2000 Gmc Sierra rough idle when cold

[Kelsey92504]

I have a 2000 Sierra with a 4.8L and when its cold the first start of the day it starts up and idles high when you first start it like it supposed to and then once it warms up a little bit and idles down it acts like it wants to stall or rough idle. Less than a year ago I had the fuel system serviced and they burned all the carbon out of the motor, and I just changed the fuel filter. I was going to change the plugs and wires but I talked to another guy with the same truck and motor he was having the same problem and he said he changed his plugs,wires and fuel filter and he still had the problem. I have no engine light or code. My buddy says he is almost 100% positive it is the temp sensor. Does the 4.8L have 1 or 2 temp sensors? Any help would be greatly appericated. Thank you

[j cAT]

Originally Posted by Kelsey92504

I have a 2000 Sierra with a 4.8L and when its cold the first start of the day it starts up and idles high when you first start it like it supposed to and then once it warms up a little bit and idles down it acts like it wants to stall or rough idle. Less than a year ago I had the fuel system serviced and they burned all the carbon out of the motor, and I just changed the fuel filter. I was going to change the plugs and wires but I talked to another guy with the same truck and motor he was having the same problem and he said he changed his plugs,wires and fuel filter and he still had the problem. I have no engine light or code. My buddy says he is almost 100% positive it is the temp sensor. Does the 4.8L have 1 or 2 temp sensors? Any help would be greatly appericated. Thank you

this is usually the IAC and a dirty throttle body .... if engine starts cold normally then idle does not regulate RPM this is iac... i have cleaned my iac 3x in 111,000mi and the idle response is very much faster after cleaning.... i use wd-40 and a tooth brush and gently clean the iac surface...also clean out the passage ways in throttle body....

[Kelsey92504]

Are you talking about the mass air flow sensor? I know the iac is a idle air control, where is that located on a 4.8L? Thanks for your help. I will post back when I figure it out.

[Kelsey92504]

I checked the mass air flow sensor and the throttle body and both were spic and span clean. Is the iac the top or bottom sensor on the throttle body on a 4.8L?

[Headnsouth]

Maybe this will help.

Service Information


2000 GMC Truck GMC C Sierra - 2WD | Sierra, Silverado, Suburban, Tahoe, Yukon (VIN C/K) Service Manual | Document ID: 547252
________________________________________
Idle Air Control (IAC) System Diagnosis




Circuit Description

The vehicle control module (VCM) controls idle RPM with the idle air control (IAC) valve. To increase idle RPM, the VCM retracts the IAC pintle from the seat, allowing more air to bypass the throttle plate. To decrease RPM, it extends the IAC pintle valve in towards the seat, reducing air flow through the IAC valve port in the throttle body. A scan tool will read the VCM commands to the IAC valve in counts. The higher the counts, the more air allowed, or the higher the idle. The lower the counts, the less air allowed, or the lower the idle.

IAC Valve Reset Procedure

1. Turn ON the ignition for 20 seconds.
2. Turn OFF the ignition for 10 seconds.
3. Start the engine.
4. Check for the proper idle speed.

Diagnostic Aids

A slow, unstable, or fast idle may be caused by a non-IAC system problem that cannot be overcome by the IAC valve. The following checks should be made to repair a non-IAC system problem.
Important: If for some reason the air intake to the engine is restricted, the VCM will attempt to compensate by backing out the IAC pintle to maintain desired idle. If the engine has another source of air to the intake, the VCM will compensate by extending the IAC pintle. Scan IAC counts may reach zero as the VCM tries to maintain desired idle.
• Vacuum leak (high idle) -- If idle is too high, stop the engine. Fully extend (low) IAC with tester.
Start the engine. If idle speed is above 800 RPM, locate and correct the vacuum leak including the crankcase ventilation system. Also, check for binding of the throttle blade or linkage.
• Sticking throttle cable or throttle plate shaft
• System lean (high air/fuel ratio) -- Idle speed may be too high or too low. Engine speed may vary up and down, disconnecting the IAC does not help. This may set DTC P0171. The scan tool will read heated oxygen sensor (HO2S) output less than 300 mV (0.3 volt). Check for low regulated fuel pressure or water in the fuel.
• System rich (low air/fuel ratio) - Idle speed too low. Scan IAC counts are usually above 80. System obviously rich and may exhibit black smoke exhaust. May set DTC P0172. The scan tool will read heated oxygen sensor (HO2S) signal fixed above 800 mV (0.8 volt).
Check for high fuel pressure or a leaking or sticking injector. Silicone contaminated HO2S scan voltage will be slow to respond.
• Throttle body -- Remove the IAC valve and inspect the bore for foreign material.
• IAC valve electrical connections -- IAC valve connections should be carefully checked for proper contact.
• Crankcase ventilation valve -- An incorrect or faulty crankcase ventilation valve may result in an incorrect idle speed. Refer to Rough, Unstable, or Incorrect Idle and Stalling .
• A/C compressor -- Refer to Air Conditioning (A/C) Compressor Control Circuit Diagnosis if the circuit is shorted to ground. If the relay is faulty, an idle problem may exist. Refer to Rough, Unstable, or Incorrect Idle and Stalling .


