Have you checked it?
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post #3 of 15 (permalink) Old Jan 28th, 04, 03:15 AM Thread Starter
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Thanks, Jim for the suggestion. IAC was replaced by the repair shop previous to my acquirement. I assume, there's the word, it was checked at time of replacement. I will check it out.

When one puts ECM into diagnostic mode, does it command:
1. the IAC to extend out?
2. EGR solenoid to energize?
3. Charcoal cannister solenoid to energize?
4. Anything else?

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post #4 of 15 (permalink) Old Jan 29th, 04, 04:09 AM
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Everett, Not sure on the Mac scanner you are using but I use a Mac Tech II and it only supports GM vehicles back to 1991. I also use a GM Tech I for the 91' & older and it's been a while but the Tech I gives you two options, Road Test (open mode) or Diagnostic (10K mode), the second one (diagnostic) defaults the engine to run at 1000 RPM's.

Couple of ideas - verify the IAC part# you have as being the correct one for that engine. I've seen different ones for the same year engine but different model car.

I believe you said the calibration (prom) was an aftermarket one in an earlier posting. I've seen problems with these.

Is the EGR valve functioning correctly?

Lastly, does that car have a mass airflow sensor?

I'll check ALLDATA later and see if anything jumps out at me that we may have over looked.

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post #5 of 15 (permalink) Old Jan 29th, 04, 08:20 AM Thread Starter
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Thanks for the comeback, drew. answers to your questions:

1. Verify the IAC part# you have as being the correct one for that engine. Yes, confirmed with GM dealer

2. I believe you said the calibration (prom) was an aftermarket one in an earlier posting. I've seen problems with these. Stock chip does the same.

3. Is the EGR valve functioning correctly? Yes, applied MiniVac to it and engine stumbled, egr reseated, ran fine.

4. Lastly, does that car have a mass airflow sensor? yes, unplug it, car dies. Leave it unplugged, engine will not start.

Idle does stay the same with Drive selected, A/C on, and lights on. But, for the life of me, it will not instantly start at a fast idle, unless I open throttle just a RCH.
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post #6 of 15 (permalink) Old Jan 29th, 04, 12:29 PM
 
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maybe check all around the air inlet for cracks, affecting the MAF? and wiggle it around to see if the idle stumbles or changes....and check for vacuum leaks?

is there a special procedure for setting the IAC on that vehicle? it may have to be cleared and reset if it wasn't set properly the first time around.

just throwing things out there to see if they stick. good luck.
you may have to find someone with a scanner to tell you all of the inputs during the problem. ick. 'specially with no code thrown. ick again.
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post #7 of 15 (permalink) Old Jan 29th, 04, 12:33 PM
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Have you tried this? Unplug and remove the IAC. Hold the IAC in your hand and plug it back into the harness. Then put the ECM into diagnostic mode by connecting your scan tool or shorting the upper right two pins on the diagnostic connector. (This shorting trick doesn't work on all generations but it should work on yours). Then have your helper turn the key on - no need to start. You should see the pintle on the IAC fully extend and then retract. If you don't then there is a wiring problem or ECM problem.

Also, there is an adjustment procedure for the throttle position sensor. I wouldn't think that would contribute to your problem, but it might. I'd recommend searching www.thirdgen.org for specifics. Basically, you have to preset the IAC motor and unplug it. Then you start the engine and adjust the idle speed stop until a specified RPM is obtained. Then you losen the screws on the TPS sensor and adjust it so a specific voltage is obtained on the TPS signal line. It would be good to ensure the TPS is calibrated before going much farther.
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post #8 of 15 (permalink) Old Jan 29th, 04, 07:16 PM
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could be a long shot .. but it sounds like the throttle plates are closed to far. Yes I know they arent adjustable (fuel injection) TPI or TBI but look around the throttle body linkage where the (stop) tab for the linkage rests on the aluminum body you may notice a screw sticking out, the end will be facing you standing in front of the car. look behind it and you will see its capped off, remove cap, turn it in a quarter turn only it will open up the plates just enough to get enough air in at idle, you want to do this because over the years and high mileage starts to wear out the screw (stop) this happens everytime the throttle is released, it takes years to happen.I tried to be as clear as I could, a pic is worth a thousand words right about now!


