Varying revs - is this normal

[johne53]

Just over a month ago I bought a Mazda 6 TS, 2.0 litre automatic. I'm quite used to automatics so I was a bit surprised to find this rather strange problem....

Let's say I'm driving along at a constant speed (not using the cruise control). Let's say, 40 mph. Although my speedo reading is constant, the engine RPM seems to be continually changing. For example, I might be doing 1800 RPM, then it will suddenly shoot up to 2000 RPM, then a couple of minutes later it goes down to say, 1700. After a few more minutes, it's doing 1800 RPM again, and so on. It isn't changing gear while this happens. It remains in 5th gear all the time. Also, it isn't the rev counter guage that's faulty because I can actually hear the revs going up & down. The variation can be as much as +/- 10%.

I've driven automatics for the past 10 years and I've never come across a problem like this before. Is there something wrong with my car?

 

[TheMAN]

sounds like there's something wrong with the computer programming in the car.... bring your car to your dealer and see if there's an update to the computer relating to this issue

 

[johne53]

Thanks for the advice. The car is still under warranty so I think I'll do that. I'm still puzzled though... once an automatic g/box gets out of 1st gear there's normally a direct link between the engine revs and the speed. In other words, twice the revs (in the same gear) gives twice the speed - exaclty like a manual gearbox. At least, that was always the case for my previous cars. I don't understand how the revs can vary without changing the car's speed...

 

[chuyler1]

Most automatics do not have a direct link between the engine and axle...the torque converter acts as a constant clutch which allows the car to shift gears while you drive.

Many automatics have the capability to lock-out the torque converter in the highest gear which helps with highway mpg. If you keep a constant speed but the terrain varies from hilly to flat and back you may find the transmission will jump in and out of this mode causing engine rpm to fluxuate.

 

[johne53]

Thanks Chuyler1, I've been monitoring this more closely for the past week and that's exactly what seems to be happening. It isn't very hilly where I live but this does seem to happen in top gear, if I drive up & down the very small hills in our area. The car doesn't change gear though - it remains in top gear. Does that seem right?

Also, when you talk about 'locking out' the torque converter, are you talking about a process that occurs automatically? I'm not manually locking anythng out.

 

[chuyler1]

Yes, it happens automatically. Some claim it feels like an additional gear. My wife thought her 87 celebrity station wagon was a 5 speed for years until I met her and explained this...of course in that car in engaged with a clunk...not exactly smooth.

 

[MS6S2K]

This actually seems common to me for some reason. Here's why. Any modern automatic car I've owned or driven, when it goes into OD, if I even step on the gas going up a hill, before it shifts the RPMS go up, then they will drop if I let off. Unless I am not in OD, this has been normal to me for a very long time. I'm not sure if this is the same thing, but my Mom's Subaru does it, my girlfriend's Cobalt does it, the loaner mazda 6 I got when my Speed 6 was in for service does it. I've just seen this as normal on automatic cars with OD, but I'm not sure if we are talking about the same thing.

 

[johne53]

That could account for it. I've never felt it happen in any gear other than 5th - but 5th gear comes in at quite a low speed. I might try using the Tiptronic to drive in 4th at around 45mph and see if it still happens.