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Discussion Starter #1
I’m looking to purchase a replacement WUR for my 1979 450SL. The PO replaced it with one I believe to be from another car (maybe non-Benz), which was made in France. The control pressure would decrease as it warmed up, and the gauge would jump around with lights taps to the housing. No good.

I have the WUR off of my 76 SLC on it right now, which solved the no start condition it was suffering from for the three years or so that it sat. Seems to run okay, but still hesitates. The SLC is crying for its WUR back, and it’s naughty to steal from others.

Considering I don’t have the original WUR for this car, I need to buy a used one to rebuild. I’m wondering about compatibility. I’ve heard that the SLC’s WUR is “incorrect”, despite it being an OE unit from a US-spec CIS non-lambda 450SLC, essentially the exact same system as the US-spec 450SL in question.

Is there any possibility of getting something like a 380 or 500 WUR and adjusting the pin to spec pressure? Or am I stuck with looking for a 450SL WUR? Has anyone decoded the 3 digit numbers on the housings and their corresponding vehicles/engines?

Thank you guys.
 

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I was lucky enough to pick up a new Bosch WUR part number 0438140145 on eBay for $350. Replaced it and my SLC is running smooth since the swap.
 

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The fuel pressure is supposed to increase as the engine warms up.

Low pressure = rich

High pressure = lean

It has to do with a system of springs, more pressure means more force against the spring letting less fuel into the system.

Let's confirm if the WUR is indeed not correct. What's the model number on the WUR housing? The last 3 digits on it are what is important.

This FSM article we need to be looking at when performing these tests: https://www.startekinfo.com/StarTek/outside/11883/PROGRAM/Engine/107/M117_45/073-120.pdf

and


Edit i should know better on fuel pressures!
 

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Discussion Starter #4
Let's confirm if the WUR is indeed not correct. What's the model number on the WUR housing?
0 438 140

The embossed number to the right is either 980 or 086 depending on orientation.

The number stamped on the left near the adjustment pin is 945.
 

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Discussion Starter #5
Oh geez! I just realized that there is another stamped number directly to the right of the main part number!

That would make the complete part number:

0 438 140 015

Sorry about the confusion.
 

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Curiously, it doesn't match what the FSM says but it does look to be OK as an alternative part number during a cursory Google search.

@rowdie, can you check if that part number is a good alternate for his 79 450SL?

My gut thinks it might just need a rebuild which you can send off to take care of the "jumping" when you tap on it.
 

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Discussion Starter #7
Curiously, it doesn't match what the FSM says but it does look to be OK as an alternative part number during a cursory Google search.

@rowdie, can you check if that part number is a good alternate for his 79 450SL?

My gut thinks it might just need a rebuild which you can send off to take care of the "jumping" when you tap on it.
So, that was the SLC WUR that is on there now. The one that was on the car before ends in 061. A quick Google search revealed it to be stock from what I can tell.

I’d love to rebuild it myself, but maybe it’s best sent off to the pros...
 

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What were your pressures (bar please) bone cold and then to warm and what was the outside temperature (in celcius) when you started?

There are two other components in the fuel system that can lead to engine difficulty. That would be the fuel distributor and the pressure regulator that also lives on him.

Have you confirmed that the intake is sealed up fully and no vacuum leaks in the vac powered accessories?

When my 380sl was giving rough running and being too rich, I had pretty much everything wrong that could go wrong. Here's a list of everything I had to touch in order to get it working:
1. Holes in the fuel distributor diaphragm
2. A failing system pressure regulator
3. A bad WUR
4. Air leaks in the vacuum and intake system
5. A mucked with air metering screw
6. A bad OVP relay (fuel management not working).

Just to give you an idea of what of it took me to get my car to pass smog ... legitimately without any denatured alcohol. Which by the way was the reason all of that stuff failed. Don't run that junk through an engine :'(
 

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Curiously, it doesn't match what the FSM says but it does look to be OK as an alternative part number during a cursory Google search.

@rowdie, can you check if that part number is a good alternate for his 79 450SL?

My gut thinks it might just need a rebuild which you can send off to take care of the "jumping" when you tap on it.
Those are Bosch part numbers not Mercedes. You guys will have to figure out which is correct for his. Engine number is needed. Here's the page from one online EPC (450SL USA) https://mercedes.7zap.com/en/us/fg/car/450/450+sl+usa/68d/107044/m/117985/67h/07/0/070/0/0/#270
 

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Discussion Starter #10
I cannot confirm if the intake has vacuum leaks or not. All I can confirm is the distributor has new seals and a new steel gasket. System pressure is within spec, but control pressure is a little wonky, although not nearly as cringeworthy as with the old WUR.

I am usually about methodical diagnosis and thoroughness. When I bought my 560SL, I immediately pulled the engine and trans, and almost everything has been resealed, in addition to new timing circuit components. This isn’t my usual style.

I bought this 450SL when I was 19 and didn’t know any better. I was about to get rid of it a couple of months ago, but my youngest brother wanted it, so I decided to get it running with him.

I think my best bet at this point is yanking the intake off and completely eliminating vacuum leaks. So many potential points for leakage down in the old valley...
 

