Engine RPM bounces at 60-70mph

zakool21

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#1
So here's the deal - I bought the 94' 325i a few weeks ago with 73k on it.

I hadn't driven it on the freeway hardly at all before I bought it, so this problem didn't reveal itself until I had to drive the car back to the bay area, 300 miles from where I bought it from.

When I'm going 60-70mph (has happened as low as 55 and has high as 73 or so), the car being in 4th gear (I think), it seems to do this strange revving thing. It happens at around 3000RPM, where when I punch the gas a little more, it'll rise and then drop 250-400RPM, each up-down cylce being about a second.

If I downshift to 3rd manually, and boost the RPM's (as it would if you floored it or switched it into "3"), you get more power, and the problem is gone.

It seems only to happen in 4th gear at the ~3k RPM range.

Now, the other thing I've just recently noticed.... Since I have not until now just left it on complete auto mode for the majority of my driving, the transmission naturally shifts into second gear at a fairly low RPM/speed, as long as you aren't really pushing the car too hard. Now, the one thing that I notice is a slight dip in RPM's/power right before the car shifts into 3rd gear. This only happens when going fairly slow, and never really happens when I'm really punching it.

Hope I am being descriptive enough. I am sure you guys can shed some light on the stated problems (and I hope it's not the transmission)!

Thanks,

-Andrew
 
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#4
I have no idea how involved or how much repairing the lockup solenoid would cost in a BMW. I know that in a GM vehicle, it is easy and cheap. However, I haven't a clue in your car.

I'm also not saying that this is definitely the problem (there are lots of potential causes) - but, a torque convertor lockup solenoid that is acting up could cause this type of behavior.

I don't know if your car has an EGR valve or not, but we had a Bonneville that had a bad EGR valve that behaved in a similar manner to what you are describing as well (the bad electronic EGR valve later ended up frying the engine computer in that car).
 

zakool21

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#7
I talked to one of the mechanics (at the place I bought it from) on the phone, and he said it could be a computer-related thing. He also said that the first thing they do before any sort of diagnostic is to flush the transmission fluids and change the filter.

My "brake light circuit" went on today as well. The mechanic told me that even though the brake lights still work, that the warning light means that the brake light switch may be prone to failing soon, so I should get it looked at.
 


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