CLK 500 got new plugs, now car runs horrible
Thread Starter
Junior Member
Joined: Nov 2010
Posts: 16
Likes: 0
CLK500
CLK 500 got new plugs, now car runs horrible
Took my car in for some routine maintenance...got spark plugs replaced and air filter replaced as well. Also had valve cover seals replaced to take care of an oil leak. Car has 110,000 miles. Upon getting my car back, it now is idling extremely rough. You can hear it in the way the engine sounds and also can feel it in the seat and the floor. This is only noticeable at idle, when driving the car it is fine. I've taken it back to the shop, and while they can feel and hear what I am talking about, they cannot find what is causing this. Everything checks out fine when they hook the car up to the computer. This is killing me, as now I literally hate to drive the car. Why would changing out the plugs cause the car to idle so rough? Any ideas from anyone would be greatly appreciated.
MBworld Guru
Joined: Dec 2009
Posts: 10,782
Likes: 1,011
NO LONGER ACTIVE
You may have a bad coil pack(s) or plug wire(s) that failed during the job. It's not really the mechanics fault, but it just happened when they disturbed everything for the other work. Is the MIL illuminated? If so, what codes are present? If not, then this may not be a coil pack or plug wore issue. One thing to check is make sure the engine cover/air cleaner housing is securely fitted over the intake on the MAF sensor housing. If it's not snapped down tightly in place, it can cause erratic idling. You can use your fingers to feel the gasket on the MAF to make sure it's properly mated to the cover.
Thread Starter
Junior Member
Joined: Nov 2010
Posts: 16
Likes: 0
CLK500
No, nothing is illuminated. I was there when they hooked the car up to the computer, and no fault codes of any kind showed up. I'm going to take it back to them a 3rd time, as like I said, I just hate driving my car now. It ran fine before I took it in, all I was doing was having routine maintenance done.
Super Moderator
Joined: Jun 2008
Posts: 19,942
Likes: 192
From: Llandudno, Cape Town, South Africa
late 2009 CLK 350 Coupe Elegance, '65 Jaguar S Type wires
Check that air cleaner housing. It is critical it seals properly over the MAF. A frequent cause of trouble. Rudeney is dead right on everything.
Thread Starter
Junior Member
Joined: Nov 2010
Posts: 16
Likes: 0
CLK500
Here's an update...took my car back again and they still could not find the problem. Told me they completely reversed their work, tried 2 different sets of plugs, plug wires, motor mounts, trans. mounts, nothing is helping to make my car run the way it did before I took it in. Is it possible that the brand of spark plug they are using is causing the issue? The place that I'm taking my car specializes in working on MB, and was recommended by someone else on this forum. I am attaching 3 pics. First two are my car...higher rpm is when my car is in park or neutral, lower rpm is when I am sitting idle in drive, this is when I experience the roughness in the idle at the worst. Seems like the idle needs to be adjusted up. Is this possible to do? Third pic is my Mom's E350, her rpm is identical at the level in the pic, whether the car is in park, neutral, or drive. It's killing me that they cannot figure out what is causing this rough idle.
MBWorld Fanatic!
Joined: Jan 2004
Posts: 2,200
Likes: 10
2004 CLK320
However, I removed this engine covers many times to clean or replace the air filters, it's hard not to do this right. I saw at least 2 large clamps in the bottom to hold the engine cover in place. There is only one way to secure the engine cover; if it is not sitting on the engine properly, you will certainly know.
My point is it's not easy to make the same mistake again and again. I am sure the mechanic did remove this engine cover many times to change the plugs, plugs wires...
Last edited by mis3; Jan 23, 2013 at 11:12 AM.
MBworld Guru
Joined: Dec 2009
Posts: 10,782
Likes: 1,011
NO LONGER ACTIVE
Except that even secured on the clips, the back of the cover can fail to properly mate to the seal on the top of the MAF sensor, and that's what causes the issues. Sometimes it takes a firm push or slap of the fist to get it in place. You can feel under the back edge of the cover to see if it's properly seated.
Trending Topics
MBWorld Fanatic!
Joined: May 2008
Posts: 1,250
Likes: 21
From: Los Angeles, CA
Supercharged CLK500,ML500,S580
Took my car in for some routine maintenance...got spark plugs replaced and air filter replaced as well. Also had valve cover seals replaced to take care of an oil leak. Car has 110,000 miles. Upon getting my car back, it now is idling extremely rough. You can hear it in the way the engine sounds and also can feel it in the seat and the floor. This is only noticeable at idle, when driving the car it is fine. I've taken it back to the shop, and while they can feel and hear what I am talking about, they cannot find what is causing this. Everything checks out fine when they hook the car up to the computer. This is killing me, as now I literally hate to drive the car. Why would changing out the plugs cause the car to idle so rough? Any ideas from anyone would be greatly appreciated.
The culprit ended up being a cracked breather hose coming out of the Driver side valve cover. The shop I used at the time (and never again) did not replace the breather hose even though they are known to crack if they have a lot of miles on them and then later disturbed by trying to replace the valve covers (same phenomena Rudey explained with the coil pack/plug wires).
