Help Diagnose: NC 2.0 Lean Under Load & Hesitation (P2177)

Technical Area Engine, Transmission, Exhaust etc
1
  1. My model of MX-5 is: MX-5 NC Sport Tech 2.0 2009
  2. I’m based near: Sheffield
  3. I’m looking for technical help or recommendations on: Engine running lean

Hi all,

Hoping to get some advice and listen to your experiences with a persistent issue on my 2009 NC 2.0 Sport Tech.

The Symptom: In general, the car feels more lively after some recent work (more on that below), but the core problem remains. When the engine is under high load – for example, going up a hill in 3rd gear and applying full throttle from a lower RPM – the car feels completely powerless for a few seconds. It seems to bog down, and then eventually picks up power.

It’s much happier and accelerates better with smooth, gradual throttle application. The issue is worst with sudden, full-throttle demands.

What I’ve Done So Far (The Diagnostic Journey):

  1. Historic Fault Code: The car had a P2177 - System Too Lean Off Idle code, which started this investigation. The light is not currently on, but the symptoms persist.
  2. Basic Maintenance: I’ve replaced the air filter and spark plugs. I’ve also run a bottle of fuel injector cleaner through the system.
  3. The MAF Sensor: I removed the sensor and found it was severely contaminated with black, crusty soot on one side. I’ve since cleaned it thoroughly with dedicated MAF Sensor cleaner spray and reset the ECU. This has definitely made the car feel more lively in general driving, but it has not solved the high-load hesitation.
  4. Live Data Monitoring: I’ve been monitoring live data on an OBD scanner, which has confirmed the problem:
  • At idle, the fuel trims are fine (close to 0% or even slightly negative/rich).
  • As soon as I rev the engine to 2,500-3,000 RPM or drive it under load, the Long Term Fuel Trim (LTFT) shoots up, sometimes as high as +20% to +30%.
  • This confirms a severe lean condition is happening specifically when the engine is asked to work hard. This pattern seems to rule out a classic vacuum leak.

Current Situation: After all the steps above, the car is better, but the main fault of leaning out and hesitating under high load is still there. My main suspects are now:

  • The MAF sensor is faulty and needs replacement (despite the cleaning).
  • The fuel pump is weak and can’t deliver enough volume under demand.
  • A failing upstream O2 sensor.

My Question for the Community: Based on your collective experience with the NC 2.0, is this specific pattern—a severe lean condition and hesitation only under high engine load—a well-known, common issue?

I’m trying to decide whether to replace the MAF first, or if these are classic signs of the fuel pump being on its way out on these cars.

Any advice or shared experiences would be hugely appreciated before I start throwing more expensive parts at it.

Thanks in advance!

2

Do you have the facility to read live data on your scan device ?
Try looking at the AFR and see if it confirms the car going lean where you think it is.

I use the TORQUE app and a bluetooth dongle to read live data from the ECU

3

Yes, I use OBDocker. It has a Fuel/Air Commanded Equivalence Ratio position. Will check that tomorrow. Do you have something on your mind? Could there be another reason for the fuel trim rising to 20-30%?

4

I would test components first before going on a swap to check method.

Have a read of this - it gives test methods and expected results :

5

Another thought - does the engine use oil ?
Possibility the oil contamination is on the pre-cat O2 sensor and corrupting the info the ecu needs for correct AFR

6

I would still want to confirm no vacuum leaks exist, a smoke test is the only way to do that really.
Not sure if this helps, there are lots of result for it if you search.

7

No, engine doesn’t use oil at all and it never had. Exhaust fumes are clear as well, no smoke whatsoever. I recently replaced the cam head gasket as I detected very minor leak of oil into the cylinder 1 spark plug socket. It was done after I started having issues with power so shouldn’t be related.
I’ll do more testing when I get home and will let you know.