2006, 130k miles.

I'm noticing the ubiquitous sulfur smell, typically when at moderate speeds like 50 or 60mph. But there's no engine light.

Fuel mileage isn't good either. I was getting around 27-28 when on a long trip, with the car absolutely packed with stuff when I moved cross country. We're talking many hundreds of pounds. More than enough to significantly lower fuel mileage.

Now I'm getting around 14mpg with a passenger and no luggage. The big difference, though, is that this is mostly city driving. Don't know if these numbers are reasonable for city vs highway, so I'm including them as a diagnostic data point.

The car gets top tier fuel. Premium. (Mostly Costco).

Is it possible that it's the fuel pressure regulator or fuel pump? I'm sort of leaning in that direction due to the lack of codes.

Thanks for reading. Any thoughts?

 

I bet new o2 sensors will improve your mileage. Apparently the sulphur smell is quite common my 04 w/ 134k miles gets it under heavy acceleration

 

Bad engine temp sensor can also lead to poor mileage. Should be replaced every 90k IMO. This sensor is not the same as the guage on the cluster.
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The sulfur smell means your catalytic converters are working. My '01 has made that smell under heavy acceleration for 40k miles. I only notice it if my windows or moonroof are open. The reason you smell it on the inside is the trunk lid seal no longer makes an airtight seal. It's pulling the exhaust into the trunk and then into the passenger compartment.
Depending on your cars service history you may need a "tune up". With your mileage I would recommend first a MAF sensor cleaning, followed by new spark plugs, a throttle body cleaning, a new coolant temperature sender, and lastly 2 new forward O2 sensors. These are all the components that contribute to correct air/fuel ratio which can effect mileage and emissions.

 

I almost replaced them in advance when I first got the car and was doing maintenance, but decided against. I'll price them out, take a look at the numbers on OBD Fusion, and go from there.

I think it was replaced along with the timing belt, but I'm not sure. I'll take a look at the invoice. Does the coolant need to be drained, or is dry/elevated? Any recommended brand?

 

Thanks for all the info!
I'll definitely start with cleaning the MAF tomorrow. Spark plugs are new as of a few months ago. Forward meaning pre-cat? How difficult is it to access the throttle body for cleaning? Is it easy enough to remove and clean off the car? (Sorry, very little experience with this car so far.) I'll take a look at the trunk seal as well.

 

Any chance you could upload Techstream data on fuel? Looks like either your o2 sensors are sending fun stuff to the ECM, filters are dirty (including fuel filter), or there is a possible contamination (throttle body, injector rail, dampers).

 

No techstream, but I do have one of the better OBD transmitters. Do you know if it's possible to obtain a pirated version and run on virtual/emulated Windows over wireless? (I think my transmitter is both Bluetooth and WiFi, though I'd have to check tomorrow.) My computers are all Mac. Barring techstream, would data from OBD Fusion be acceptable? I know I can get o2 sensor data, not yet sure what else is available for this car.

 

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Yes, the pre-cat O2 sensors.
The throttle body is easy to remove once the air intake is out of the way.
Make sure when you are cleaning your MAF that you are actually cleaning the sensing wires. They are the 2 small parallel wires deep inside the tube. You can see them with a bright light. The red bulb that is visible is the air temp sensor. Clean that too.
If you have access to obd data the most important data to diagnose issues are the fuel trims. There are 4 total. Short term banks 1 and 2, long term banks 1 and 2. The long term trims should be at or near zero.

 

I can relate to your city driving mpg. I do 95% city driving and get approximately 14.5 mpg with the 5 speed trans on my 01. I was told for city driving it’s the norm. I use only premium gas.

 

I picked up some MAF cleaner, but didn't have time to do the actual cleaning.

OBD data after a 20 min drive. Trans in park, engine on.

https://imgur.com/a/rSRGlN7

 

Don't forget winter fuel and cold air also leads to less economy. Are you consistently getting bad mileage on the highway? This is the best way to test, flat road cruising at 70 mph or so. What is your mileage after 4 or 5 miles? You should be able to get at least 23.

As mentioned, the main sensors that determine correct air to fuel ratio are the pre-cat O2 sensors. Post
-cat does nothing but monitor for the cats themselves. An exhaust leak can also be an issue.



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Poor overall mileage started before winter. IIRC, my highway mileage, under the condition you recommended, was in the mid 20s. I'll check again when I get a chance.

Any thoughts on the OBD data?

 

I'll just put to this way, in my old 04 UL I replaced the O2 sensors, engine temp sensor, plugs, egr, pcv, cleaned everything thoroughly as mentioned on this thread, and when I lived in Atlanta on the mild rolling hills I could average as high as 33 MPG at best cruising on the highway. It was very impressive for a car with 100k miles. Usually about 29 MPG cruising around 1800 rpms.

The UL does lower on the highway and I think this helps a bit. In the city it was 15 or so like everyone else. ​​​​​​I moved to NJ and the mileage dropped 4 - 5 MPG... I tank average 20.5 in the SC430. I have yet to see more than 27 mpg on the highway up here. But it's hard to drive the SC slow.
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Those long term fuel trims are too high. Either you have a vacuum leak somewhere adding extra air or your O2 sensors aren't reading correctly. Since they are close bank-to-bank I would say it's a vacuum leak. The easiest way to diagnose this would be a smoke machine.