Fluctuating Idle

[Phone Man]

Hello everyone. Here is a problem for ya! The idle on my truck fluctuates in drive and reverse but not in neutral or park. In gear it's steady for a short time and then slowly goes up and down and then, goes up and down faster and faster until it starts to surge and then stall.

No codes. SES light is off. Here is a list of things I have done.
Replaced:
TPS
MAF
MAP
EGR
IAC
THROTTLE BODY
COIL
TEMP SENSORD (BOTH)
OIL PRESS
CAM POS SENSOR
COMPUTER
NEW LOWER INTAKE, NEW ARP BOLTS AND FELPRO
CRANK RE-LEARN
TRANSMISSION W/NEW TC
BATTERY IS GOOD AND ALT OK

According to the GM TECH II, the engind is running fine. VSS is good as well as o2 sensors. I had a plenum spacer that I removed and put back on bc there was no difference.

The only thing I can think of is what, if any electronis are transfered over from the old transmission to the new one. I had someone else do the swap and are no longer available to talk to.

Any Ideas????

 

[retorq]

TPS and IAC adjusted properly?? Check with a vacuum gauge?? Leaks??

 

[Phone Man]

No vacumme leaks. TPS only goes on one way and IAC resets when key is turned froward. I am not aware of any adjustments for the IAC. Is there one? The idle barely drops when going from P to D

 

[retorq]

I realize TPS "only goes on one way" but if the TPS voltage at idle is off it will cause some issues as the computer will think you have your foot on the pedal all the time.

Yes there is an IAC adjustment procedure.

 

[steamroller]

No adjustments for the Vortecs

What is the idle RPM? It may need to be bumped up slightly or a little timing added in the tune.

 

[bowtiefreak]

did you ever get a custom tune? if not drop me a PM....I was a good test subjuect for the tune for the HT383

 

[Phone Man]

Hey thanks for the input. I bumped up the idle via the screw on the TB and nothing. I did elongate the holes on the TPS to adjust it and it resulted in a SES light with no improvement. I also replaced the computer with a stock tune and also have one with a BB tune.... all the same.

I didn't know there was a IAC procedure this is actually good news to me! how does it go?

I did get a BB tune but havn't been able to get anything out of it bc of this problem so I went back to a stock computer. When I get this cleared up I will get some info from you ...thanks

 

[SunlitComet]

Without Scan Tool


Start the engine for 20 seconds .
Turn OFF the ignition for 10 seconds .
Start the engine.
Check for the proper idle speed.

Badeep badeep badeep.. thats all folks.

---------- Post added at 12:04 AM ---------- Previous post was Yesterday at 11:46 PM ----------

Just thought about something. Since in only drive or reverse have you thought that may be the line pressure in tranny is in the "to damn high party" range. Bogging the engine down and fluctuating because the the drive-train is trying its darnedest to "keeping it real"?

Had this happen on a torqueflite A904 straight from rebuild once.

 

[steamroller]

^^If that's the issue there's a pressure test port above the neutral safety/gear selector switch on the side of the trans that a gauge could be hooked up to. Not sure what ''normal'' operating pressures are though

 

[SunlitComet]

Line pressures are calibrated for two sets of gear ranges—Drive-Park-Neutral and Reverse. This allows the transmission line pressure to be appropriate for different pressure needs in different gear ranges.



Before performing a line pressure check, verify that the Pressure Control (PC) solenoid for the transmission is receiving the correct electrical signal from the PCM:

Install a Scan Tool. Caution: Keep the brakes applied at all times in order to prevent unexpected vehicle motion. Personal injury may result if the vehicle moves unexpectedly. Notice: The transmission may experience harsh, soft or mushy shifts for up to two days later.
Start the engine and set the parking brake.
Check for diagnostic trouble codes, including a stored diagnostic code for the PC solenoid.
Repair the vehicle if necessary. Include the following areas:

Inspect the fluid level
Inspect the manual linkage at the transmission
Install or connect the Scan Tool.
Install or connect the oil pressure gauge at the line pressure tap.

Put the gear selector in Park and set the parking brake.
Start the engine and allow the engine to warm up at idle.
Access the PCS Control test on the Scan Tool.
Increase DESIRED PCS in 0.1 amp increments. Read the corresponding line pressure on the pressure gauge. Allow the pressure to stabilize for five seconds after each current charge.
Compare your data to the Drive-Park-Neutral line pressure table below. If your pressure readings differ greatly from the line pressure table, refer to the Diagnosis Tables. The Scan Tool is only able to control the PC solenoid in Park and Neutral with the vehicle stopped. This protects the clutches from extremely high or low pressures in Drive or Reverse ranges.



Refer to the Line Pressure table. The pressures in the table assume an engine speed of 1500 RPM and a temperature of 66°C (150°F). The pressure will vary with a change in temperature.