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Mike S.
Mike S., Chevy Mechanic
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I have a 1991 chevy 2500 p/u with a 5.7 and throttle body.

Customer Question

I have a 1991 chevy 2500 p/u with a 5.7 and throttle body. When I got the truck it had an exhaust leak I got it fixed. It also has a bad head in the air conditioner.
The problem that I am having is that it runs fine until it warms up and then when at a stop the idling becomes rough and then when you go to take off it is almost like it is trying to flood itself out. It never does die. Also, it does this when going slow like when I am in town. What could be causing this? Does this vehicle have a MAF sensor?
Submitted: 5 years ago.
Category: Chevy
Expert:  Mike S. replied 5 years ago.

Mass Air Flow Sensor

1991 Chevrolet Truck C2500 3/4ton P/U 2WD 5.7L TBI 8cyl

This vehicle is not equipped with selected component.

 

It does have a MAP sensor though.

 

Causes of a rough idle.

Improperly Functioning or Faulty EGR Valve.

 

Causes of erratic idle.

Priority Action Part Type Cause
1 Inspect M.A.P. Sensor Faulty M.A.P. Sensor.
2 Inspect EGR Valve Improperly Functioning or Faulty EGR Valve.
3 Inspect Fuel Injector Dirty or Worn Fuel Injectors.
4 Inspect Idle Air Control Valve Damaged, Loose, or Faulty Idle Air Control Valve.
5 Inspect Idle Speed Actuator Damaged, Loose, or Faulty Idle Speed Actuator.
6 Inspect Throttle Position Sensor Faulty or Incorrectly Connected Throttle Position Sensor.
7 Inspect Fuel Filter Clogged or Dirty Fuel Filter.
8 Inspect PCV Valve Plugged or Damaged PCV Valve.
09 Inspect Valve Burned, Worn, or Sticking Exhaust Valves.

 

Testing.

Manifold Absolute Pressure (MAP) Sensor

OPERATION



See Figures 1 and 2

The Manifold Absolute Pressure (MAP) sensor measures the changes in intake manifold pressure, which result from the engine load and speed changes, and converts this to a voltage output.

A closed throttle on engine coastdown will produce a low MAP output, while a wide-open throttle will produce a high output. This high output is produced because the pressure inside the manifold is the same as outside the manifold, so 100 percent of the outside air pressure is measured.

The MAP sensor reading is the opposite of what you would measure on a vacuum gauge. When manifold pressure is high, vacuum is low. The MAP sensor is also used to measure barometric pressure under certain conditions, which allows the ECM to automatically adjust for different altitudes.

The ECM sends a 5 volt reference signal to the MAP sensor. As the manifold pressure changes, the electrical resistance of the sensor also changes. By monitoring the sensor output voltage, the ECM knows the manifold pressure. A higher pressure, low vacuum (high voltage) requires more fuel, while a lower pressure, higher vacuum (low voltage) requires less fuel.

The ECM uses the MAP sensor to control fuel delivery and ignition timing.



Click image to see an enlarged view

Fig. Fig. 1: Common Manifold Absolute Pressure (MAP) sensor used on 4.3L, 5.0L and 5.7L engines



Click image to see an enlarged view

Fig. Fig. 2: Common Manifold Absolute Pressure (MAP) sensor used on 7.4L engines

TESTING



See Figures 3, 4 and 5

  1. Backprobe with a high impedance voltmeter at MAP sensor terminals A and C.
  2. With the key ON and engine off, the voltmeter reading should be approximately 5.0 volts.
  3. If the voltage is not as specified, either the wiring to the MAP sensor or the ECM may be faulty. Correct any wiring or ECM faults before continuing test.
  4. Backprobe with the high impotence voltmeter at MAP sensor terminals B and A.
  5. Verify that the sensor voltage is approximately 0.5 volts with the engine not running (at sea level).
  6. Record MAP sensor voltage with the key ON and engine off.
  7. Start the vehicle.
  8. Verify that the sensor voltage is greater than 1.5 volts (above the recorded reading) at idle.
  9. Verify that the sensor voltage increases to approximately 4.5. volts (above the recorded reading) at Wide Open Throttle (WOT).
  10. If the sensor voltage is as specified, the sensor is functioning properly.
  11. If the sensor voltage is not as specified, check the sensor and the sensor vacuum source for a leak or a restriction. If no leaks or restrictions are found, the sensor may be defective and should be replaced.




