rough idle, loss of power with a/c
 

2005 Hatchback, Manual Transmission, 60k miles (I've had it for about 10k)

I've noticed a bit of a rough idle for a few months now. It happens whether the engine is warm or cold. It'll run rough for a couple seconds, then run smoothly for a little while, then rough for a couple seconds, repeat. It's not missing. There's no check-engine light. I just kind of ignored it since it didn't really make much difference in the way it drove.

Here's where things get frustrating. Now that it's heating up and I'm using the AC while I drive, the problem is manifesting itself in a new way. I don't think the roughness while idling with the AC on was any different, but I admittedly didn't think to check until now. Now, while driving down the road, it's like I let off the gas for a split second. The car loses power for just a moment, whether I'm driving 30 or 70, accelerating or constant speed. It sort of "jerks" because of it then continues as if nothing had happened. The timing between each event doesn't seem to be constant. Again, I'm quite sure it isn't missing; I know what that feels like. That should throw a CEL anyway.

I bet it was doing the same thing before. I just didn't notice it because the power-robbing AC wasn't on. Even with it on, if I'm not on a reasonably smooth road, the incident is almost indistinguishable from a bump.

I've already tried cleaning the MAF sensor, since that seems to be the first suggestion for every problem with this car. Spark plugs look normal according to NGK's guide. The air filter is clean and fairly new. I've been getting fuel from the same place since several months before the problem started. I'm not really sure what else to check...

Any ideas would be greatly appreciated. I'd like to solve this before it becomes something major.

Really? Not even a guess?

Maybe I'll just have to suck it up and take it to the dealer.

if you can stand waiting, i'm sure it will set the CEL soon. If you have a scanner, you may be able to see a fluctuation in the real time data that matches the timing of the roughness. I'd bet there's an idle control solenoid that's sticking or something, but i'm not familiar with what solenoid it is for this control.

If you use the auto setting on the HVAC, it will cycle the AC - which may cause the change in idle, though it shouldn't feel rough. I know that I left my AC off since it would cause a slight jerk when it switches on and off that drove me nuts, especially when I didn't want the AC on anyway, defrost will cycle it even with the selector for AC turned off.

I first noticed it during the winter while waiting for the engine to warm up a bit before driving, so it's not just the AC cycling. I would think the CEL would have come on by now if it was going to. This must be the only time in my life I've hope for a CEL. I don't have a scanner, but I've considered buying one just to try solving this issue. Kind of pricey for an experiment though...

Thanks for your response.

Loss of power
 

I had a similar problem with the Mazda Demio, when aircon was on it would lose power, especially up hills, and sometimes stall in traffic. Turns out it was the high tension wires that lead to the spark plugs, the rubber was split and electricity was shorting out. Wrapping them with electrical tape solved the problem until they got replaced with new ones a few days later.

Oh
 

So it was a purely electrical problem?

The 1st thing I'd do is check the plugs and wires.

Yes, that was the second time. The first time I had a similar problem it was the air filter. So it looks like different causes can give similar symptoms.