I’ve got a thinker if any of you folks are mechanically inclined. 2004 Honda Accord V6 Auto 156000 miles, starts strong, then idles rough after a couple of seconds, but when given throttle at idle or in gear, it runs beautifully. Tune up has been done, no obvious vacuum leaks, fuel pressure not verified but all plugs smelled of fuel. CEL is on with only one code, P2195 B1S1 O2 Sensor Lean. When looking at scan tool info, O2 sensors and fuel trim do reflect a lean condition. Throttle body was cleaned. Cleared codes, no change whatsoever and the code soon returned. All 6 cylinders are slowly affected. If watching scan tool data, one can see each cylinder in no particular order start to skip. The car never wants to stall, but it steadily misfires at idle only.

I’ve seen a TSB for this code on this year model, but do not know if This vehicle has had the ECU reset or not, nor do I know if the original TSB concern was only a CEL, or poor engine performance because of such as well. I also see that a valve adjustment is a common necessity for these cars, and may cause this code. Any and all suggestions are welcomed, this one is throwing me off. Thank you all!

Update after working on it more this morning. There is no Idle adjustment that I know of, it’s all the PCM through the electronic throttle body. The O2 sensor code has not come back since I rechecked spark plug torque. At the moment, I have absolutely no codes whatsoever, not even pending. However, when idling, I get about 1 misfire per cylinder, per minute, but only at idle. When on throttle (sitting still or under load) the car runs beautifully.

I am unaware of any idle adjustment on this vehicle. The throttle cable runs from the driver firewall to a plastic covered box on the passenger firewall with an electrical connector. The signal then appears to be sent to the electric throttle body which would control all action from the throttle blade. At the moment, the vehicle idles around 850 rpm with a steady but very slight misfire registering on all cylinders. About 1-2 misfires per minute at idle, none whatsoever when throttle applied.

 

Usually a misfire at idle only, is a valvetrain problem. Sometimes a leak down or compression test will show this, but not always.

The only surefire way is an oscilloscope and pressure transducer.

 

Could be a fuel problem or bad injector(s). Could also be a vacuum leak. I'd look at the manifold absolute pressure sensor readings @ idle and see if there is a fluctuation of manifold pressure or air getting into the engine that shouldn't be. A small vacuum leak could cause the fuel pressure regulator to fluctuate fuel pressure just enough to cause the fuel in the injectors to not deliver consistent amounts of fuel.

 

A bit of an update: compression test has been run, all checked out fine (although I do not have my books in front of me with the actual numbers). Vacuum gauge was steady 18in at idle, fuel trims did not jump out at me although again, no book in front of me with the numbers. I did notice that the CKP Pulser Learn readings on the scan tool show not completed, indicating the CKP hasn’t recognized the crankshaft profile? I’ve also heard once that because this vehicle has an aftermarket starter, not an OE one, that it may not be spinning the motor fast enough at start up? Has anyone heard of this?

Also MAP reads stead .9-1.1 volt at idle, while misfiring. Steadys our nearly altogether with throttle, and drives beautifully.

Because of the strange CKP readings, and my absolute lack of a check engine light sense cleaning my ignition coil grounds (but my misfire is still just as present as ever at idle only), I’m leaning toward the Honda ECU update?

Again, thank you all for your input.

 

Could also be ignition related. If its on all 4 cylinders and only at idle I doubt it's the coils, you might be on the right track with the ecu. Honda ecu's rarely fail but they can have internal electronics that burn out like resistors, capacitors etc.