BMW e46 330cd, diesel, automatic
157 000 miles
no OBDII codes on cheap bluetooth scanner
Car history: I own the car 3 years. I mended many provisionary repairs of previous owners. In the past 25 000 miles, I have done following: new turbo charger, all new vacum suction hoses and vacum pump, EGR valve, starter module (small computer regulating startup). Completely rebuild suspension and steering. Transmission has new fluid, mechanic said the transmission is okay. Engine has new heating plugs and during change was inspected. Mechanic confirmed the engine is strong and no issues. Engine does not leak oil nor consume oil. Oil change every 6 000 miles. Fuel filter change upon purchase and 4000 miles back.
Problem: rough RPM bouncing under these conditions
- automatic cruise control ON
- cruising at slow speeds around 70 MPH
- prolonged uphill / downhill
Only if all these conditions are met, then the RPM bounce 200 - 300 RPM up/down intermittently. It bounces only sometimes. The car gets kicks into the back as the RPM change.
After longer highway journey (more than 70 miles driven), the RPM disappears even on uphill/downhill. If I drive "manually" via acceleration pedal, there is no RPM bounce. If I drive above 90MPH, there is no RPM bounce. If I drive unaccelerated leveled cruise, there is no RPM bounce. If I accelerate / brake / idle there is no RPM bounce.
It does not seem to be intake, it does not seem to be fuel, nor the engine.
My mind goes towards catalytic converter / muffler. Any other opinions?
Nope, can't be the catalytic converter or the muffler because if they are clogging up you will have the worst problems the faster the engine spins because it has to exhaust more gas and a clog muffler Cadillac converter will make it worse. The faster the engine goes. Yours is kind of the opposite so I would guess it has some type of minor fuel injection problem such as worn or clogged injector that once you get spin and faster it works fine. Also realize something else. Those are extremely complex vehicles and if let's say the transmission is causing that problem, no one's ever going to be able to figure that one out unless it gets worse and then the transmission has to be completely rebuilt. I have seen that happen
That makes a lot of sense. You are right. It can't be the catalytic converter. It would not drive 70 MPH needles to say 100. I had absolutely wrong logic.
So far the transmission oil was clean. There was a small seepage but nothing to worry about.
Yeah, it is automatic which is always a problem. Maybe it is the beginning of the end or maybe there is a worn cruise control sensor.
I did consider purely mechanical E36 or E30. E46 provides better comfort, speed, fuel mileage and handling for what I need. It ain't any dragster nor tyre burner. It is more like an "executive fun toy". And I got it bargain for 1k 😉 so it's worth it.
Thank you again