Idle Problems on 99 GMC 4.8L V8

Hello,
I am seeking some additional advice/assistance to this odd problem that I have been having recently and don't really know who to trust.
My '99 GMC 4.8L V8 has been idling very poorly lately, the engine almost never dies but goes down to maybe 200 rpms catches itself and then goes up to about 800 and this continues to fluctuate till the engine is completely warmed to normal temp. The engine does this only when the outside temp is below about 40 degrees and the colder it gets the worse the idling. Also in the cold mornings is like the engine needs to be choked. I have already changed fuel filter, plugs and wires and still this continues. I taken it to a few shops and they don’t seem to have any answers. The S.E.S. (Service Engine Soon) light has recently come on and took it to a shop and they replaced the Idle Air Control Valve, this still has not fixed the problem. One shop thought that it might be a temperature sensor for the computer or something, also a possible vacuum leak. I guess I’m leaning towards cleaning the fuel injectors as a next step.

Any help would be much Appreciated.

temp sensor sounds like a good idea to me.
if the computer was reading normal, but it was cold out, it would not supply enough fuel.
that is one possibility.

Could be 2 things. 1, the first thing that came to mind was your Temperture senor is gone. or 2, Your Mass air flow intake is very Dirty. There is a "Flap" in there that can/ does get stuck.

Thanks for your thoughts.
This morning the S.E.S light came on AGAIN and ran the codes and the it said that both banks were running lean. So I guess I go with trying to replace the temp. sensor. Does anyone happen to know where that is located?

There are a few things that stand out with that engine and idle problems..
Unfortunately my service information is taking forever to load tonight so I can't give you the info I have..
If you PM me, I'll check in the morning and hopefully I'll be able to send you some useful information

Brad,
Here is some information that may be helpful..it is a GM tech bulletin that may address your concern.
Increased Accelerator Pedal Effort, Idle Instability (Clean Throttle Body and Adjust Blade) #02-06-04-054B - (03/17/2003)
Table 1: TPS Voltage Chart
Increased Accelerator Pedal Effort, Idle Instability (Clean Throttle Body and Adjust Blade)
1999-2002 Chevrolet Silverado

2000-2002 Chevrolet Suburban, Tahoe

2002 Chevrolet Avalanche

1999-2002 GMC Sierra

2000-2002 GMC Yukon, Yukon XL

with 4.8L or 5.3L V8 Engine (VINs V, T, Z -- RPOs LR4, LM7, L59)

This bulletin is being revised to add condition information. Please discard Corporate Bulletin Number 02-06-04-054A (Section 06-Engine).

Condition
Some customers may comment on an idle instability and/or a higher than expected accelerator pedal effort from the idle position.

Cause
Condition may be caused by deposits in the throttle body bore and on the throttle plate.

Correction

Important
This procedure only applies to cable actuated throttle bodies on the models listed above. This procedure should not be performed on electronically controlled throttle bodies or on any vehicle or engine that is not listed above in the Models section.


Verify that the cause for this condition is not a damaged or binding throttle cable.
Remove the air intake duct. Refer to Air Cleaner Duct Replacement.
Inspect the vehicle for installation of a fixed orifice PCV valve, refer to Corporate Bulletin Number 01-06-01-029B for PCV valve identification. Vehicles undergoing this throttle body procedure MUST use a Fixed Orifice PCV Valve, P/N 12572717.
Clean throttle body bore and throttle valve plate of carbon using a shop rag and an appropriate cleaner. Refer to Engine Controls Repair Procedures - Throttle Body Cleaning Procedure.
Select the correct plug for the size of the air bypass hole in the throttle body being serviced. Use a 1/8-inch (.125") drill bit to determine which plug to use.
Vehicles built before introduction of the Fixed Orifice PCV Valve will have a 3.75 mm (.150") air bypass hole in the throttle plate. The 1/8-inch drill bit is smaller than the air bypass hole in these throttle bodies and can be inserted into the bypass hole. To plug the air bypass hole in these vehicles requires the yellow plug, P/N 12580749.
Vehicles built after introduction of the Fixed Orifice PCV Valve will have a 2.6 mm (.104") air bypass hole in the throttle plate. The 1/8-inch drill bit is larger than the air bypass hole in these throttle bodies and cannot be inserted into the bypass hole. To plug the air bypass hole in these vehicles requires the red plug, P/N 12581011.




Insert the appropriate plug (1) into the air bypass hole in the throttle plate. Insert the "tail" end of the plug through the throttle plate air bypass hole.



Open the throttle plate to allow access to the tail end of the plug. Pull the tail section to securely position the plug into the air bypass hole and trim excess material (1) from plug.
Turn ignition key to the on position, with engine off.
Use the Tech 2 Scan tool to read initial TPS voltage.

Important
TPS voltage cannot exceed .69 V. If the vehicle has an initial TPS voltage greater than .61 V, replace the Throttle Body Assembly. This part is currently on parts restriction. Contact the General Motors Powertrain Quality Center at 866-654-7654 for assistance.


Using the Tech 2 Scan tool and a T15 driver to rotate the Minimum Air Rate screw, increase TPS voltage by .08 Volts (Refer to TPS Voltage Chart below).
Turn ignition key to the off position. Verify that the accelerator pedal moves freely - Depress the accelerator pedal to the floor and release.
Install the air intake duct. Refer to Air Cleaner Duct Replacement.
Start engine.
Using the Tech 2 scan tool, monitor TPS voltage and verify the TPS voltage is no greater than 0.69V.
Road Test Vehicle.

Important
If an increased accelerator pedal effort condition still exists after performing this bulletin, throttle body replacement may be required. Please contact the General Motors Powertrain Quality Center at 866-654-7654 for assistance, as the throttle body is currently on a parts restriction program.


TPS Voltage Chart Initial TPS Voltage
TPS Voltage After .08 Volt Increase

0.45 V
0.53 V

0.47 V
0.55 V

0.49 V
0.57 V

0.51V
0.59 V

0.53 V
0.61 V

0.55 V
0.63 V

0.57 V
0.65 V

0.59 V
0.67 V

0.61 V
0.69 V


If initial TPS voltage is greater than .61 V Replace Throttle Body Assembly

Parts Information
Part Number
Description
Qty

12580749
Plug, Throttle Plate - Yellow
1

12581011
Plug, Throttle Plate - Red
1


Parts are currently available from GMSPO.

Also, I believe there is a calibration update for idle instability also.
Lean Hesitation, Sag or Stumble on Light/Moderate Acceleration (Reprogram PCM) #01-06-04-006 - (02/09/2001)
Lean Hesitation, Sag or Stumble On Light/Moderate Acceleration (Reprogram PCM)
1999 Chevrolet and GMC C/K Pickup Models (Silverado, Sierra)

with 4.8 L, 5.3 L or 6.0 L V8 Engine (VINs V, T, U -- RPOs LR4, LM7, LQ4)

Condition
Some customers may comment on the following condition:

A lean hesitation, sag or stumble present during light to moderate accelerations.

Usually only present on the first one or two accelerations.
The condition occurs when the coolant temperature is between -6°C and +21°C (20°F and 70°F).
Fuels with a high driveability index can also be a contributor.
Cause
Calibrations not optimized for various operating parameters.

Correction
Verify that the driveability condition is not the result of an inoperative component, or a related wiring/other condition.

As far as a lean condition..check for vacum leaks especially where the plastic intake bolts to the head...also the plastic intakes will crack and cause a leak.
Fuel pressure being a little low will also cause this.

damn!
good help.