Mitsubishi 3.5L V6 Rich mixture

1995 Mitsubishi Pajero 3.5L V6 Evolution running extremely rich.

I was called out to this vehicle after the garage had discovered a bad oxygen sensor and replaced it, only to still have an excessively rich mixture. (>10% Co).  They were working on the car for a import specialist and so had no history of the car.

When I got out to this vehicle, I experienced the rich mixture condition and noted that the oxygen sensor was reading low voltage and not responding.  On close inspection, once I clambered under the vehicle, I noticed the colour of the wires and identified it as incorrect specification.  It was a 4 wire broadband sensor and not a standard Zirconia narrowband.

So the sensor was replaced, again, with the correct type this time.  Unfortunatley the rich mixture was still present, but now the sensor was reporting a rich mixture.

I was able to read serial data, but this was pretty old technology, so I didn’t want to rely on serial data alone.
The ‘long and the short’ of it, was there were no stand-out sensor inputs.  I checked for injection time regulation, in response to the rich mixture and confirmed activity trying to lean the mixture, until the ECM ‘gives up’.

I coupled my scope to one of the injectors, so that I could confirm fuel trim regulation. On this engine, I could only get to one injector directly and that was No.2 cylinder, (front right as you look at the engine).

From this sample I could confirm active fuel trim adaption from start-up, until the ECM stops trying.  Below is a sample from the injector.

Injector pattern 1
Injector voltage signal from Pajero, for injection duration analysis.

I was really only interested in the injection duration and control of the duration during this test and was happy all seemed good, leaving the problem, what’s causing the rich mixture.

Unfortunatley I didn’t have any suitable adaptors to physically check the fuel pressure, plus access to any couplings was extremely limited.  (I very rarely have to work on vehicles of this age and type).  So this meant I couldn’t easily do an injector flow test.

I then went on to make sure the intake system was clear and breathing correctly, I found no problems there, (sorry no scope captures of intake pressures etc.).

I then noticed a harness connector that housed all the wires for the injectors, wish I’d seen that earlier.

This meant I could properly profile the injectors, with both voltage and current.  Take a look:

Injector pattern 2
Full injector profile analysis, voltage and current.

I have sampled two injector voltage signals, (no identification on which injectors they are), and the current clamp was hooked up to the common supply.

I really wished I’d seen that connector before!

See how the current signal has really shown up a problem, but which is bad?  Take a closer look.

Injector pattern 3
Voltage and current profile close up.

If you look at the current ramp of both injectors and compare to my reference page, you will see how the ramp is steeper and higher than normal for the bad injector, peaking at a little over 1.3 amps, and the voltage ‘pintle hump’ is smoother than the good one, indicating a softer closing.  Yes!  There is only one good injector in the engine.

After concluding the injectors were a problem and needed to be replaced, we discussed the potential cause.  Maybe the vehicle had been sitting around with bad fuel in it.

However, when the diagnosis was reported to the customer, they mentioned that the injectors had been replaced!  Only they were replaced for a misfire fault, when the vehicle was first imported into the country.  The customer had used a local garage to them, to rectify the misfire.  So digging deeper, it turned out that this garage had decided to replace all 6 injectors first, (I can’t imagine why anyone would choose to do that, as that involved removing the inlet manifold assembly), and then replaced a coil which actually fixed the problem.

Then it turned out, the injectors had come from a very cheap aftermarket source and in-fact only 5 of the injectors were actually replaced and so 1 was left original.  (Lucky for me).

I wonder which pattern belonged to the original injector… I think we know…

This outcome is the expensive result of the original ‘Guess Diagnostics’ technique.

Be warned..

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