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I had a problem with the coolant light going on for few minutes after running the car from a cold engine and then it would turn off by itself. I took it to the mechanic and they added coolant and, while in there, I also put some premium gas from a pump that I have been so far using its 'plus' grade. Then aftewards I noticed that the engine struggles and makes a muffled noise when accelerating. The noise is more apparent when on low speeds specially on uphills; you can feel that there is an strain on the engine. Also since then the engine heat up very fast, but the coolant light hasn't come up again. The mechanic says there is probably something wrong between the engine and the exhaust but they don't know what it is and they suggested I take it to a BMW service center but that is not an option for me since there is non nearby and I can't take the car on the road in this condition for long distances.

I would really appreciate any suggestions that you might have.

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I would bet your mechanic is probably near the issue. I'm betting your catalytic converter(s) is/are starting to be plugged. You'll probably notice over time that it will become increasingly more difficult to get the car to perform. You'll have to engage the gas pedal further and further to get the desired results. It will get to a point where the engine will not exceed 1500-2000 rpm without wanting to sound like it's dying. If this is the issue, the only real fix is replacement. Depending on the year, this may be covered under warranty.

  • Thank you, that is exactly how it feels like. I press the gas pedal more to get the ssme acceleration out. Do you mean the Catalytic converter has to be changed or the entire engine? It's a 2003 model, first car, and I'm on a student budget. Do you know if it is the catalytic converter, how much would it cost me? | Sorry that I can't vote up. – oligilo Sep 13 '14 at 14:53
  • If it's a 2003 and the converter already needs replaced, then it's likely you have some other issue causing. Catalytic converters will last for a very long time when there aren't contaminants in the exhaust. Common issues that cause contamination are coolant getting into the cylinders due to the head gasket leaking, or oil getting into the combustion chamber, for various reasons. Catalytic converters can be very cheap or very expensive. BWM service will use BMW parts, so look to spend $$$. – dobey Sep 13 '14 at 15:08
  • Agree with @dobey. And just the cat(s) would need to be changed. 2003 is an 11 year old car, so not unheard of it might need a cat. You can take it to BMW and get them to put it on, but would highly suggest (if this actually is the issue) you take it to a muffler shop and have them put an aftermarket one on there. Ensure you get a "3-Way" converter. 3-Way indicates it works on all three types of gasses: Hydrocarbons (HC); Carbon Monoxide (CO), and Nitrogen Oxide (NOx). 2-Way converters do not do anything with NOx gasses, which to me is by far the nastiest of the three (think acid rain). – Pᴀᴜʟsᴛᴇʀ2 Sep 13 '14 at 15:37
  • If this turns out to be the actual cause, you might want to consider getting your O2 sensors looked at and changed if necessary. If they quit working optimally (front ones mainly) it can cause the engine to dump too much fuel and end up plugging your cat. If you don't get them looked at, it can plug a new cat as well. Just thought I'd mention. – Pᴀᴜʟsᴛᴇʀ2 Sep 13 '14 at 15:38
  • Before deciding on any replacements or work being carried out, have the vehicle scanned for faults. A simple scan from an app like Torque Pro on a smart phone, search on-line for it, can be very revealing to what the problem area is. – Allan Osborne Sep 13 '14 at 16:05

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