1992 TBI 454 4L80E. Idle hunts in Park/Neutral but is smooth in drive.

[Cheyennek1500]

Hi Everyone, I have been chasing down a idle hunting problem recently. My issue is that in park/neutral the engine idle will hunt for the a stable engine speed. This happens after the engine warms up. When I shift into drive the engine speed stabilizes. The AFR in drive is rich around 11. The AFR in park/neutral goes lean and then it catches itself and goes to stiochiometric as I can see on a wideband o2 sensor I installed. I recorded both events with TunerPro RT which is attached in the Zip file. I would appreciate some guidance of how to solve this issue. Thanks!
Vehicle Details:1992 K2500 454 TBI 4L80E (All Stock)
Fuel Pressure: 26 PSI
Base Timing: 4 BTDC
New and checked parts:
-O2 Sensor
-IAC Valve
-TPS
-CTS
-MAP Sensor
-Distributor
-Exhaust Manifolds and Y-pipe
-Fuel Pressure Regulator
-Fuel Pump and lines
-Fuel Injectors
-EGR Solenoid

 

[Wolvrine82]

When you replaced the IAC, did you clean the passage in the throttle body? They can become clogged with carbon and affect idle. Did you replace the EGR valve/verify operation? If the valve is not working correctly it will affect idle. Have you replaced the fuel filter? What about the usual tune-up items? Do you have the service manuals? If not, they are available for free download as a sticky on this website under Forums/Engine Performance and Maintenance. There is a pretty good section on diagnosing poor idle.

 

[Cheyennek1500]

Hi,
Thanks for the suggestions! The EGR is functional. The fuel filter was replaced with the fuel lines. I do have the service manuals too. I did not clean the passage for IAC I will check that.

UPDATE: My fuel pressure was too high at 26PSI for a 1992 TBI. I replaced the regulator and am at 11 PSI. My IAC counts are now 14-15 in park. I still have a fluctuation in engine speed of ~100RPM. New data files attached.

 

[Schurkey]

Does this engine have the vacuum-operated throttle kicker?

Have you ever unsealed the minimum-idle speed screw and messed with the setting?

 

[Cheyennek1500]

It does not have the vacuum-operated throttle kicker. I have not unsealed the minimum idle speed screw.

 

[Schurkey]

If you have not dicked with the minimum-idle screw, there's hope for fixing this.

Is it supposed to have the vacuum throttle kicker?

 

[Cheyennek1500]

I have not touched the idle screw and it is not suppose to have the vacuum throttle kicker.

I have done a lot of data logging and it looks like my MAP voltage in park/neutral is low (~.88/.90 volts) it is suppose to be 1-2V at idle. When I am in gear the MAP sensor voltage is within spec (~1.25 volts). I checked the sensor reading at barometric pressure and it reads 85kpa which is accurate for where I am in Colorado. I even created vacuum leaks by pulling the brake booster hose and the MAP voltage increased above 1 volt. I do no think I have a vacuum leak because of this.

My wideband O2 sensors also read lean (~16) when in park/neutral but in gear (~14.5).

From this I think I am chasing a high vacuum/lean issue. This seems counterintuitive. Thoughts?

 

[scott2093]

UPDATE: My fuel pressure was too high at 26PSI for a 1992 TBI

I thought the big blocks were 30psi?

 

[Cheyennek1500]

I thought the big blocks were 30psi?

'94 and up TBI big blocks required the higher fuel pressure

 

[Schurkey]

I have not touched the idle screw

Excellent.

and it is not suppose to have the vacuum throttle kicker.

Also, good.

I have done a lot of data logging and it looks like my MAP voltage in park/neutral is low (~.88/.90 volts) it is suppose to be 1-2V at idle. When I am in gear the MAP sensor voltage is within spec (~1.25 volts).

Easier for me to think about this in terms of actual pressure, not voltage.
IF (big IF) this chart is accurate for your MAP sensor:
www.autozone.com/znetrgs/repair_guide_content/en_us/images/0900c152/80/08/c6/ae/large/0900c1528008c6ae.gif

You must be registered for see images attach

Your idle pressure is low, meaning high vacuum.

I checked the sensor reading at barometric pressure and it reads 85kpa which is accurate for where I am in Colorado.

Fairly high altitude, I guess.

I even created vacuum leaks by pulling the brake booster hose and the MAP voltage increased above 1 volt. I do no think I have a vacuum leak because of this.

Open a huge vacuum leak, manifold pressure should go WAY up, leading to much higher MAP voltage. 3+ volts not "above 1 volt".

My wideband O2 sensors also read lean (~16) when in park/neutral but in gear (~14.5).

What is the narrow-band O2 sensor--the one the computer is reading--showing for voltage and cross-counts?