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wur vacuum question

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


hi all

ye might be able to help me, i have been having a cold start idle problem.

on a cold start the car starts fine but idle quickly rises to 1800 2000 rpm, i had assumed it was the aav but i have replaced it twice, hit it, squeezed the bulb soaked it and ended up with 3 aav's which do not open enough but closes at 60/70 degrees.

i put in a second hand aav ( thanks arcijack) the other day and the car behaves as it did when i bought it, perfect start but after 5/10 seconds idle rises if your driving it feels as if the choke is out.

i think the aav is good i gave it a soak cleaned it a bit and it openned and closed fine maybe 80+ degrees

i think i have a vacuum leak but only when cold? when i went looking for a leak i pulled the large vac hose from the top of the wur as it was quite loose in the aav hose and put my finger on the openning in the aav hose and the cold idle dropped to 1300 /1400. i put a screw in temporarily and it idled great i returned the hoses and the high idle returned.

the other vac hose from the side of the wur connects to the intake on the other side of the aav.

would this suggest that the wur is faulty, it is certainly enriching on cold start or could the leak be elsewhere.
the car drives great although sometimes i feel it could be more responsive if i push it hard between 60 and 80 kph after that it flies

one other wur question my wur lies on its side on a bracket as opposed to on the flat is this a feature of early 116s? mine is a 450 1976.

thanks in advance


Hi.  I had wur hassles on my 79 k jet 280sel.  The problem was that the main chamber of the wur had filled with oil over three decades which led to slow warm up and running rich.  I don't know if the same gravity/plumbing problem exists with the v8's. I carefully dismantled it (watch the fragile diaphragm and pin) and cleaned it with brake cleaner. All good thereafter.

Maybe none of this applies to D jet.
Sydney, Australia


its a kjet , im just not sure if its a vacuum leak , cold start valve or the wur, my local mechanic doesn't have much time for old cars
i will bring it to a good guy i know in january.

i would be nervous to take the wur apart without a spare the car is a daily driver.


gerry l

Hi John
I lived in Galway until I was 24, But have been in the US for 30 years. I just got a 77 450 SEL It was off the road for about 10 years I am having problems with WUR, AAV, etc, etc Looking forward to hearing more about your problems. PS I also have a 71 Rover 3.5 RHD I got in London.


hi Gerry
good to hear from you, i hope the car works out well for you they are great fun when working.
in the u.s you will be able to get parts much cheaper than here which will help, i am not very mechanically minded although i will do easy jobs and i over think problems and these cars are quite simple.
my prob is high idle till warm and sometines it wants to die for a few seconds at start up apart from that it drives very well, some rust which i hope to sort this year.

i used to love those rovers its a 3500s? a rarity in new jersey I'd say great looking car .

i think you might have a fuel mix issue did you set idle speed when warm ? getting  the aav and warm idle screw correct can be a bit of a balancing act. 
best of luck


gerry l

Hi John
I bought a used WUR on ebay last night for $35 I will take it apart to familiarize myself with how it works. I am a mechanic (air compressors), But I did serve my time at Galway Motors in 1976 Datsun and Hillman , So I enjoy the challenge. by the way the Rover is a P5B, off the road at the moment. There are people (wife)who reckon I would do better getting the Rover going before buying the Merc. But I always wanted a 116. I think you said you had taken your WUR apart is there anything I should look out for? Also John if it is not too much trouble could you measure the distance from centre of the back wheel to wheel arch. I am not sure if my car is a bit low in the back, I posted this question a few days ago but have not got a responce yet. Thanks for your help.


hi gerry

My car is in getting the alternator fixed, should have it back today, I will measure height then. The cars have a slight wedge shape ie higher at the back and my previous 116 was, my current one appears lower but not by much. the rear springs do get tired after 30 years and I think are a pain to change.

i never took my wur apart although i have doubts about mine, getting one for a 450 for 35$ is pretty good
like the aav they are basic the strainer can get clogged, there is an element that heats up and the unit can fill with oil / gunge, to test you need a pressure guage to measure fuel pressure.

when you changed the aav did you open the idle screw it may have been closed to compensate for a bad aav
which happens a lot, is it when the car warms up it dies?

ps my brother had a 120y were they from your time at datsun

gerry l

Good morning John
Yes $35 + $15 for shipping, The guy had 3 of them, said they had been sitting around for a while for that price it is worth it even just for parts, By the way if you see something over here, and you need help with shipping or anything let me know. I live in NJ but I work in New York, so I get around. Yes I worked on the 120Y. Thanks again John.


hi gerry

just got my car back after getting alternator sorted, i measured the height from the centre of the hub to the arch and it is 31cm, thats not scientific mind. i imagine they vary a bit, they are a lower looking car than modern cars.

