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WUR or something else?

Started by vlv8vic, 01 March 2011, 01:54 AM


Hi all,
I've had a cold acceleration issue since I've had the car.  It starts first go, but while cold it'd splutter and choke if i stab at the accelerator.  This seems to fix itself after a few minutes of warm up with a few stabs at the pedal until it clears.
The idle has been changed, i suspect, to compensate for the cold idle issue as once the car is warm and has been driven for a few minutes the idle comes up quite a bit, to something close to 2000rpm.  It drops back significantly when in drive on the auto, but when shifted to park it'll come right back up.
I tried a cold start with the electrical connection of the WUR pulled off and the car started exactly the same as it would any other day.  DOes this indicated the WUR might be an issue? Might be mind over matter but the car did seem to run better with the WUR disconnected, but it might also have been because it got a proper warm up this time, not just me getting in and pulling out to get to work in a hurry.
I've pulled the vac lines from the distributer while the car is running, one of them seems to do nothing and the other causes another increase in RPM so i think that means the vacuum is operating as it should (to some degree).
Timing looks pretty close if not correct - from other posts you'll know mine seems to be missing the pin used to check the timing against so I'm working on a rough estimation of where the pin should actually be.

I'm scared of attempting to pull the WUR from the car unless i have to.  Besides it being in a less-than-ideal location the pipes look old and fragile and that makes me nervous.

Car has new fuel filter at the tank, new leads, new plugs, dissy 'looks' fine but I've no idea what to look for other than clean connections and points.

Lastly, on the exhaust side of the engine there's a bell shaped device attached to the exhaust manifold with two tubes coming from it.  It's never had anything connected to it but appears as thought it should have.  Not sure if this could form part of the picture also.
All thoughts and opinions appreciated.



Hi Japes, my 450SEL 4.5ltr would cold start (especially on cold days) and idle like a pig barely keeping itself ticking over until it had warmed up.

Whilst doing the timing chain replacement i sent the WUR to MB spares for a rebuild and when fitted to the car there was a complete transformation beyond my best expectations! She starts and idles smoothly up to around 1500rpm then gently comes down to around a 1000rpm and as she warms she settles to a smooth idle of around 700-800rpm.

No adjustment was needed to the fuel system which was a relief (some say this may need to be done)

I remember explaining the cars poor start up on cold starts to WGB (Bill) and he said straight up it looks like the WUR, he was on the money  8)

I had replaced the cold start valve two years ago and it actually made the cold starts worst to the point i thought i had a dud, then i fixed the WUR and everything is sweet. They can give symptoms of high cold start rpms. Search of cold start valves and it gets plenty of coverage on the Org.

For a 450 like mine a cold start valve will set you back around US$500 (MB Classic Center Irvine California) as they are no longer produced and are thinning out in supply. A WUR rebuild set me back recently AU$290 from memory (MB Spares Canberra Australia). i have seen in the USA websites places that can rebuild cold start valves so that could be an option in the future for us.

12/1979 450 SEL 148K on clock (museum piece)
12/1986 Lotus Esprit Turbo 87K on clock 'Darling, look what Q has brought for us, isn't it nice' :)


Idling low and coming up as it warms up is opposite of what should happen suggesting the auxilary air valve is not working.

Remove it and test it in boiling water, see if it closes.

Giving it a good clean out with carb cleaner might help.

But also by the sound of it the engine is not getting sufficient fuel when cold which could be a blocked screen in the WUR.


Boogity, Boogity, Boogity, Amen!


a couple of things I'm not sure on (remember i'm still a learner on the MB).

Is the cold start valve different to the WUR?  I'm not sure what the cold start valve is or where to look for it.
While we're nutting out the acronyms, where do i find the AAV and what can i do to it?

I've also worked out the 'thing' on the manifold is the EGR and have searched a little about it - it doesn't have ANY vac line attached despite having two tubes where it appears they could attach.  I read that if stuck open this can cause some vac and idle grief. Thoughts?

I think right now I'm chasing my tail a bit...

Edit - found the cold start valve - is this self serviceable? It comes off nice and easy.  Have also ascertained that the EGR, despite having no vac attached is operational by hand and i guess through lack of vac is always in the open position? The engine was already warm so i couldn't really muck with it much.

double edit - it's dark out now but i reckon i have a fair idea where the AAV is now and will attempt to give it a clean up when i get time.

TJ 450

The cold start valve isn't really serviceable, other than cleaning it. The good thing is that they are near ubiquitous and used ones can be found easily.

1976 450SEL 6.9 1432
1969 300SEL 6.3 1394
2003 ML500


Cold start valve is the extra injector at the front  of the inlet manifold which runs for a few seconds after cold starts - it doesn't sound like the cause of your issue.

The Auxuliary air valve is to the right of the WUR when you're standing at the radiator, and can be tested by checking that it's open when cold (extra air), and that it closes over a few minutes when 12v is applied to its connector (extra air not required when engine warms up). If it's closed all the time, but the fuel is still enriched when cold (good WUR), then you'd have an over-rich mixture generally, when cold, and when you hit the gas when cold it would be even worse.

However, it could also be a bad WUR (control pressure is too high, leading to a too-weak mixture when cold) - hitting the gas then would lead to a really weak mixture (too much air, not enough fuel).

Some testing is required to see which one it is.
[URL=[url=""]]My[/url] 1976 350SE W116 in Silbergrün[/URL]


thanks crew, once again invaluable advice.

Bit of an update, i've located the AAV and it's not quite where I picture from your description Type17, and i doubt that the location allows me to check that it operates in situ but I'm confident that i can pull it out on Sunday for a look.

I got the WUR off and cleaned out the screen. there was some shit in there but not huge amounts.  The WUR itself didn't feel right so i disassembled it and it was chock full of oil.  I have given it a clean out which would make even the most fastidious  mother in law proud but didn't attempt to do any tests on it.
The oil ingress, I assume, is probably a result of years of blow-by and vacuum hoses being arse-about.  The hoses connected seem to suck the right way now so maybe the oil has been dribbling back down from the inlet manifold.
Either way the oil can't have been doing the operation of the WUR any good and may have wrecked it properly.
The result of given this a good clean out seemed to bring the RPM down a bit at idle when warm, something closer to 1500 rather than 2000.  The car was already slightly warm but it still coughed a bit on startup with a jab of acceleration so i doubt i've made any great impression there.

I wondered if i could pull the RPM down a bit more with the little air knob thing which i suspect it attached to the AAV but i think it's already all the way down because it didn't want to turn beyond a few mm more - though i didn't apply a great deal of force.

Any thoughts on the EGR and where the vac connections to/from it should go?  It has two tubes with nothing connected and coincidentally there are two vac hoses over near the electronics box (for lack of knowing what it is...) that have been capped off with a small bolt each.  I wonder if those are supposed to circumnavigate the engine and make functional the EGR?


s class

The AAV and WUR are really inaccessable on the 280SE, and their location below the inlet manifold differs from the v8's where they are in plain sight on top at the front of the inlet manifold. 

[color=blue]'76 6.9 Euro[/color], [color=red]'78 6.9 AMG[/color], '80 280SE, [color=brown]'74 350SE[/color], [color=black]'82 500SEL euro full hydro, '83 500SEL euro full hydro [/color], '81 500SL


Quote from: Japes on 03 March 2011, 04:20 AM
I've located the AAV and it's not quite where I picture from your description ...

Apologies, I was assuming a V8 layout, I forgot that the 6-cyl would be different.
[URL=[url=""]]My[/url] 1976 350SE W116 in Silbergrün[/URL]