1997 Chevy Truck 1997 Chevy Z-71 engine vacuum problem -

  • V8
  • 4WD
  • 157,800 MILES
Have a 1997 5.7 liter V8 stepside pickup. Symptoms include high idle while running down the road, 40mph and release gas pedal and truck very slowly lowers in speed unless brakes depressed then it slows the engine idle down. examination shows a vacuum line port with no hose and I cannot determine what (if anything) should be in there? I have never replaced the MAF or serviced it in the last 6 years that I know. Dont know where to start or where to get a diagram to check the hose issue. Is there instructions on cleaning or replacing the MAF, or replacing the thermal sensor they speak of myself? Hard to justify another expensive dealer visit for this old but dependable truck. Will ADD a tip and another donation for a good advised solution - first time user. I have a picture to upload. [added: ALSO there is pressure in gas tank when gas cap is removed, hisses significantly when opened to fill - maybe related??



Do you
have the same problem?
Sunday, August 1st, 2010 AT 2:39 PM

3 Replies

There is no hose that goes to that solenoid, it is a vent. The solenoid you have pictured is the evap purge vacuum switch, I believe the one next to it with the red connector is the evap purge solenoid. Here is a brief description of its purpose. A vacuum switch in the purge line detects when the system is purging. The normally closed switch will open when less than 1 inch Hg is present in the purge line.

Pressure in the gas tank is not necessarily an indication of a problem, it can be considered normal. The MAF can be cleaned by spraying the hot wires with an electronics, or contact cleaner aerosol. If you had an active vacuum leak, you should have an engine light and corresponding lean fuel trim codes
Was this
Monday, August 2nd, 2010 AT 10:49 PM
So you are syaing that a vacuum leak is probably NOT this issue and not causing the high idle situation?
It does not seem to be a sticky throttle cable or anything - once vehicle is slowed to a stop at a light, the engine slows down with it. Perplexing.

I see in another post there is an idle controller. With no codes present, is there a way to test that unit? So far, I have not located it. It must be deep inside the black plastic top works.

I do not see any idle adjustment screw (you are probably LOL)

Thanks Coconut Chuck
Was this
Tuesday, August 3rd, 2010 AT 6:56 AM
There is an idle air control valve on the RH rear of the throttle body. Aside from replacement, the only other thing is to remove it and clean the passage. It can develop deposits. It's wire colors are green/white, green/black, blue white, and blue/black. There is a service procedure for rough, unstable, or incorrect idle. It is pretty generic in nature. It is included.

Rough, Unstable, or Incorrect Idle and Stalling

Definition: The engine runs unevenly at idle. If severe enough, the engine or vehicle may shake. The engine idle speed may vary in RPM. Either condition may be severe enough to stall the engine.

Preliminary Check
Refer to Important Preliminary Checks.

Sensor Checks
" Check the Heated Oxygen Sensor (HO2S).

" Check for silicon contamination from fuel or improperly used sealant. The sensor will have a white powdery coating. The sensor will result in a high but false signal voltage (rich exhaust indication). The Control Module will reduce the amount of fuel delivered to the engine causing a severe driveability problem.

- Refer to DTC P0131 HO2S Circuit Low Voltage Bank 1 Sensor 1.

- Refer to DTC P0132 HO2S Circuit High Voltage Bank 1 Sensor 1.

- Refer to DTC P0133 HO2S Slow Response Bank 1 Sensor 1.

- Refer to DTC P0134 HO2S Circuit Insufficient Activity Bank 1 Sensor 1.

- Refer to DTC P0135 HO2S Heater Performance Bank 1 Sensor 1.

- Refer to DTC P0137 HO2S Circuit Low Voltage Bank 1 Sensor 2.

- Refer to DTC P0138 HO2S Circuit High Voltage Bank 1 Sensor 2.

- Refer to DTC P0140 HO2S Circuit Insufficient Activity Bank 1 Sensor 2.

- Refer to DTC P0141 HO2S Heater Performance Bank 1 Sensor 2.

" Check the Throttle Position (TP) sensor. If a sticking throttle shaft or binding linkage causes a high TP sensor open throttle indication, the Control Module will not control the idle. Monitor the TP sensor voltage. A scan tool and/or voltmeter should read less than.85 volts with the throttle closed.

" Check the Engine Coolant Temperature (ECT) sensor using the scan tool to compare the engine coolant temperature with the ambient air temperature on a cold engine. If the coolant temperature reading is more than 5 degrees greater than or less than the ambient air temperature on a cold engine, check for a high resistance in the coolant sensor circuit or the sensor itself. Refer to DTC P1115 Engine Coolant Temperature (ECT) Sensor Circuit Intermittent High Voltage Diagnostic Aids or DTC P0118 Engine Coolant Temperature (ECT) Sensor Circuit High Voltage Diagnostic Aids.

