1999 Tahoe Starting Problems

[TalPal]

My 1999 Tahoe 5.7 will start and then die repeatedly. If you leave it set after trying to start, will start in about an hour. If you start it every few hours and let the temperature come to normal it starts and runs fine. If you let it sit more than eight hours it won't start, it happens more when the weather is damp.

 

I have 59.5# fuel pressure. I have changed the Fuel Pressure Regulator, MAP Sensor, Coolant Temp Sensor, Fuel Filter, PCM, Intake Gaskets, Cap Rotor & Plugs, and The Timing Cover.

 

It does not leave any DTC's. I tested the voltage to all the sensors and they all dropped a DTC.

[98 Z71 SLT]

Ignition Coil

 

Classic complaints - what you listed...

[bobcat5544]

Pull off the distributor cap and rotor. Look at the distributor and see if there is any moisture of rust build up.

 

If it checks out good unscrew the Idle Air Control Valve/Motor on the passenger side of the throttle body make sure carbon has not built up on it. Also check it or get it check for proper operation.

 

Idle Air Control (IAC) System Diagnosis

 

Circuit Description

The control module controls idle speed with an idle air control (IAC) valve. The desired RPM is based on sensor inputs and the actual engine RPM. Four circuits are used in order to control the IAC valve, allowing a precise amount of air into the intake manifold. In order to increase the idle RPM, the control module will command the valve pintle to move away from its seat, allowing more air to bypass the throttle plate. To decrease RPM, the control module commands the IAC pintle to move closer to its seat, causing a lower airflow to bypass the throttle plate. A scan tool will display the control module commands to the IAC valve in counts. The higher the counts, the more air is allowed to bypass the throttle plate. Lower counts indicate that less air is commanded by the control module.

 

IAC Valve Reset Procedure Without A Scan Tool

Turn ON the ignition, with the engine OFF for 5 seconds.

Turn OFF the ignition for 10 seconds.

Start the engine and check for proper operation.

IAC Valve Reset Procedure With Scan Tool

Turn ON the ignition, with the engine OFF.

Install a scan tool.

Select F4 Misc. Tests.

Select F3 IAC System.

Select F1 IAC Reset.

Use up arrow to reset IAC.

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 engine. If idle speed is above 800 RPM, locate and correct vacuum leak including crankcase ventilation system. Also, check for binding of throttle blade or linkage.

System lean (high air/fuel ratio) - Idle speed may be too high or too low. Engine speed may vary up and down, disconnecting IAC does not help. 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 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 (8 volt). Check for high fuel pressure, leaking or sticking injector. Silicone contaminated HO2S scan voltage will be slow to respond.

Throttle body - Remove IAC valve and inspect bore for foreign material.

Crankcase Ventilation Valve - An incorrect or faulty crankcase ventilation valve may result in an incorrect idle speed. Refer to Symptoms .

A/C Compressor - Refer to Air Conditioning (A/C) Compressor Control Circuit Diagnosis if circuit is shorted to ground. If the relay is damaged, an idle problem may exist.

If intermittent poor driveability or idle symptoms are resolved by disconnecting the IAC, carefully recheck connections, valve terminal resistance, or replace 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 check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis in Wiring Systems.

 

If a repair is necessary, refer to Wiring Repairs or Connector Repairs in Wiring Systems.

 

Test Description

Number(s) below refer to the step number(s) on the Diagnostic Table.

 

This test determines if the VCM is capable of closing the IAC valve.

 

This test determines if the VCM is capable of opening the IAC valve.

 

All engine mechanical related conditions should be checked and repaired before testing the IAC system.

 

Step

Action

Value(s)

Yes

No

 

1

 

Important:

Before clearing the DTCs, use the scan tool Capture Info to save the Freeze Frame and Failure Records for reference. The control module's data is deleted once the Clear Info function is used.

 

 

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

Set the parking brake and block the drive wheels.

Turn OFF the A/C.

Start the engine and allow it to idle in Park (A/T) or Neutral (M/T).

Install a scan tool and go to Special Functions.

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?

1200 RPM

 

100 RPM

Go to Diagnostic Aids

Go to Step 5

 

4

Check for the following items:

 

Vacuum leaks

Throttle plates not closing properly

Damaged PCV valve

Restricted air induction system

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

Turn OFF the ignition.

Disconnect the IAC valve harness connector.

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

Connect the J 37027-A IAC motor driver to the IAC valve.

Turn OFF the A/C.

Set the parking brake and block the drive wheels.

Start the engine and allow it to idle in Park (A/T) or Neutral (M/T).

Install a scan tool and monitor the engine RPM.

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

Install the appropriate IAC node lamp onto the IAC valve harness connector.

Cycle the IAC motor driver and observe the lamps.

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

Remove the J 37027-A IAC motor driver.

Measure the resistance between the IAC motor terminals A and B with a J 39200 DMM.

Measure the resistance between the IAC motor terminals C and D with a J 39200 DMM.

Are the readings within the specified value?

40-80 ohms

Go to Step 11

Go to Step 22

 

11

Measure the resistance between the IAC motor terminals B and C with a J 39200 DMM.

Measure the resistance between the IAC motor terminals A and D with a J 39200 DMM.

Are the readings within the specified value?

infinity

Go to Diagnostic Aids

Go to Step 22

 

12

Turn OFF the ignition.

Disconnect the VCM C2 connector.

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

Remove the IAC node lamp.

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

Turn ON the ignition leaving the engine OFF.

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 valve circuits. Refer to Wiring Repairs in Wiring Systems.

 

Is the action complete?

--

Go to Step 24

--

 

18

Repair the short to voltage in the IAC valve circuits. 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 (IAC) Valve Replacement .

 

Is the action complete?

--

Go to Step 24

--

 

23

Replace the VCM.

Program the new VCM. Refer to VCM Replacement/Programming .

Perform the Passlock Reprogramming Procedure.

Perform the CKP System Variation Learn Procedure. Refer to CKP System Variation Learn Procedure .

Is the action complete?

--

Go to Step 24

--

 

24

Install the scan tool.

Start the engine and allow the engine to idle .

Turn OFF all of the accessories.

Command the engine sped 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?

1200 RPM

 

500 RPM

System OK

Go to Step 2