High fuel consumption on highway 2.0 RWD

[dincaalin]

Hello everybody,

I have a 2019 GT-line 2.0 RWD Stinger, 71k miles (115k km) that has all of the GT2 (3.3l) options except for the brembo brakes (yes, I have the adaptive suspension as well, it's a German version of the car with all of the options).

I see very high fuel usage during my daily commute and I did some tests at night on the highway, both in summer and winter. Premium tires for summer and winter, at the recommended pressure, no luggage / people in the car, warm engine and transmission:
- 110 km/h = 8.0l/100km // 69MPH = 29MPG (8th gear at 1850-1900 rpm)
- 130 km/h = 10l/100km // 82MPH = 24MPG (8th gear at 2150-2200 rpm)

This happens on ECO, Comfort and Sport mode.

This is tested exclusively on a straight highway road and it doesn't take into consideration the accelerations to get up to speed when entering the highway, it's strictly constant accelerator pedal and speed.

From what I could find on the forums, even the 3.3 Stinger gets better fuel economy when cruising like this. So something must be wrong with mine. I've drove another Stinger exactly like mine and that one had better fuel economy, on the same road I did the tests (29-30MPG at 85MPH // 8l/100km at 135km/h). Both cars have the same ECU and TCU software version, the one after the GPF issue was fixed in the software, so it cannot be that popular GPF logic bug that was happening on early softwares.

- replaced the spark plugs, the old ones had around 30k km (20k miles) and were black (attached photo)
- I did a smoke test for the intake, no leaks detected.
- The fuel rail pressure stays around 20000 kpa (2900 psi).
- The O2 sensor reports fluctuations between 0.95 and 0.98, where 1 is stoichiometric A/F ratio and 2 is 100% oxygen and 0% fuel, so it reports just a bit rich mixture, too small I think to count. Also the other Stinger that I drove had the same values and not the consumption issue.
- LTFT stayed at 4.5% and STFT stayed around -1% to 1%
- intake manifold absolute pressure was at 97-100kpa
- timing advance between 15 and 18 degrees
- cat 1 temp 780°C and cat 2 temp 680°C
- engine coolant stays around 100°C, engine oil is around the same temperature

It drives me crazy cause I use it as a daily and burns through my pocked with that fuel usage (20-25% more than it should). In mixed driving I get around 15l/100km (15MPG) without any hard accelerations.

I am planning to do the following hoping I might find the issue:
- replace the fuel pressure sensor (a few times I got the P0191 error and was showing fuel rail pressure at 500-600 kpa, so 10-40 times lower than normal, goes away after turning the engine off and on again)
- try to see if the dealership can override the current ECU fuel map with the stock one, to make sure the previous owner did not try to tune it or anything
- replace the upstream O2 sensor
- take down the GPF and have it checked physically at a specialized service to make sure it is not clogged
- replace the MAP sensor
- replace the HPFP

If anyone can help me in this matter would be great!

 

[Snicklefritz]

I want to hear what gets shared, so, subbed.

Most of what you said is "Greek" to me. Ha hah.

 

[Smeeps]

Have similar issue on my 3.3L. Subbed to see the response

 

[Thomby]

even the 3.3 Stinger gets better fuel economy

29mpg at ~70mph, and 24mpg at ~82mph, do seem a little low. In my 3.3 I can break 24mpg on 50-55mph country roads with minimal braking, and on the highway at 80 I'll gradually work up to 28-29mpg.

I'd expect the 2.0 to do a little better due to friction & pumping losses, but maybe not as dramatic for steady-state cruising, because you need the same amount of power (as opposed to driving around town, where accelerating with less power from a smaller engine means additional savings).

Have you calculated mpg via tank mileage & amount to refill, or are you just going off the display? Have you checked hoses etc under the hood? I noticed low mileage for a tank of gas last year, and after re-situating the catch can hose in the PCV circuit, it went back to normal. Not sure if it was kinking, or maybe just a bad batch of gas. Can't hurt to try another station.

 

[dincaalin]

29mpg at ~70mph, and 24mpg at ~82mph, do seem a little low. In my 3.3 I can break 24mpg on 50-55mph country roads with minimal braking, and on the highway at 80 I'll gradually work up to 28-29mpg.

I'd expect the 2.0 to do a little better due to friction & pumping losses, but maybe not as dramatic for steady-state cruising, because you need the same amount of power (as opposed to driving around town, where accelerating with less power from a smaller engine means additional savings).

Have you calculated mpg via tank mileage & amount to refill, or are you just going off the display? Have you checked hoses etc under the hood? I noticed low mileage for a tank of gas last year, and after re-situating the catch can hose in the PCV circuit, it went back to normal. Not sure if it was kinking, or maybe just a bad batch of gas. Can't hurt to try another station.

I did the calculated mpg on the screen vs the real usage at the pump from one full tank to the next and the computer shows correctly the fuel usage.

Recently I had to replace the air intake part (brittle cracked plastic that goes on the turbo) and then I rechecked all the hoses including PCV, nothing was wrong and the issue persists.

The fact that I drove another identical Stinger and that one got better fuel economy (same as many other 2.0 owners reported to me) is a big indicative that only my Stinger has an issue.

I tried all kinds of gas from lots of places (from normal 95 octane up to premium 100 EU octane, that is around 91 up to 95.5 US octane), the fuel economy issue remains.

I've attached another picture of the previous spark plugs. Clearly the engine is burning a lot rich, but seems to only report 3-5% rich over the stoichiometric A/F ratio during driving.

 

[GT Elite]

This could be an issue of faulty fuel injectors (sticky - staying open).

 

[Thomby]

This could be an issue of faulty fuel injectors (sticky - staying open).

Maybe, but I'd expect the O2 sensors to pick this up, try to trim fuel back to correct it, and eventually throw a code if it can't. Also, @dincaalin reported normal lambda (A:F) readings:

The O2 sensor reports fluctuations between 0.95 and 0.98, where 1 is stoichiometric

Where are you pulling these values from? Averaged over some time window? When you're cruising off- or light-throttle, the car will lean way out. It's been a while but I think I used to see 16-19:1 in the JB4, which would be like lambda 1.2, 1.3.

I'd expect an AFR issue to throw some kind of code, but maybe an O2 sensor is acting up without failing completely?

 

[dincaalin]

Where are you pulling these values from? Averaged over some time window? When you're cruising off- or light-throttle, the car will lean way out. It's been a while but I think I used to see 16-19:1 in the JB4, which would be like lambda 1.2, 1.3.

I'd expect an AFR issue to throw some kind of code, but maybe an O2 sensor is acting up without failing completely?

Please check the images attached below. I took these readings when I was holding constant accelerator pedal to maintain speed on the highway. The thing is that the O2 readings were the same on the other Stinger that I drove, the one without the fuel economy issue. I never see it going over the value of 1 when driving, it always burns a little bit rich.

From my research I concluded that the O2 reports just that, the remaining oxygen that's getting out of the cylinder, not the amount of unburned fuel. So maybe there is a problem with the injectors or HPFP that give the car more fuel, but not more air (as per the A/F ECU map), so not all fuel is getting burned.

Also I got again that pesky P0191 code yesterday while driving in the city (and also reporting 500-600kpa pressure, which basically are the low-pressure pump values), but couldn't check the HPFP physically (touching it to feel vibration, listening for the sound it makes). I hope that I can do that the next time to see if anything seems different with the HPFP. If it does, then I guess it is failing. If is the same, then maybe the pressure sensor is malfunctioning.

I'll keep you guys posted in case I have a breakthrough.