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ECM/IAC - Not an ideal idle


RiggerParish
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Hello,

 

My '90 Coupe after starting shoots up to about 1600rpm and then over the course of about 5 seconds lowers to rest at approximately 1000-1100rpm. This is the lowest the idle gets to in park/neutral. Obviously, when driving it lowers somewhat but still feels high.

 

Since I've had the car I have replaced a lot of things that were original for general maintenance (and other reasons), such as: TPS, IAC, EGR, CTS, ECM, Air temp sensor, alternator and belt, air filter, battery, and all vacuum lines. It has no codes.

 

I am able to live with this high idle situation but it's not an ideal idle! I'm almost certain that the fault lies somewhere between the IAC and the computer because I can unplug the IAC, manually adjust it and bring the idle down. I have multiple IACs and the computer keeps trying to make them idle at 1000. I've traced the wires from the IAC to ECM with my meter and they are good.

 

Any ideas?

 

Thanks

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Years ago I was working with some people at GM engineering........they made a comment below. Not sure if it applies to you exact application, but it could be the cause. If possible, swap out the ignition module with a known good one. I don’t like parts changing, but it’s my understanding there is no way to test it.
 

 

 

“The ignition module has a timing and idle flair at startup........it only applies during initial start and is not adjustable or programmable.“ They even went into detail how the module had a chip in it just for this application separate from the rest of the unit. I’m trying to remember more of what they said..........maybe something like it hands over to the computer after a certain amount of RPM’s time, or other criteria. It’s been too long.

Edited by edinmass (see edit history)
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TPS reading should be .38-.42 at idle and 4.0+ at WOT.

90-91 also has diagnostics, just through HVAC control rather than CRT.

IAC command is at ED-22, if command drops and idle says up, it is probably sticking.

When replacing the IAC did you also clean any gunk out of the hole ?

Ed: didn't think you did cars of the 80s ?

Edited by padgett (see edit history)
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10 hours ago, RiggerParish said:

I can unplug the IAC, manually adjust it and bring the idle down. I have multiple IACs and the computer keeps trying to make them idle at 1000. I've traced the wires from the IAC to ECM with my meter and they are good.

Any ideas?

 

That sounds like the ECM thinks the RPM needs to be high at idle.  I agree that the TPS should be adjusted properly first.  That might be the problem.

 

There are other things that could cause ECM to raise the idle.   One of them that is often overlooked is the power steering pressure switch. It is mounted on the steering rack.  If that switch is sending a signal to the ECM that the pressure is high, the ECM will command the IAC to raise the idle slightly to compensate for the extra load on the engine. 

 

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1 hour ago, padgett said:

TPS reading should be .38-.42 at idle and 4.0+ at WOT.

90-91 also has diagnostics, just through HVAC control rather than CRT.

IAC command is at ED-22, if command drops and idle says up, it is probably sticking.

When replacing the IAC did you also clean any gunk out of the hole ?

Ed: didn't think you did cars of the 80s ?

 

 

I must admit........I always liked the Reattas..........don't tell anyone I like anything from the 80's!🤫

Edited by edinmass (see edit history)
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4 hours ago, Ronnie said:

 

That sounds like the ECM thinks the RPM needs to be high at idle.  I agree that the TPS should be adjusted properly first.  That might be the problem.

 

There are other things that could cause ECM to raise the idle.   One of them that is often overlooked is the power steering pressure switch. It is mounted on the steering rack.  If that switch is sending a signal to the ECM that the pressure is high, the ECM will command the IAC to raise the idle slightly to compensate for the extra load on the engine. 

 

Yes, what he said.

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1 hour ago, RiggerParish said:

My ED022 for IAC reads 0 when engine running and idling at 1100rpm. Is that normal?

 

I will grab another PS pressure switch. The idle lowers a touch when I turn the steering wheel so I thought it might be OK.

 

Thanks guys

If I remember right [I have never done this], it is so hard to remove the switch that the easy way out is to just jumper the pins to "trick" the car into thinking it is working.

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31 minutes ago, rogold99 said:

Maybe a small vacuum leak under the dash? Do the climate control work correctly?

 

I traced all vacuum lines back to dash and all are good and connected. The climate controls all work well.

 

3 minutes ago, DAVES89 said:

If I remember right [I have never done this], it is so hard to remove the switch that the easy way out is to just jumper the pins to "trick" the car into thinking it is working.

I saw where the switch was today and it does look like a nightmare to get at, hahaha. If it comes to it, its good to know this trick. Thanks.

 

Still wondering what the ED22 should be showing at idle.

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1100 rpm is not caused by a small vacuum leak. Intake manifold or PCV failure maybe. Tossing parts at it isn’t a good idea............time to break out the DVOM and start proving sensors..........and checking plugs and harnesses. Sometimes it just isn’t easy.