• If intermittent poor driveability or idle symptoms are resolved by disconnecting the IAC, carefully recheck connections, valve terminal resistance, or replace the IAC.
An intermittent may be caused by any of the following conditions:
• A poor connection
• Rubbed through wire insulation
• A broken wire inside the insulation
Thoroughly inspect any circuitry that is suspected of causing the intermittent complaint. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.
If a repair is necessary, refer to Wiring Repairs or Connector Repairs in Wiring Systems.

Test Description

The numbers below refer to the step numbers on the diagnostic table.
2. This test determines if the VCM is capable of closing the IAC valve.
3. This test determines if the VCM is capable of opening the IAC valve.
4. All engine mechanical related conditions should be checked and repaired before testing the IAC system.

Idle Air Control (IAC) System Check
Step
Action Value(s) Yes No
1 Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? -- Go to Step 2
Go to the Powertrain On Board Diagnostic (OBD) System Check

2
1. Set the parking brake and block the drive wheels.
2. Turn OFF the A/C.
3. Start the engine and allow it idle in Park on an automatic transmission or Neutral on a manual transmission.
4. Install a scan tool and go to Special Functions.
5. Using RPM control in the IAC system test, command the engine speed to the first specified value.
Is the actual engine speed within the second specified value of the commanded RPM? 500 RPM
100 RPM Go to Step 3
Go to Step 4

3
Using the RPM control in the IAC system test, command the engine speed to the first specified value.
Is the actual engine speed within the second specified value of the commanded RPM? 1,200 RPM
100 RPM Go to Diagnostic Aids Go to Step 5

4
1. Check for the following:
• Vacuum leaks
• Throttle plates not closing properly
• Damaged positive crankcase ventilation (PCV) valve
• Restricted air induction system
5. Refer to Diagnostic Aids.
Did you find a problem? -- Go to Step 20
Go to Step 6

5 Check for obstructions or excessive carbon deposits in the throttle body IAC passages.
Did you find a problem? -- Go to Step 21
Go to Step 6

6 1. Turn OFF the ignition.
2. Disconnect the IAC valve harness connector.
3. Check for a poor connection at the IAC valve. Refer to Diagnostic Aids.
Did you find a problem? -- Go to Step 19
Go to Step 7

7 1. Connect the J 37027-A IAC motor driver to the IAC valve.
2. Turn OFF the A/C.
3. Set the parking brake and block the drive wheels.
4. Start the engine and allow it to idle in Park on an automatic transmission or Neutral on a manual transmission.
5. Install a scan tool and monitor engine RPM.
6. Extend and retract the IAC valve using the IAC driver.
Does the engine RPM decrease and increase as the IAC valve is cycled? -- Go to Step 8
Go to Step 22

8 Does the RPM change smoothly with each flash of the IAC driver lamp? -- Go to Step 9
Go to Step 22

9 1. Install the appropriate IAC node lamp onto the IAC valve harness connector.
2. Cycle the IAC motor driver and observe the lamps.
3. Both lamps should cycle green to red but never OFF as the RPM is changed over its range.
Are the lamps flashing red to green but never OFF? -- Go to Step 10
Go to Step 12

