Until a couple months ago, I had no idea what "fuel trim" even was. It wasn't until I got a Bluetooth OBD2 monitor and the Torque App that I took a look at it. This website explains it pretty well:

Fuel trim can be a valuable diagnostic tool - Eastern Manufacturing

So basically, being at "0" indicates that the ECU is doing a perfect job of providing the exact amount of fuel and air into the cylinder. However, in an imperfect world, it's almost always going to be at least +/- 3% since adding an absolutely perfect amount of fuel to the cylinder isn't an easy task for the ECU, even in the day and age of computerized throttle bodies and fuel injectors. Staying -10% to +10% is usually considered acceptable for older engines that naturally have older sensors and cylinder ring or vacuum leaks. If you're above (+/-) 10% or so there is a significant problem which could affect drivability and should be addressed. A positive fuel trim indicates that you're running too lean (too much air) with a negative fuel trim indicating that you're running too rich (too much fuel). There's a short-term fuel trim which is more of an immediate reaction based on your O2 sensor readings while long-term fuel trim is where the ECU "learns" over time, what the common condition is inside the respective cylinder bank. I believe Long-Term fuel trim is one of those things that the ECU has to re-learn when the battery cable is pulled. But it's constantly adjusted on a more minor scale based on driving style and the health of the engine.

Before I had an oil control piston ring fixed, I was pulling about a -12% to -15% fuel trim on bank 2 and a +8 to +12 trim on bank 1. So basically out of spec on both, especially considering the mileage on the engine and the fact that the J-series is a pretty well-tuned machine. Too rich on bank 2 and too lean on bank 1. I assumed bank 2 was running rich due to oil burning on cylinder 5. I had no idea why bank 1 was registering lean though. After the mechanic tore the engine apart and replaced the rings as well as all the seals, both trims now only range from about -1.6 to +3.0 which is well within normal operation. After speaking to him about it, he said that removing the intake manifold or runners may cause an air leak in just one cylinder if the bolts are not torqued correctly or if the gasket isn't in good condition, allowing a bit of air to infiltrate into the runners. I'm guilty of both. I've removed the runners from the head and removed the intake manifold from the runners, never having replaced the gaskets and torqueing by feel rather than with a torque wrench. I also learned that overtorquing can be as bad an undertorquing since it can cause the gasket to flex or squish too much in one area, letting in whatever is supposed to stay out.

The reason I added this to the performance sub-forum is because of the significant power increase that the engine put out after having the problems fixed. We tend to concentrate on power adders but also need to make sure the engine is healthy in the quantitative regard - more than just making sure the oil is clean and we don't have a clogged air filter (see bolded items below) I wasn't expecting it, but the difference in power, especially low end torque, was definitely significant. Based on the very scientific butt dyno the difference was at least the same as after I did the full exhaust (high-flow cats, j-pipe and magnaflows).

Remember that the ECU's first priority is drivability and reliability, with performance being 2nd after those other items are addressed. The ECU will rob performance all day long if it's necessary to make sure the engine runs somewhat "correctly" and doesn't cause damage (ie. spark knock damaging the engine, bad fuel/air mixture damaging catalytic converters, etc.). The ECU attempts to compensate when running too rich or lean, but having your fuel/air ratio off can result in a loss of power due to a less-than-ideal mixture of fuel and air in the cylinder which produces less power output. Not to mention potential damage to the catalytic converters over time and lower gas mileage.

It's super easy to pick up a Bluetooth OBD2 adapter on Amazon or eBay. Mine cost $15 on eBay. I got the Veepeak and it works great. Then download the free Torque app which connects to the adapter and provides a ton of info. I purchased the "Pro" version of the Torque app for $10 which provides a lot more features. For $25 total, it's a great tool to have, and can give you some good info that will help performance with monitoring the following and making changes where needed:

Fuel Trim - Already discussed. Probably one of the biggest things you want to be in the proper range for both performance and engine longevity.
Intake Temps - Helpful if you want to modify your intake to see if you can pull cooler intake air compared to ambient. I've reduced my intake temps by around 5-10F by making some changes to the 4G stock "cold air" intake.
Coolant Temp - See how hot the engine is getting. Too hot may indicate a bad thermostat or cooling system clog. This can also be used if you're looking to do any engine-bay cooling mods.
Timing advance - Pretty big deal, especially in Honda engines and VTEC. If the engine is pulling timing due to low octane fuel or spark knock, this value will come down significantly, which of course affects engine output.
Spark Knock - The "Knock Detector" is a free add-on to the Torque app which will tell you if the knock sensor is detecting any problems, as well as how much timing is being pulled due to knock. When the ECU pulls timing it's not black and white. It may pull just a little bit or a whole bunch based on the frequency and amplitude of knock that is being detected. Before I had my oil ring fixed I was routinely getting knock, presumably from carbon buildup which was very thick on the cylinder head and exhaust valves. Sometimes it was pulling up to 12 degrees on timing. After fixing it, I've managed to get it to knock only a couple of times at very high RPM's and once with full throttle at low RPM's with the transmission locked in a high gear using sport mode. The highest I've been able to get it to pull timing from knock has only been 0.5 degrees. At that low of a level, performance is hardly affected. Heck, at that low of a level, it may very well be background noise setting off the sensor.
Diagnostic code identification and reset - Not so much a performance issue but is obviously helpful in detecting CEL codes and resetting them without pulling the battery or fuses. Anyway, CEL issues often affect performance (ie. faulty sensors, misfires, stuff like that)

