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Mike Kelly
Mike Kelly, Auto Mechanic
Category: Car
Satisfied Customers: 5224
Experience:  17 years experience as a auto mechanic,foreign and domestic.5 years running my own repair business.
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I have a 1995 Isuzu Rodeo. 3.2l MPFI with a DIS system.

Resolved Question:

I have a 1995 Isuzu Rodeo. 3.2l MPFI with a DIS system. The truck has been running rich, getting about 10 mpg on the highway. I have also noticed a black soot being discharged from the tailpipe at start up. I have replaced all the plugs and plug wires (one was shorting out), Replaced the O2 sensor, I have run fuel injector cleaner in each tank of gas (three times)and just recently changed the oil. I have no "check engine" light on and the truck runs pretty smooth. Can you give me any advice on what to else could be causing this problem? I tried looking for a mass air flow sensor, but this engine doesn't look like it has one.
Submitted: 4 years ago.
Category: Car
Expert:  Mike Kelly replied 4 years ago.
Hi,how are you?From what you are describing this sounds like an issue with the map sensor,which is what this vehicle has instead of a mass air flow sensor.

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 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.

TESTING



Sensor

See Figures 1 and 2



Click image to see an enlarged view

Fig. Fig. 1: MAP sensor terminal identification-except 2.8L, 3.1L and 3.2L engines



Click image to see an enlarged view

Fig. Fig. 2: Terminal identification is stamped on the MAP sensor-except 2.8L, 3.1L and 3.2L engines

  1. Backprobe with a high impedance voltmeter at MAP sensor terminals A and C on 2.8L, 3.1L and 3.2L engines. On other engines, backprobe between terminals 1 and 3 .
  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 impedance voltmeter at MAP sensor terminals B and A on 2.8L, 3.1L and 3.2L engines. On other engines, backprobe between terminals 1 and 2 .
  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.

 

  1. Unplug the connector from the valve.
  2. Using an ohmmeter, check the resistance between the terminals. It should be between 30-40 ohms. If not, replace the valve.

REMOVAL & INSTALLATION



See Figure 3



Click image to see an enlarged view

Fig. Fig. 3: Common MAP sensor mounting found on Isuzu trucks

  1. Disconnect the negative battery cable.
  2. Tag and disconnect the vacuum hose(s).
  3. Disengage the electrical connector.
  4. Release the locktabs or unfasten the bolts and remove the sensor/valve.
  5. Installation is the reverse of removal.
Mike Kelly, Auto Mechanic
Category: Car
Satisfied Customers: 5224
Experience: 17 years experience as a auto mechanic,foreign and domestic.5 years running my own repair business.
Mike Kelly and 9 other Car Specialists are ready to help you
Customer: replied 4 years ago.
Thank you very much for leading me to the MAP sensor. With alomst a 30% reduction in fuel economy that really makes sense. I will verify the voltage . Aside from that sensor is there anything else that I should be looking for that might cause the same problem if the sensor checks out OK.
Expert:  Mike Kelly replied 4 years ago.

I really believe that this will turn out to be your issue,but other than this i would check out the egr valve,which is Under the hood, center, rear engine area, passenger side of distributor, mounted in block .Thank you.

Customer: replied 4 years ago.
Turns out it wasn't the MAP sensor. Tank of gas with new MAP sensor 10.8MPG and most of that was highway driving. The EGR is next on my list to remove and test. I always thought the EGR would affect the idle, and my idle is smooth. The truck really runs smooth and if it weren't for the gas mileage problem I would say the engine is running perfectly. Aside fromt the EGR could the coolant sensor cause the same kind of reduction?
Expert:  Mike Kelly replied 4 years ago.
Hi,i don't believe the coolant sensor would cause this issue,i think you are on the right track with the egr valve.The egr valve is located Under the hood, center, rear engine area, passenger side of distributor, mounted in block.
Customer: replied 4 years ago.

Mike,

 

Sorry to keep bugging you with this Rodeo issue. I have now verified that the EGR is operating correctly and didn't seem to be clogged. I was hoping that it was.I also pulled the MAP sensor tube and noticed that the intake plenum has some black gasy soot in there. I also verified if the ignition system is working and each cyclinder is getting the proper spark.

Here is what I have replaced so far:

Plugs, wires, O2 sensor, air filter, Oil, oil filter, MAP sensor. I have checked the vacuum lines, PVC valve and EGR valve, along with checking for vacuum leaks, none of the hoses seemed clogged

With such a major reduction in MPG (10mpg), I really thought it would be something easy to find.

If you have any other ideas, i would greatly appreciate them.

Expert:  Mike Kelly replied 4 years ago.
Hi,other ideas are a leaking manifold,or a leak in the exhaust before the catalytic converter,both of these will cause the vehicle to run rich.Thank you.

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