Resetting the ECU

[Treachery]

After disconnecting the battery for about 30 minutes, I unplugged the idle bypass valve on the throttle body and started the car. I then adjusted the engine rpm to 850rpm via the idle stop screw.

At this point then engine idles at 850rpm then goes up to around 1500rpm... then drops back to 850rpm... then up again to 1500... then back down to 850...

Do i let this continue until it stablizes? Or will it stablize when i plug the idle bypass valve back in?

[Jeff Chambers]

Back the idle set screw off until you can fit a .010" feeler gauge between the screw and the blade; now turn the screw in 1 full turn. This sets the proper air flow past the throttle blade in the closed position. Check your TPS at this point. It should be between .93 and .99 volts, no greater than 1.0 volt. With the IAC plugged in, start the car and let the idle stabilize. This is the proper idle setting for the car.

[82 GT]

BTW, that'a NOT the correct way to re-learn the ECU.
It's done this way:

After resetting the ECM,

1) Start the engine, and allow it to idle for 1 minute.
2) Shut it off, and wait 10 seconds.
3) Start the engine, and turn on the headlights on high, and your a/c, with the fan on high, and let it idle for 1 minute.
4) Turn all the accessories off, and turn the engine off, and wait 10 seconds.
5) Start the engine, and make it idle @ 2000 rpm for 1 minute.
6) Shut it off, and wait 10 seconds.
7) Start the engine, and turn on the headlights on high, and your a/c, with the fan on high, and make it idle @ 2000 rpm for 1 minute.
8) Turn all the accessories off, and turn the engine off, and wait 10 seconds.

This is the best way to relearn the ECM, but you should take a 10 minute test drive afterwards to make sure all is well.

[Jeff Chambers]

Yeah right, if it were that simple I'd never have to touch idle and part throttle tuning on a car.

[82 GT]

That's what PKRWUD once said on here to someone else.
He is/was one of the most informed people on this board.
His knowledge about cars is unquestionable so I say, yes, it is that simple.

[Capri306]

Calm down, guys.

Touching the stop screw was probably the first mistake, IMO. But since it's already been screwed with (haha, I made a funny), it's going to need 'resetting' to the factory position. Jeff's method is correct IIRC, straight from the service manual, for doing this. You need to be at the factory setting before tweaking anything else; this is because the throttle blade must be open enough to not bind and yet not diminish the efficacy of the IABV/IAC.

Now that it's in the proper position, the TPS voltage should be measured. I find the easiest way is to use hat pins to pierce the insulation, but hey, to each his own.

As for 'learning idle'? Please, it's a preprogrammed microcontroller there in the EEC-IV guys! It's not 'learning' anything! It's programmed with a value for the engine idle speed, and uses a series of maps and logic to carry out instructions; if the computer can't achieve that idle speed through manipulation of its outputs (actuators in this case), there is SOMETHING else causing it. My money is on a faulty IABV/IAC. I've had them go bad so many times it's not even funny. Too bad you can't just buy that stepper motor on the end of the assembly, but that's the way it goes.

[Jeff Chambers]

The PCM does learn idle corrections. In fact, it learns them very quickly. However, the idle circuit is a very complicated system and affects driveability more than any other routine(s) IMHO. The biggest mistake that I see is people trying to set the idle up using the set screw, to some idle value that they think makes the car run better....say 850 in this case. However, the PCM has an idle setting for both nuetral and drive conditions (as well as decel/coast and others) that it is always going to try and achieve. For most calibrations, this neutral idle setting is in the 640-740rpm range. The computer will do whatever is necessary to get to this setting, including removing air, fuel and spark as necessary. So setting the stop screw for 850 rpm will cause more problems than it cures. The PCM learns idle quickly, but there are limits to the adjustments that it can make.

[Capri306]

I think we're on the same wavelength, Jeff.

