I've been having fuel trim issues lately. No CEL, but at idle trims for both banks are around 20%-22%. Under load it drops to 10%-14%. I could have had these issues all along but I only started to monitor after I had headers installed.

I'm still under CPO warranty so I took the car into the dealership and asked them to check the intake manifold, IM gaskets, PCV, and other Vac lines because I suspect a vacuum leak. Since LTFT is higher at idle my guess is a vac leak.

I then told them if they can't find a leak to check the MAFs and possibly a leak at the header flange.

Well...they came back and told me that since the secondary O2 sensors are turned off it's causing my fuel trim problems. They also asked why I have extension cables on the rear sensors...I had to explain to them that the rear O2 sensors are to monitor cat converter performance and don't feed into the fuel trims. Of course I also had to explain that the sensor wires aren't long enough to plug in so extension cables are required.

First, can someone validate that the rear sensors don't impact fuel trims? Second, assuming that statement is correct, what should I tell the tech/SA to look for? Do dealerships do smoke tests to check for vac leaks? This is my first Merc so I'm not sure how deep they go into diagnosis.

I'm still on the V5 header tune so I told them I'd swing by and flash the stock tune back to show them that even with the rear o2 sensors on, the fuel trim issue still persists. Any other advice other than pull the car from the dealership and pay a performance shop?

As an aside, I also have ROW boxes with AFE filters. Again, no CEL at this point.

 

Most modern cars these days can use the secondary O2 data to alter fuel trim. Not sure what disabling them completely would do though.

Turn them back on, use a defouler and see what happens.

 

Certainly not questioning you, but then wouldn't most people running the v5 header tune have the same fuel trim issues I have?

I'm going to flash it back to stock to see what the trims do. I already tried it and had the same issues but I'll do it and let the tech's diagnose. At the same time I'll do as you suggest and have Jerry turn them back on.

Last question, if the rear sensors provide data used for fueling, wouldn't pulling them out of the exhaust stream using defoulers cause sampling errors? Meaning they wouldn't be sending accurate data?

 

I don't even have rear O2s anymore. Mine were removed completely and I made simulators to plug in their place (to get rid of the heater errors). No fuel trim problems here.

 

Maybe they do have high FTs, but since they're within spec with no CEL they don't know about it.

From what I have read, the secondary O2 data can affect fuel trims if there's something drastically wrong. Plugged cat etc.

That's why I wasn't sure if having them disabled could do something wonky.

 

Closing the loop on this (sort of) because I hate searching a topic and finding an old thread where the OP didn't come back and state what they found out.

In any case, the dealership couldn't find anything wrong with the car, other than the fuel trims being too high (but not high enough to throw a CEL). We put the stock tune on and had the same issue which told us on or off, the secondary O2 sensors weren't the issue. They had their AMG Tech inspect everything without ripping apart the car and throwing parts at it. And, since it didn't have a CEL they didn't want to do expensive work which can't be claimed under warranty, thus screwing me over.

So, their advice was "just drive it. If a CEL comes up bring it back in". Obviously with fuel trims well over +10%, hitting over 20% at times, "just drive it" didn't sit well with me. Phillip from Eurocharged provided the solution for me, however.

He recommended that I put my stock airbox and filters back on since I had ROW boxes with AFE filters. We were fairly confident there were no exhaust or intake leaks since that was all tested. The approach was to return the car back to stock, starting with the cheap parts first. He had also seen threads where some had issues with ROW boxes. I put the stock boxes and filters back on and took the car for a drive.

With the V5 header tune still on the car, within minutes the fuel trims began to migrate back down to "normal" levels. Within a couple driving cycles the car now sits around 0% for both banks. It doesn't stay at 0 but hangs out between +/-3%.

I'm going to do some test to determine if it was the combo of ROW and AFE or if it was isolated with one of the products. But for now, it seems I'll be running with the stock boxes and filters.

 

try oem filter on row

 

This car doesn't have secondary o2 sensors. The only sensors are pre/post primary cats.

Has the car had headers since you bought it or was that something you added?

Also, put your stock intake/filters back together and that will fix your problem.

Edit: You already did and fixed it sooo yeah, sorry I missed that in your post.

 

Pre primay cat = primary O2
Post primary cat = secondary O2

 

You know, I had never considered that. My brain has been wired so hard to using pre/post and secondary being the secondary cats. Huh. Man, I probably confused a lot of people hahahahaha :. Thanks though, the lightbulb finally went on to the idea of people calling the "post o2" (in my mind) the secondary which makes perfect sense.

 

Sorry if I mixed up terminology. Looks like we got that sorted.

And I recently installed the headers with a catback so I was interested in checking the fuel trims. I monitored them religiously in my previous car but since my C63 was mostly stock (ROW + AFE) prior to the headers I didn't bother to monitor. It's very likely that my trims were messed up prior to the headers...which makes me wonder how many other people are running around with wonky trims and don't know it...

 

Trims like that are odd with just an airbox, as it shouldn't flow that much more air unless it isn't sealing well somewhere and allowing air to sneak in after the MAFs.

It's not dangerous to drive it like that though, people that panic over high trims just don't understand why they are there. It is the car properly reacting to extra air, in your case. The correct solution is to get re-tuned for the mods you have that added more airflow, if you checked and there were no leaks/faults. The stock tune is designed for stock parts, and can't always apply to any different part you add on.

FWIW, it usually takes 25% trims to trigger a CEL, so you were almost there. MB has more sensitive codes on the DI engines but according to OBD2 standards, it has to turn the light on when any trim exceeds 25% I believe.

 

Thanks. I appreciate the input, sincerely. I understand how trims work and was searching for a post MAF leak. However there were no leaks found, including checking that the lids were seated correctly. The car was tuned for the mods I have, albeit a canned tune from Eurocharged. The stock tune was tried so that the rear O2 sensors would function as intended. This of course was to eliminate them as a variable. Jerry at Eurocharged did not want to try to tune around this because based on the info we had, it definitely seemed like a leak.

Now, while running trims that high isn't the end of the world, I didn't like knowing that they were at the point of almost triggering a CEL. No reason to panic, as you stated, completely agree. But also not something I personally wanted to ignore. Given that it wasn't "normal" it was something I wanted to "fix".

 

You have headers and the Dealership didn't try to void your warranty or blame your issues on aftermarket parts ? I need to find a dealership like this ..

 

That was a really valuable addition to a 7mo old thread. Good job!