They replace all the breather hoses and that fixed the problem.
MB World Stories
The Best of Mercedes & AMG
Manual Mercedes? 6 Times Sindelfingen Let Drivers Have All The Fun
Verdad Gallardo
Mercedes SLR McLaren 722 S Is Extremely Rare Example Modified by McLaren
Verdad Gallardo
8 Classic Boxy Mercedes Designs That Have Aged Like Fine Wine
Verdad Gallardo
Flawlessly Restored Mercedes 190E Evo II Heads to Auction
Verdad Gallardo
Electric Mercedes C-Class Unveiled: 11 Things You Need to Know
Verdad Gallardo
Mercedes EQS Gets A Major Update: Everything You Need to Know
Verdad Gallardo
5 Underrated Mercedes-Benz Models That Don't Get the Love They Deserve
Verdad Gallardo
Mercedes 300D Has Pushed Well Past 1 Million Miles and It Ain't Stopping
Verdad Gallardo
10 Most Reliable Mercedes-Benz Models You Can Buy Used
Verdad Gallardo
Thread Starter
Junior Member
Joined: Nov 2010
Posts: 16
Likes: 0
CLK500
After Rodney gave me the suggestion about the air cleaner housing being properly sealed over the MAF, I mentioned this to the mechanic and he assured me the seal was good. I was reading another thread on this forum about a couple of other people who had the same "symptoms" of my car. Not sure if the same problem, but the symptoms seem to be the same. Rodney, if you're reading this, you had some input on that post as well. It seems that changing the EGR ultimately is what fixed the problem of the rough idle for both of these people. The question is, is that is what's needed to fix my car? All I did was take it in for routine maintenance. It was running just fine, and now it's terrible when I sit at a light. I tried a fuel injector cleaner from the auto parts store, but as I suspected, that did not make any difference. The shop who changed my plugs has absolutely no idea why my car was running fine beforehand and now is not.
MBworld Guru
Joined: Dec 2009
Posts: 10,782
Likes: 1,011
NO LONGER ACTIVE
It is very possible that a vacuum hose was "disturbed" during the plug change and it broke, and that is causing the problem. I really wouldn't blame the mechanic for that. As these rubber hoses age they become very brittle and it doesn't take much to break them. I literally just touched the power brake vacuum hose and it cracked the check valve. Luckily, these parts are not too expensive - less than $75 for all of them.
As for the EGR valve, if it's leaking it can cause problems. I think the test for this is to remove the vacuum hoses, cap them off and see if that helps. Of course if the valve is stuck open, then that won't change anything.
As for the EGR valve, if it's leaking it can cause problems. I think the test for this is to remove the vacuum hoses, cap them off and see if that helps. Of course if the valve is stuck open, then that won't change anything.
MBWorld Fanatic!
Joined: May 2008
Posts: 1,250
Likes: 21
From: Los Angeles, CA
Supercharged CLK500,ML500,S580
After Rodney gave me the suggestion about the air cleaner housing being properly sealed over the MAF, I mentioned this to the mechanic and he assured me the seal was good. I was reading another thread on this forum about a couple of other people who had the same "symptoms" of my car. Not sure if the same problem, but the symptoms seem to be the same. Rodney, if you're reading this, you had some input on that post as well. It seems that changing the EGR ultimately is what fixed the problem of the rough idle for both of these people. The question is, is that is what's needed to fix my car? All I did was take it in for routine maintenance. It was running just fine, and now it's terrible when I sit at a light. I tried a fuel injector cleaner from the auto parts store, but as I suspected, that did not make any difference. The shop who changed my plugs has absolutely no idea why my car was running fine beforehand and now is not.
- Visually check them by removing the engine cover and at the end of the valve covers (toward the firewall) there will be a hoses that routes toward the MAF. Check the condition, if they look old or are cracked, they were never replaced and probably has an air leak.
- Look at your mechanics invoice and on the parts list, it should say breather hose somewhere. If you don't see it there, they were never replaced and probably has an air leak.
- Ask you mechanic is he replaced them when he did the valve covers, if he says no, then they were never replaced and probably has an air leak.
If you are confident that the Breather Hoses are good and the engine cover is seated correctly (these two things should be easy to verify), THEN you should look at the EGR (troubleshoot easy stuff first, then get into the harder stuff).
GL
Thread Starter
Junior Member
Joined: Nov 2010
Posts: 16
Likes: 0
CLK500
You both mention the hoses. The first time I took the car back, he checked and there was actually a hose that was cracked. He replaced the hose with a new one but that did not do anything at all in making the problem any better. I'm not faulting the mechanic, in fact, I took my Mom's E350 to them for a service C on her car. It's just very frustrating to me that they cannot figure out what is causing this issue. Is there any way to bump up the idle a little bit when the car is sitting in drive? I realize that this may just be masking the problem, as opposed to fixing it, but at least I would not have to hear the roughness and feel the car shaking when I'm sitting at a light.