Click image to see an enlarged view

Fig. Fig. 3: Location of the MAP sensor-TBI system shown



Click image to see an enlarged view

Fig. Fig. 4: Probe the terminals of the MAP sensor to check for proper reference voltage



Click image to see an enlarged view

Fig. Fig. 5: Manifold Absolute Pressure (MAP) sensor wiring diagram

 

The EGR Valve is located:

Under hood, center, upper engine area, mounted in rear of intake manifold

Exhaust Gas Recirculation (EGR)

OPERATION



To lower the formation of nitrogen oxides (NOx) in the exhaust, it is necessary to reduce combustion temperatures. This is done in the diesel, as in the gasoline engine, by introducing exhaust gases into the cylinders through the EGR valve.

The Exhaust Pressure Regulator (EPR) valve and solenoid operate in conjunction with the EGR valve. The EPR valve's job is to increase exhaust backpressure in order to increase EGR flow. The EPR valve is usually open, and the solenoid is normally closed. When energized by the B+ wire from the Throttle Position Switch (TPS), the solenoid opens, allowing vacuum to the EPR valve, closing it. This occurs at idle. As the throttle is opened, at a calibrated throttle angle, the TPS de-energizes the EPR solenoid, cutting off vacuum to the EPR valve, closing the valve. Two other solenoids are used for EGR valve control. The EGR solenoid allows vacuum to reach the EGR vent solenoid under certain conditions. The vent solenoid then controls the EGR valve to regulate the flow of gasses into the intake manifold.

TESTING



Exhaust Gas Recirculation (EGR) Valve

See Figure 1

Apply vacuum to the EGR valve with a hand vacuum pump. The valve should be fully open at 11 in. Hg (75 kPa) and closed below 6 in. Hg (41 kPa).



Click image to see an enlarged view

Fig. Fig. 1: Some EGR valves may be tested using a vacuum pump by watching for diaphragm movement

EPR Valve

  1. Apply 11 in. Hg (75 kPa) vacuum to the EPR valve tube with a hand vacuum pump. Observe the valve actuator lever for movement.
  2. If it does not move, spray a penetrating lubricant on the lever and try to free the valve.



CAUTION Make sure the valve is not hot.

  1. If the lubricant will not free the valve, it must be replaced.

Fuel injectors, look for signs of them dripping.

 

Idle Air Control (IAC) Valve

OPERATION



See Figure 1

The engine idle speed is controlled by the ECM through the Idle Air Control (IAC) valve mounted on the throttle body. The ECM sends voltage pulses to the IAC motor causing the IAC motor shaft and pintle to move in or out a given distance (number of steps) for each pulse, (called counts).

This movement controls air flow around the throttle plate, which in turn, controls engine idle speed, either cold or hot. IAC valve pintle position counts can be seen using a scan tool. Zero counts corresponds to a fully closed passage, while 140 or more counts (depending on the application) corresponds to full flow.



Click image to see an enlarged view

Fig. Fig. 1: The IAC valve can be on the throttle body, usually next to the throttle position sensor

TESTING



See Figures 2, 3 and 4

  1. Disengage the IAC electrical connector.
  2. Using an ohmmeter, measure the resistance between IAC terminals A and B. Next measure the resistance between terminals C and D.
  3. Verify that the resistance between both sets of IAC terminals is 20-80 ohms. If the resistance is not as specified, the IAC may be faulty.
  4. Measure the resistance between IAC terminals B and C. Next measure the resistance between terminals A and D.
  5. Verify that the resistance between both sets of IAC terminals is infinite. If the resistance is not infinite, the IAC may be faulty.
  6. Also, with a small mirror, inspect IAC air inlet passage and pintle for debris. Clean as necessary, as this can cause IAC malfunction.




Click image to see an enlarged view

Fig. Fig. 2: Using an ohmmeter, backprobe terminals of the TPS sensor to check for proper resistances



Click image to see an enlarged view

Fig. Fig. 3: The TP sensor and IAC sensor are usually located at the side of the throttle body



Click image to see an enlarged view

Fig. Fig. 4: Idle Air Control (IAC) valve wiring and terminal identification

 

Idle Speed and Mixture Adjustments

GASOLINE FUEL INJECTED ENGINES

GASOLINE FUEL INJECTED ENGINES



The fuel injected vehicles are controlled by a computer which supplies the correct amount of fuel during all engine operating conditions and controls idle speed; no adjustment is necessary or possible

 

The Throttle Position Sensor is located:

Under hood, center, upper engine area, center of intake manifold, mounted on passenger side of throttle body unit

 

Throttle Position Sensor (TPS)

OPERATION



See Figure 1

The Throttle Position Sensor (TPS) is connected to the throttle shaft on the throttle body. It is a potentiometer with one end connected to 5 volts from the ECM and the other to ground.

A third wire is connected to the ECM to measure the voltage from the TPS. As the throttle valve angle is changed (accelerator pedal moved), the output of the TPS also changes. At a closed throttle position, the output of the TPS is low (approximately .5 volts). As the throttle valve opens, the output increases so that, at wide-open throttle, the output voltage should be approximately 4.5 volts.