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If the WUR on a 450 works the same way as the WUR on a 380... here's what it does.

The FD receives fuel from two sources... the fuel pump and the WUR. They are called System Pressure and Control Pressure respectively.

The Control pressure is supplied to the FD by the WUR.

Low Control pressure = richer mixture.

Normal control pressure (notice I didn't say High control pressure) = leaner mixture.

The WUR works by a simple electric heating mechanism.

When cold, the Control pressure is low which allows the fuel plunger in the FD to ride higher in its channel which exposes more of the fuel metering slots which in turn allows more fuel per unit of air to reach the injectors. In other words ... richer mixture.

As the WUR warms up, it increases the Control Pressure applied to the top of the FD which in turn restricts the fuel plunger inside the FD from rising quite so high, which causes less of the fuel metering slots to be exposed which makes the mixture leaner.

On cold starts the Control pressure should be low, maybe 5 or 10 psi. I'm not certain of the exact value. It's adjustible but the method is crude, involving a hammer and a drift. Don't ask..

When fully warm the WUR should increase the control pressure to around 45 or 50 psi.

That's really all the WUR does... and it doesn't even rely on coolant temperature. It only relies on the electric heating element inside that expands when 12 volts is applied to it's terminals. The heating element begins to expand due to heating and slowly allows more of the System fuel pressure to reach the Control Pressure at the top of the FD. Kinda simple actually. Not much can go wrong.
:) :) :)
 

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The fuel pressure is supposed to decrease as the engine warms up. That means the WUR is doing its job.

Low pressure = lean

High pressure = rich

...
Are you sure of that?

Higher control pressure counteracts the airflow requiring more air for the baffle to move down and let more air.
On my car it starts at approx 2bar at 20C and goes up to ~3.8bar at warm.


Redundant response
 

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Discussion Starter #13
Are you sure of that?

Higher control pressure counteracts the airflow requiring more air for the baffle to move down and let more air.
On my car it starts at approx 2bar at 20C and goes up to ~3.8bar at warm.
Right. More pressure=less deflection of metering plate=lean condition.
 

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Right. More pressure=less deflection of metering plate=lean condition.
Actually that is not correct. Control pressure does not respond to airflow. Control pressure has nothing to do with airflow... airflow is a function of throttle setting. Open the throttle wider and more air flows into the engine. The air entering the engine passes by the air metering plate which moves the fuel metering plunger higher or lower inside the FD. The air metering plate does not control airflow... it reacts to it. The throttle controls airflow.

That is how the WUR can control the mixture via the Control Pressure. The air metering plate is not moving due to increased airflow during WUR enrichment, it's moving because the Control pressure is allowing the fuel metering plunger to move slightly upward inside the FD, thereby allowing more fuel to flow to the injectors without more air getting through the throttle opening. This is the very definition of enriching the mixture

Keep in mind, the air metering plate you see when you remove the air filter is not the throttle... it's a device that reacts to the air flowing past it which varies as a result of changing the throttle position. The throttle controls air... the FD controls fuel... the WUR provides the Control Pressure that sets the base value of the air/fuel mixture during warmup. When the WUR is cold the Control pressure it supplies to the FD is low. When the WUR is all warmed up, the control pressure it supplies to the FD is whatever is the optimum for that engine, but usually between 45 and 55 psi.

I hope that clears things up. :) :) :)
.
 

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Discussion Starter #18
Actually that is not correct. Control pressure does not respond to airflow. Control pressure has nothing to do with airflow... airflow is a function of throttle setting. Open the throttle wider and more air flows into the engine. The air entering the engine passes by the air metering plate which moves the fuel metering plunger higher or lower inside the FD. The air metering plate does not control airflow... it reacts to it. The throttle controls airflow.

That is how the WUR can control the mixture via the Control Pressure. The air metering plate is not moving due to increased airflow during WUR enrichment, it's moving because the Control pressure is allowing the fuel metering plunger to move slightly upward inside the FD, thereby allowing more fuel to flow to the injectors without more air getting through the throttle opening. This is the very definition of enriching the mixture

Keep in mind, the air metering plate you see when you remove the air filter is not the throttle... it's a device that reacts to the air flowing past it which varies as a result of changing the throttle position. The throttle controls air... the FD controls fuel... the WUR provides the Control Pressure that sets the base value of the air/fuel mixture during warmup. When the WUR is cold the Control pressure it supplies to the FD is low. When the WUR is all warmed up, the control pressure it supplies to the FD is whatever is the optimum for that engine, but usually between 45 and 55 psi.

I hope that clears things up. :) :) :)
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Right. But the metering plate is still less mechanically deflected with a higher control pressure for a given throttle position, right?

At 25% throttle, the plate will physically be able to be drawn down further at 35psi than at 45psi.

At 80% throttle, the plate will theoretically be drawn down further at 55psi than at 35psi at 25% throttle.
 

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Right. But the metering plate is still less mechanically deflected with a higher control pressure for a given throttle position, right?

At 25% throttle, the plate will physically be able to be drawn down further at 35psi than at 45psi.

At 80% throttle, the plate will theoretically be drawn down further at 55psi than at 35psi at 25% throttle.
Perhaps we are both saying the same thing but from two different points of view. :)
 
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