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Sudden Drop in Fuel Economy & Exhaust Smell on 2017 S90 T6

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Mike T. · rep 0 · May 22, 5:12 AM · 1 replies · solved · Question

Vehicle: 2017 Volvo S90 T6 Inscription

Hi all,

I've noticed a significant and sudden drop in fuel economy on my 2017 Volvo S90 T6 over the past few weeks. I used to get around 28-30 MPG on the highway, and now I'm barely hitting 22-23 MPG. On top of that, I've started to smell a faint, somewhat sulfur-like odor from the exhaust, especially when the car is idling or after a cold start.

There's no check engine light on yet, but I'm concerned about the fuel consumption and the smell. It doesn't feel like the car is losing power, but it's definitely not running as efficiently. I'm wondering if this could be related to the catalytic converter or oxygen sensors.

Has anyone experienced similar symptoms with their S90 T6? What was the diagnosis? Any advice on what to look for or suggest to my mechanic?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 5:12 AM

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ClearTheCode admin answer:

A sudden drop in fuel economy combined with a sulfur-like exhaust smell on your 2017 Volvo S90 T6 often points towards issues with the emissions system, specifically the catalytic converter or oxygen sensors.

Common Causes:

  1. Failing Catalytic Converter: The sulfur smell (often described as rotten eggs) is a classic symptom of a catalytic converter that is failing or overloaded. When the converter can't efficiently process hydrogen sulfide in the exhaust, you get that distinct odor. A failing catalytic converter also restricts exhaust flow, which can reduce engine efficiency and thus fuel economy. While a check engine light for P0420 (Catalyst System Efficiency Below Threshold) usually accompanies this, it might not trigger immediately.
  2. Faulty Oxygen (O2) Sensors: Upstream and downstream O2 sensors monitor the oxygen content in the exhaust and help the engine control unit (ECU) adjust the air-fuel mixture. A failing O2 sensor can send incorrect readings, causing the engine to run too rich (too much fuel), leading to poor fuel economy and potentially overloading the catalytic converter. This often triggers codes like P0171 (System Too Lean) or P0172 (System Too Rich), or specific O2 sensor codes (P013x, P015x).
  3. Engine Running Rich: Other issues causing the engine to run rich, such as leaky fuel injectors, a faulty mass air flow (MAF) sensor, or a stuck-open thermostat (preventing the engine from reaching optimal operating temperature), can also lead to increased fuel consumption and the sulfur smell.

What to Do:

  1. Professional Diagnostic Scan: Even without a check engine light, a professional diagnostic scan can reveal pending codes or live data from the O2 sensors and fuel trims. This data is invaluable for pinpointing whether the engine is running rich and why.
  2. Inspect for Exhaust Leaks: While less likely to cause the sulfur smell, an exhaust leak could affect O2 sensor readings and overall efficiency.
  3. Check MAF Sensor: A dirty or failing MAF sensor can cause incorrect air measurements, leading to an improper air-fuel mixture.

When to See a Shop:

It's advisable to take your S90 to a Volvo specialist or a trusted mechanic as soon as possible. While the car may seem to drive fine, prolonged driving with a rich condition or a failing catalytic converter can lead to more severe engine damage or complete catalytic converter failure, which is an expensive repair. A professional can accurately diagnose the issue by analyzing O2 sensor data, fuel trims, and potentially performing an exhaust gas analysis.

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