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wur vacuum question

Started by w116john, 18 December 2013, 10:59 AM


I will add to what Brian has said. WUR is really a misnomer and does not adequately describe what this important component does. It is responsible for mixture control the entire time your car is running. When cold it is responsible for enriching the mixture by lowering the control pressure. When the engine reaches operating temp it is responsible for maintaining steady state control pressure which directly affects your fuel air ratio the majority of the time. During acceleration it is responsible for lowering the control pressure to enrich the mixture. Each WUR has specs for what those control pressures should be under each of those conditions. If any are out of spec and everything else is perfect your engine will not run right. While the WUR is on the engine the only was to determine that is to connect a CIS test gauge and carefully test and record those control pressures and compare them against factory specs for that specific WUR. While doing this you also need to check the fuel pressure that the WUR sees against spec. If you can not do this find a shop to do it for you and make sure you have the mechanic record what the test gauge says and give the numbers to you.

Any other monkeying around, taking the WUR apart, trying to clean it, or trying to swap in a used WUR is just a waste of time IMO. You might get around your immediate problem but your engine is not going to run right. The CIS tester is not that expensive and not that difficult to use. The specs are available in the small blue Mercedes tech books which are not expensive and will contain specific specs for the exact model of WUR the factory engineered for your engine.

Another tip that I was unaware of for a very long time until Larry Fletcher made me focus on it. That is the very precise requirement for the rest position of the air metering disc. Even 2-3 mm can make a big difference if it is not perfectly positioned. I never believed it until I got it perfect on my 6.9. Why does it matter? Because that position establishes the relativity for the fuel air ratio for the entire range of different air flows and operating conditions based on the precisely engineered shape of the walls of the air metering device. If is not exactly right you will be too rich or too lean under different operating conditions and there is no way to adjust or modify the WUR to account for that. It's a difference that makes a difference. Check yours against the diagram in the Mercedes service manual for your car and make sure it is right if you want your engine to run as the factory intended.

Hope this helps!


Great explanation!  Thanks for the info   :D
1979 6.9 #5541 (Red Bull)
1978 6.9 #4248 (Skye)
1979 6.9 #3686 (Moby Dick)
1978 6.9 #1776 (Dora)
1977 450SEL #7010 white -P
1975 450SEL #8414 gold -P

gerry l

Hi guys
I have a 1977 450 SEL 4.5. It was not a running car when I got it but someone had played with it, the WUR has vacuum lines going to the top and bottom, the line going to the bottom is connected to the manifold side of the Auxiliary air valve, and the one connected to the top is going into the big hose on the other side of the AAV  I just got a book Mercedes-Benz CIS theory and operation.  This shows the vacuum port on the bottom open to atmosphere. Mine is the one that stands up (not on its side). Can anybody tell me witch is correct?  Thanks for in advance for any responses.


Do you have any factory workshop manuals and the small tech book for your car? If not, they are well worth the cost. Mercedes made changes in the WUR specs from time to time and you need to check the number on yours first to see if it is the right one for your car and then check the manual for the connections. As I recall the larger port is usually the vacuum line and the smaller one is for atmosphere. But, check to make sure. If you don't have this documentation write down the Bosch number for the WUR and pot it on this thread and I will try to find it for you. You could also pull the intake side vacuum connection to the WUR with the engine running at idle which will put the WUR in acceleration enrichment mode and you should hear that difference. The WUR sees vacuum most of the time which keeps it in steady state but when the throttle opens the vacuum goes away and that causes the control pressure to drop which makes the mixture richer for acceleration. If nothing happens you have the lines backwards on the WUR so swap them and test again. I like to be meticulous and use the factory manuals so I know exactly what I am doing. You can also get a year's subscription to StarTek EPC for $ 12 and get a lot of the same info. Those small blue soft cover tech manuals have a ton of good info and they also cross ref the Mercedes and Bosch numbers for the WUR and Fuel Distributor. Hope this helps!!


On reading the original thread I wonder if the WUR is really the problem at all.

In some cars the AAV lets in too much air when cold and you get an exaggerated fast idle when cold which is useful under very cold conditions but not so useful in milder climates.

The solution was to fit a restrictor into the inlet side of the AAV to restrict the airflow and reduce the idle.

As concerns the WUR cvacuum leads there are several dfifferent options depending on markets, smog gear and year.

I think the lines were reversed sometimes during the 1978 year which makes it even more confusing but all will be revealed in the correct manuals.

