[darraghodonnell]

I am regularly on the 129 forum and it is such a fountain of information and money saving tips- but as i have an SL320 M112 (95000 miles) engine maybe any of you E320 owners might help-(most of the 129ers are sl500 and sl600)-
Problem-since i own the car (bought at 65,000 miles) about ten seconds from cold I get a constant vibration when stationery. It rarely is even noticeable on the rev counter and the revs are correct as per the engine spec. With aircon on it sometimes worsens. When steering wheel is locked it worsens. Sometimes I get a rhythmic surge. If I tough the gas pedal it disappears. Several well qualified mechanics as they don't know where to start have suggested I live with it. No fault codes come up. Otherwise, the car is wonderful and drives beautifully. It sounds as it should. I just want to get rid of the idle.

Looking at the engine in P- it is rough at around 560rpm gets more rough up to 1000rpm and is completely smooth at 1500rpm and above


Replaced - oil oilfilter air filters spark plugs ht leads three engine mounts cps MAF sensor throttle body and egr valve cleaned no obvious vacum leaks(but i don't know how this was tested-by visual inspection?) Gearbox has been rebuilt and works perfectly. Valve cover gaskets replaced and breather hoses from valve covers. ............i am no mechanic but i did the valve cover gaskets and spark plugs myself and am learning all the time -


ANY IDEAS PLEASE?
Thank you very much -

DARRAGH>

 

[Check Codes]

I'd start by popping off the serpentine belt just to see if it's one of the rotating accessories that is giving you fits. That takes just a couple of minutes. Then fire it up and see if the vibration is the same. From cold you could run it like that a couple of minutes without any trouble. If the vibration's gone then after you turn it off and before putting the belt back on you could spin and side-load everything by hand to see if you can locate the offender.

There was a recall in the US on harmonic balancers; that seems unlikely from your description BUT it should still be inspected.

Who changed the HT leads? These are sequential-fire engines, so those go on in a specific order per cylinder. In the wrong order they might not trigger codes, but would cause problems.

Clogged cats can also contribute to a rough idle. You usually will get a loss of economy, but that might be hard to calculate given the type of driving you most likely do in your area. That can be tested by removing an O2 sensor and inserting a pressure sensor, a well-equipped shop can do that. Cats on the 210 were junk; I don't know if MB used the same supplier for the 129 with the 112, but I'd suspect they did.

You mention "fault codes". Has this actually been tested by plugging into the diag port, or just that nothing has come up on the dash? If just the latter, then the diag port would be my next step.

Good luck.

 

[darraghodonnell]

Gregs210,
Thank you for the reply and tips-

Yes it has been diagnosed many times by the 38 pin diagnostics by merc independants and i have been told to just live with it or leave the car with them so they might spend hours and hours diagnosing and replacing parts...thats why I am picking the brains at this forum- and what a great forum it is.

I am aware that the cats are rubbish, they were replaced by the dealer under warranty before i bought the car. I doubt cats are the source but i am aware they might become damaged if this source problem continues.

I did the HT leads in the same order as the originals were and have re-checked them and double checked with my mechanic in case. However, what if the original leads were tampered with and inserted wrongly??? How can i find the insertion order?

My mechanic inspected the harmonic balancer and said it was fine - pardon my limited knowledge but which one is the serpentine belt so i can do that check - how do i 'pop it off?'
Am i looking for a shaky cog or something?


I'll bet you are now sorry you tried to help me.....thank you
regards
darragh

 

[jlomon]

My mechanic inspected the harmonic balancer and said it was fine - pardon my limited knowledge but which one is the serpentine belt so i can do that check - how do i 'pop it off?'
Am i looking for a shaky cog or something?

