Suzuki Swift / Honda Vezel (Fit) Rich Condition

Hello,

I'm trying to figure out aboutslightly high MAF and MAP readings at idle, which is making the engines run rich both when idling and driving. The total fuel trim is around -20% to -25%.

Car details: a Honda Vezel (Fit) 1.5L GDI and a Suzuki Swift 1.3L MPI. Both cars show slightly higher MAF/MAP values compared to other cars that are running normally.

There are no leaking injectors, and the fuel pressure on the Suzuki Swift is within the normal range. I thought the problem could be a stuck closed fuel pressure regulator in the tank, causing the rich condition. But with the GDI engine, I'm not sure if it's an electronic fuel system or a mechanical returnless system. Another possibility could be that the EGR valve is slightly open.

Thanks in advance for any help!

Updated: 11/07/24
The customer returned for a follow-up appointment, still concerned about poor fuel mileage. We found that the car was running rich, with fuel trims showing about -20%. After closely inspecting the MAF sensor, we discovered a slight bend in one of the hot resistors. We carefully readjusted it, which resolved the issue. However, we recommend that the customer replace the MAF sensor to prevent future problems.

As for testing for engine oil contamination with fuel (gasoline), is blocking the PCV hose and observing a positive shift in the short-term fuel trim a valid approach?



Suzuki swift was fixed by cleaning MAF harness & reading drop down to known good value. Car was still running rich may be because oil contamination. oil has 500 kms mileage on this rich condition.
Is it better to change oil or shall i drive the car on high cruise for some time, to burn out that gasoline.

fuel trims got better while on a short trip of 20 kms on 80 km/hr speed. Drop from -20 to -10 , Sometime while driving it move to +ve fuel trim as well. But on deaccel it goes back to -20 and took time to back positive.

Thanks in advance.