My MY04 engine seems to take a long time for rpms to drop (even when at operating temp). What controls the engine deceleration and is it adjustable?

 

Do you mean that it idles fast for longer than seems necesary to warm up?

 

No, if the engine is at 5000 or so rpm, it takes a while (too long I think) to reach idle after I take my foot off the gas. This was (in the 80,s) contolled by a decelereation valve. I expect it is now electronic.

 

Ah, I know exactly what you mean. My '04 does the exact same thing. (So did my '88 Accord. Go figure. The Integra and '00 S2000 did not do this at all.) I asked this very question a while ago and didn't get anything I could work with. My guess is that it's a function of the ECU Honda decided to add in for some reason. I guess we'll just have to learn to live with it - although I'm curious how it'll run with an '03 ECU...

 

The reason this is an issue with me is that it affects my shifting. I instinctively wait for the engine speed to match the transmission on upshifts and this delay is annoying.

 

Indeed, it's not exactly to my liking either.

Here's my previous question on this subject:

https://www.s2ki.com/forums/index.ph...pic=214049&hl=

 

I see there was no resolution in the earlier thread. Odd that R11 has an 04 and didn't notice the delay. Like you said in your thread, it seems to be about half a second delay. PIA

 

Not sure if this helps any...
my fiance's RSX used to do this. It was VERY annoying... then we changed over to the AEM CAI and I noticed it stopped doing that. Atleast on the RSX, the airbox seems to have something to do with this.

 

Both an intake and lightened FW help greatly reduce this. I have a late year '03, and when I went to the Comptech intake and FW, there was a dramatic change in the delay. It's not to the point that it's harder to shift due to rapid RPM drop either.

 

I think it may have something to do with the Idle Air Control mechanism. If you take your foot off the gas, the throttle plate closes on the TB. However, in order for the rpms to stay elevated as long as it does on some '04s, air has to get into the intake manifold somehow. This is the job of the IAC - this has an air bypass that gets around the closed throttle plate. Notice the hole just in front of the throttle plate? I believe this is where the IAC gets its air from to maintain idle. I'll bet the '04 ECU tells the IAC to function slightly differently. Why? Maybe only Honda knows.

ps. Shifting rapidly with elevated rpm has never been an issue for me. In fact, in some instances, this is desirable (not letting off entirely on the gas). It does make the syncros work a bit harder but I've never found this to be damaging.