By monitoring the output voltage from the TPS, the ECM can determine fuel delivery based on throttle valve angle (driver demand).



Click image to see an enlarged view

Fig. Fig. 1: Common Throttle Position Sensor (TPS) found on GM trucks

TESTING



See Figures 2, 3 and 4

  1. Backprobe with a high impedance voltmeter at TPS terminals A and B.
  2. With the key ON and engine off, the voltmeter reading should be approximately 5.0 volts.
  3. If the voltage is not as specified, either the wiring to the TPS or the ECM may be faulty. Correct any wiring or ECM faults before continuing test.
  4. Backprobe with a high impedance voltmeter at terminals C and B.
  5. With the key ON and engine off and the throttle closed, the TPS voltage should be approximately 0.5-1.2 volts.
  6. Verify that the TPS voltage increases or decreases smoothly as the throttle is opened or closed. Make sure to open and close the throttle very slowly in order to detect any abnormalities in the TPS voltage reading.
  7. If the sensor voltage is not as specified, replace the sensor.




Click image to see an enlarged view

Fig. Fig. 2: Using a DVOM, backprobe terminals A and B of the TPS sensor to check for proper reference voltage



Click image to see an enlarged view

Fig. Fig. 3: Using the DVOM, backprobe terminals C and B of the TPS sensor, open and close the throttle and make sure the voltage changes smoothly



Click image to see an enlarged view

Fig. Fig. 4: Throttle Position Sensor (TPS) wiring diagram

 

The PCV Valve is located:

Under hood, center, upper engine area, top of engine, mounted in passenger side valve cover

 

That can usually be checked by putting your thumb over the valve and you will here it snap when good.

 

Valve Burned, Worn, or Sticking Exhaust Valves.

Take a dollar bill and place it over the tailpipe flat, it will get sucked in if the exhaust valve is leaking.

 

Expert:  Mike S. replied 5 years ago.

Expert daves71, adds a bad coolant temp sensor on this year truck will cause the same symptom that the customer has

 

The Coolant Temperature Sensor is located:

Under hood, center, upper engine area, passenger side of water outlet, mounted next to valve cover

 

Engine Coolant Temperature (ECT) Sensor

OPERATION



See Figures 1 and 2

The Engine Coolant Temperature (ECT) sensor is mounted in the intake manifold and sends engine temperature information to the ECM. The ECM supplies 5 volts to the coolant temperature sensor circuit. The sensor is a thermistor which changes internal resistance as temperature changes. When the sensor is cold (internal resistance high), the ECM monitors a high signal voltage which it interprets as a cold engine. As the sensor warms (internal resistance low), the ECM monitors a low signal voltage which it interprets as warm engine.



Click image to see an enlarged view

Fig. Fig. 1: Engine Coolant Temperature (ECT) sensor location-4.3L, 5.0L and 5.7L engines



Click image to see an enlarged view

Fig. Fig. 2: Engine Coolant Temperature (ECT) sensor location-7.4L engines

TESTING



See Figures 3, 4, 5, 6 and 7

  1. Remove the ECT sensor from the vehicle.
  2. Immerse the tip of the sensor in container of water.
  3. Connect a digital ohmmeter to the two terminals of the sensor.
  4. Using a calibrated thermometer, compare the resistance of the sensor to the temperature of the water. Refer to the engine coolant sensor temperature vs. resistance illustration.
  5. Repeat the test at two other temperature points, heating or cooling the water as necessary.
  6. If the sensor does not met specification shown in the temperature versus resistance chart, it must be replaced.
  7. The sensor may also be checked in the vehicle. Unplug the sensor and attach a digital ohmmeter to the two terminals of the sensor.
  8. Using a calibrated thermometer, compare the resistance of the sensor to the ambient air temperature.
  9. Repeat the test at two other temperature points, heating or cooling the water as necessary.
  10. If the sensor does not met specification shown in the temperature versus resistance chart, it must be replaced.




Click image to see an enlarged view

Fig. Fig. 3: The ECT sensor is usually located near the thermostat housing



Click image to see an enlarged view

Fig. Fig. 4: Using a thermometer, a DVOM and some jumper leads, check the resistance of the ECT sensor and compare your readings to those in the chart



Click image to see an enlarged view

Fig. Fig. 5: Submerge the end of the coolant temperature sensor in cold or hot water and check the resistance



Click image to see an enlarged view

Fig. Fig. 6: Engine Coolant Temperature (ECT) sensor wiring diagram



Click image to see an enlarged view

Fig. Fig. 7: Engine Coolant Temperature (ECT) sensor temperature vs. resistance values

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