I noticed this morning that when I'm coasting in neutral, my RPMs jump between about 800 and 1200. It's pretty fast, maybe once per second. I haven't driven much lately but I feel like I would've noticed if this was happening in the past. It only happens when I'm in neutral, and seems roughly correlated to my speed. When I stop, the needle stops jumping. Is this something I need to worry about? I'm about to depart on a 5 hour road trip. Also my car is loaded down with stuff that I'm moving, if that makes any difference. 95 Civic VX.

Thanks!

12/2/2008 10:56:05 AM

can you determine if it is the tachometer only, or if indeed the engine speed is fluctuating?


was the car at operating temperature? Might not have been warmed up all the way

12/2/2008 11:18:05 AM

faulty MAP sensor. you should be safe for your trip, though. just get it fixed when you get where you're going. be safe.-

[Edited on December 2, 2008 at 11:52 AM. Reason : -]

12/2/2008 11:50:48 AM

I don't think it's a problem to worry about. Traditionally, Hondas have had issues with fast idle or idle control once the car gets some miles on it. However, it's usually a problem that manifests itself when the engine is cold; the problem is related to the idle air bypass control.

The system regulates idle speed based on a whole host of inputs, as you well know. The problem lies in the fact that, although today's powertrain control systems are designed to "learn" an engine's optimum operating parameters, wear and degradation of input quality and/or quantity do take their toll...you've had your fair share of controls theory and know a bit about how feedback algorithms work. Think like this: the PID control algorithms that worked optimally when the car/engine were new(er) may not be as effective now. While ideally the ECM adapts to an engine's changing conditions, these conditions may fall outside of its ability to compensate. As a result, loop control instability may result where the ECM overshoots when correcting to achieve design parameters.

There are a bunch of physical factors that can come into play (carbon fouling of the throttle body and many of its orifices, EGR fouling (I don't think your car even has an EGR system; uses cam lobe overlap and flow reversion to achieve the same results - I THINK), deviation in calibration of the MAP sensor (which is a Wheatstone bridge and could possibly vary in calibration due to membrane/strain gage fatigue), even plain ol' fouling of the oxygen sensors.

I'd love to have one of those VXs. 50mpg is a beautiful thing.

12/2/2008 11:54:57 AM

thanks guys. Can't tell if the tach is wrong or not. Just drove a bit and the problem has gone down some. Maybe it was temperature.

12/2/2008 12:26:04 PM

CIVIC

VX

12/2/2008 1:11:15 PM

You can try a can of throttle body cleaner. Take the air filter off and just spray it in there. Don't get any on the sensors. Pay special attention to whatever type idle air control mechanism that engine uses, could be gunked up. That's always worked for me in the past, but I don't know anything about Hondas.

12/2/2008 2:40:05 PM

If nothing else, stop coasting in neutral.

12/2/2008 5:07:07 PM

shit. she's hypermiling.

12/2/2008 10:10:51 PM

^^ true

12/2/2008 10:15:54 PM

I've also been having a problem with my idle. On Friday and Sunday it was extremely low. Like 500rpm, low enough that the engine shook a lot and the lights would dim. Today the idle was high, around 1600 (after work). Interestingly enough, when I turned off the climate control fan the idle would return to normal.

Yes I know I need a new car.

12/2/2008 11:00:04 PM

yeah, sell that thing to an overpaying fanboy and get something else before it suffers a major, catastrophic failure.

[Edited on December 2, 2008 at 11:43 PM. Reason : asdfs]

12/2/2008 11:43:24 PM

On cars that use a pulsewidth modulated solenoid valve like that for idle control, surging can be the result of too much bypass air entering the system due to the adjustment of the throttlebody screws. On many cars there is usually an air bleed screw and a stop screw of some sort for the butterflies (Civic included). Too much air through these and the IAC will overcompensate under some conditions, causing a sticking or surging idle. On Hondas equipped with a chipped ECU (such as Hondata), idle surging/hunting can be fixed by decreasing the idle air control valve duty cycle in the ECU itself. I've done it before on those applications with some success (it was a D16Y8 on a 98 civic EX with an OBD 1 conversion and Hondata S300). Of course that doesn't help a stock vehicle.

You really should just do the idle adjustment procedure in the factory service manual. The ones I'm posting here are for 96+ but it's basically the same thing. I've used the very first one here (for the US spec Y7 motor in my friend's 96 LX), but I am not sure what motor the VX came with and thus what throttlebody it had. I'm guessing the Y7 procedure would probably work. If there's nothing else going wrong (MAP sensor or a mechanical problem), setting the idle speed properly with the adjuster screw will have the idle air control valve operating within the range it designed for.





[Edited on December 3, 2008 at 5:04 PM. Reason : this is also a common problem on 93-95 Rx-7's FYI...]

12/3/2008 4:56:24 PM

all bets on the engine having not reached op. temp yet

12/3/2008 4:58:46 PM

^ Do you really think Civics had a surging idle from the factory when the engine is cold?

If there are no check engine lights and no obvious mechanical problems (throttle cable adjusted properly etc), follow the service procedure first.

12/3/2008 5:01:19 PM

RPMs= Revolutions Per Minutes

Does not compute...

12/3/2008 5:07:10 PM

12/3/2008 5:11:14 PM

arghx, what I am suggesting is that tww not go on a witch hunt (which it looks like), chances are Britta just didnt let her car warm up completely. If indeed it was at operating temperature, then sure, I agree there's something else to blame


I'd say anyone with a 10+ year old car will experience some rough engine vibes before it's warmed up in the winter

12/3/2008 6:11:26 PM

i don't agree

12/3/2008 6:15:55 PM

since I last posted, I haven't seen any more jumpy RPMs. should I still be worried? it only happened on the day I made this thread

12/3/2008 7:45:15 PM

no

12/3/2008 7:47:21 PM