Bad idle when warm - not Aux Air Valve

[Jakob]

Hello all,

My '78 450SEL has developed a really bad idle after it warms up.  When the car is cold it idles normally but when it's warm it will feel like it is about to stall every 30 seconds or so (but it won't).  It will also bog down on takeoff until 1500rpm or so.

The ignition leads and distributor cap and rotor have recently been replaced.  I thought it might be the aux air valve since the symptoms fit descriptions on the forum but I took that off over the weekend and put it in a pot of boiling water.  The valve closed completely until it got cold again, and then it appeared to open as per normal.

The car is on LPG.  Does anyone have any suggestions of other things to check?

[Jenno1981]

Same vehicle, same fuel, same problem!

Although I have given my AUX valve a good clean and am ready to re-install, for my sake I hope that is the solution, but if it's not I will be in the same boat.

[Jakob]

Looking forward to your experience Jenno.  I talked to MB Spares (merc shop in the ACT) yesterday and they suggested it might be a vacuum leak to the manifold, so if your problem still persists, it would be worth checking that too.

[Jenno1981]

Sorry for the delayed reply. I did actually re-install the aux valve and started running fine. Unfortunately I neglected to get myself a aux valve gasket and it very slowly started leaking coolant,  so thought it would be best to shut her off anx savd it until the gasket arrives.
Any luck on the manifold vacume Jacob?

[daantjie]

Had same issue on my 6.9
Make sure your AAV top section in flush with the body of the valve. Centre "head" should be raised, but the "shoulder" section must be flush with outer body.
If not the valve will close at higher temp causing rough hot idle.
There is good post by member Koan on this as well.

Take valve out and tap head section with hammer and plywood to protect head of valve.
Once flush, reinstall, hot idle should be much improved.
Cheers

[Jenno1981]

Daantji, ive heard of this before but never attempted it. Could you please post a photo to show us or the link to Koan's post?
Thanks!

[w116john]

hi guys

get hot idle sorted first

when engine is warmed up i would open the idle screw until engine idles at 750 ish in park, drive it a bit and see how the idle is if its ok then see how it idles when cold, it takes a long time for these engines to get cold so wait till the next day and if it idles very high 1500+ then i would tap the aav.

i think the best way to do it is when engine is cold and revs are high gently tap the aav and the idle should come down, dont be tempted to bring it down too much there is no going back. i have done that twice which resulted in sluggish cold starts

then get it a tuned up,  you should notice a big improvement my hot idle was very lumpy and i had the car tuned and its been great since

best of luck

[Jakob]

The driveshaft has been out getting rebalanced so I haven't put the AAV back on to check other things yet. I hadn't thought of the fact that while the valve might close in boiling water on the stove, it might not close in only 90 degrees C coolant.  I might just buy a new one and see if it makes a difference.

[Jenno1981]

Apologies for the delay in reply!

I've had the AAV back in after the clean of it's life and testing it so it closes at the correct temp.... and there was only a small difference but I still get this lumpy, spluttery hot idle.
So I'm up to this 'tapping the head of the AAV' as a solution but want to make sure it's what's needed before I do it.  This is how it's happened thus far:

1. Let the car warm up as normal in park and observed anything under 1000rpm get that lumpy/spluttery idle.
2. Opened the idle screw to alleviate the issue as she warmed up to about 80 degrees
3. Went to take her for a drive and as soon as I put her into drive the rpm dropped to about 600-700rpm and it was lumpy, got out and opened her up a bit more
4. Took her for a drive and noticed at the lights in drive it was a lot better
5. Got home and let her cool over (2) nights then started her up and rpm rises consistently to about 1600rpm
6. Observed the 'shoulder' of the AAV to be raised slightly from the outer edge as described above.

Is there anything else I should do before tapping the AAV that may be the cause of the problem?

Any thoughts are welcome!

[gavin116]

Hi Jenno


I had very similar issues to you.  I too changed my AAV for a new unit, and although it was better, the idle was still lumpy and would surge. 


I thought the next logical component would be to refurbish or replace the WUR (warm-up regulator: a misnomer, I prefer the American term: control pressure regulator, which describes the job it does more accurately).  I am lucky I didn't get this far. 


Deep in my subconscious I realised that in my 'lets replace the 35 year old rubber components' I had only thus far changed most of the coolant hoses and the air hoses.  I still need to replace the heater hoses, and am working on collecting suspension parts...


You may wonder where I am going with this?  Well, I decided to replace all the rubber hoses that connect the vacuum lines, including all vacuum check valves (they help isolate systems like interior cabin ventilation, cruise control and central locking from the vacuum system specific to the functioning of the engine).  There's a lot of rubber connections, and nearly all were perished. 


It has made a huge difference to the running of the car!  One or two of the hose connections were tricky to replace, but for the rest, they were straight forward.  Take lots of pictures before you start in case you cannot remember where they all go.  Occasionally the hoses slip out of the 3 and 4 way connectors as you are doing them, so be mindful of their positions.  I used a little bit of di-electric silicone grease to help marry the vacuum lines to the new hoses.  Also, be careful with the old vacuum hoses, they are very brittle...


To get my car absolutely smooth at idle now, I will need to replace the engine mounts, and then there should be little to no engine shake.


Try this first,


Gavin


P.S.  I bough a repair kit, complete with most of the hoses, and a good length of continuous hose that you can cut to length from MercedesSource.
https://mercedessource.com/store/616-617-diesel-vacuum-connector-service-kit

http://forum.w116.org/test-drive/my-first-w116-here-at-last/msg117593/#msg117593


Edit: found some pictures to show the stark difference, and added a link for the vacuum service kit, although you may need to buy more of the connectors, readily avaliable from any M-B dealer cheaply.

[Jenno1981]

Thanks a heap for that Gavin!  I'm moving house soon so not getting too adventurous under the hood but will get to this for sure.
By the way, got the AAV in and although it works fine it has not alleviated the problem so I'm moving on from the AAV.
Thanks again!
Jenno