[GhettoPop]

just wanted to add that what i do to reset the computer is disconnect the battery and then turn the headlights on. leave like that for 20 minutes and the capacitors will be fully drained. then turn the lights off and reconnect the battery. then:

1) Start the engine, and allow it to idle for 2 minutes.
2) Shut it off, and wait 10 seconds.
3) Start the engine, and turn on all accessories (fan on high), and let it idle for 2 minutes.

note that i have noticed that after doing all this, my stang usually takes a day or two to run well as the idle will be crap until i put the car through a good 5 or 10 minute highway ride with some accelleration. guess the computer must relearn everything and it can't learn it all in 4 minutes. just my 2 cents.

[Treachery]

Thanks guys. VERY informative stuff. I think i resolved the problem just as Jeff and 82 GT advised. I used a combination of the 2 methods. And yes... I shouldn't have touched that idle stop screw in the first place! I've also replaced the idle bypass valve for good measure.

After resetting the ECU, adjusted idle & TPS, the engine still sound like it was not idling correctly. But after about 85 total miles of driving to and from work, it was as if the EEC-IV "woke up" and idle was OK.

Ok... So now i got the darn thing to idle somewhat correctly.

Now i'm noticing that engine will idle at 850rpm or so... perfect. But after a few minutes, the idle will DROP to around 700-750rpm and begin to sputter as if power was cut in half. I would then have to rev the engine a little to prevent it from sputtering and eventually stalling out.

Did i miss something?

PS- I posted new message about another weird issue. Subject: "Smoke Blowing Through The Vents!" I don't know if all these issues are related.

[Jeff Chambers]

Originally Posted by Treachery

Now i'm noticing that engine will idle at 850rpm or so... perfect. But after a few minutes, the idle will DROP to around 700-750rpm and begin to sputter as if power was cut in half. I would then have to rev the engine a little to prevent it from sputtering and eventually stalling out.

As I mentioned, the engine is trying to achieve its programmed idle setting which is 672 in nuetral while sitting still (no vehicle speed). See the attached image for the idle settings. These are just a few of the scalars, there are multiple tables and even more scalars that determine idle quality.

The computer will ALWAYS strive to reach these settings. If these settings don't provide the driveability that you're after, then you need to have a qualified tuner set you up with a custom program that does provide the idle quality, driveability and performance that you want.

[Treachery]

Very good Jeff and thank you.

During my last dynotune, i told the [SCT] tuner to set idle at 850rpm which he said that he did.

He got an extra 44hp out of the engine but the idle was still bad. He said that he set it for 850rpm but i think he did not know how to tune the idle correctly.

Assuming i'm correct, to who can i send the chip to? All i need is the idle tuned correctly. The 44hp gained from the SCT tuner chip is impressive.

[Jeff Chambers]

Idle tuning via mail order is a tough call. It may take some patience on your part while your tuner gets it right since he doesn't have the car directly in front of him to assess how it responds to the tuning changes. I've done 4-program chips with four different idle tunes, let the owner drive each program for a while, return the chip with feedback on the programs and take another shot at it. As you can see, you've got to have some patience going about it this way. Otherwise, if I've got the car in my shop, the idle/part throttle can usually be nailed within an hour, assuming there are no underlying mechanical issues.

Realize though that there is no way for a tuner to 'back out' the tuning that is on your chip. So any idle tuning done on your chip will overwrite the performance tuning. There's no way to merge the two from two different tuners. Now if you can get your old tuner to send the actual files to your new tuner, then it can be picked up from there.

[PKRWUD]

Originally Posted by 82 GT

That's what PKRWUD once said on here to someone else.
He is/was one of the most informed people on this board.
His knowledge about cars is unquestionable so I say, yes, it is that simple.

Thanks for the kind words, but allow me to elaborate; there are two different topics being discussed. What you posted, that I had told someone else, is the best method for relearning the ECM after resetting it, but that has nothing to do with setting the idle speed, which is what this thread was really about. I'm guessing the thread title is what threw you off.

On a side note, I'm kinda surprised any of the things I said in here way back when are still remembered. Thanks.