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1980 450SL, 2003 BMW 530i
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35 Posts
Discussion Starter #1
I finally fixed the hesitation problem with my car and wanted to share what I found; maybe it will help someone else with a similar problem. My car ran well when cold, but sputtered and hesitated badly on acceleration once it warmed up. It felt like the car was running overly rich. Here is the long version of how I finally figured out the problem:

My car has the K-Jetronic with lambda fuel injection system. The lambda feature was first put on the 450SL in 1980. This system’s main components are the electronic control unit (ECU), oxygen (lambda) sensor and frequency valve.

The ECU gets it signal from an oxygen sensor in the exhaust pipe. This sensor signals the ECU if the engine is running rich or lean. Based on this signal, the ECU modulates a frequency valve. The frequency valve makes minor adjustments to the air/fuel ratio and by making these adjustments, can richen or lean the mixture.

The way the ECU modulates the frequency valve is by rapidly turning the voltage to it on and off, which opens and closes the valve. You can actually hear the buzzing noise the frequency valve makes when it is working. The ratio of on time to off time is called the duty cycle and the ratio is indicated by a percentage. As an example, a 50 % duty cycle means the frequency valve is on half the time and off half the time over a given time period.

When the duty cycle goes above 50%, the frequency valve is on more than it is off and the mixture is richened. Conversely, when the duty cycle goes below 50%, the frequency valve is off more than it is on and the mixture is leaned.

A diagnostic plug is located in the engine bay that will allow you to measure the duty cycle. Sears sells several volt-ohmmeters with a duty cycle feature for around 20 to 30 bucks.

The ECU initially fixes the duty cycle at a constant 60% when the engine is cold, which lets the engine start and warm-up easier and run smoother. The oxygen sensor signal is ignored by the ECU during this period. Once the engine is warm, the ECU starts using the oxygen sensor signal to determine if the engine is running rich or lean. If the engine is running too rich, the ECU will lower the duty cycle to the frequency valve to lean the mixture. If the engine is running too lean, the ECU will raise the duty cycle to the frequency valve to richen the mixture.

The ECU continuously adjusts the duty cycle to maintain the proper mixture. The duty cycle should fluctuate between 40 to 60 percent after the engine is warm.

My duty cycle measured 10% after engine warm-up, which indicated the engine was running overly rich and the ECU was trying to compensate by lowering the duty cycle. I wanted to rule out a bad oxygen sensor, so I replaced the old one, which cost about 20 bucks. I then adjusted the mixture via the adjustment screw next to the fuel distributor until the duty cycle fluctuated around 50% and took the car for a test drive.

The hesitation problem was almost completely gone. I thought I solved the problem. However, after I parked the car and the engine got cold, I found the car was extremely hard to start. The mixture needs to be rich on any car when the engine is cold in order for it to start well. By adjusting the mixture screw, I apparently made it too lean for starting. The only way I could get it to start reliably again was to richen the mixture, which brought the hesitation problem back.

I finally bit the bullet and purchased a fuel injection pressure gauge and checked the pressure per section 073-120 of the Mercedes manual. The system pressure was to spec at 5.4 bar, but the control pressure was also at 5.4 bar, when it should have been around 1.5 bar when the engine is cold and around 3.5 bar when the engine is warm.

The warm-up regulator (WUR) is the device that controls the control pressure. When the control pressure is around 1.5 bar (when the engine is cold), the engine runs rich. This rich mixture allows the engine to start easier and run smoother during warm-up. After about 3 minutes, the engine doesn’t need the rich mixture anymore, so the WUR raises the control pressure to around 3.5 bar and thereby leans the mixture. Since my control pressure was not doing any of this, being stuck at 5.4 bar, the WUR was not doing its job of richening the engine when cold.

I suspected that the control pressure was too high due to a blockage in the warm-up regulator or one of the fuel lines connected to the WUR. I took the WUR apart (not for the faint of heart!) and found that there was an inlet screen that was completely plugged.