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1 hour ago, RiggerParish said:

My ED022 for IAC reads 0 when engine running and idling at 1100rpm. Is that normal?

 

No. I just started my engine. On cold start it is idling at 1125 RPM. ED022 reads 71 at 1125. I think your '90 should read about the same.

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I just went thru the same problem. Replaced all the sensors and still had the same problem.

!st make sure the TPS is set correct. Then remove the plug form the IAC and check the readings

using a voltmeter.  You can find the readings in the manual.

I had a new IAC and half of it was not working. I tested anther and found it to be good.

It solved my problem.

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2 hours ago, edinmass said:

1100 rpm is not caused by a small vacuum leak. Intake manifold or PCV failure maybe. Tossing parts at it isn’t a good idea............time to break out the DVOM and start proving sensors..........and checking plugs and harnesses. Sometimes it just isn’t easy.

Aw, this really bums me out to hear. I'm not really a mechanic, I just found this cool car cheap and wanted to get it back on the road. I dont even know what DVOM means.

 

2 hours ago, Ronnie said:

Go into ECM diagnostics -> Overrides and see if you can change the idle RPM up and down using ES09.

 

 

Another issue this car has is its reluctance to enter diagnostics. I can press OFF and UP all day and about one  or two times it will work, the other thousand times it will flash the IPC and do nothing. 

 

1 hour ago, 63viking said:

I just went thru the same problem. Replaced all the sensors and still had the same problem.

!st make sure the TPS is set correct. Then remove the plug form the IAC and check the readings

using a voltmeter.  You can find the readings in the manual.

I had a new IAC and half of it was not working. I tested anther and found it to be good.

It solved my problem.

I am on my 3rd IAC, I really hope there aren't that many defective parts out there. I have ordered a manual and I will check these readings. Thanks.

 

I am going to sort this car out if it kills it!

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"My ED022 for IAC reads 0 when engine running" this means the ECM thinks the idle is too high and is trying to lower it. This can be from a vacuum leak, sticky IAC, or throttle blades not closing all the way. Given what has gone before I'd look for a vacuum leak both under the hood and under the dash (brake switch, HVAC programmer). What is the idle vacuum ?

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10 hours ago, RiggerParish said:

Another issue this car has is its reluctance to enter diagnostics. I can press OFF and UP all day and about one  or two times it will work, the other thousand times it will flash the IPC and do nothing. 

 

10 hours ago, RiggerParish said:

I am going to sort this car out if it kills it!

 

Without being able to use the onboard diagnostics it's probably going to kill you... or at least break your pocketbook if you keep throwing parts at it.  The Reatta's onboard diagnostic system is the most important tool you can have to figure out a problem like yours. I'd recommend getting diagnostics to work properly before moving on. Maybe the buttons you are pushing are dirty. Here are instructions for cleaning them.  How To Clean 1990 Climate Control Panel

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6 hours ago, padgett said:

"My ED022 for IAC reads 0 when engine running" this means the ECM thinks the idle is too high and is trying to lower it. This can be from a vacuum leak, sticky IAC, or throttle blades not closing all the way. Given what has gone before I'd look for a vacuum leak both under the hood and under the dash (brake switch, HVAC programmer). What is the idle vacuum ?

I checked the vacuum at the HVAC but not at the brake switch, I wasn't aware of this one. The vacuum system seemed healthy, when tested, at 19 inHg.

 

4 hours ago, Ronnie said:

 

 

Without being able to use the onboard diagnostics it's probably going to kill you... or at least break your pocketbook if you keep throwing parts at it.  The Reatta's onboard diagnostic system is the most important tool you can have to figure out a problem like yours. I'd recommend getting diagnostics to work properly before moving on. Maybe the buttons you are pushing are dirty. Here are instructions for cleaning them.  How To Clean 1990 Climate Control Panel

You're not wrong, Ronnie 🙂 That will be tonight's repair job, thanks for the link. I'll report back.

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With some cleaning of the contacts the diagnostics stays on longer and I have more success accessing it, but I think it can still do with more cleaning.

 

With the car in spark and at idle this is what it was telling me.

 

TPS/ED01 - .40

CTS/ED04 - 21

Injector pulse width/ED06 - 5.3

Oxygen sensor voltage/ED07 - .50

Spark Advance/ED08 - 33

Battery voltage/ED10 - 14.5

Engine RPM/ED11 - 1087

VSS/ED12 - 0

Spk angle/ED16 - 0

OLD PA3/ED17 - 178

Oxygen sensor cross counts/ED18 - 20, 10, 6 (This value was not steady and changed between these numbers)

Fuel integrator/ED19 - 128

BLM fuel (Block learn mult)/ED20 - 128

Air Flow/ ED21 - 9.0

IAC/ED22 - 3

MAT/ED23 - 13

Ignition cycle counter/ED98 - 9

 

I'm going to pull the climate controls again to clean more as this is progress. Any glaring discrepancies?

