Closing in on a problem

[lionelhutz]

But in neutral with no engine load wouldn't the rpm rise unless the IAC motor closed more, not opened more???

At any rate, I would try closing the throttle blades more and see what happens.

[EagleMark]

But in neutral with no engine load wouldn't the rpm rise unless the IAC motor closed more, not opened more??? .

Your right, lower IAC... never let go to 0 IAC counts though.

[1great40]

Lionel, I'm not having a problem with a high idle speed, the idle is too low. Right now the IAC numbers are around 12 in park and a little higher in drive. From what I have observed, I want to have the largest quantity of the idle air coming by the throttle plate and a minimum admitted through the IAC without the IAC ever going to zero. When the throttle on a TBI engine is closed too much, it will still idle by virtue of the IAC admitting air but that air then has to mix with a dribble if fuel dripping past a closed throttle plate, not the best scenario for getting good vaporization. The more fuel spray that gets by the throttle plates, the better it mixes with the air.
So, the minimum air setting seems to be just fine, the ECM commands a high idle when the engine starts, the engine also goes to the correct speed when in 10K mode. As of now I'm pretty confident that the IAC is working, the ECM is working and the last piece of the puzzle is to use TunerPro to see what the actual values for idle speed are in the PROM. The stock PROM seems to make the engine idle at 600 RPM. The Prom that I'm using now that EagleMark made for me is supposed to have the idle set to 725 but with this PROM, the engine idles at about 550. First chance I get, I'm going to fire up the truck and look at the values. I understand there are many idle speed values for different conditions so, I'm at the beginning of another learning curve. Hopefully, this will be as simple as getting the PROM re-programmed but if the values are correct and the speed is still not right, I don't know where to look next!

[EagleMark]

Idle RPM is not accurate on this ECM, so at this point it needs to read higher. I need to send another chip with an additional 100 RPM. But until I get home I don't know of anoter way.

[1great40]

EagleMark, do you need me to mail the other chip to you?

[EagleMark]

Problem is I have been in hospital and rehabilitation since I did your chip and have no way to burn and mail another. But have someone who will, just need to contact him.

[1great40]

It's been a few weeks since I last posted. Eagle Mark said that the speed control on the ECM that I'm using is not too precise and that another bump in the idle speed would be in order. That got me to thinking. Once the truck is in gear if I'm stopped and I crack the throttle with my foot, and get the engine up to about 675 to 700 RPM, everything feels fine.

From what I understand, the ECM looks at the TPS voltage when the engine is started and uses that value as the idle TPS voltage. If you fiddle with the screw you can raise the idle speed, only for that one start/run cycle. If you stop and restart the engine after moving the screw, the ECM looks at the "new" TPS voltage and calls that the idle TPS voltage. So, once the minimum air is set, you can't play with it any more to try to set the idle speed.
But, once the engine is running, any movement of the throttle shaft has a corresponding change in the TPS voltage so the ECM doesn't try to force the engine speed, you're doing it with your foot.

My next step was to make a secondary idle stop so that I could "fake" the idle speed. I made a small block of aluminum with a second idle stop screw and attached it to the throttle body, right next to the minimum air screw. Once the engine is started and the ECM knows the idle TPS voltage, I could raise the idle speed with my new "idle stop screw". I could set the idle speed manually and it would return to the same speed every time the vehicle came to a stop. I would let the engine idle for several minutes in gear to see if the ECM tried to undo what I was trying to accomplish. Fortunately, I faked out the computer.

Obviously, it's highly impractical to back the screw off every time you shut off the engine and then reset it once the engine is restarted, but I did prove that the computer can be faked out and the idle speed can be set manually, at any speed I needed.

The next step is to automate this process. I ordered a continuous duty 12V "push" type solenoid from McMaster-Carr industrial supply. The solenoid has 30 oz. of force. I fabricated a bracket that attached the solenoid to the throttle body assembly. The actuating rod of the solenoid was set up to push on the boss on the throttle arm where the throttle cable attaches. I machined an eccentric cam that fit over the boss of the throttle arm and locked in place with a set screw. Rotating the cam with the solenoid actuated opens or closes the throttle plate. I have actuated the solenoid by connecting it to the battery and it moves the throttle no problem. Once the correct speed is obtained, the set screw can be locked.

My plan is to utilize the P/N switch that currently grounds the B-10 line to the ECM in park. I will add a SPDT relay and tie one side of the relay coil to a 12V signal that is switched with the ignition. When power is applied to the relay and the transmission is in park, the relay will actuate.
The B-10 line to the ECM will be connected through the normally open contacts to ground, so as soon as the ignition switch is turned on, the contacts ground the B-10 line to the ECM, just like a normal setup. Once the transmission is shifted out of park, the ground to the relay coil goes away, and the B-10 line is ungrounded, just like a normal set up. The other set of contacts in the relay now close and apply power to the idle stop solenoid, kicking up the idle when the truck is in gear.

As of now,the solenoid and the mechanical part of this set up is working. The next step is the wiring. Stay Tuned.