Strange issue with a BMW 325I(M56 Engine) RPM drop & strange noise


Old 01-06-18, 05:47 PM
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Join Date: Sep 2007
Location: USA
Posts: 305
Strange issue with a BMW 325I(M56 Engine) RPM drop & strange noise

SO, I have posted this in two BMW forums and have received some help, BUT I have yet to solve the issue. Just thought I would throw it out here and see if anyone has any input. I have a a 2005 E46(325I) with the M56(SULEV) engine. The car has a a 104,XXX miles. I have recently done they following; plugs, pulleys, thermostat, water pump, belts, coolant system, oil change and coolant change. Here is the issue, when driving the vehicle, and I hit 40 mph and about 1600 RPM, the RPM's will drop and the car will bog down(loss of power). The interesting part is the car makes a DEEP THROATY TYPE SOUND from the rear, possible exhaust. I can power through it with no issues. Any thoughts on what could be going on? THANKS!
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Old 01-06-18, 06:02 PM
Join Date: Jun 2010
Location: Canada (near The Motor City)
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Did the car have this issue BEFORE you changed the parts ??
Old 01-06-18, 06:04 PM
Thread Starter
Join Date: Sep 2007
Location: USA
Posts: 305
And that's where the problem is. I DON'T believe so. I couldn't pinpoint when the issue exactly started. Also, should of added , NO CODES being thrown.
Old 01-08-18, 11:24 AM
Join Date: Dec 2016
Location: Canada
Posts: 181
My line of thinking is to deal with anything that would temporarily lag the delivery of fuel under load, with continued load the computer is adapting to the anomolies and adding more fuel, recovering your acceleration.

When the car is lugging with the auto trans seeking best fuel economy by operating at lower rpm, the computer would send more fuel, but somehow that's not happening and you are getting the bog.

Check each of the spark plugs, since they are recent they should all look pretty good, light tan colour. If one looks different than the others, might indicate a coil on plug failing. The throaty sound might just be the result of a cylinder not firing, and the engine sound would change. Check the electrical connectors to each coil, might have bumped things with the other service items you did.

Could be the various sensors, the intake air temp, crank position and cam position sensors specifically. All of these feed data to the computer that changes the fuel metering. These sensors are wear items, and tend to fail slowly, ie. give incorrect readings to the computer, rather than outright. But first check the connectors to the sensors as well. Use some electrical contact cleaner to make sure the connections are as sound as can be.

Could be a maf sensor, but that usually results in some rough idle and other symptoms beyond what you are reporting. Oxygen sensors on the cat might also be suspect, but the drivability problems should be a bit broader than you report. Does the car lag if you hit the accelerator at any speed? Is the idle ok? Same issue if the car is warmed up or cold?

Not sure if you have emissions testing in your area, but a sniffer in the exhaust pipe with the car operated under various loads might tell you something if the emmissions are out of spec.

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