The serpentine belt is the one rubber belt that drives all of your accessories (power steering, air conditioner, water pump, alternator, etc). The belt tensioner provides tension to keep that belt in place. By rotating the tensioner anti-clockwise you release the tension and remove the belt. This thread shows how to replace a tensioner - the only portion you need to concern yourself with is how to release the tension to remove the belt (and also the diagram for re-installing the belt properly).

http://www.benzworld.org/forums/w210-e-class/1380201-belt-tensioner.html#post2929658

Please keep in mind that you may have the "new style" belt tensioner, which has the tension release nut in a different spot (about the 7 o'clock position - you'll see in the pictures there).

As Greg said, if you run the motor for a minute or two without the belt in place, and your vibration disappears, then you just have to find out which accessory drive pulley is the culprit. Once you have the belt off you can spin each accessory drive pulley by hand (engine off, of course).They should spin quite freely. Any one that sticks could be causing you a problem. That is what you are looking for.

 

[Check Codes]

Gregs210,
Thank you for the reply and tips-

Yes it has been diagnosed many times by the 38 pin diagnostics by merc independants and i have been told to just live with it or leave the car with them so they might spend hours and hours diagnosing and replacing parts...thats why I am picking the brains at this forum- and what a great forum it is.

I am aware that the cats are rubbish, they were replaced by the dealer under warranty before i bought the car. I doubt cats are the source but i am aware they might become damaged if this source problem continues.

I did the HT leads in the same order as the originals were and have re-checked them and double checked with my mechanic in case. However, what if the original leads were tampered with and inserted wrongly??? How can i find the insertion order?

My mechanic inspected the harmonic balancer and said it was fine - pardon my limited knowledge but which one is the serpentine belt so i can do that check - how do i 'pop it off?'
Am i looking for a shaky cog or something?


I'll bet you are now sorry you tried to help me.....thank you
regards
darragh

In reverse order...

I'm never sorry to try and help someone who appreciates the attempt and who provides so much terrific information in their posts and profile. Besides I drove through Cork so much when I visited your delightful country, so I owe you. Or something like that.

If you have a "normal" M112, you should only have one large "fan" belt that is about 25mm wide, it runs around everything and is driven from the crank. The tensioner is a simple mechanism (if you look in the stickies you'll see DIYs for it and if it hasn't been changed to the new design I'd suggest doing so as they can and do fail...what's the new design? you may wonder, there are pics and descriptions in the stickies) that allows you to easily release the tension on the belt and remove it. Look at the stickies to see which kind you have and go from there but I bet if you just take a look at it you'll see how tension is reduced. Compress the belt and you'll see it move and from there you'll see which direction you need to move the roller pulley to get it to release.

The leads are marked at the coil and head/head cover, A goes to a and B goes to b for each cylinder.

Vacuum leaks are always culprits for a rough idle, and I recall one poster discussing that he had his engine decarboned at the dealer and that finally cured his rough idle. In the states that treatment is about $150, might be worth a consideration.

Good luck and stay with it!

 

[darraghodonnell]

jloman and Gregs210,
Thank you so much for the info and excellent write-up. I looked at my tensioner and it is the old style T10 at the front and a hex shape closer to the centre of the wheel. I will loosen the tensioner and take it out, and check the smoothness of the wheels around which the serpentine belt goes and then i will report back. My car has 95000 miles - should i order a new tensioner?
In eurocarparts there is a photo of the belt tensioner - It looks the same as the old one except for a 'female' Torx head almost flush with the wheel - would this be the modified part?? - see link below

http://www.eurocarparts.com/ecp/c/Mercedes+Benz_SL+Class_SL320_3.2_1999/p/Car-Parts/Engine-Parts/Belts-Chains-and-Tensioners/Tensioners-Idlers-and-Dampers/?326220290&1&890ab78d969b7811a68ca6f0944efc143912d579&TENS

Kindliest regardations from Cork,
Darragh.