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post #9 of 15 (permalink) Old Jan 30th, 04, 12:49 AM Thread Starter
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Good suggestions from all of you. dnult: 1. TPS voltage has been set alittle high, 600 mvolts, this makes it run alittle smoother at idle than at 500 mvolts. There is no holes in the TPS when slowly opening the throttle while measuring ohms. 2. IAC should extend AND return, On other vehicles, it extends, but, never returned. No wonder it idled at 2500 rpm when IAC was unhooked.

ohcscott, I have done everything except take a hammer to the MAF meter short of destroying it....LOL. I filled the bellows with water and it held it, not while it was on the engine, but, on the table.

late68, I have the throttle plates opened up to keep the idle at least from falling to a min 600 rpm. Before it would die.

To all, I have unplugged the IAC connector and looked with a DVM, the signals coming from the ECM for controlling the IAC. They are constantly changing across their respective coil, ie, A-B & C-D.

I could always put a manifold & carb onto it with an older HEI......Thanks for your help. Depending upon weather, I'll check it out this weekend.

Oh, I forgot...GO PATRIOTS !!!!!!
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post #10 of 15 (permalink) Old Jan 30th, 04, 01:43 AM
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Everett, I looked on ALLDATA and saw one bullentin that may affect you.
#85-183 1985 Camaros with LB9 5.0 V-8 Engines (Vin code F) - this is the fuel injected motor.
Subject
Prom change to correct TCC engagement speeds, hesitation on acceleration during cold drive away & repeat incidents of code 34 (mass airflow sensor).
A new service prom # 1227739 with A/C and # 1227740 without A/C has been released for 85' Camaros 5.0 TPI engines.
This new prom will raise the TCC engagement point to 41 mph and eliminate hesitation on acceleration during cold driveaway.
Minimum idle speed should be reset to 500 RPM as descibed in the 1985 Shop Manual, Page 6E3-145.
At the bottom there is a Notice: These service PROMS contain calibration changes designed to enhance vehicle driveability. Engine idle speed will be raised to 650 RPM's when the IAC motor is connected, and the TCC engagement will be raised to 41 mph.

Dave mentioned checking the TPS to see if it's adjustable. With key on engine off you can back probe the wires with your DVOM and see the output voltage, should be about .5 volts. One wire is 5 volts, one is ECM ground, and the other is output voltage. Could try raising output to no more than .8 volts and see if that affects a change.

To increase idle speed the ECM retracts the IAC motor and allows more airflow through the idle passage and bypass the throttle plates until idle speed reaches the correct RPM's. This will be seen on a scanner as increased "counts". Each time the engine is started and then turned off the ECM will reset the IAC valve. This is done by sending enough counts to seat the valve. The fully seated valve is the ECM reference zero. A given number of counts are then issued to open the valve and normal ECM control of the IAC will begin from this point. The number of counts are then calculated by the ECM. This is how the ECM "knows" what the motor position is for a given idle speed. The ECM uses the following information to control idle speed:
Battery voltage.
Coolant Temperature
Mass Air flow
Throttle Position Sensor
Engine speed
A/C clutch signal
Power Steering Pressue Switch (on a 2.8L engine)

If the IAC connector is removed with the engine running the idle set RPM may be lost. To reset idle RPM turn ignition ON, then OFF.

Hope this is some help. I'll look and see what the procedure is for setting minimum idle speed to 500 rpm's. ALLDATA didn't have the procedure for minimum idle speed on a fuel injected engine. Suggest the library for the GM service manual unless someone here has access to one.

One last thing, does the scanner you have show what gear the car is in? This is also an input to the ECM for controling idle speed. This would be the Park/Neutral switch.

[ 01-30-2004, 07:08 AM: Message edited by: drew69 ]

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post #11 of 15 (permalink) Old Jan 30th, 04, 05:58 AM Thread Starter
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Thanks drew for the info. Will take it all into consideration. Here's what I believe has happened. Car was/is an 85 w/TPI, VIN F = 305.