" Check the Manifold Absolute Pressure (MAP) sensor response and accuracy. Refer to DTC P0106 Manifold Absolute Pressure (MAP) System Performance.

Fuel System Checks
" Check to determine if a Rich or Lean system causes the condition. Drive the vehicle at the speed of the complaint. Monitoring the Fuel Trim will help identify the problem.

- Lean - The Long Term Fuel Trim is greater than 150. Refer to DTC P0132 HO2S Circuit High Voltage Bank 1 Sensor 1 Diagnostic Aids. Refer to DTC P0138 HO2S Circuit High Voltage Bank 1 Sensor 2 Diagnostic Aids.

- Rich - The Long Term Fuel Trim is less than 115. Refer to DTC P0131 HO2S Circuit Low Voltage Bank 1 Sensor 1 Diagnostic Aids. Refer to DTC P0137 HO2S Circuit Low Voltage Bank 1 Sensor 2 Diagnostic Aids.

" Check the fuel injector driver circuit.

- Disconnect the injector harness connector at the injectors.

- Connect an injector test light between the terminals of each injector connector and note the light while cranking.

- If the test light fails to blink at any connector, it is a faulty injector drive circuit harness, connector, or terminal.

" Perform the fuel injector coil test. Refer to Fuel Injector Solenoid Coil Test.

" Perform the fuel injector balance test. Refer to Fuel Injector Balance Test for the 5.0L, 5.7L. Refer to Fuel Injector Balance Test for the 7.4L.

" Check for fuel in the pressure regulator vacuum hose. If fuel is present, replace the regulator assembly.

" Check the Evaporative Emission (EVAP) control system. Refer to Evaporative Emission Control System Diagnosis.

" Perform a cylinder compression test. Refer to Engine Mechanical.

" Check for leaking fuel injectors. Refer to Injector Circuit Diagnosis

" Check the fuel pressure. Refer to Fuel System Diagnosis.

" Check the scan tool for an injector fault which could indicate one or more faulty injectors or wiring.

Ignition System Checks
" Check the ignition output voltage using the spark tester J 26792 or the equivalent. Refer to Enhanced Ignition System

" Check for the following conditions:

- Wet plugs

- Cracks

- Wear

- Improper gap

- Burned electrodes

- Blistered insulators

- Heavy deposits

" Check the spark plug wires by connecting an ohmmeter to the ends of each wire in question. If the meter reads over 30,000 ohms, replace the wires.

Additional Checks
" Check for vacuum leaks. Vacuum leaks can cause a higher than normal idle and low Idle Air Control (IAC) counts.

" Check the IAC operation. Refer to DTC P1121 Throttle Position (TP) Sensor Circuit Intermittent High Voltage. Refer to DTC P0507 Idle Speed High. Refer to DTC P1509 Idle Speed High - Idle Air Control (IAC) System Not Responding.

" Check the Control Module grounds for being clean, tight, and in their proper locations. Refer to Engine Controls Component Location for the 5.0L, 5.7L, 7.4L.

" Check the Automatic Transmission Fluid Pressure (TPF) manual valve indicator. Refer to Transmission Fluid Pressure Switch Assembly.

" Check the scan tool to determine if the Control Module is receiving an A/C signal. Refer to Air Conditioning (A/C) Compressor Clutch Control Diagnosis. If a problem exists with the A/C ON, check the A/C system operation. Refer to Section 1B of the appropriate service manual.

" Check for the EGR being ON while idling which will cause roughness, stalling, and hard starting. Refer to Exhaust Gas Recirculation (EGR) System Diagnosis.

" Check the battery cables and ground straps. They should be clean and secure. Erratic voltage will cause the IAC to change its position resulting in poor idle quality.

" Check the IAC Valve. The IAC will not move if the system voltage is not within 9 to 16 volts.

" Check the A/C refrigerant pressure for being too high or for a faulty high pressure switch.

" Check the Crankcase Ventilation Valve for proper operation by placing a finger over the inlet hole in the valve end several times. The valve should snap back. If not, replace the valve. Refer to Crankcase Ventilation System.

Engine Mechanical Check
Check for the following:

" Broken motor mounts.

" Improper valve timing

" Low compression

" Bent pushrods

" Worn rocker arms

" Broken or weak valve springs.

" Worn camshaft lobes

" Refer to Engine Mechanical.
Was this
Wednesday, August 4th, 2010 AT 10:55 PM

Please login or register to post a reply.

Sponsored links