Keep the engine happy and it will keep you happy

By the way, if I'm off with any of that information, please feel free to correct me or add clarification.

 

Thanks for this reminder - I'll need to dig up the old ELM and check out some things on the TL

 

I’m running between -4% and -7% for both of my banks for a 2011.

 

How many miles do you have on the engine? You're not past the -10% threshold so it's not that big of a deal. You might want to look into some of these possibilities though:

Running too rich – High negative fuel trim corrections can be caused by MAF sensor problems, high fuel pressure, leaking fuel pressure regulator diaphragm, faulty evaporative emissions components, leaking injectors, defective O2 sensors, exhaust leaks/pinholes before the O2 sensor, coolant temp sensor problems, and base engine issues such as low compression and incorrect camshaft timing.

 

I'm fairly sure it might be related to the evap, I got a code for low evap pressure or something similar a while back. Could also be my intake manifold out of torque spec or the tune that I'm running, but I'll be replacing my manifold this weekend. About 140,000 on the engine.

 

Not a bad idea. Might as well give it a stab.

It looks like we have identical setups with trim, exhaust and tires. I haven't done the Ktuner and J37 IM and TB. Did you find that the IM and TB made any sort of difference? How significant was the tune?

 

Couldn’t tell you yet, I had to take it off to get some studs for the the mounting points on the side of the IM. I think it’s something to do with the EGR. Should be on sometime this weekend. I’m curious to see if that will have an effect on my fuel trim. Right now I just have a basic tune, definitely drives better than before. Raises the redline to 7100 and disables the limiter.

 

Just bought one, can't wait to see it working. I want to know my engine temp when I am in cold climates and this is a perfect tool to do that. To bad acura won't give us on board diagnostic stuff in the navigation. Well maybe they do but I sure don't know how! Just give me oil, engine temp and tranny!

 

Eh, now a days it seems auto manufacturers don't provide any info on engine conditions other than the basic alternator light and coolant gauge - which is totally inaccurate since it sticks at the same gauge level as long as the coolant is between 170F-230F. The coolant temp is interesting to monitor. It rarely dips below 168F since the thermostat starts to close at around 170F or so. I get it up to 210F-220F when going up hills. And interestingly enough, it still doesn't go over about 220F even in the summer. Once the fans kick in, the coolant temp drops very quickly.

The fuel trims are a bit interesting as well. They often drop below 0 during city driving but head back to 0 when cruising. I'm guessing the constant stop and go causes the engine to run slightly rich at times. I monitor intake air temps as well along with catalytic converter temps for both banks since I have high flow cats from RV6 which I've wrapped and want to make sure they don't get over 1800F. So far the hottest I've gotten them was about 1750F when I was racing a turbo Kia Optima SX up a hill for about a mile. I got about two car lengths ahead of him but he started closing in pretty good before I shut it down. 100mph in a 55

 

 

Maybe it was due to the turbo delay on his part. That and I upshifted to 6th sooner than I needed to. Probably should have kept it in 5th.

 

 

I thought the 4G top speed is governed at 130.

 

^ Couldn't say. The highest I've gotten a tad above 100mph. And only that one time. I've turned off the speed limiter using K-tuner but I don't know under what circumstance I'd go over 130mph.

 

 

Thanks Losiglow for the info. Are Long Term fuel trims only available in the Torque Pro app? I have the Torque free app and cannot see any Long Term Fuel Trims, only Short Term fuel trims. Would like to know before I purchase.

 

PMd you a link

 

Will data logging with KTuner give you a great deal of the same info?

 

I can't recall if KTuner shows fuel trims or not. But if it does, then logging would indeed provide some data that might be useful. Long term fuel trim is normally what I keep an eye on, since short term can change considerably from one driving condition to another. Short term could still be useful if trying to detect a specific problem but otherwise, monitoring long term to make sure you're not consistently running too rich or lean seems sufficient.

 

Yes