Previous owner before my son-in-law, replaced 305 w/89 Corvette 350 TPI. But, they kept the 305 ECM rather than change out to the new box due to adding two wires and repinning ECM harness to match 89 ECM as 86 was changeover to new ECM, no removeable prom.

Therefore, I believe I may have to get a 350 TPI chip, Corvette maybe? This intake manifold does have a Bosch cold start switch, 40 watts ON for 8 seconds, next the the ECM coolant sensor in the thermostat chamber. Camaro's & Firebird's wiring diagrams do not show this switch. But, there is a connector on it and it does go to the firewall.

Yes, I will check manual for procedure on adjusting base rpm and hopefully, it will learn.

Scan tool is a Mac Tool P/N ECM-1 is a Code Card with A-B jumper on card and codes on reverse side, my thinking, and a DVM......LOL
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post #12 of 15 (permalink) Old Feb 2nd, 04, 12:41 AM Thread Starter
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Bring everybody up-to-date, I found it had the wrong spark plugs. Some ACDelco's with a 3/4 inch reach.

I looked at the heads to make sure they were SBC, cause I don't remember SBC using anything but 3/8 inch length. So, some new Autolites went in. What a pain in the behind to change. I can see I'll be cutting an access hole in the wheelhousing for number 8 plug.

I went and applied ECM diagnostic mode with the IAC out of car. I didn't have an assistant, so I lost the pintle and spring. Found the pintle, spring must be somewhere in NC, I never found it. I bought a new one from Advance Auto. Installed same and now I'm able to back off the 'curb idle' setting of the throttle plates.

Advanced base timing a couple degrees, sounds alittle better.

All in all it does run better, but, I think I have a faulty injector. When the engine is really hot, 200*F, it has a miss. I unplug this one injector, and no change from rpm. I swap injector harness with the next cylinder, and no change, ie, #4 harness onto #2 injector, #2 still works. Swap back to original set-up, #2 w/h on #2 inj, #4 w/h on #4 inj, #4 cyl is dead. I am running a can of Techron cleaner thru the tank.

I did this because the Mitchell CD repair schematic shows the odd number injectors are powered from one source of ECM, and even injectors are powered from another ECM source, batch fire(?) if you will. All even inj's get fired once and all odd inj's get fired separately, regardless what cylinder is where.

There ya go.....
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post #13 of 15 (permalink) Old Feb 2nd, 04, 06:08 AM
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[QUOTE]Originally posted by Everett#2390:


Previous owner before my son-in-law, replaced 305 w/89 Corvette 350 TPI. But, they kept the 305 ECM rather than change out to the new box due to adding two wires and repinning ECM harness to match 89 ECM as 86 was changeover to new ECM, no removeable prom.

I agree, this is most of the problem with the battle your fighting. At least your making some headway. Good luck.

Drew<br />69' X-11 Coupe
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post #14 of 15 (permalink) Old Feb 3rd, 04, 12:23 AM Thread Starter
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Yes, definitely a battle. But hey, so is life and it goes on. Started the car last might before dark and cranked right up. No fast idle when cold, but, it ran well. Did not die when put under a load.

Injector came back to life, maybe the cleaner is starting to take effect. I didn't let it get very hot, ie, 200*F, dinner was on the table. I did advance timing more, 4*B to 10*B and an improvement overall. Doesn't sound as lopey, idled smooth and very responsive.

I have found it does not have 89 heads. It has 68-72 heads, spark plugs require a sealing washer, but, heads have accessory holes. Could be a good set of 492's, but I'm wishing.

Thanks drew for your help.
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post #15 of 15 (permalink) Old Feb 3rd, 04, 03:18 PM
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The lost pintle and spring reminds me of a funny story. An honest friend of mine recalls a day when the hot-shot of the shop went over to show a lady the IAC that needed replacement. In the process of the demonstration, somehow the pintle and spring shot out and went down the front of her dress. John said that everyone kind of covered their faces to laugh and scampered off. [img]smile.gif[/img] Poor smuck.

[img]graemlins/beers.gif[/img]

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