clk430 intermittent rough idle - Mercedes-Benz Forum

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post #1 of 3 (permalink) Old 02-19-2007, 10:55 AM Thread Starter
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clk430 intermittent rough idle

I have a 2001 CLK430 that had the Check Engine light come on a few months ago. Thanks to the help I got on this forum, I checked the codes and replaced the MAS, which shut the light off and helped the car. Now the Check Engine light does not come on, there are no codes other than normal operation, but the engine, at idle, will run really rough for about 2 seconds every minute or so, sometimes actually stalling. Acceleration is good with no hesitation. The gas in the car [it's seldom driven] is probably 5 months old and we have never had a tune-up of any kind [it has 35k miles]. The plugs, wires, air filters, and fuel filter are all original.

What could be causing this intermittent rough idle? It certainly seems fuel related. Wouldn't an electronic malfunction cause the Check Engine light to come on?

Thanks to all for any help.
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post #2 of 3 (permalink) Old 02-19-2007, 01:29 PM
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Hi, Doc.

You might try posting in your car's native forum, but here's a couple of thoughts.

Five month old gas isn't necessarily bad, but it's certainly getting there. The first thing I'd do is go on a nice long drive and get it down near empty, then only put in a few gallons. If you're only going to drive it rarely, leave it near empty, then put in fresh gas when you take it out. It's also possible you have accumulated moisture in your fuel tank so a can of dri-gas or a premium treatment that includes a gas drier may help as well.

I'm not sure if the V-8's CPS is as occasionally buggy as are those in the 6s, but that may be something to check if it's not just old moisture-laden fuel.

FWIW, the CEL is tied only to emissions-related things. There are lots of things that can cause driveability issues that don't illuminate the light (the afore-mentioned CPS being one of them). Also, the light doesn't illuminate automatically; there are some sensors that when they go out of range, a code is stored, but unless there are enough repetitions or combined with other codes, the CEL won't illuminate. However, you can still download those codes with an OBD-II code reader.

Were it mine I think I'd drive out the old gas, add the additive and some fresh fuel, then drive it some more. At that point I'd plug in the reader and see if there are any codes, and then go from there.

Take care and enjoy the ride,

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post #3 of 3 (permalink) Old 02-19-2007, 03:27 PM
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The plugs, air filter, and fuel filter should be changed. The plugs are only good for five years. The air and fuel filters are only good for four years. If this is the seventh year the car has been in use, the fuel and air filters have been in service for close to twice as long as their rating allows.

Bad plugs plus bad injector spray patterns (due to a disintegrating fuel filter) may be the cause of the problem. If the car idles bad enough to stall, I believe a code should be logged, but I could be mistaken.

I hope this helps.
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