Is a sligthly not idle RPM not normal or bad?

bmw
330

#1

I noticed sometimes my RPM seems to move a little bit, maybe by 100 at the most. I also feel a slight vibration when it happens. Is this ok/normal?

2004 BMW 330ci, 233k KM. No maintenance records apart from what I have done to it yet which is fuel pump relay, fuel pump, crankshaft sensor.


#2

Also, it seems to happen after some driving, at least an hour.


#3

If I knew what ( unidle ) meant then I might have an answer.


#4

If we knew this BMW’s model year, odometer mileage, and maintenance record, some of us would probably be able to provide responses with some genuine value.

:confused:


#5

Yeah it sounds stupid how I wrote it.

I meant the RPM is something not idle, but not by a lot.


#6

2004 BMW 330ci, 233k KM. No maintenance records apart from what I have done to it yet which is fuel pump relay, fuel pump, crankshaft sensor.


#7

Unfortunately this one is going to take some diagnosis. There are a myriad of possible causes.
The good news is that there’s a real good possibility that your ECU has stored a fault code or two that can help point a mechanic in the right direction… even if the Check Engine Light isn’t illuminated.

Your comments on the lack of maintenance records suggest to me that this car is new to you. Since it apparently has an unknown history, a good “going over” by a qualified mechanic is a good idea anyway. Besides, you might only need new sparkplugs!


#8

Does this happen when the car is in gear and in motion, or only when bumping the revs slightly in the driveway in neutral? Don’t recall, is this a manual xmission or automatic version?


#9

I checked the codes and got nothing, except a crankshaft code that came and went away, wasn’t in confirmed state but that was after the opther issue, that I should mention here to that I had. Happened 2 nights as of yet. Basically I was driving and randmonly sometimes stopped or driving my RPM dips down and my battery icon lights up (prob as a result of going to stall). Had real low power up to ~4000 RPM. Shut off car 5 minutes everything was fine.

I did a PPI but he didn’t check the car mechanically, more suspension and such. I’ll check the spark plugs. I need to do an oil change and coolant flush before winter…


#10

I noticed it happening when stopped. Manual transmission. And it’s not bumping, sometimes it going lower and not doing a bump.

I noticed I had this issue before my fuel relay and crank pos changed.

I’m thinking it’s because I did not use the OEM crank sensor and that can cause issues? I’m suspecting that can be my issue to the massive RPM dips but like I said, it only happens after 2 hours of driving at least.


#11

Crank sensors do have a tendency to misbehave when hot, and work ok otherwise. So it could be that. I’d feel safe myself to use an aftermarket crank sensor though. I mean if I owned a car which uses a crank sensor. I think I mentioned before that the gadget the crank sensor senses can cause a crank sensor failure too.

I’ve had a similar problem on my Corolla before, caused by the idle speed set too low. Next time at the shop ask your mechanic to double check the idle rpm is to spec. Overly lean mixtures can cause this, but that usually shows up when the engine is cold, and you are seeing it when the engine is hot, so that’s unlikely in your case.

A compression check might be in order too.


#12

Just did a start and the car was doing the slight idling issues. Just started car, go in my trunk and I hear the exhaust change every time the car was doing it. So I guess first things first scan and spark plug check?


#13

Here’s an easy experiment. Take a $1 bill out of your wallet and hold it near to the exhaust pipe, let it dangle there. Does the bill ever move toward the end of the pipe? It shouldn’t.


#14

No 1 dollar bills… in Canada! But I’ll try it with a bill when I come back from work and let know.


#15

I suggest first assume none of the basic maintenance in the Owner’s Manual has been done.
Replace all fluids with what BMW recommends.
Replace spark plugs. Check for vacuum leaks.
Clean/replace the idle air control valve (if it has one) and the throttle body.
Pour a bottle of Chevron /w Techron fuel system cleaner in the gas tank.


#16

That’s a very good idea. For the engine to idle & run smoothly all the spark plugs, injectors, and pistons have to be working equally. If one injector isn’t spraying as much as the other, that could cause a symptom like this. So an injector cleaner treatment is a good idea. If it seems to help, but not enough, try it again.

There’s a fuel system test OP could have done too, called in injector balance test. Howw it works in principle, imagine you had 2 small leaks in one of those metal tanks used to hold compressed air. You wanted to know which was the bigger leak. So you’d block both up with your fingers, then pump the tank to 100 psi, then remove the block of leak 1 and time how long it took for the tank to go from 100 to 90 psi. Then you’d pump the tank back to 100 and do the same thing for leak 2. If it took 1 minute for leak 1, and 2 minutes for leak 2, you’d know leak 1 was twice as much as leak 2. Same thing can be done to test the injectors by pressurizing the fuel rail and using an injector pulsing gadget.

Checking the compression for the proper amount, and balance cylinder to cylinder is a good idea along these lines too.


#17

Yeah I did plan on changing the oil and coolant flush before winter. As for ATF in power steering and brake fluid I don’t know, no issues so.

I really should check change plugs.


#18

I didn’t had chance to try the dollar bill trick is was raining. But yeah it did it again and my engine was cold so change that, it happens anytime.


#19

By the time there’s issues it’s too late.