I attempted to clean the filter by soaking the top half of the WUR in various solvents and carburetor cleaner, but failed to remove the blockage. I also tried to remove the filter from the WUR without damaging it, but couldn’t do that either. I finally gave up and dug the filter out of the top of the WUR with a pick. This procedure destroyed the filter, but did remove the obstruction.

I put the WUR back together, installed it, tested pressure and measured 3.5 bar. I adjusted the WUR by lightly tapping on the recessed brass plug next to the electrical connector with a punch and hammer until the pressure lowered to 1.5 bar. I started the car and after about 3 minutes, the pressure rose to 3.5 bar. This is exactly how the WUR is supposed to work, so I thought I was making progress. However, once the engine cooled, the pressure stayed at 3.5 bar instead of lowering to 1.5 bar like it was supposed to. I tried several times to adjust the WUR, but is seemed that it was way too touchy to adjust between 1.5 and 3.5 bar; only the slightest tap with the hammer on the brass plug would toggle the pressure from 3.5 to 1.5 bar.

I took the WUR apart again. The WUR is basically a fuel valve that is open when the engine is cold and slowly closes as the engine warms. The valve consists of several parts, including a very thin stainless steel diaphragm that rests on an o-ring, a round steel plate with a hole in the center, a small metal button that sits inside the hole of the round steel plate, and 2 screws that holds the whole assembly to the WUR body. I noticed that the o-ring was flattened instead of round. I guess 30 years of being compressed by the round steel plate did the o-ring in. I could now see why adjusting the WUR was so difficult; the flattened o-ring left no adjustability in the valve; it was basically fully opened or fully closed, but there were no intermediate positions.

I took the WUR down to the local rubber and gasket supply house and got a replacement o-ring, made of fuel resistant Viton material. I put the WUR back together again, installed it, tested pressure and after some minor adjustments to the WUR, finally measured the control pressure at around 1.5 bar when the engine was cold and 3.4 bar when warm. After the engine cooled, the control pressure dropped back down to 1.5 bar. Success!

With the control pressure now correct, the WUR was now doing its job by allowing the car to run richer when cold like it was supposed to. I tried leaning the mixture again until I measured a duty cycle of 50%. The hesitation problem was vastly improved and the car would start when cold.
I then replaced the ignition coil, spark plugs, spark plug wires, distributor cap and rotor and the hesitation is now completely gone and the car runs great.

The one thing that baffled me was why the mixture was set so rich in the first place. Apparently I stumbled across an old technique that mechanics would sometimes use on these cars to get them up and running in a limp-home mode when the WUR failed. When the WUR fails, the car is extremely difficult to start. A mechanic can richen the mixture with the mixture screw as a short term solution. The car will hesitate badly, but it least it will start and run. Apparently this is a problem I inherited when I took ownership of this car. Hope this story helps others.
 

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82 380SL 96 SL500 03 SL500
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6,154 Posts
Congratulations on a job well done. :thumbsup:

Sounds like you did your homework, did a thorough diagnostic of k-jet w/lamda operation and a proper repair.
 

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82 380 SL ; 78 450 SLC ; 84 BMW 533i
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263 Posts
Congrats, a very nice job of troubleshooting. :thumbsup:

Which two pins on the diagnostic plug did you use for the duty cycle? I am guessing you used the Dwell setting for 4 Cyl on the meter for the duty cycle?
 

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2004 CLK 240 Coupe
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11,367 Posts
I will doff my cap to you, Sir.

This is a well-written report, showing true determination and doggedness in spite of setbacks. Just the stuff I love to read here..... :bowdown:
 

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1980 450SL, 2003 BMW 530i
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35 Posts
Discussion Starter #5
Congrats, a very nice job of troubleshooting. :thumbsup:

Which two pins on the diagnostic plug did you use for the duty cycle? I am guessing you used the Dwell setting for 4 Cyl on the meter for the duty cycle?
I used pins 3 and 6 on the diagnostic plug to measure the duty cycle. I used a digital volt-ohmmeter with a duty cycle feature, which allowed me to measure the duty cycle directly.

