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Mazda MX5 NB, Old Timey Diagnostics

The MX5 is still running rich, and I’m still not happy. Without ODB2 though, or any proper diagnostic system, it’s time to bust out the multimeter and get probing.

I’m making slow improvements with the MX5 over fuelling. When I got the car it was bad, with black smoke and carbon on the ground. Now it’s still got a bit of black smoke, but no carbon on the ground. What it does have though is a lumpy idle and the ability to completely blacken spark plugs in a short drive.

In order to rule out another component I decided to change the spark plugs out. The old ones didn’t look too bad, except for the fact they were completely jet black and covered in carbon. Not oily though, which is a good sign.

I shot to Supercheap and picked up some new NGK BKR6E-11 plugs, and set about changing them.

I love older, simpler cars. No plastic covers or other rubbish to remove

Just pull up carefully on the plug boots, pop them aside, and away you go

Mazdaspeed was even nice enough to make sure the strut brace doesn’t block the No.3 plug

This is a new plug vs one of the old ones. The camera had trouble focusing on the old ones because it is just solid black on the end. All four look like this.

Pop them back in, pop the leads back on and bam, job done. Easy.

I took it for a quick drive, but noted I still had a rough idle (especially straight after start-up, it runs on 3 cylinders for a little unless you give it some revs). It drove OK, but didn’t really feel any different. Unfortunately a quick check of No.1 plug when I got back showed that it was still running rich enough to blacken the new plugs too. I’ll need to take them out and give them a clean.

So, what next?

The NB1 MX5 doesn’t have OBD2, so no flashy plug-in diagnostics here. The best I have is a DIAGNOSIS port under the hood, that I can stick an LED into and read the flashes. Kinda “flashy” i guess 😄

So that’s where I started. I whipped up a quick LED and resistor combo (the port is 12v, it’ll pop an LED if it isn’t 12v and has no resistor), and bridged the LED between FEN and +B ports. I also needed to set the ECU into diagnostic mode by bridging TEN and GND again.

Its messy, but worked

This is the result. Key on, and read the flashes.

There is a bit of a trick to it. A long flash is a space. So from the start of the video, I get this,

space 1x FLASH space 5x FLASH space 2x FLASH space 3x FLASH

And then since I only have the one code, it repeats over and over.

From that, my code is P (all codes start with P) 1523 – VICS Solenoid Open Circuit.

This is the VICS solenoid, tucked away at the back of the engine, on the end of the inlet manifold

What is VICS? It stands for Variable Inertial Charging System. Basically, there are a set of flaps inside the inlet manifold that open at a certain RPM, which opens up another part of the inlet manifold; a big resonance chamber. VICS increases the inlet manifold volume and allows more air into the engine. This allows the engine to flow well at high RPM but also have good low-end torque and power.

Heres the workshop manual details on VICS, for all you nerds like me.

Under 5250RPM, the flaps are shut, over that, the flaps are open and the air can enter the chamber. So anyway, I have a code for the solenoid that controls this wizardry.

I whipped the solenoid out to test. Its easy, only one 10mm nut on the back and out it comes.

Using some small terminals and some spare wire I created a pair of test leads. When a solenoid is powered, it should emit a solid click. I connected it to a car battery, and nothing. I tried a bigger battery, still nothing. The solenoid is dead.

I have ordered a replacement solenoid off eBay, so that should fix that issue. I don’t believe it’s causing me to run rich though. From my testing, the actuator on the inlet manifold will be in the “open” position when vacuum isn’t being applied to it, so my chamber is always open. I’ll be losing some down low power, but that should be it.

So where to from here?

Old school testing. Out came the Digital Multi-Meter and my favourite back-probing T-Pins.

The two most common sources of over fuelling are a failed engine temperature sensor, and a failed O2 sensor. These are what were tested.

I started with the temp sensor. The book calls for draining the coolant and removing the sensor so it can be tested in a pot of water. I don’t have spare coolant to refill the system (I’ll need to flush it shortly as I’m not sure what’s in there), not to mention if I’m going to remove it I may as well replace it, so tested it in situ. The coolant temp was about 27c, give or take a couple of degrees.

According to the workshop manual this should give me a resistance reading of around 2.27-2.73K Ohms. Is that what I have?

Not even bro.

Maybe that’s the “warm” reading? No, the manual states at 80c the reading should be 0.29-0.34K Ohm. The coolant was nowhere near warm enough to be that low. I’ll test it again tomorrow before running the car (so it’ll be ambient temp which won’t be over 20c at the moment) to be sure, but I suspect the sensor is out of spec.

What about the O2 sensor then?

This is a bit harder to test at 10:30pm, as I can only test half of it. I can’t test the actual sensor signal readings without the engine running, and I think my neighbours will stab me if I run the engine up to temp and then at 3000rpm to test the sensor.

In the meantime I can test the resistance of the heater circuit. Before testing I did notice something that concerns me a little; the sensor has been replaced in the past and wiring spliced. It’s a common practice, but easy to do wrong, or to fail over time. The splice is under the red zip tie. The JDM NB1 only has one O2 sensor.

In go the T-Pins, and with the meter on resistance, lets test.

Are we good? No.

The Oxygen Sensor Heater Circuit… is open-circuit. This means the meter can’t complete a circuit through the heater element in the sensor. Either the wiring is damaged (looks visibly OK), or the sensor has failed.

I need to test the signal side of the sensor tomorrow, but I suspect we will be down for a new O2 sensor too.

And then, we will cross our fingers that it finally runs OK.

So, still convinced the car was in “as advertised” condition?

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Rico Suave
Rico Suave
6 years ago

It’s great that you’re fixing this. If it were me I would’ve just walked away and called the previous owners bluff. Unless it was cheap cheap 🙂