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2006 E320 CDI rough idle while cold

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lauraf13's picture
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While at work I have to park outside and the light to get out of the parking lot sometimes takes several minutes. This means that I drive a short distance at low speed before having to sit at the light. On cooler days ( < 60 F) when I have to wait for the light my car will start idling rough after 1 - 2 minutes and when I take off the car doesn't accelerate smoothly. After I get up to speed then everything is OK. Also, if I don't have to wait long for the light then everything is OK.

I have tried twice to get the dealer to look into the problem, but they can't find anything. They kept the car overnight to see if it would do it after sitting out overnight, but still couldn't find anything.

Any thoughts on what it might be and how I can describe it such that the dealer can find and fix the problem?

Thanks,
Laura

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lkchris's picture
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Re: 2006 E320 CDI rough idle while cold

Ours recently developed a fairly annoying knock at idle (and a poor idle) and the dealer can connect its computer and verify the performance of each individual injector.

By "can't find anything" do you know if they actually hooked up diagnostics?

Since your problem occurs when cold, it may be glowplug related.

Did they check glow plugs?

CDI glowplugs do MUCH more than just come on/go off based on a timer.

It was correct to do the "overnight" test, as that's of course how they get a cold engine.

For our injector test, there is a value beyond which Mercedes (warranty) will agree there's a problem. This doesn't mean the engine runs perfectly when getting close to that value, however. The test results are submitted to Mercedes along with the warranty claim. "Can't find anything" could perhaps mean "problem not bad enough yet."

Just some thoughts--it's the only problem we've had and a new $755 injector fixed it.

I'd verify if nothing else that they can show you test results and explain them to you. It's maybe more important that they have them and will explain them to you than that you actually understand them. I didn't.

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Re: 2006 E320 CDI rough idle while cold

Laura,

Here's a simple test that may narrow down the problem. Next time the idle gets rough after sitting at that stop light too long, make sure you are in a safe spot to do so, and shut off the engine and then re-start it.

I know very little about your engine other than it uses diesel fuel and has common rail injection, but like Kent I suspect a glow plug problem. My (rough) theory is that a timer which was keeping the glow plugs energized after a cold start has expired while you sit there idling, and if you re-start the car you will re-set the timer.

I hope someone with more knowledge of the glow plug system on your car will chime in and enlighten us.

Let us know how it works out.

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Max Dillon
Charleston SC
'87 300TD, 334k miles (need to pull the head again...)
'95 E300 Diesel, 335k miles (daily driving duty)
'73 Balboa 20

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Re: 2006 E320 CDI rough idle while cold

I think that it didn't happen for them, so they didn't checked out anything. I don't know that for a fact, but that's the impression I got.

I'll try the restart of the engine. It won't be today since it's supposed to be in the upper 70's today. I didn't know that the glow plugs did more than help start the engine.

I've driven diesel cars since 1979. I had one gas car during the time MB didn't import diesels and I went back to diesels as soon as I could.

Thanks for the help,
Laura

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lkchris's picture
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Re: 2006 E320 CDI rough idle while cold

Just for fun, here's three pages from MBUSA's 73-page training viewgraphs on the OM648. The pages covering the glow plugs, that is.

If you want to download the entire 22mb document, click
[url]http://www.mercedestechstore.com/pdfs/331_0405update/03-331%20pp%20diesel%20print(FAH)%205-5-04.pdf[/url]

AttachmentSize
om648 glow plugs.pdf 212.68 KB
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Re: 2006 E320 CDI rough idle while cold

Thanks Kent, BUT that is more than fun, it's downright educational!

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Max Dillon
Charleston SC
'87 300TD, 334k miles (need to pull the head again...)
'95 E300 Diesel, 335k miles (daily driving duty)
'73 Balboa 20

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Re: 2006 E320 CDI rough idle while cold

These cars use an "afterburn" system while the engine is running to keep the combustion chamber a bit wamrer to encourage lower emisisons and smoother idling.
If this issue does occur at lower temperatures, then we can say 2 things-
the combustion chamber is not hot enough to keep the diesel combusting correctly at low ambient temepratures and low engine temepratures

and also, one or more inejctors may not bhe suppling fuel at an adequate pressure when cold. Keep in mind that the OM648 uses solenoid tpye injectors- they fire off three times for each compression cycle and can act up if the solenoid valve does not open on time or properly.

Time for a trip to the dealer.

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Re: 2006 E320 CDI rough idle while cold

Pierre,

I've been to the dealer twice with no luck.

What can I say to the dealer that will help them look in the correct direction? They seem to have to have it happen. I'm not sure the service adviser is accurately passing on the information I give him. Last time I gave him a written description, but that didn't help. I used to have a really good service adviser who would listen, but he retired.

Thanks,
Laura

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Re: 2006 E320 CDI rough idle while cold

We finally had a cool day and I had a chance to try restarting. I sat and waited for the rough idle to start and then I turned off the engine and started it. After the restart it idled just fine and accelerated fine.

I think maybe I can explain it to the dealer better, but unless we get a long stretch of cool weather I may have to wait until fall.

Thanks for everyone's help

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Re: 2006 E320 CDI rough idle while cold

Laura,

After reading the material that Kent supplied and hearing the results of your re-start test, I suspect that what you have going on is this: The N3/9 module is 'timing out' the after-glow function and shutting off the glow plugs. When the glow plugs are shut off by N3/9 during a cold engine idle, combustion degrades (which you experience as a rough idle and low engine power). So, the question, I think, is whether this is a 'feature' or a fault in the after-glow function of N3/9. I'd ask your dealer to check the temperature sensors that supply data to N3/9, and see if that module is providing any error codes. They may also be able to research the design after-glow time and tell you how long you should be able to idle on a cold engine before the after-glow is shut off, and you can use that to judge whether or not the N3/9 module is performing properly.

I suspect this is a 'feature' that you'll have to live with, and perhaps the best thing to do is re-start or shut-down and wait when you are in a situation causing a prolonged cold engine idle.

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Max Dillon
Charleston SC
'87 300TD, 334k miles (need to pull the head again...)
'95 E300 Diesel, 335k miles (daily driving duty)
'73 Balboa 20

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