I did a search on "rough idle" and got a lot of posting, of course. However, it seems that the response of what to do were pretty varied, so I'm going to post in as much detail as I can what is wrong.

- Starts fine.
- Usually a rough idle. Starts out with a "jump" of sorts in the engine that is not visible on the tach. This happens every few seconds at first, but gradually turns into a shake after driving.
- Driving under 30-40, the engine feels the same, i.e. feels rough.
- This roughness sometimes goes away, but I honestly could not say when that happens, i.e. how long I've been driving, etc.
- No noticeable problems with the car is in park. Does shake when in reverse.
- Oil pressure guage jumps around with the rough idle. When driving, it's all the way up to three.
- Gas mileage is good. Probably about 300m/tank on the highway.
- Above, 30-40, car runs PERFECT.
- I just drove from New York to Ft. Lauderdale, doing 80-85 the whole way. Accept for the idle and the low speed roughness, zero problems.
- Not burning any oil.
- No noticeable leaks.

Could this be a problem with second gear start?

I obviously don't know much about these cars (new owner), so I would greatly appreciate any help. If you can, list things in the order that you would do them to elimate possible sources of the problem, from easiest to do up to just taking into the dealer.

Please give estimates on costs if you can.

I would likely go to the Pompano Beach Dealer (heard bad things about that??)

Year/Make/Model? If we know that, someone experienced in that vehicle can jump in with an opinion.

Aaaaahhh!! Forgot the most important part..

86 420SEL

Sounds like a repeat of all the problems I'm having..sounds like a 420SEL thing only differnecr mine is a 1987 model. Loo9k forward to somebody coming up with the answer...Almost forgot my Seatbelt light is stuck on instead of going out after 6-7 times of blicking. Anybody have an answer for that one? There is no wires to the seatbelt assembly and this sticking just started. Thanks!

david-having similar experience with my '86 560SEL. What i am finding out is that the engine mgt system on this vintage of car is amazingly complex and subject to component failures that are hard to diagnose. In our model year online diags were/are not avialable as I understand it. I have not been able to solve my hard start , cold rough idle problem but patience is key I guess. A few thoughts on your situation ( I am no expert) -
-have u been through routine "tune up"?? The high mile cars need a certain amount of ignition mtc - esp plugs, rotor,cap etc. I am not a big wire replacement fan (replace em when they fail) but that could be done too.
-depending on mileage (greater than 100K miles) new injectors might be another thought - preventive as well as addressing your problem .
-check the coolant temp sensor for resistance and wiring continuity.
-I am starting to lean toward fuel distribution on my car (165K miles) but want to look further. It is a very expensive replacement. Also , check and replace fuel filter near the fuel pump and inspect what junk comes out of the old one.

My $.02. GL and keep us posted.

Steve Brotherton responded to anoter 420SEL issue today. It may be worth looking at. See the following post:

Help - Moderator? - Idle

I didn't read this post earlier because picking idle problems from general problems of running are hard enough.

I get a totally different opinion of this case. I almost suspect a single cylinder misfire, usually caused by plug wire ends. The key is always to isolate the problem. In the other post the key was two intersecting symptoms. In the shop, if I didn't have experience as happened the first time I found that problem, I would see if there was a single cylinder misfire first.

All of the CIS systems have a couple unique features that can assist in the engine diagnostics with no tools at all (well you do need a finger and a twelve millimeter open end wrench.

The first thing I do is remove the airfilter and gently push down on the airflap while the engine is running. If this totally smooths out the engine there are a couple possibilities. First is there could be a general mixture problem, second the injectors could be variously restricted; a motor running in closed loop (figure out what that means because it is the heart of modern control) will be running so close to lean misfire that any difference in injected quantity will cause such a misfire in the weaker cylinders.

Unfortunately. it might also lead one to some inproper decisions as in the other car with a single cylinder lean misfire caused by vacuum leaks to one cylinder. When the common mixture is raised enough to get that one cylinders mix in range it will also start running again.

The other feature of CIS is the ability to do cylinder balance testing by openning the injection line similar to a diesel. By cracking the lines loose at the fuel distributor one at a time, the relative power of each cylinder can be evaluated. Cylinders that don't cause further roughness when opened are not running strongly.

A little sensitivity here and some follow up electronics (see the DIY article; evaluating engine controls) can get to the bottom of this.

In the vacuum line case, the above testing would have found the cylinder that got the leak (#8 if I recall). The further testing with the airflow plate would probably made the cylinder run again showing it to be a mixture problem. It then might have been tricky to decide that the mixture problem was vacuum leak related. Not too difficult though, because the problem goes away at higher speeds. This kind of mixture problem is almost always a vacuum leak because the amount of leak stays constant and as the engine speed increases it becomes proportionally smaller till of no consequense. Single cylinder mixture problem caused by fuel distributors or injector flow restrictions don't go away at greater flow rates, often they get worse.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Loads of high tech stuff there but no one has suggested the most basic and simple check, that is, to check all the vacuum lines for leaks/splits/poor connections.

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Mick J
'08 Chrysler 300CRD (MB OM 642 engine)
'95 E220 estate
'89 230TE (R.I.P.)

What a great thread! Now for the reverse problem. What do you advise checking when you have a good idle, but two or more cylinders run lean under acceleration? Indie mechanic says ignition system is fine and recommends changing fuel distributor. An expensive gamble, in my opinion. Any ideas would be appreciated.

