Tech: Troubleshoooting the EFI system in the 3.0l V6 Short Cuts

Author/ Photographer: Phil Hansford, October 27, 2000 


Editor's Note: 
The information in this article is specific to the 1989 3.0l V6 motor. Many aspects of this procedure are common to all 3.0s, but it is advisable to consult a service manual before you troubleshoot your own engine.

The biggest advantage that the Mitsubishi V6 had over the existing four cylinder when it was introduced in 1989, aside from the obvious power gains, was its method of fuel delivery. Where the four cylinder relied on the trusty Mikuni carburetor to dump fuel according to a complex linkage of springs and vacuum lines, the new V6 adopted a multi-port fuel injection system to precisely spray fuel into the combustion chamber. This system used a computer, or Electronic Control Unit (ECU) to monitor all the engine's vital systems, as reported by different sensors, and adjust them accordingly. This translated into an arguably more efficient, more reliable engine, which could be maintained more easily. 

Getting To Know the 3.0's Sensors

The components under the hood (see table)
The "cockpit" components (see the table below)

What does all this technology mean for the owner? With a few simple tools, the system's ECU can tell you if there is a problem, and where to look to fix it. Before you "talk" to your ECU however, you should familiarize yourself with the components that make up the  V6's Fuel Injection System. 

Fuel Injection Component Identification

Name Symbol Name Symbol
Air Conditioner Relay A Inhibitor Switch (auto) L
Air-Flow Sensor B MPI control relay M
Idle speed control servo (ISC) C Air conditioner switch N
Throttle position sensor D Engine check indicator O
Air conditioner temp sensor E Power steering oil pressure switch P
Engine coolant temp sensor F Ignition timing adjustment terminal Q
Ignition coil G Self-diagnosis check connector R
Crank angle sensor H Fuel pump drive terminal S
Purge control solenoid valve I Electronic control unit (ECU) T
Injector J EGR temp sensor U
O2 sensor K EGR control solenoid valve (m/t) V

Required Tools

Now that you have identified the various components that make up the system, you are ready to communicate with the ECU, to find out if all these parts are working as they should. With many vehicles you will need to purchase an expensive "code-reader" to translate the language of the computer but with the Mitsubishi V6 all this can be accomplished with a common high-impedance voltmeter or a test light, both available at your local hardware store.

How Do I Know There is a Problem in the System?

tn_dash2.jpg (2133 bytes)
The "Check Engine" light self tests when you start your vehicle.

The first thing that should alert you to a potential problem in the system is the illumination of the "Check Engine" light.  Under normal operating conditions the light self-tests when you first start your vehicle, lighting up for about 3 seconds, and then going out. If the light stays on, it indicates that the computer has one or more trouble codes stored in its memory. A word of caution here: don't assume the trouble lies elsewhere if the light doesn't illuminate. My "check engine" bulb was burned out, so I was unaware of a trouble code waiting for me in the ECU until I had eliminated everything else as a source of trouble.

"Harnessing the Power"

tn_glove_comp3.jpg (3210 bytes)
The connector on this vehicle was held against the back by a wire-tie
tn_glove_comp4.jpg (3031 bytes)
Remove the clip on either side of the glovebox, and let it hang down

If you have determined that a trouble code is waiting for you in the ECU's memory, the first order of business is finding the "Self Diagnosis Check Connector". This is a wiring harness that looks like it should be attached to something, but was left dangling. On my 1989 two door this harness was behind the glove box, but some owners have reported finding it below the glovebox, by the firewall. If you do find it behind the glovebox, the diagnosis is made easier if you remove the side clips and let the door hang down, before manipulating the connector into position.

"Cracking the Code"

terminal.jpg (1653 bytes)
The black (-) lead goes to the terminal indicated by black, and the red (+) lead goes to the red terminal. The gray area indicates an open spot.

Once you have found the connector, set up your voltmeter on the 15V setting, so you can easily detect the sweeps. Hook up your meter as indicated by the diagram:  the positive lead goes to the top right terminal, and the negative lead goes to the bottom left hand terminal ( assuming that you have oriented the harness with the open section on the top, as illustrated). In attaching the leads to the harness, I found the easiest way to ensure a good contact was to stick the probes in through the "back" of the harness, assuming that the "front" is the open end pictured. Now that you have attached your voltmeter, you are ready to talk to the ECU! 

The Trouble Codes

The trouble codes for this motor are sent to the voltmeter in a series of needle sweeps, either long or short, immediately after the ignition is turned on (If you have had previous FI troubles, disconnect the battery for ten seconds to clear the memory). 

The chart below indicates the code and its corresponding component:

Engine Fault Codes

Component Malfunction Code Output Order
Oxygen sensor 11 1
Air-Flow Sensor 12 2
Intake Air Temperature Sensor 13 3
Throttle position sensor 14 4
Engine coolant temp sensor 21 5
Crank angle sensor 22 6
Top Dead Center (TDC) Sensor 23 7
Speed Sensor (Reed Switch) 24 8
Barometric Pressure Sensor 25 9
Injector 41 10
Fuel Pump 42 11
Exhaust Gas Recirculation (EGR) 43 12

The two digit trouble code is displayed by the meter as follows: The first digit is indicated by long sweeps, and the second digit is indicated by quicker pulses. For example, a problem with the Coolant Temperature Sensor would display the following:

This indicates the code 21. If more than one code is stored in memory, they are sent out in numerical order, starting with the smallest, and punctuated by a brief pause, then the next code, and so on.  When all codes have been displayed, the readout repeats. Thus a malfunctioning Speed Sensor and Throttle Position Sensor would be displayed as 14, pause, 24, pause, repeat.

If there are no codes in memory, the meter will simply repeat a fast sweep. If the ECU itself is malfunctioning, the needle will simply sweep up, and stay up.


After diagnosing the problem, if any, the next step is to check any associated wiring to the affected sensor. If no short can be determined, you have the option of replacing the sensor, having it tested, or bringing the vehicle to a qualified mechanic armed with the knowledge of what is malfunctioning in your FI system. This easy procedure may save you hundreds on wild goose chases to track down a problem which the ECU was more than happy to share with you. Note that if you get all the codes being sent to your meter, check for an open ground/power circuit. 

For an excellent in-depth discussion on the workings of the Fuel-Injection System in the 3.0l V6, refer to John Parkinson's three- part series for the Mitsubishi 4WD Owner's Club of Queensland: Pajero V6 Fuel-Injection Systems. 

Contacts: Related Links:

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