Fuel trim question - 98 Civic

[98civex]

Have a 98 civic EX all stock. Today out of curiosity I wanted to check the fuel trim levels. Currently car runs with no issues but a slight exhaust leak before the catalytic converter.

With engine warm in closed loop at idle I observed fuel trim and short term was fluctuating between +3-7%. Long term was at +4-5%

I was wondering if the long term trim was within a "normal" range. I read a while back somewhere that these cars are meant to run lean at idle for fuel economy, but not sure how true that is.

Currently looks like it's running lean so fuel is being added thus the +% values.

Curious for inputs. At idle the car isn't 100% smooth you can feel tiny vibrations but at 185k I attributed that to normal

 

[98civex]

The exhaust leak is causing the primary O2 sensor erroneous high O2 readings, which in turn cause the ECU to tell the fuel injectors to increase fuel to the cylinders.

Fix the exhaust leak immediately.

Are you sure? The leak is at the front flange of the catalytic converter.

Outside air is making its way to the primary o2 from there?

Also observing the primary o2 voltages they fluctuate from .2-.8v. Isn't that normal fluctuation?

 

[98civex]

Upstream at the exhaust manifold


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[98civex]

Yes and the leak is at the flange where exhaust pipe A connects to the converter

 

[98civex]

I'll check next time I drive on the highway.

Only idle "problem" I'd say is slight vibrations felt inside the car. Very slight as in you'd have to pay attention to notice but other than that it idles 620-650 rpms


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[98civex]

Manual


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[98civex]

Thanks I've done that before. It says 670 +/- 50rpm so looks like I'm within specs


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[98civex]

Is it unsteady? You should be able to set the idle speed to read a constant/steady 670 rpm.

On the stock tach it looks steady. If I have an OBD2 connected to read rpms is when it shows the slight fluctuations

 

[98civex]

I can set my Civic's idle speed to read a solid steady 670 rpm when reading live data on my OBD2 tool. My experience is that an unsteady OBD2 tool reading generally indicates a dirty IACV and/or the idle speed set slightly too low.

Might be interesting to see if a steady idle speed also changes the fuel trim readings at idle.

When was the last ignition system tune up?

I've played with the idle setting using same procedures you attached and I could never get a steady/constant idle it would always fluctuate 10rpms.

Just rechecked and my idle goes from 655-675 for brief moments it'll be at 670

Also after driving for a while I rechecked fuel trims and now the long term trim was steady at 0.8%. Short term still +5%. Then I revved engine to 2500-3000 rpms and short term fluctuated -5% to +3%

 

[98civex]

Cool thanks. Does that idle screw open the throttle body when adjusting it?


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[98civex]

No. It bypasses air from the IACV hole in the TB to the IM.

Does the IACV adjust to changes made by the screw?


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[98civex]

The ECU (brain) adjusts by slightly opening or closing the IACV passage way.

What would cause the need for the Idle screw needing to be adjusted?

The extent of cleaning the IACV was cleaning the screen and both holes and corresponding holes on the throttle body.

Would it need to be cleaned beyond this?


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[98civex]

After trying to adjust the screw I just put it back to where it was from the factory. Is this considered adjusted too low?

From what I remember there are two holes on the IACV and one of them has a metal mesh/screen.

I cleaned it with throttle body cleaner, q-tips, and an old toothbrush

 

[98civex]

Did you remove the IACV from the TB? There's more to an IACV than a screen. In fact, my IACV has no screen.

Can you post pics?

I did not take it off the TB. I found a pic online of it. Looks the same as mine. The screen is on the left

 

[98civex]

The screen is built into the gasket. Did you remove the gasket and thoroughly clean the valve mechanism. A sticky gunk covered valve mechanism is what causes idle problems.

Oh right it is the gasket. I remember doing that now. It's been about 2 years.

I sprayed the mechanism but hard to really clean it out. Think cleaning it with an ultrasonic cleaner would help or cause issues?

Also if you remove the coolant lines from it do you need to bleed the system after?

 

[98civex]

Thing is even when I cleaned it two years ago the idle still fluctuated the same as it does today.

I'll try cleaning again though. How do you confirm the mechanism moves freely?


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[98civex]

Two years ago when I adjusted the idle. PCV valve was changed then too with an AutoZone brand. Does that matter?


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[98civex]

Only code is p1456 for evap that I can't figure out. Put it on a smoke machine and everything. It'll pass evap check if outside temps are 70+ but anytime they're not the code gets thrown


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[98civex]

Food for thought. A very small vacuum leak could throw that code and unsteady the idle speed.

View attachment 29327

Hmm I've looked for vacuum leaks with a homemade smoke machine that I injected into the intake but didn't see anything.

This was all when I was trying to steady the idle two years ago. Code's been coming on since

 

[98civex]

Have you tested the fuel cap for a leaky seal?

Yes and I've replaced it along with the filler neck.

Also replaced the two way valve and bypass valve for the evap