I thought this was a fluke last year or somehow caused by me, but now I know it's not me.

The weather here has been hovering around 0º to 10º for a couple weeks. I've noticed multiple times that the engine speed is fluctuating while driving. This is generally at moderate speeds, such as around 30 MPH, light to moderate throttle and when the car is cold to cool. (I haven't noticed it happening when fully warmed up, but by then I'm generally on auto-pilot mode.) Last year I thought my foot was not pushing on the accelerator enough, but now I notice it will sometimes continue to do it if I press a little further.

I suspect it's caused by the transmission. It's a similar 'seat-of-the-pants' feeling to the torque converter engaging between shifts during light throttle. If you're not paying attention you wouldn't ordinarily notice it. The tachometer can be seen going up and down repeatedly, although it's not by much.

Has anyone else experienced this or have an idea of what the cause could be? Sound weird? I'm not going to take it to the dealership because it's something that isn't consistent and I'm not going to pay them $xxx to "diagnose" something this small.

 

I get Engine Fluctuations but usually only right after start up on colder days. It hasn't happened while driving but I do know that if I don't let the engine warm up enough, the idle sits high enough (about 2k rpm) where I can just let out the clutch and drive without pressing the gas. The rpms do bounce a little at that point as well but I have to say I haven't noticed it while driving though.

What RPM range are you seeing it at? (and can you see it at multiple rpm ranges? such as around 2krpm, 3krpm, 4krpm?)

~SB

 

What's the frequency of the up down flutter? Every couple of seconds, once every few minutes?

It might be the AC compressor kicking on/off. If you've got the air headed to the defrost position, that engages the AC.

 

The fluctuation occurs about as fast as you say the words, up, down, up, down." It's certainly not A/C because I disabled that on A/C mode. (Not to mention it probably wouldn't engage at 0°F.)

It happens only while cruising. Acceleration seems to make it stop. And it doesn't happen while stopped. It seems to appear around the 1k to 2k RPM areas which is at roughly 30 - 40 MPH.

If it seems to happen even on a manual transmission I may have nothing to worry about. Maybe the computer just has trouble getting things 'just right' when it's butt cold outside.

I'm taking a long drive out past Sarah Palin Land today so it will be a good "leg stretch" beyond my usual 10 mile commute.

 

Its called engine surging. What Causes Surging in Car Engines? | eHow.com

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I am leaning toward bad gas. I have the same problem when my tank went down past a third of a tank. I am running it down and hoping it goes away.

 

I would say it's the super cold gas as you mentioned it goes away when warmed up.

I wouldn't give it much thought for concern, although if you are in an experimenting mood, you could try fuel additives to see if that changes anything.

 

Hmm, interesting stuff. I guess if it gets any worse I'll know something might be amiss. Thanks for your replies everyone. :-)