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High Negative Fuel trim

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Hey Scotty, I’ve been having issues with high negative fuel trim. 2002 Honda civic ex coupe D17A2 205k miles. I’ve been using a Bluetooth obd2 scanner to monitor live data and have noticed my short term fuel trims are running negative, even at idle they fluctuate around -11% up to -15% at times. My idle feels rough and almost like an occasional misfire but no CEL. When I’m driving for example going 45mph and I let off the gas I will sometimes feel a jerk and I notice when the engine does that the fuel trims will drop even more negative. I have changed my oil and filter, changed all my spark plugs, checked vacuum, rebuilt entire exhaust to endure no leaks. I noticed my upstream o2 sensor was not switching as fast as what I think it should comparing it to videos I’ve seen on YouTube. I have a new upstream o2 sensor on its way soon. If this doesn’t fix the negative fuel trim, what should my next step be? Thank you!

4 Answers
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Scotty answered at 4:42

1

have you checked the engine air filter?

Yea my filter still looks fairly new. Did the test by holding it up to sun and could see the light passing through

1

Maybe one or more fuel injectors aren't spraying properly. Have an injector balance test done by a mechanic or run some cleaner through it first.

 

I installed the new upstream o2 sensor today. I set my live data on obd2 scanner to show upstream o2 & Downstream o2, short term fuel trim & long term fuel trim, intake manifold pressure Psi @ Kpa. Wish I could upload the screen recording. After starting it up the short term fuel trim fluctuated from -3, up to -24 at times. Intake Manifold Preasure was fluctuating also I noticed when it would jump as high as 90kpa the short term fuel trim would drop closer to 0 but then go back to high negative again. I have a bottle of tune up I could add but I don’t like driving with it running like this. I will take your advice on the fuel injectors. I have listened to them with a screwdriver and noticed 1 of them doesn’t click as loud as the others. I have not been able to smell any gas in the exhaust it around the injectors. I always track my mileage and my last fill up I averaged 27mpg city/hwy. that’s the best I’ve gotten in quite awhile. I would think that if an injector was leaking I’d smell gas and my mpg would tank. Thanks for the advice though it’s been ok my radar as a possible culprit.

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Posted by: @trevorlane

Wish I could upload the screen recording.

just get the youtube app on your phone and upload it there.

Posted by: @trevorlane

After starting it up the short term fuel trim fluctuated

 

STFT is supposed to fluctuate. It's the long term you should pay attention to.

You also didn't say if this was idling, or driving or what.

When you change O2 sensors or anything like that, the computer has to perform a fuel trim relearn which takes a bit of driving.

Posted by: @trevorlane

Intake Manifold Preasure was fluctuating also I noticed when it would jump as high as 90kpa

This isn't terribly useful information without knowing the throttle position, engine RPM, etc.

90kPa is still a partial vacuum.

 

There’s a link to live data I recorded.

I ended up taking my injectors out and bench testing. Was shocked to see that only 1 appeared to be working correctly. 1 had a split v spray pattern, one was just dribbling, and the last one was completely blocked. Not sure how this wouldn’t have set a CEL. New injectors are on the way.

interesting. How did you test the injectors?

I used a car battery. Took 2 wires with alligator clips, attached one to the positive and one of the terminals on the injector.
I had a water bottle held in place with a jack stand, and placed the injector tip through the top of the water bottle.
I used multiple pieces of tubing starting with one that fit snug around the head of the injector. Used gradually smaller tubing down to the size of the red straw that comes with the can of carburetor cleaner.
I clipped one end of the other wire to the other terminal. Then I just sprayed a couple bursts of the carb cleaner while simultaneously tapping the other end of the wire to the negative terminal on the battery.

I was trying to mimic how the injectors would be acting as if they were installed on the car.

I know there’s equipment that’s made to test way these things more
accurately but unfortunately it’s not in my budget! Also I really enjoy learning how things work on my car.
My transmission is the infamous BMXA and it’s starting to fail so my next task is going to be to rebuild it once it finally kicks the bucket.

Nice. I like it.
just be careful not to energize them too long or you might damage them. Injector pulses only last fractions of a second.
Also keep in mind they usually get 50ish PSI (or more). If only one injector worked the engine wouldn't run at all.

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