GMC Terrain Diesel Running Rich, Power Loss Issue

Had almost the exact same issue with my 2017 GMC Acadia diesel last year. As someone with some hands-on experience working on cars, those symptoms immediately pointed to fuel injector problems. The symptoms you describe match perfectly, poor fuel economy, rough idle, and power issues are classic signs of faulty fuel injectors. In my case, one injector was completely failed, causing an imbalanced air-fuel ratio and throwing off the entire fuel delivery system. Took it to a workshop where they diagnosed a defective fuel injector. They had to remove the intake manifold to access and replace the problematic injector. Total repair came to 515 Euro including parts and labor. After the fix, fuel economy improved dramatically and the rough running completely cleared up. Before jumping to conclusions though, could you share: Current mileage on your Terrain? Any recent maintenance history? Are you noticing any smoke from the exhaust? Have you scanned for specific error codes? This info would help confirm if your issue matches what I experienced. Could also be worth checking the catalytic converter, as similar symptoms can show up when its getting clogged.

Thanks for the feedback. My Terrain has 53433 KM and last service was done just 2 months ago. No smoke from exhaust but I am seeing black residue around the tailpipe. I should mention the engine seems to run better after warming up, suggesting the oxygen sensor might be the culprit. I checked with an OBD scanner and got codes P0172 and P0102, pointing to a rich fuel mixture and possible air filter or intake manifold issues. The rough idle tends to be worse in the morning, gradually improving as the engine reaches operating temperature. When the fuel injectors were checked during my last service they appeared to be functioning normally, which is why I am leaning more toward an air intake or sensor problem affecting the air-fuel ratio.

Interesting you mention the codes and warm-up behavior, this actually aligns perfectly with what I discovered later with my Acadia. While I initially thought it was just the injectors, I ended up finding out the Mass Air Flow (MAF) sensor was the root cause, causing similar rich mixture issues. The P0172 code combined with the black residue and improved performance after warm-up strongly suggests an air measurement problem rather than a fuel delivery issue. The rich mixture is likely happening because the engine computer is getting incorrect air flow readings. After replacing my MAF sensor (120 Euro part), the check engine light cleared and fuel economy returned to normal. The installation took about 30 minutes, its located between the air filter and intake manifold. Would recommend checking: 1. Air filter condition 2. MAF sensor for contamination 3. Intake manifold for vacuum leaks Given your recent service cleared the injectors, focusing on the air intake side makes sense. The black tailpipe residue is a clear sign of rich mixture, which perfectly matches my experience before fixing the MAF sensor. The P0102 code specifically points to low MAF sensor input, which explains the rich mixture as the computer compensates by adding extra fuel.

The codes and improved warm-up performance are giving me more clarity on my issue. Really appreciate the detailed feedback about the MAF sensor experience, makes a lot of sense given my symptoms and matches what I suspected about the air-fuel ratio being off. Going to focus on the air intake system first. The black residue around my tailpipe definitely confirms the rich fuel mixture issue. Planning to check the MAF sensor and air filter before moving forward with any repairs. The oxygen sensor could still be involved, but the P0102 code suggesting low MAF input seems like the more likely root cause. Will clean the MAF sensor first since its a simple fix, then look at replacement if needed. Good to know its a straightforward installation and relatively affordable at 120 Euro. The check engine light and rough idle are definitely my biggest concerns, so hoping this addresses both issues. Thanks for helping me narrow down the problem. Will update once I check these components and see if cleaning or replacing the MAF sensor resolves the rich running condition.