 

[Check Codes]

jloman and Gregs210,
Thank you so much for the info and excellent write-up. I looked at my tensioner and it is the old style T10 at the front and a hex shape closer to the centre of the wheel. I will loosen the tensioner and take it out, and check the smoothness of the wheels around which the serpentine belt goes and then i will report back. My car has 95000 miles - should i order a new tensioner?
In eurocarparts there is a photo of the belt tensioner - It looks the same as the old one except for a 'female' Torx head almost flush with the wheel - would this be the modified part?? - see link below

Mercedes Benz - Euro Car Parts UK?s No.1 Car Parts Retailer

Kindliest regardations from Cork,
Darragh.

The one in your link is the new and improved design. Although it does bear some similarity, it is wholly and completely different. The hex face that is part of the casting is actually where you affix a wrench to release tension.

The old design looks like this (pictures from G-AMG's great sticky):

If that's what yours looks like...personally I would replace it (unless of course your frustration level with the car is such you're going to sell it!). It's unlikely to be the cause of your vibration, but the older design has been known to fail rather catastrophically into many pieces, damaging other things and, of course, leaving you stranded. (Same for the idler pulley, but it's a cheap and very easy DIY, might as well do them both at the same time.) If it's the OEM belt, heck, do all three and you're good.

Please re-read what we advised. Do not just check the smoothness of the pulleys; rather, also check for end play and side play...but most importantly, start and run the engine with the belt off to see if the vibration is, perchance, gone.

 

[darraghodonnell]

Gregs210,

Sorry I cant post pics- On page 2 of the link to the write up from which those photos are taken, a character called 'netdvn' posted a photo(with red arrows indicating A and B) of his tensioner and you reply to him that it is the old version - netdvn's tensioner is the same as mine.

Which one is the idler pulley?

I'll get back,

Thank you
Darragh.

 

[Check Codes]

Gregs210,

Sorry I cant post pics- On page 2 of the link to the write up from which those photos are taken, a character called 'netdvn' posted a photo(with red arrows indicating A and B) of his tensioner and you reply to him that it is the old version - netdvn's tensioner is the same as mine.

Which one is the idler pulley?

I'll get back,

Thank you
Darragh.

Sorry, it took me a bit to find netdvn's post. You can set your page length preference in the control panel and mine is longer than yours, so it was still on page one.

Yep, though, that is the old one, no matter which way you look at it.

The idler is the topmost pulley on the left side as you're looking at the engine with the bonnet raised. Here's a link to details and a pictorial DIY:

Are you OBSESSED about your Benz? | DIY, M112 and M113 Idler Pulley Replacement

(There is a DIY in the stickies but the pics are corrupted.)

 

[darraghodonnell]

gregs210,
I located that pulley in my engine and read the thread on how to change it thanks. Observing the tensioner with the engine on (and therefore vibrating) I note that behind the wheel of the tensioner, the alloy 'sticky-out' bit (see photo you posted above indicating the two bolts with the red arrow drawn- half way along that red arrow is the sticky out bit) shakes as the tensioner wheel spins - is it shaking 4mm right and left - either in a springy way with the shaking of the engine or I wonder if the tensioner itself is shaking and if it could cause the engine to shake? The source of the problem? Doubtful I'd say.

Thanks for listening.
DArragh.



I am aware that ''sticky-out bit'' is not a technical term.

 

[Check Codes]

gregs210,
I located that pulley in my engine and read the thread on how to change it thanks. Observing the tensioner with the engine on (and therefore vibrating) I note that behind the wheel of the tensioner, the alloy 'sticky-out' bit (see photo you posted above indicating the two bolts with the red arrow drawn- half way along that red arrow is the sticky out bit) shakes as the tensioner wheel spins - is it shaking 4mm right and left - either in a springy way with the shaking of the engine or I wonder if the tensioner itself is shaking and if it could cause the engine to shake? The source of the problem? Doubtful I'd say.

Thanks for listening.
DArragh.

I also doubt that is the source of the problem. But if it has that much play, I'd suggest changing it, regardless what else you do (that's why it was redesigned). And you can pull it and the idler pulley off, then start it for the test, and install the new parts when you go back.

Good luck, keep after it and let us know how you're getting by.