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Pete, Mercedes Mechanic
Category: Mercedes
Satisfied Customers: 28717
Experience:  Over 15 years experience as a Mercedes dealer technician
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We have a 2003 ML 320 with 138k on it. It lacks power and

Customer Question

We have a 2003 ML 320 with 138k on it. It lacks power and struggles going uphill since about a year. No engine light ever came on to indicate issues.We took it to AAMCO and they suggested to change transmission oil as it was due and as a first step to follow a process of elimination and check if the engine power issue was arising out of insufficient tranny oil. The last tranny oil change was done at 80k. All A & B services have been done on time. Some 4 years ago we had a cracked exhaust which was welded and fixed. The engine sound changed with some kind of slight leaky sound but did not loose any power. Going uphill that sound seems to increase these days.
Submitted: 1 year ago.
Category: Mercedes
Expert:  Pete replied 1 year ago.

Hi my name is ***** ***** I will be happy to assist with this today.In some cases I may need more info to assist better.I apologize for any delay.

The most common cause for the lack of power and sound you hear under load or when power is needed is a restricted catalytic converter.I have seen the catalytic converters restrict on this model here requiring replacement when they due restrict.If you have not had an exhaust back pressure test done at all to see if this is the cause here this needs done.If found restricted the affected catalytic converter needs replaced.Your exhaust back pressure should not exceed 1.5 psi at any rpm range.If it does then this indicates the catalytic converter being tested is plugged needing replaced.

You can rent an exhaust back pressure tester from a local parts store and thread it into the pre cataltytic converter oxygen sensor port to test this pressure.

If found to be o.k. the other cause is a restricted fuel filter needing replaced or a weak fuel pump that isnt supplying the correct amount of fuel pressure or fuel volume.The fuel system would then need tested to see if a component is found to be at fault needing replaced.

If more help is needed, use the reply tab to continue our conversation. If no further assistance is needed, kindly rate my service.You can rate at any time and we can continue to work on your question as this will not close out your question.Keep in mind in some cases it can be difficult to fully diagnose or help repair your vehicle without seeing it and I work on delivering the best possible answer from what I see to work with.

Thanks Pete