Hi everyone,

I have a 1992 Eunos Roadster (Japan import ) 1.6 Turbo with 120.000km and I’ve been fighting a very strange rich-running issue for a long time.

Current setup:

• GT2560 turbo
• Kraken Top mountbManifold
• ECU Master Black ECU
• Custom ECU wiring
• Bosch EV14 530cc injectors
• K24 electronic throttle body
• Mazda Demio electronic pedal
• AEM fuel pump
• Tomei FPR
• Bosch 4.9 LSU wideband
• Bosch oil pressure sensor
• Bosch oil temperature sensor
• Bosch coolant temperature sensor
• SkidNation radiator + dual fans

The car runs extremely rich most of the time. AFR usually drops to around 9-10, there is a very strong fuel smell from the exhaust, cold starts become inconsistent, and the tune keeps “drifting” richer over time.

The strange part is:
This exact problem already existed when the car was naturally aspirated years ago. Even before turbocharging, the car had a heavy fuel smell from the rear and would randomly start running rich.

When I first turbocharged the car, everything was actually fine for a while. AFRs were stable and the car drove properly. Then after some time, the rich condition slowly started appearing again and became permanent.

The car has been remapped multiple times from scratch, but after 1-2 days it starts running rich again.

One thing I suspect:
After the turbo setup, I replaced the cooling fans. Originally the Miata has one main fan and one A/C fan. With the new setup, both fans are powered from a single connector/trigger source.

Also, sometimes the fan relay sticks ON and the fans continue running permanently even with the car cooling down.

Could this possibly be causing electrical interference, grounding issues, voltage instability, sensor reference problems, or ECU compensation issues that could affect fueling?

At this point almost everything has been changed or checked, so I’m trying to think outside the box.

Has anyone experienced something similar ?

 

Post the tune and a datalog if you want actual help.

Yes, the fans could be driving temps down and causing the ECU to compensate, but nobody will be able to tell for sure if we can't look at the tune.

 

I don’t have the tune or the data logs myself — the car is being mapped by a professional tuner.

 

Okay then time to start guessing... so you said it was doing this before the turbo went on, which was before the fans went on, so probably not the fans.

The cars been remapped multiple times. Is it always the same tuner?

Not that your phone screenshot tells a ton, but are you actually idling at 28 degrees of advance? 50 KPA at idle also seems way high.

Is your gas mileage consistently horrible, or does it seem normal?

 

Before the turbo setup, the only issue was the fuel smell — the car itself was running fine. Of course, back then I couldn’t really monitor the values. Around three months after going turbo, the AFR values started becoming inconsistent. And yes, it has only been mapped by the same tuner. Also, I’m starting to suspect the electronic throttle.

 

3 months is a pretty long time. It could be ambient temps causing corrections in the tune to add more fuel. I would expect more fuel to be added going from summer to winter as the air temps get colder, but who knows how the tune is set up.

Any reason you suspect the ethrottle?

 

Sometimes, although rarely, the car gives throttle by itself… especially when car cold.



As for the weather conditions, it’s pretty much always hot here. We only have about a 3-month winter, and even then the temperature is usually around 10–15°C.

 

Sometimes, although rarely, the car gives throttle by itself… especially when car cold.



As for the weather conditions, it’s pretty much always hot here. We only have about a 3-month winter, and even then the temperature is usually around 10–15°C.

 

Going from 10 C intake temps to 40 C intake temps can change a lot depending on how things are set up.

Have you talked to your tuner about this? What does he think the issue is?

 

This is crazy. If you are paying somebody to tune (not map), they should be able to diagnose this problem very quickly. If they can't, you need to pay somebody else to tune the car.

 

That's what I was alluding to. I'm always surprised when someone is paying a "professional" and then comes to the forums to diagnose why their car doesn't run properly. Even if it's a mechanical issue a competent tuner would likely have at least some idea of what's going on.