Persistent Rich Condition (-26 Fuel Trim) on Suzuki APV 2009

Question:

I’m working on a 2009 Suzuki APV (1.6L, gasoline) that has no DTCs stored, but when checking the fuel trims, I’m seeing Total Fuel Trims around -26, indicating a consistently rich condition. I’ve known this vehicle for years, and it has always had a noticeable fuel smell. The client recently asked me to check it out, mentioning that this issue has been present for years and no one has been able to solve it.

Fuel pressure at idle is about 34 PSI and increases slightly under throttle. I tested the fuel pressure regulator by disconnecting the vacuum line—it responded as expected. I ran the engine with the vacuum line disconnected for about 30 minutes and didn’t observe any fuel entering the intake, so it doesn't seem to be leaking internally.

I cleaned the injectors and checked the ignition system—plugs, coils, and spark are all functioning well. Engine timing also seems fine, and the engine runs smoothly. While it may need an oil change, I doubt that’s the root of the rich condition.

Interestingly, the client reports that fuel economy seems normal and that the car feels well-balanced during daily driving. However, they do experience some power loss when going uphill.

Given this long-standing issue—with no fault codes, proper fuel pressure regulation, no injector leaks, and solid ignition performance—what other causes would you consider for this persistent rich condition with slightly reduced uphill power?

David, I am not familiar with this car. Does it have a MAF sensor?
Skewed MAF could account for a mixture issue and loss of power.
The same could be said for a bad O2 sensor.
If it has had a fuel smell for a while, the oil may be contaminated with fuel. Since it is overdue it may be a good idea to get it off the table. Not that I think it is going to be the fix, but depending on if/how contaminated it may have some effect on fuel trim

Hi Noah,

Thanks for your response. Yes, this APV engine doesn't seem to rely on any MAF sensor. It runs with the MAP sensor only. I disconnected and reconnected it to see if there would be any significant change in fuel trim behavior or general engine response, but surprisingly, it remained pretty much the same. The vehicle actually has a decent fallback strategy—it still runs reasonably well even with the MAP unplugged.

The MAP values I’m getting on the scanner appear relatively normal, nothing out of the ordinary.

As for the upstream O2 sensor: the current one is a new OEM replacement. The previous sensor was still switching, but it had a fault in the heater circuit. I personally confirmed the heater was open internally, which is why I replaced it.

Here’s what’s interesting: when the issue begins to manifest more clearly, the O2 sensor stops switching altogether—it flatlines, just as if it were no longer reacting. This behavior was present with both the old sensor and the new one, which reinforces my suspicion that the issue isn’t with the sensor itself but something else affecting combustion or exhaust flow.

Now that I think about it, I didn’t check post-catalyst sensor readings—because this vehicle doesn’t have one. It looks like the catalytic converter has been removed entirely, so there’s no post-cat sensor installed.

The oil was changed about a month ago, but due to the strong fuel smell, it’s obvious the engine is still contaminating it. So yes, I agree this thing is definitely running rich.

Unfortunately, I don’t have access to OEM diagrams or data for this model. I’ve had to rely on whatever I could find via Google. None of my software includes detailed APV info, so I don't have reference values or TSBs for this case.

The van itself is one of those microbus-style units that holds around nine people. What makes the case more difficult is that according to the client, this issue has been going on for years, and no one’s really been able to sort it out. He’s just been adapting to the car as it is. I wouldn’t be surprised if there’s something mechanical at play.

Fuel pressure seems stable. It held for 5 minutes, and during injector pulse testing, the drop across the rail was consistent across all cylinders. That seems to rule out fuel delivery issues.

Honestly, it's not the kind of case that keeps me up at night—but it definitely makes me curious.

Thanks again for your time.

Does your APV have a specific engine code? Mostly trying to see what US market vehicles came with the same 1.6L engine, if any.

What is the MAP reading at hot idle with no loads? What's your elevation? Does the MAP reading agree with a manual gauge?

Timing belt engine? My first instinct is that the belt is off. MAP systems tend to react with rich trims when there's a mechanical issue, in my experience.

I just wanted to pop in here and share an experience car I just finished up this afternoon .
I had a Mercury Grand Marquis with rich trouble codes and negative fuel trims across the operating range. Fuel pressure was too high. 80psi, spec. is 45psi.
This car ended up having a restricted return line. I removed the line from the rail and the pump and blew shop air through it. There was an audible "pop".
Afterwards, the fuel trims fell into single digit correction.
I'm not saying that's going to fix your issue, but I wanted to share it as this is the first time I have encountered this problem.

Soon the customer is going to bring the vehicle back to me, because I haven’t seen it again since then. One of the things I haven’t measured yet is the engine compression. But the customer says that the mechanics who have worked on it in previous years told him it’s in good condition. However, we’re talking about a vehicle that consumes about a quart of oil every month, so the engine can’t really be in good shape.
I did measure the fuel pressure with my pressure gauge, which is a cheap Chinese one ?. It’s not very reliable, but it read around 34 PSI. And when I accelerated, the pressure went up a bit and then dropped again. So it's actually a bit lower than expected—even though there isn’t much information available on these Suzuki APVs.
One thing that really stands out is that, despite the fuel trims being so high, the vehicle isn’t throwing any codes.
But thank you very much for your responses. I’m going to try everything you suggested, and now I know that a blocked return line could also be another possible cause.

The Suzuki APV likely uses the G16A (or G16AID) engine, a 1.6-liter inline-4 gasoline engine with the following specifications: Engine Code: G16A Displacement: 1,590 cc (1.6 L) Configuration: Inline 4-cylinder, SOHC, 16 valves, multipoint fuel injection (MPFI) Power: 91 hp (68 kW) @ 5,750 rpm Torque: 127 Nm @ 4,500 rpm  This engine belongs to Suzuki's G engine family, which has been in use since the mid-1980s and is found in several Suzuki models. The APV has used the G16A engine from around 2005 up to at least 2018.