Vacuum Related Rough Idle? (R129/SL500, M119.972)

[jsmulhollandiv]

Hello 500eb,

I searched through the various areas about vacuum lines but wasn't able to identify this part. I have had a persistent slightly rough idle when in drive and I couldn't find any issues with the vacuum lines until this today. I'm not even wholly sure what it is but it looks like it was sucked flat like you would with a straw if you covered it on one end. It leads to a grouping of other vacuum lines near the modules (also attached). Can anyone share what this mystery part it's attached to is? I'm wondering if this could be the miracle of getting rid of the persistent uneven idle.

It's an M119.972 engine.

Any thoughts?

 

[gsxr]

I don't know what that is, since the SL500 has completely different vacuum systems than the sedans.

However, that small of a vacuum leak should not cause a rough idle. I'd look elsewhere.

Can you explain exactly what is wrong? Is it a rough idle, or misfiring? How old are the engine mounts, and are they OE/OEM if recent? Have all the large vacuum/PCV hoses been checked? Etc.

 

[jsmulhollandiv]

Hi Dave,

Ha, it was a long shot; bizarre to look at though right? I've never seen a vacuum hose crumble in on itself. I've found little help at the BW (a dirty word around here, i realize) so I figured I'd throw it out there.

As to rough idle:

On start up the there don't appear to be many issues. There is a little movement in the tach (+/- 25 RPM if that) but generally not too bad in P or N. However once the engine warms up and is in D or R like at stop light or in a parking lot, the tach can dance almost 100 RPM at times but is completely smooth when gas is depressed. If it's bad on a given day, i will give it 200 rpm or so of gas just to keep it smooth at a stoplight. Where it is really problematic is if I come to a relatively hard stop after moving down the highway at speed, suddenly the tach will dance 500 rpm or so for 3-4 seconds, oscillating between 250 rpm and 750 rpm as if something is settling down. Normally i put into neutral quickly to stop it from stalling.

Engine mounts next order of business. They are original and have never been replaced. I had the trans mount replaced when i need it dropped for some other work but not the engine mounts. I do notice movement if I do the torque test (foot on the brake, gentle increase of revs) but I don't have much of a benchmark to know if thats normal or not. My thought is potentially that the engine is oscillating after stopping at high speed and causing the extra rough idle until it re-settles a few seconds later.

Would this theory pass a 500eb smell test?

 

[gsxr]

That helps! Sounds like a significant vacuum leak is a good possibility. If you can't spot anything obvious, try a smoke test. Engine mounts won't cure a 500rpm oscillation, but you'll need to shell out for OE/dealer if you want them to last another two decades ($400+/pair).

Any fault codes on the E-GAS module or LH? What is the date code on the ETA?

 

[jsmulhollandiv]

I have an appointment at the dealer (god bless me) to get the mounts looked at and done. Honestly its probably not the worst idea for preventative maitenance at this point anyway since they're original.

I guess i'm going to have to take it for a smoke test as I don't have the proper equipment to do it myself. I find it interesting that the vacuum leak would only cause large issues when coming down from speed - the rough idle is irritating but not car ending of course. I wonder why that would be.

No faults on E-Gas or LH. As to the ETA... ugh its original. Upper wiring harness is brand new, 2017.

 

[gsxr]

If the ETA is original, getting it rebuilt by RFC / Don Roden would be a good idea, even as preventive maintenance. I don't think that's the cause of your issues but it's hard to say without viewing live data from the computers while the RPM's are oscillating. No fault codes makes it tougher - I assume no codes on the EZL, or other powertrain modules (BM, DM, ASR).

 

[jsmulhollandiv]

Good to know. I'll have to reach out to him when things settle down a bit. For now I'm going to do a leak test and will report back on anything I find. Never hurts for the benefit of others (even if its a very different vacuum line set than w124).

 

[gerryvz]

I spent 30 minutes on the phone with Don today. He is very grateful for all of the business from members of this forum. Cost for an ETA rebuild is $379.

 

[jsmulhollandiv]

Gerry, I probably won't get around to it for at least 6 months. What is his contact information?

 

[gerryvz]

Gerry, I probably won't get around to it for at least 6 months. What is his contact information?

Quick search ......



Gives this as third result....

 

[jsmulhollandiv]

As promised, I would like to report back. I ended up sending it to the dealer this time instead of my local Indy. I've tamped down on 90% of the idle instability. Results as follows:

1. No vacuum leak could be found except for the one that I posted up above which is apparently an EGR Recirculation Changeover valve. Unclear what the line connects to inside the coffin still but regardless... Line is changed and I still don't know what it is or what it does. Not sure if this would actually have had any effects.

2. Engine Mounts/Trans Mounts: Completely, totally, 100% collapsed. You could see where the fluid had escaped from them a while back when looking at underside.

While there is still some minute instability when in D or R, the change is profound. When braking hard the Tach no longer dances. When speaking with the tech he explained this isn't unusual. As you're coming down from speed the engine and the trans if their mounts are shot will vibrate and can cause that symptom. It also feels more responsive to acceleration though that might be a placebo effect... Either way its much better. I would imagine this experience is readily transferable to most of the mercedes from that age. If it feels like its vibrating too much, change the darn mounts... (and also any collapsed vacuum lines.. maybe).