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I have a 1998 Civic EX with D16Y8 engine and standard transmission. Have owned it since September 2005. I don't remember it doing this back than, but for the past year at least, it has had a hesitation/stumble when accelerating. It's almost like someone learning to drive a standard when accelerating, but no matter how hard you try, you can't keep it smooth. Seems to be worse when cold but I can't guarantee it. I have 2 other cars so this Civic literally only gets driven like 300 KM's a month. It's basically a to work and back car and that's it. I live 6.3 KM's from work so it's 12.6 KM's a day for 6 days a week equaling around 305 KM's a month. It's been driven this way for about 3 years now so I always make sure to de-carbon the throttle body, passages, IAC valve etc. since I know driving such a short distance all the time is not very good on an engine. I work for a dealership in service so I try to keep up on maintaining all my cars. My Civic doesn't set any SES light at all. But that could be because it never drives far enough to set a code or run all the on-board tests?
Anyways, I have multiple scan tools and understand all the parameters and working of fuel management etc. The only thing I find is that at idle, the Fuel Trims are good, within 6% or so, but when I rev to 2500-3000 RPM's, the Short Term Fuel Trim really drops to -15 to -20% and the rear 02 stays Rich. So for some reason, it's not getting the amount of air into the engine that it expects, or, it's getting more fuel than it expects, either way it's the same thing. Too much fuel is the same as not enough air. So my question would be why? Air filter is brand new, throttle body nice and clean, IAC nice and clean, etc. My throttle cable is adjusted to have exactly 1.0cm of deflection as per the specs.
The only thing I haven't been able to do or verify is Fuel Pressure. It's possible that Fuel Pressure is increasing too much when there is no vacuum on the Regulator.
The last piece of the puzzle that may or may not be related is that sometimes, after warmed up, if I step on the clutch pedal to disengage completely, the engine will sometimes ALMOST stall and then pick itself back up again and idle fine. So it's almost like the IAC is not picking-up the change in engine load quick enough or something? This would kind of indicate again, that the ECM is expected MORE air based on the TP and MAP than the engine is actually getting, once it realizes this via the rear 02 it starts to cut the Fuel Trim down by a lot.

Anyone with some ideas here? I was thinking I might just follow the Trouble Tree for Code P0172 even though it hasn't actually set yet.
THANKS!

JEFF

 
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Never mind watching the rear O2 - what is the front O2 saying? The rear O2 (on any OBDII vehicle) is only ever monitoring the catalytic convertor's efficiency. Also, make sure you do a fuel pressure and volume check. Without knowing what the front O2 is reading and what the fuel pressure/volume specs are, I would guesstimate the front O2 is your culprit - it usually is!

Good luck!

 
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I notice my MAP Reading on a Scan Tool is 30 KPA at idle, which converts to only 9 inHG. I thought a car should be AT LEAST 15 inHG at idle!?