Do all w126 shakes at idle?

[fpharryc]

I mean V8 W126.

I have two of these W126s for years, interestingly they idle with the same pattern. While idling, the rpm gauge is very stable. However, sitting in the driver seat, I can feel the whole chassis shakes/moves slightly left and right. These V8 engines do not run perfectly still. They "twitch". Both of them otherwise run excellent.

This phenomenon simply can't compare to my LS400s. LS400's engine idles perfectly, it does not vibrate or shake. I can't feel the motor running while sitting in the car.

Is this observation correct? My W126s are not some ignored old Benz. They are well taken care of. In fact, the 500SEL engine was just completely rebuilt, new fuel distributor, new motor mounts, new motor shocks, new vacuum lines, everything. Duty cycle is set correctly. I plan to drive it for another 200,000 miles before next rebuild.

I never have chance driving other W126. How satisfied are you W126 owners about your car in terms of the way it idle? How do I improve it? Or maybe simply that's just the way it is? Am I too picky? I use Chevron only.

My 300CE idles as smooth as silk though.

I have seen this on MB v8 cars, and usually they need a little fine tuning.

I would have someone put the car on a scope and see what is going on, the likely culprit are bad ignition wire ends(too much resistance) and/or partly clogged CIS injectors, assuming that the valve guides are not worn, letting excess air into the fuel mixture. A vacuum guage will help to diagnose a valvetrain problem.

If you don't have someone with a scope, I would look real hard at the secondary ignition components(esp. the plug wire ends and proper non- resistor plugs), and also consider replacing the injectors. They run around $30.00 each, and after twenty years, the spray pattern is probably poor. The fuel mixture will have to be reset after replacing the injectors, though I would use a can of BG44K first if it was my car.

I knew someone who bought a 500SEL new in 1985, after three years it also had a funny idle. The factory trained tech blamed the fuel injectors, and he was right. It turns out the cheap regular unleaded his wife used caused the injectors to gum up, a can of BG44K fixed it.

I would make sure the ignition is right first, and then move on to the rest.

[fpharryc]

John, the whole ignition system is new installed by myself. I will try BG44k.

Originally Posted by fpharryc

John, the whole ignition system is new installed by myself. I will try BG44k.

The BG44k will do wonders, I usually don't believe in "snake oil" products, but it really works if used properly.

The local MB dealer stocks it for the service department if that tells you anything.

[86560SEL]

Well, I have a 1985 380SE and even before my car developed the engine "miss at idle when in park and when first shifting into 3rd and 4th" problem I would notice an occasional side to side movement when I was setting at a stop light or something. Sometimes it does not do it at all. Heck, I may need some of that BG44k stuff.

I am still working on mine.... I have heard my problem could be a vacuum leak, fuel pump relay, spark plug wires, etc. So, my next purchase is a FPR, or at least try to test my car to see if that could be the problem. I also have an occasional stalling of the engine, but only when outside temperatures are above 70°, but spring and summer are coming, so I need to fix whatever this is.

John said, "assuming that the valve guides are not worn" - thats probably my luck and whats wrong with mine, as I have 269K and people wonder why I do not want to spend $$$ on this car.

I almost bought a 84' 300SD, with 335K. It had a horrible engine shake.... it would violently shake at idle, but with even the slightest acceleration, it would smooth out completely. I had no idea what what was wrong with it, as I came here and I was told it could be bad news (bad compression), vacuum leaks, bad injectors, or as simple as a bad motor mount..... more of that "wild goose chase" info.

SO- I bought a 1990 Lexus LS400, so I can relate to your statement about it being smooth as silk. My car had 245K when I sold it and the engine and transmission were still as smooth silk.... flawless. I would have to look at the tachometer to see if the engine was still running when I was at stop lamps. I sold it because my A/C evaporator went out, as was my radio (it would only work when outside temperatures were above 90° for some reason). I sold it to my grandfathers pharmacist, who crashed it a few weeks later.

I currently have a 2000 Toyota Tundra pickup, with the 4.7L V8, which is supposed to be based on the Lexus V8 design (which is actually the same engine used in the Lexus LX470 SUV). It too is as smooth as silk... you cannot even hear it run at idle and I have to again, look at the tach to see if its still running.

[420 SE]

Nope, a well tuned, and as you have already replaced the engine mounts - rule that out, 126 does not shake, you could balance a coin on the dash if they are running right.

Mine is quiet and still at idle, you have to check the tach to see it is on.

[gmercoleza]

My 85 380SE with approx. 131,000 miles idles smooth as silk. Can't even tell the engine is running. I know the PO replaced all the engine and tranny mounts and shocks. Even if yours are new it is possible you have one or more defective parts. Even new parts are occasionally defective.

[blueeagle289]

I have both of these cars and the same experience as the Thread writer; the 89 420 has a slight roughness in the idle, not a shaking or anything -- but runs very well otherwise. (perhaps I need to run a can of the super cleaner through it? My 300CE idles smoothly.... Ben Carter

[Monomer]

Mine (when I had it running) ran perfect.

New plugs, dist., wires, air filter, cam oilers, Resoldered Idle control unit and OVP.


now, if the fuel lines didnt rust out...

[Hatterasguy]

My diesel I6 W126 is silk smooth at idle, you can feel vibrations through the steering wheel but thats it.

Change your engine mounts, make sure the engine is idleing right.

You should almost not know the engine is on with a V8 W126.

[86560SEL]

Oh joy. Aren't you the lucky ones?

If I cannot figure out why my car is missing, when it is in park and when it first shifts from 2nd to 3rd and 3rd to 4th, I am going to redline it until she bites the dust......

[cool]

No ALL V8 126 are not suppose to shake.

[hk20000]

Another interesting thing to consider is that LS400 has hydraulic subframe mounts that eliminates any NVH the engine wants to give you versus the old Benz that has the engine bolted onto the chassis where you sit on.

makes a world of difference. Newer Mercedes probably has similar design too for NVH blocking.

[420 SE]

Originally Posted by 86560SEL

Oh joy. Aren't you the lucky ones?

If I cannot figure out why my car is missing, when it is in park and when it first shifts from 2nd to 3rd and 3rd to 4th, I am going to redline it until she bites the dust......

86, It really makes me sad to hear you continually say you are going to drive your car into the ground.

Seriously, if you are not going to keep up the maitenance it (and all cars) requires, give it to someone who will keep a fine piece of engineering on the road.

I don't mean to be harsh, but I am afraid your frequent postings of ignoring what amounts to normal ongoing preventative maintenance are getting sorry.

There is an old saying that there is no such thing as a cheap Mercedes. I don't know (nor do I want to know) your budget, but if it doesn't stretch to looking after what sounds like a fine car, allbeit needs a little work, pass it on.

Said with the best of feelings and the love of great cars.

Cheers

[blueeagle289]

I agree that there is some cause for really rough idle on these cars. I found that two plug wires were on the wrong cylinder when I bought the car; probably a mechanic who crossed hands. It ran basically fine, but had a more than normal idle roughness. Sometimes it is just something basic like that; fuel or air or vacuum related. As others have said in this thread, perform basic maintenance in those areas and see what the cure is. Also, take it to a GOOD mechanic who can check it out and also add some good injector cleaner; that kind of thing...... Ben Carter