Most of the available manuals are produced for the US market only which is not so handy if your car was produced for elsewhere but there are supplements but there are yearly supplements that give market differences.

Some years ago I digitised my manuals and I am sure they were sent to this site as well as the other Sydney Site and they will give this level of detail.



hi bill

totally agree with you, i had changed the aav twice and both times i had this very high cold idle 1700-2000 rpm so i thought it must be something else. when looking for vac leaks i did notice that blocking the large vac line from the wur dropped the idle, so i was wondering if the wur was causing the problem.

i took out the aav again, tested and subsequently squeezed the bulb assuming that an aav that closed too early was less of a problem than one which didn't close fully.

my idle is pretty good it can still rise to 1500 rpm but drops back to 900-1000 quickly.
i do think i have a vac leak which is more evident when cold, i will get the mechanic check it out next time its in


gerry l

Hi Guys, I am near the end of my rope. Bought the car in Oct. as a non runner, the guy I bought it from was a mechanic who had tried to get it running with no luck (it had been sitting for about 10 years) he had drained and replaced the fuel, changed fuel pump and filter. When I got it, it would run only by spraying starting fluid in the fuel distributor. I suspected the fuel distributor, I bought a rebuilt kit took it apart did not find anything obliviously wrong put it back together and the car ran very well for about an hour, then started to splutter and died, if I remover the distributor from the car and replaced it, it did the same thing ran well for a short time then died. I decided to have it professionally rebuilt, this guy also did not find anything wrong with it, but when I reinstalled same thing again, then I turned my attention to the WUR checked incoming voltage disassembled and checked ohm rating cleaned and reassembled again ran great for about a half hour then spluttered to a stop, while I was working on the WUR I noticed one of the ballast resistors was cracked so I took a brake from the fuel and went to work on the ignition new cap, wires, plugs, and two ballast resistors (waiting for a new rotor) again started up and ran so well for about an hour I decided to take it to the gas station broke down after about 2 miles. I was able to get back home by letting it sit for 1-2 minutes then it would run 3-4 hundred yards. I did buy a CIS test kit, control pressure between WUR and top of Distributor is 3.6 bar, fuel pressure to Distributor from pump is about 6.5 bar. my VIN # is116 033 12 070243 the WUR #is 0438140010 The StarTek site does not support Mac. so I will have to join at work. What are the small blue soft cover tech manuals? do you mean the Mercedes workshop manuals?. PS I am a mechanic with about 30 years experience but not familiar with this system, So any and all help is greatly appreciated, and yes I know I repeated some stuff from early posts, But I had to vent a little. I still love the car and know it is just a matter of time.


As far as technical documentation, here's close to the complete set.

M110 Engine
2 volumes Complete info on all aspects of Service, and adjustment. The best technical manuals I have ever seen in my 50 years of working on cars. The M110 manual came out as an 1 volume manual which doesn't apply.

Chassis 116 2 Volumes

Technical Data for Passenger Cars (year) This is the small softcover book which contains setting data, torque values, adjustment info. Extremely valuable - the go-to reference.

Mercedes Benz Model Year (year) Introduction Manual for Passenger Cars These don't come out every year, and detail changes from earlier manuals + info on newly introduced models.

Mercedes Benz Maintenance Manual USA Model Year (year)

Parts Book Mine is Catalog H. Good for seeing parts and assemblies. Very clear drawing that match up to EPC.

It's taken me a long while to accumulate all of these, but eBay is an excellent resource - just don't overpay.  Also if there are any I have missed, someone let me know.

Note: I have a Mac and use Parallels. Pain but it works.
190sl 1957 rusting away
250S 1968 long gone
280SE 1976 got hit, parts
280SE 1979 running fine
C320 4Matic 2005 for wife -Mercedes after MIT


It is Sisyphus's work to adjust K-jet without pluging all the vac leaks first, make sure ignition timing is default and there is no significant stretch in the timing chain.

In case like OT, after vac leaks plugged  and timing is set at default, I'd adjust CO value at 80*C engine temp (no more, no less) using 98 fuel and with idle rpm forced to be 750 at idle screw
All the other problems should be addressed from such starting point and then CO and rpm revisited after any adjustment. IMO, AAV  adjustment should be the last.

Most of the problems are purely due to vac leaks but most of the know-it-all mechanics start to fiddle elsewhere, messing up the whole system so even if you end up plugging the leaks, they introduced bucketloads of other off-tunnings making your task  a nightmare.
Vitamin C for SL... the SLC