1990 chevy 5.7 tbi Idle Trouble-Learning how to use a scanner!

My pickup has developed a rough idle, it is most noticeable idling in gear with the ac running, it is less noticeable when cold! Driving down the road towing my lawncare trailer it runs great, plenty of power, no missing under load.

I purchased a bosch 1300 scanner because it can do live data with obd1, i got it hooked up and can read data while the engine is running, it does cause the motor to run a little high rpm, I am not sure what to look for really when viewing data, I just dont have access to a good idling truck to compare with!

Any help is appreciated ! I just want my truck to idle good again!

When you plug the scanner in, your idle goes high?

Chances are due to wear on the throttle shaft, you might have to tweak the idle screw.

Remove the Air Breather and look for a small circular plug on the front of the TBI unit on the throttle linkage side. Pop the plug out and use a Torx-10 to slowly turn the adjusting screw clockwise. This will open your throttle plate just a bit to improve the idle.

Make sure the IAC passages are clean if you remove the TBI unit too.

yes, it seems as if somehow the scanner causes the engine to idle higher, as i drive with it hooked up reading live data it idles higher and higher, this only happens with the scanner hooked up!

I have found that plug, it does seem like years of use has sorta worn both the throttle screw and plate that meets up with it, could a tiny amount of wear really cause this rough idle issue? I have not removed the throttle body, is there much that can be cleaned by removing it?

Put a feeler guage in between the screw and the plate, Just opening that throttle plate a few thousandths might improve the idle. Yes it can cause a rough idle.

If you run smooth off idle, you might not be getting enough idle airflow when the plate is closed.

The IAC passages are the biggest thing to be cleaned.

On the scan tool what are the IAC counts at idle? Do the IAC counts increase with TPS voltage?

I am not sure what those iac counts, I did monitor the tps to make sure it went up and down smoothly as I drove, tomorrow morning ive got to drive the truck on my mowing route, ill try and hook up the scanner and see what it says on the iac counts. Last time I watched it i did see the fuel trims dropping at idle to maybe 98 on the short term , Im wondering if that throttle plate is closed to much causing it to run rich then the computer tries to compensate by lowering the fuel trim? , but if the throttle plate is closed to much would that also limit the fuel as well?

The problem is that the TPS is reporting a minimum voltage, and the PCM believes the throttle plate to be at the correct angle. In your case it is highly likely the plate is closed a little to much meaning less air.

Less air does mean Richer mixture, and once that engine is in closed-loop, the trims might go negative Block Learn and Integrator.

This simple test to do is when the truck is 'acting' up slide a feeler gauge between the screw and the plate and see if the idle improves. If it does you need to adjust the screw a bit clockwise.

The caveat is that the min TPS voltage will rise and may have to be adjusted as well. But these old systems were good for a few millivolts before the PCM would see a demand for more fuel.

Here is a little update, I removed the plug on the idle screw, I marked my screw driver with paint, I increased it about 1/4 of a turn, I misplaced my feeler guage so i figured a 1/4 turn would do ok, on my evening drive it seemed like the truck idled a little higher but still rough! once again anything other than idle she runs great! I also sent bosch an email regarding my high idle when the scanner is hooked to the truck if the issue cant be figured out I may return it of possible and go another route for live data, If the truck wont idle like normal idk how I can use the scanner to help me!

Is there more to adjusting that idle screw than just turning it little, like does the ecm need to relearn it or re calibrate it somehow?

Is it actually running rich?
You can spray some gas near throttle plates and observe whether fuel trims still decrease trying to subtract more fuel. You can also see O2 sensor voltage raising at the beginning.

Is PCM jumping into closed loop?
You must observe O2 sensor bouncing between 0.1 and 0.9 volts (02 sensor PID). I suppose is jumping because you don´t have rough idle during engine warming up.

Is PCM getting desired RPM?
That scanner allows you to see desired and actual RPM. Let us know if there is any discrepancy between those PID's

Is IAC actually at 98 step?
You can perform a IAC relearn procedure. Also bear in ming that at steady vehicle speed (i think more than 45mph), PCM recalibrates IAC automatically. My guess is that rough idle is not related to IAC position.

You can rule out IAC issue by completely blocking off IAC bypass passage. You just put your finger on it (very slowly) and engine should idle at about 400 rpm without stall.

Is fuel pressure between specs?
Bear in mind that system fuel pressure spec is 12psi

Is Map pressure between specs?
Let us know your readings at idle and barometric pressure.

Does your engine have EGR?

I can answer a few of those questions, fuel pressure is 11psi at the throttle body, my engine does have an egr, I have made sure it seals good and does not leak at idle.

I have contacted bosch customer support, they told me the bosch 1300 scanner I bought causes issue with idle itself on these older trucks, they told that it would not be possible to change that in any way, since that is the issue im trouble shooting I boxed it up and sent it out. I am Looking at buying a cable to use with my laptop to view data that way, ive read its possible to work around the high idle issue that way.

Hello all, i am still battling my rough idle issue, i have gotten setup with a moates cable and winaldl. The truck now can be monitored while the idle issue is present.
I am wondering were to go from here! I have observed it seems like the ecm is trying to lean the mixture out at an idle!

Hello hbryant,

All my questions/suggestions were related to rough idle. That's the path i suggest you follow up to give us direction.

It's not clear until now whether is a fuel, spark, calibration or mechanical issue

One thing to check as previous reply said is mechanical. Observe Harmonic Balancer.