Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Wavering RPMs

My wife’s 1998 Honda Civic has a funny quirk. When you are coasting down a hill or coming to a stop the rpms seem to waver up and down. It has an automatic transmission. My 5 Speed Saturn never seems to do this unless I start changing gears- which I usually don’t do if I am coasting or almost at a stop. What causes the rpms to go up and down for seemingly no reason? Is my transmission nuts? Or could it be some Idle control issue? Once the car is at a dead stop the rpm issue seems to go away and they remain steady.

If the transmission is downshifting as you slow the rpms will vary, just as they would if you manually downshifted your Saturn.

How much does it waver? A few rpms or a few hundred rpms?

I am guessing it jumps up and then down a couple hundred rpms- It sounds a little like the motor is kind of “whirring”- As if someone was giving it a little gas and then letting off. It will do this even if my foot is completely of the throttle.

Does she use cruise control when the idle seems to move like this? The AT has different modes and is under computer control. The most likely issue is the lockup on the tourque converter. This is an electro magnetic lock on the turbine that matches the crank speed. When it is off the AT allows the tourqe plate to slip in relation to the crankshaft. This can cause a few hundred rpm shift in the rpm gauge. This is normal.

Clean your idle air control valve & throttle body, and test your throttle position sensor.

No we do not have cruise control that I know of-- if we do we don’t use it. It’s interesting that you mention the torque converter as we have a very intermittent slight shudder that seems to be related to a transmission problem. All I know is both of these issues started about the same time (about 6 months ago). This was shortly after the car blew a heater hose and overheated- plugs and plug wires and radiator were replaced and the bad hose of course.

Yeah- We had an idle air control valve with a Ford Escort and we cleaned it a couple of times and that seemed to help- but the symptoms were different- When that one was dirty it would lose power and run rough as I recall.

That’s funny I meant to say idle air control valve issue. The throttle position sensor I would have to ask my mechanic about- don’t know anything about it.