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M. Sandler

1986 560SEC: 150,000 miles, runs great, but I've got to sell it (too many cars for one man)
1987 560SL: 122,000 miles, used to run poorly, now (thanks to forum), runs great!
1997 GMC Jimmy: Turned out to be a turkey.
1989 T-Bird Super Coupe: 150,000 miles, still runs great. Ford got it right.

Before discussing the loaded misfire, I should place a warning to my previous post.

Be sure that when cracking lines loose, to catch the fuel lost with rags and don't let it accumulate. I also get chills up my back thinking of the people who will run right out trying to identify a ignition misfire with gas accumulating on the top of the motor. Be sure you have no spark arcs happening any where near. BE CAREFULL!

As to the loaded misfire. This unfortunately streches the tooling of many shops and most individuals. The technique becomes replacement diagnostics.

I have two tools, no one in my town has, to solve that problem on this car. the first is a programmable load chassis dyno. I can watch loaded misfires while sticking my head right in there.

The other tool and specifically the tool necessary to accurately test the current hypothesis: needs a fuel distributor. That tool is a differential flow meter. With this tool hooked to the injectors like a "milking machine" the fuel pump is hotwired and the airflow plate is held with a variable fixture to any depression including that which would happen at full throttle maximum speed. The flow rates of each cylinders injector/distributor path can accurately be measured and problems that will show up in performance are easily viewed. This tool is the factory tool made by Bosch and shown in early manuals discussing K-Jetronics. I know our local dealer doesn't have one. I'll wager very few do.

Without similar tooling its a guessing game, but I would start with injectors. I might also reverify ignition with a different tech. Ignition problems of misfire under load are real hard to see. In this case a portable secondary ignition scope is the answer. The best is the Vetronis MTS5100 (I know because I had the rest and just got the 5100). other good products are the Fluke 98 and the Interro PDA100 (got both of them). Find someone with the equipment and the experience!

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

As usual Steve, you demonstrate your complete knowledge of Mercedes. As a former foreign car mechanic (in the late 70's) let me express my gratitude for your advice and my respect for your knowledge and ability.

Makes me kind of wish I had stayed with it................

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M. Sandler

1986 560SEC: 150,000 miles, runs great, but I've got to sell it (too many cars for one man)
1987 560SL: 122,000 miles, used to run poorly, now (thanks to forum), runs great!
1997 GMC Jimmy: Turned out to be a turkey.
1989 T-Bird Super Coupe: 150,000 miles, still runs great. Ford got it right.

Steve,

On the fuel distributor problem, I forgot some important information. My mechanic suspects the fuel distributor because three of the spark plugs show those cylinders running lean. Compression is fine. He believes that these cylinders may "drop out" under load. Is this a possible condition with malfunctioning fuel distributor? Ordinarily, I would suspect a valve adjustment problem, but I believe this engine has hydraulic lifters.

As a means of peformance comparison, I have my 1986 560 SEC, same basic drive train, with no drivability problems whatsoever. At the risk of screwing them both up, I considered switching the fuel distributor from the SEC to the SL. Probably not a good idea.

Any thoughts?

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M. Sandler

1986 560SEC: 150,000 miles, runs great, but I've got to sell it (too many cars for one man)
1987 560SL: 122,000 miles, used to run poorly, now (thanks to forum), runs great!
1997 GMC Jimmy: Turned out to be a turkey.
1989 T-Bird Super Coupe: 150,000 miles, still runs great. Ford got it right.

If the three cylinders are identified, a number of comparitive tests can be run. Onr is to swap a poor cylinders injector with a good one and see if the problem moves. We keep extra lines around so we can also change the firing order (bg) so to speak. Move the poor cylinder feed to a good cylinder.

Swapping the two distributors for comparison would be a very efficient testing move. I would have no problem just switching them and leaving it that way if the problem didn't follow. Be sure you are capable of setting up a new distributor before you attempt this. Its not just R & R.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Boy you are quick, Steve! The injectors are all brand new, so I've kind of eliminated them. One of the alternative suggestions from my mechanic was to use a good fuel system cleaner in the gas tank and see if that helps. I threw in some Chevron Techron, and it seems to run a little better. What's really aggaravating is to drive my SEC, which starts on the first turn of the starter, and is silky smooth at all rpm's, and then jump into the SL, which suffers in comparison.

Anyway, I'm going to drive it for awhile and see if it gets better. Decided to abandon the switching fuel distributor idea. Thanks again for all the technical and common sense advice.

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M. Sandler

1986 560SEC: 150,000 miles, runs great, but I've got to sell it (too many cars for one man)
1987 560SL: 122,000 miles, used to run poorly, now (thanks to forum), runs great!
1997 GMC Jimmy: Turned out to be a turkey.
1989 T-Bird Super Coupe: 150,000 miles, still runs great. Ford got it right.

Join the club of the poor idling 86 420SEL's . I have a bad idle in my 86 420SEL as well. I bought a new fuel distributor but that didnt help. Turned out to be a bad exhaust valve.

So in your diagnosis, If you find you need a new fuel distributor, let me know. I will let it go for a very reasonable price, since it is only accumulating dust on my shelf.

-Andy

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93 300D 2.5 Turbo, Black/Palomino 273K
09 E350 Black/Black 41K