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I have a 2000 Mercedes CLK 320 with the M112 V6 engine at 125 000 kms. The car has rough unsteady idle and is lacking a lot of its power - to illustrate, 0-100 km/h takes exactly 10 seconds, while before it took around 7.5 seconds.

I had to replace the MAF sensor, because the car barely had any power. I guess it also caused quite a bit of raw fuel to be dumped in the exhaust, because shortly after that the upstream O2 sensors also needed to be replaced. That restored the fuel economy, which is nice, but had no effect in terms of power. Here's a list of what else I have replaced with zero effect:

  • spark plugs
  • vacuum hoses
  • intake manifold gaskets
  • egr valve gaskets
  • injector seals
  • fuel filter

Some observations:

  • there's a hissing sound during acceleration, which (coupled with the lean condition) points me in the direction of a vacuum leak or exhaust restriction
  • muffler is rattling, but I don't know is it's clogged, causing the engine to run poorly, or if the poorly running engine vibrated it to destruction. I'd like to diagnose it in some way before replacing it
  • car is running hotter, I guess due to the lean condition
  • both short term fuel trims are fluctuating around zero
  • both long term fuel trims are steady around 6-8 % positive
  • upstream O2 sensors are maxing out at wide open throttle, so I guess fuel delivery is not an issue
  • downstream O2 sensors are steady, so catalytic converters must be doing something

That's all I can think of now. The engine also has a MAP sensor but I don't know the procedure to test for exhaust restriction using that.

Any suggestions are highly appreciated, I can do scans with my basic obd unit.

Thanks in advance!

1. This is a reference graph from my brother's car with the same engine. This is a reference graph from my brother's car with the same engine.

2. This is the first graph from my car. Looks really similar, a tad higher overall. This is the first graph from my car. Looks really similar, a tad higher overall.

3. This is the second graph from my car, taken a few minutes later. The values dropped siginificantly overall for some reason. This is the second graph from my car, taken a few minutes later. The values dropped siginificantly overall for some reason.

  • Welcome to Motor Vehicle Maintenance & Repair! Have you checked the cats? If you've been dumping a lot of fuel, you could have clogged them. This would restrict exhaust flow and slow the acceleration down. Are there any codes? – Pᴀᴜʟsᴛᴇʀ2 Jul 8 '18 at 15:09
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    Is it a turbo intercooled engine? If so, you should check the turbo to intercooler and intercooler to engine pipes - could well be the hissing sound you hear... Had a leak on mine and down on power... – Solar Mike Jul 8 '18 at 16:23
  • It's a naturally aspirated engine, thank God :) – J. Doe Jul 8 '18 at 18:21
  • There are no codes. How can I test the cats? I know there is way to do it by using the MAP sensor reading, but I couldn't find anything definitive. – J. Doe Jul 8 '18 at 18:23
  • Holding at about 3000 rpm, MAP should slowly climb a bit if there is a significant exhaust restriction as pressure builds up. I would prefer a vacuum gauge on it as a preliminary test, otherwise the definitive test is an exhaust back pressure test through either the front o2 sensor hole or tapping in a port to connect a gauge to. – Milison Jul 8 '18 at 19:03

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