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2002 S500 4 door sedan 5L V8
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Discussion Starter · #1 ·
Hi all,
I bought a 2002 S500 about two years ago with 78,000 miles and I love it. It has some minor flaws, but what older car doesn't. My recent issue is that I have what appears to be a low idle miss. When the engine is cold or when I am stopped at a light or in park, I can feel a miss. At times it is audible, and is more pronounced when the air cleaner is off. There is a popping noise coming thru the air mass meter. Occassionally there is a hesitation on acceleration from a stop, but not a big issue. The odometer just turned 100,000 miles. By the way, this has not always been a problem, seems like it started about 3 months ago, before that it ran as smooth as glass.

Recently I have taken it to two experienced Mercedes specialists, ( not a dealer ), and they have done the following in efforts to solve the problem. First a computer hookup to diagnose the miss, and found several codes to attack. First the air mass meter was replaced, then the coil packs were moved from place to place, and the miss did not change. We found two small vacuum leaks and replaced those gaskets; the spark plugs have all been replaced ( with 9652 Bosch Iridium plugs ) plus two spark plug wires that tested bad were replaced. The miss continues, and the popping noise from the air mass meter is still audible, sometimes louder than others.

The fuel pressure is good in the line and at the injectors. I use only premium gas, 91-93 octane, and now BP or Mobil only. In the past I used Sunoco 91. I am now also very aware of the "Top Tier" brands ratings. I can not attest to what gas was put in the tank by the prior owner. Also I twice put the STP 10-X fuel system cleaner in, but made no difference and was told to stop.

The 3 of us are very frustrated to say the least. I can't believe that these two mechanics with all their fancy and "special" equipment that dealers don't have, are stumped. I need help !! has anyone had this problem ? and if so, what is causing it and how do we fix it. Sorry to run on, but I wanted to tell all I could about this issue so everyone could see what has transpired. Thanks.

Best regards,


· Outstanding Contributor - Always Remembered, RIP
Zotye Auto 1.5T T600 2016
52,890 Posts
The most common cause of this is a badly seating valve allowing burning gasses to flow back into the inlet manifold and this is the pop you can hear. Could be a bad valve or it could be the valve timing is out.

I would advise starting with basics and compression testing all 8 cylinders and let us know what the pressures were for each cylinder.
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