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Car running lean with headlights/thermo fan on!

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My Mazda runs fine whilst idling without any accessories on (14 Afr).

As soon as I turn my headlights on, Afr goes upto 16.

When I turn my thermo fan on it goes upto 18afr. (Fan is connected to switch in cabin).

Voltage is at 14v without accessories, goes to 13.8 with them on.

Has constant 3bar fuel pressure with acc. on or off.

Alternator, battery, fuel pump, injectors etc. are new...

Driving is fine until I stop at a set of lights, then car nearly stalls.

Car is running Microtech mt-8 (don't laugh please) :)

Has only just started happen, car's only been on the road for a few months now... :(

Where, and how, are you measuring the voltage? How do you have the lights and fan wired into the electrical system?

I suspect there's a greater drop in the actual voltages to the injectors and that's affecting their effective opening time.

I haven't run into this in a long time, but long ago before the days of Injector Dynamics and ECU and reflash options with more thorough injector characterization, I used to run into this often. As Gord said, this may simply be the nature of not having physical injectors which match full characterization data you can enter in that ECU. Dead time, low IPW non linearity, or both could be a factor here.

I would test the alternator, battery, and verify all main grounds and the battery terminal connections. It's possible your grounding isn't sufficient for high electrical load and that's disrupting engine operation. A loose battery terminal connection, or battery that's on its way to death can cause this sort of thing too.

Also check your fuel value on the cells right above the "unloaded" idle load. With additional load, you may be interpolating more from those cells you may have missed in the tuning.

So I had a look at the car, seems like headlights are wired up-to battery directly (through a relay).

I checked voltage with a multimeter to injectors and there's a .2v drop when fans/headlights are on.

I've checked all grounds I could see, they seem fine.

The voltage reading is displayed on the ECU's handset.

I'm not really good at tuning (had it tuned by a shop), but whatever the Ms means on handset (guessing opening time for injectors), it goes from 2.0Ms with accessories off to around 1.85ms with them on (that's when it's running leaner)...

Took the car for a spin and it's only boosting to 5psi then running super lean and stuttering/misfiring. Not sure if this is related to this issue or another problem entirely...

I was running 20psi a few weeks back. Although, car was starting to buck/misfire at higher revs...

The other day I had a look at the fuel pump strainer as it was dirty so drained e85 and replaced the strainer for a new one (which is larger) and put fresh e85 in car. Thought it would fix the issue, but has made it worse it seems...

As David suggested, I would check the tune to make sure the additional electrical load doesn't get the ECU in a cell that wasn't tuned, or interpolating in between.

As there is only a .2V drop, I don't think it could be an injector deadtime issue, but it could be worth checking anyway.


It sounds like there may be 2 different things going on. It sounds like you're getting into another cell in the fuel table when there's high electrical load, so tweaking that cell would help that condition, but the lean and misfiring in boost seems more likely related to a fuel pump/filter or other fuel delivery system fault.

Hi guys,

I've went into the load map and changed injector time to 2ms instead of 1.82ms for 20hg/25hg/30hg cells.

Car runs a little richer now, but with fans/headlights on it's all good! :)

Will try to fix this lean issue into boost now.

Thank you so much for the help! I really appreciate it.

Don't forget to also check the ground/earth path voltage drops - I was thinking, earlier, a lower voltage across the injectors may slow their opening, and reducing the net opening time, which could compromise the amount of fuel passed into the ports.

Thanks Gord, I've tried my best to check earth points on the car. Having issues in boost now. But I think I will make another thread for that.

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