all the best


If you have CIS test gauge I is pretty easy to test control pressures and to compare cold control pressures during warm up with the chart given the ambient air temp. I think it is a bad idea to take one apart yourself or to by a used unit not meant for your car with the hope you will fix your problem. Larry Fletcher at CIS Flow Tech in Texas can check your WUR and rebuild it if needed. I have sent him a WUR to check on Monday and had it back on Friday of the same week! He has all the equipment and knowledge to test and rebuild WUR and fuel distributors at reasonable prices. I have no financial interests in his business. He has rebuilt several WUR's for me and checked one of my fuel distributors with perfect results. He is a good guy and will treat you right. I am trying to fix a maddening cold high idle problem on my 1985 Euro 280E and have replaced a huge number of parts and still have the issue. Only a couple possibilities left. Thought it would be easy to put right but I was wrong. I also had a new AAV installed from MBCC and sent it back when my mechanic thought it was not closing all the way. Didn't make a bit of difference. We also smoked the intake hoping to find a vacuum leak and could not find anything except other than EGR valve leak and I removed all of the EGR stuff a couple days ago.


Quote from: w116john on 07 January 2014, 03:58 AM
My car is in getting the alternator fixed, should have it back today, I will measure height then. The cars have a slight wedge shape ie higher at the back and my previous 116 was, my current one appears lower but not by much. the rear springs do get tired after 30 years and I think are a pain to change.

With the right compressor the rear springs are an easy job. I had mine in and out in under two hours, including fitting new shocks. Removing the fronts in my parts car was like a roller-coaster ride. One actually went airborne (but luckily I did take precautions, knowing it's a risky job).

The rears in my 69k miles, 1979 450SE were shorter by just over 11mm, if memory serves right, compared to the new Bilstein B3 springs.
1993 "Pearl Blue" W124 280TE
1988 "Arctic White" W124 200T
1979 "Icon Gold" W116 450SE


hi ptashek

so there was only 11 mm difference new v old my rears look as if they could do with change but its not a priority

etmerritt  if my idle issues conyinue thats what i will do,  thanks


gerry l

Thanks for all the info guys.
I just went out and started the car and it started and ran as it should, and it is very cold (10 degrees f) but when it got warm (170), it started to run bad again I mean- not drivable - will not take acceleration at all. A few day's ago it ran OK when hot for a short while, then the same problem. Does the WUR affect the running when hot? How about the cold start valve?


In those temps, without a properly operating cold start valve, you'll have a devil of a time getting the car started. Once the car is running, if the cold start valve is leaking (not closing fully), you'll have a rough/rich idle. If I recall, the CSV is only supposed to cycle for about 10 seconds at startup, and only when the engine temps are below a certain value.

If you understand carburetors, think of the CSV as the equivalent to pumping the accelerator pedal a couple of times before cranking the engine. All (I think) carburetors have an "accelerator pump" which is a mechanically actuated plunger that squirts additional fuel down the carb when you press on the accelerator. "Pumping the pedal" squirts extra gas down the carb to help with starting.

The WUR (Warm Up Regulator) actually has two purposes. As the name indicates, it helps with fuel delivery when the engine is cold by increasing the control pressure in the system, which causes more fuel to be injected into the engine. Think if the WUR as the equivalent to the choke plate on a carburetor. As the car warms up, the WUR slowly reduces its effects on control pressure. An additional feature of the WUR is to provide full throttle enrichment. That's the purpose of the attached vacuum line. Under full throttle, virtually all vacuum in the intake is eliminated due to the fully-open throttle plate. The WUR senses this and increases the control pressure to cause more fuel to be injected.

WUR's on cars of your vintage are installed on the side. Later models have the valve standing up. There has been a lot of discussion as to whether the "laying down" versions are swappable with the "standing up" versions. There is a difference between the two as to where the vacuum line attaches (one on the side, one on the top).

If you can get some assistance, I'd try to see if squirting a little petrol down the throat will help compensate for your "will not take acceleration at all" problem. Take a small spray bottle with gasoline - make sure it's configured to spray a stream, not fine spray. Have your helper step on the gas and when the engine stumbles, squirt a couple of pumps at the big air intake plate and see what happens. If the motor responds favorably, then you're too lean.

Frankly, I think that's what's happening anyway, based on your issues. You said in one of your early posts the car seems under powered until you get past 80kph (are your really talking kph or mph?) - either way, the faster you go, the more the WUR is going to enrich the fuel delivery.

I have an entire manual that deals with diagnosing fuel problems on our generation MB's. I can send you a PDF version of the document if you'll send me your eMail via the message boards "send email" feature.

W. Brian Fogarty

'12 S550 (W221)
'76 450SEL 6.9 Euro #521
'02 S55 AMG (W220) - sold
'76 450SEL 6.9 Euro #1164 - parted out

"Bond reflected that good Americans were fine people, and most of them seemed to come from Texas..." Casino Royale, Chapter V


Good explanation of the WUR, Brian. I'd like a copy of that manual, if if you don't mind

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