 

Thanks

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1 hour ago, RiggerParish said:

IAC/ED22 - 3

 

Since that number is so low compared to mine when the engine is cold I believe you have extra air coming into the engine other than through the IAC. Maybe a bad manifold or some other gasket. You have cleaned the IAC opening haven't you?

 

You said you adjusted the idle and could get it down. Were you turning the screw that moves the throttle plate? If so did you adjust it back properly? It should be adjusted so the throttle plate is closed but doesn't stick closed in the throttle bore.

 

Also, you did you turn the pintel of the IAC by hand while you had it out in an effort to change the idle. If you did you could have it out of it's operating range.

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51 minutes ago, Ronnie said:

 

Since that number is so low compared to mine when the engine is cold I believe you have extra air coming into the engine other than through the IAC. Maybe a bad manifold or some other gasket. You have cleaned the IAC opening haven't you?

 

I suspected this, so a friend smoke tested the intake and any smoke was really negligible, the most i saw came from the EGR so I replaced that gasket. The first thing I did was clean the IAC opening as per the instructions on ROJ. The IAC that I messed with to bring the idle down was a junk one that I never connect. The one in the car now is new and never been adjusted by hand. When I said I was able to get the idle down a bit, it was by moving the IAC pintle by hand on that old IAC I mentioned. I am unfamiliar with adjusting the throttle plate. I have observed it when at idle and it appears fully closed.

 

I will study some literature to see how to use ES09 tonight. 👍

 

Thank you Ronnie, Padget et al, I appreciate all the time you spend helping.

 

 

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With car running IAC value is 0 so all I could do in ES09 was increase the RPM up from 1100. Could not bring it down. Coolant was at 87 so commanded RPM should be 625.

 

On 12/17/2020 at 6:27 AM, padgett said:

 Given what has gone before I'd look for a vacuum leak both under the hood and under the dash (brake switch, HVAC programmer). What is the idle vacuum ?

 

There is vacuum to a brake switch? Im looking around and I cannot find it anywhere. Is it the brakes or parking brake?

 

Thanks

 

 

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I found the hose and it looks good. I even blocked the vacuum off before the check valve by the intake and plugged it to rule out any leaks under dash and it idled the same at 1100rpm, unplugged it increased to 1300rpm. I have sprayed around all gaskets and I cannot find any leaks. I think I can rule out vacuum now as being the culprit. 

 

This is the current condition:

 

 

 

InkedVacuum_lines-3.JPG.8c2a8b259a596e06b6a782607314cb95_LI.jpg

Edited by RiggerParish (see edit history)
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I wouldn’t rule out a vacuum leak yet, with the computer commanding the IAC to zero, seems it has logic, and is trying to overcompensate. It was fun to see under hood on a car I haven’t seen in 30 years. 👍

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On 12/16/2020 at 1:02 AM, RiggerParish said:

I can unplug the IAC, manually adjust it and bring the idle down. I have multiple IACs and the computer keeps trying to make them idle at 1000.

 

On 12/16/2020 at 8:42 PM, RiggerParish said:

My ED022 for IAC reads 0 when engine running and idling at 1100rpm.

 

8 hours ago, RiggerParish said:

With car running IAC value is 0 so all I could do in ES09 was increase the RPM up from 1100. Could not bring it down.

 

I went back and read this whole thread again and the above is what stands out.  Since you can run the IAC pintel out by hand and manually bring the idle down is telling us the problem is likely caused by the operation of the IAC and probably isn't a vacuum leak... contrary to what most of us have been suggesting up to this point. Sorry I missed this the first time I read your post.

 

The fact that the ECM is reading 0 tells us it knows the idle is too high and is trying to lower it by extending the IAC pintel in as far as possible to shut off the air.

 

The fact that you can increase the RPM with ES09 but you can't lower it with ES09 tells us the ECM is able to run the IAC motor to retract the pintel (add more air) but it can't extend it to shut off the idle air. Now the question is why?

 

You said you have tried multiple IACs. And and you have traced the wiring all the way from the ECM to the IAC so it should be good.  If you are certain the connector at the IAC is good, logic tells me the problem must be caused by a bad connection where the harness plugs into the ECM or you have a bad ECM that isn't capable of sending a signal to the IAC motor to extend the pintel and shut off the idle air passageway completely.

 

You could try cleaning the ECM connectors and see if that helps. Or replace the ECM if you have a spare. Maybe someone else has a better suggestion but that is what makes sense to me based on what you have said so far.

Edited by Ronnie (see edit history)
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