OK, you might want to try for the looping idle problems.
Read on if you have an idle that goes from 900 up to 1000-1200 and back.

I am learning the ecm prom code so I can make some changes when I get them figured out. The first change I want to do is drop the coolant temp down to maybe 220*. Also have tuning to do on my sons Firebird 383 stroker.

Anyway I have often asked people with idle problems if they have ever checked their ecm battery voltage (supply voltage) to see if it is changing. Everyone always blames it on vacuum leaks, egr, etc but I don't alway buy that. Not saying it is not the problem in some cases, but many vacuum leaks are constant and don't vary so they should not cause an up and down idle but will cause a fast idle.

Some have reported that adding ground wires have help their idle. Well their on the right track in may cases.

This week while going thru the code I found what follow.

In the ECM prom there is actually a section that controls idle speed depending on battery voltage.(supply to the ecm)
The one table I looked at if the voltage dropped to 8 volts the idle speed would be adjusted to 1200 rpm.(on another 1000) At 14.4v it drops back to 900 rpm. I believe the idea is if the voltage gets low then they wanted to up the speed to get more power out of the alt.
Here is a table.

Volts Idle
8.0 1000
9.6 1000
11.2 900
12.8 900
14.4 900

Anyway you can see that if the power to the ECM changes you could get into an loop where voltage drops, idle is increased, voltage comes up, idle is decreased. Sound familiar?

I have suggested to owner before that they try putting a battery charger on the car while it idles to see what happens. The idea being that it would stablize the voltage and make the hunting idle quit.
Other ideas for you to concider:
Maybe feed a stable 12v to the ecm.
Put a Volt meter on the power to the ecm.
Or try what BraveRon did and turn on your headlights, blower, whatever. In his case the motor started to die, then would increase speed to 1000 rpm. If you see an idle change with electrical load it could be this function in the ecm, or even occuring under normal load conditions.

I have never heard anyone say they knew about these entries in the prom, but they seem to be there.
(update: there is a idle post in the archive by Jazzman that did mention this in 2004. found by 3800Superfast)

We may have more issues with grounds, power, connections etc. then we think.

OK, I'll get off my soap box, give it a try and see if it does anything for your problem.

Would be interested in feedback on any fixes or outcome.

Dodgerunner

[This message has been edited by Dodgerunner (edited 12-28-2005).]

This is good info, I want to test it out, how would I supply a constant 12V to the ecm ? Un-less you just meant to make sure the battery was suppling enough? And where would I place a Volt meter at , to check the power to the ecm. Or maybe I`m over looking something here??

Bump

Related information...

https://www.fiero.nl/forum/Forum2/HTML/043240.html

[This message has been edited by Dodgerunner (edited 12-26-2005).]

I just wanted to bump and add in my $0.02. I have recently noticed that my idle is effected by my left turn signal. When the signal is on my idle goes up, when off it goes back down. I bought new bulbs to replace it with but haven't had the chance to do so yet. I also added a ground from my engine and it helped some but not with this problem. Thanks for the heads-up, it definitely motivates me to go out and replace that bulb.

You just reminded me of a situation that a friend of mine was having.
His car was doing the "bouncing idle speed" routine. It only happened when the engine was cold. He tried all sorts of things to try to resolve it, to no avail.

The problem "mysteriously" cleared up when he swapped in another battery, for an unrelated reason.
Perhaps the first battery had a problem, although it still started the car just fine.

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Raydar
88 3.4 coupe...........

Praise the Lowered!
Read Nealz Nuze!

Someone asked how they could monitor the 12v on the ecm. I am not sure how the connector on the ecm is made (yet) but would hope a person could slip a stright pin, safety pin or some thin wire into the back of one of the ecm plugs to monitor the 12v to the ecm with a meter.
I would do my checks in this order.
Steps 1 and 2 are crank tests. There there have been some post on PFF on how to does these.
1. Measure for any voltage between the battery neg. terminal and good body ground. If not a solid 0V's or <1V correct.
2. Measure across the battery Neg. to Pos. terminals while cranking the engine. The battery should be able to hold no less than 11.5-12V's. If your battery goes very much below 12 you should consider replacing your battery.
3.(All readings should be taken to a good body ground.)
Looking at the pinout of the ecm plugs I would monitor J2 - B01 (+12V Batt) or J1 - C16 +12V Batt and see what they do when varying loads on the electrical system.(Lights, blower, AC, windows, etc) Then as one additional check do J2 - A06 +12V Contact (IGN) and see that it is a good +12. Your looking for the voltage to very several volts. If it is then this idle table would be in effect and you need to find the cause of the voltage fluctuation.

Here is a link to a pinout of the connectors.

http://www.euronet.nl/users/fo_elmo/techno.htm

If vary large variations in voltage are see on these pins you will need to determine where the problem is related to battery, alt. bad connections, possible bad ignition switch.

In addition you might want to also check J2 - A12 and J1 - D1 and to see if they are at a good ground. Should be 0 v's on these pins.

I have always felt GM electrical systems where under engineered. They use lighter gauge wire then some other car companys do for many of their systems. When new the wire handles the loads OK, but as the system ages and wires move a broken strand in a wire here and there start to add up and problems occurr.

I'll update information on this post as I hear or learn more.

DR

[This message has been edited by Dodgerunner (edited 12-26-2005).]

this is why good grounds & good clean connections at the power distibution (where the fuseable links are) point is important to smooth idle. and, a newer CS alternator helps ALOT, which has much better output at idle. and its not only what the ECM does. the sensors getting fluctuating voltage will report with fluctuating voltage - most are resistance based, and report a percentage of of either 12v or 5v. this makes the problem even worse. the ECM end up chasing its tail trying to get a good idle.

Pyrthian

That is what I have been saying all along.

I'm not sure the sensors are affected as much since they are on a regulated 5v supply and if the power supply in the ecm was designed correctly should not be affected by a moderate voltage drop. But without testing can't verify that.
I figured the voltage to the sensors could be a big part of the idle problem until I found the idle/voltage table in the prom.

The ECM also changes the fuel map when voltage is too low and maybe when too high.

This is because system voltage will also affect fuel pump flow and injector function. You'll also have weak spark with low votage and could even burn up the ignition module as it works harder to generate the spark.

The easiest way to check ECM voltage is scan the ECM. Far as I know all of them report the voltage they see. This can be more important than the voltage you measure on ECM terminals because bad parts inside the ECM can result in the ECM reading the voltage wrong. If the volts you measure check out but you still have problems scan the ECM to make sure its reading the same thing as your meter.

GM and others have had multiple bulitins/articles over the years that say just this sort of thing... when tracking idle problems on computerized engines you've got to check the battery alt and all that. Computers in your house don't like crappy power, same goes for your car.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurasic Park)

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