10 1. Remove the J 37027-A IAC motor driver.
2. Measure the resistance between the IAC motor terminals A and B with a DMM.
3. Measure the resistance between the IAC motor terminals C and D with a DMM.
Are the readings within the specified value? 40-80 ohms Go to Step 11
Go to Step 22

11 1. Measure the resistance between the IAC motor terminals B and C with a DMM.
2. Measure the resistance between the IAC motor terminals A and D with a DMM.
Are the readings within the specified value? ∞ Go to Diagnostic Aids Go to Step 22

12 1. Turn OFF the ignition.
2. Disconnect the VCM C2 connector.
3. Check for a poor connection at the VCM. Refer to Diagnostic Aids.
Did you find a problem? -- Go to Step 19
Go to Step 13

13 1. Remove the IAC node lamp.
2. Check for an open in the IAC driver circuits between the IAC valve harness connector and the VCM C2 connector.
Did you find a problem? -- Go to Step 19
Go to Step 14

14 Probe the IAC valve harness connector terminals with a test lamp connected to B+.
Is the test lamp ON at any of the terminals? -- Go to Step 17
Go to Step 15

15 1. Turn ON the ignition, leaving the engine OFF.
2. Probe the IAC valve harness connector terminals with a test lamp connected to a ground.
Is the test lamp ON at any of the terminals? -- Go to Step 18
Go to Step 16

16 Check for a short between the IAC valve circuits.
Did you find a problem? -- Go to Step 19
Go to Step 23

17 Repair the short to ground in the IAC circuit. Refer to Wiring Repairs in Wiring Systems.
Is the action complete? -- Go to Step 24
--
18 Repair the short to voltage in the IAC circuit. Refer to Wiring Repairs in Wiring Systems.
Is the action complete? -- Go to Step 24
--
19 Repair the circuit as necessary. Refer to Wiring Repairs in Wiring Systems.
Is the action complete? -- Go to Step 24
--
20 Repair the condition as necessary.
Is the action complete? -- Go to Step 24
--
21 Clean the IAC passages.
Is the action complete? -- Go to Step 24
--
22 Replace the IAC valve. Refer to Idle Air Control Valve Replacement .
Is the action complete? -- Go to Step 24
--
23 1. Replace the VCM.
2. Program the new VCM. Refer to VCM Replacement/Programming .
3. Perform the Passlock Reprogramming procedure. Refer to PASSLOCK Reprogramming Auto Learn in Theft Deterrent.
4. Perform the CKP System Variation Learn Procedure. Refer to Crankshaft Position System Variation Learn .
Is the action complete? -- Go to Step 24
--
24 1. Install the scan tool.
2. Start the engine and allow the engine to idle.
3. Turn OFF all of the accessories.
4. Command the engine speed up to the first specified value and then down to the second specified value two consecutive times with the scan tool.
Does the actual RPM closely follow the commanded RPM? 1,200 RPM
500 RPM System OK Go to Step 2

[Kelsey92504]

I unplugged the Iac and started my truck and it seem to run better. I checked it for resistance and it was reading between 40 and 80 ohms. While the truck was running with the Iac unplugged it seemed like the gas pedal was sticking a little bit. If you hit the gas the rpms would go up and slowly come down, but with it plugged in it didnt do that. With it unplugged I also got no check engine light, should it have came on? Because of the way the rpms would slowly fall with it unplugged does that mean it is dirty or need to be replaced?

[Headnsouth]

Gas pedal sticking? Check the TB butterfly for carbon build up and clean it with some TB cleaner
TB= Throttle body

[Kelsey92504]

I took out the iac, it was covered in black soot as well as the passage. I cleaned the throttle body, passage and iac with throttle body cleaner. Also replaced the pcv valve, it to was dirty. At first after I got it back together and started my truck, it was acting like the iac was bad. When you rev it up, the engine rpms would slowly fall. So I unhooked the negitive battery cable and unplugged the iac and then hooked it back up. Then re-started truck and it was idling good. Took it for a test run and it ran better, more responsive throttle. Don't really know if it did fix the cold idle, because it usually only does that in the morning or late at night. Im pretty sure it did, because u can tell a difference. Thank you guys for all your help. Very much appreciated

[Headnsouth]