I don't have a dwell meter, but you can also measure the duty cycle with it too, setting it on the 4 cylinder scale as you stated. Using the dwell meter in this fashion gives an error of approximately 10%. If greater accuracy is desired, then just multiply the dwell meter reading by 1.1 to get a closer result.
 

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Always Remembered RIP
1979 280sl 4 sp w/ac 1957 MGA 1998 volvo xc/70 2004 F150
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6,050 Posts
this is what being a mechanic is supposed to be, GOOD JOB !!!!!!!
 

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82 380 SL ; 78 450 SLC ; 84 BMW 533i
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263 Posts
I learn something new every time I read these forums, I did not know that there was a duty cycle available meter. That really simplifies getting the AF settings since it's pretty hard to get at the connector for the O2 sensor on my car, I had to take apart the lambda unit connector last time.

By the way, nice orange paw :D
 

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Premium Member
1986 560SL with M120 V12 Engine, 1988 560SL Stock
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9,732 Posts
Excellent trouble shooting and repair. 2 points off for starting with the shotgun O2 sensor but for $20 what the heck. This is why everyone with CIS should get a fuel pressure gauge prior to doing any work on these systems.
 

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560SL,380SL
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4,012 Posts
If you can get more of those O-rings, I'd like to get a few, and so would others, I'll bet. I have a WUR that I made adjustable but didn't rebuild that part of the unit.
 

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1985 380SL; 2004 E320; 2007 GL450
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296 Posts
I used pins 3 and 6 on the diagnostic plug to measure the duty cycle. I used a digital volt-ohmmeter with a duty cycle feature, which allowed me to measure the duty cycle directly.

I don't have a dwell meter, but you can also measure the duty cycle with it too, setting it on the 4 cylinder scale as you stated. Using the dwell meter in this fashion gives an error of approximately 10%. If greater accuracy is desired, then just multiply the dwell meter reading by 1.1 to get a closer result.
Questions from one Tiger to another:

After reading your story, I am also curious on the duty cycle in my '85 380SL. So I used the freq/duty cycle function of my digital multimeter for this test. I also used the 3/6 pins of the diagnostic plug (see attached picture). However, the reading is around 700 Hz. Did I use the wrong pins for this car, or should I interpret the reading differently?
 

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1980 450SL, 2003 BMW 530i
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35 Posts
Discussion Starter #13
Questions from one Tiger to another:

After reading your story, I am also curious on the duty cycle in my '85 380SL. So I used the freq/duty cycle function of my digital multimeter for this test. I also used the 3/6 pins of the diagnostic plug (see attached picture). However, the reading is around 700 Hz. Did I use the wrong pins for this car, or should I interpret the reading differently?
Not sure what type of meter you have. On mine, I first have to set the dial to duty cycle/frequency. I then have to press a separate button to toggle between duty cycle and frequency. When the meter is displaying units in Hz, then it is set to measure frequency, When the meter is displaying a percentage sign, then it is set to measure duty cycle. Hope this helps.
 

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1985 380SL; 2004 E320; 2007 GL450
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296 Posts
Thanks Tiger for the quick response. I found the toggle button in my meter and now it displays %. It displays 75% if the positive lead of the meter connects to the #3 pin and the negative lead to #6. If I reverse the positive lead to #6 pin and negative to #3 pin, I get a 25% meter reading. So which one is the correct reading?

When I turn the mixture control screw clockwise or counter clockwise, it does not change the duty cycle reading. And, if I turn the mixture control screw more than 360 degrees in either directions, the engine will stop. Seems like something is not working properly. Any ideas? Thanks in advance.
 

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560SL,380SL
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O-ring Size

Do you have any idea on the O-ring size? I can tell you that Harbor Freight's is too thick - 14mm I.D., OK, but 2.5 mm wide. The best I've been able to find is 1.78mm wide. With my O-ring smashed I was not able to get truly accurate measurements but I think that this is very close. The Porsche people, who also have to deal with WUR's, warn of using too thick of an O-ring. The unit has to seal AND the plate has to lay right on top of the plug when assembled. Too thick of an O-ring will prevent this.

On removing the filters, I have found that once you get the thin metal plate off, it's pretty easy to get a pick to poke it out from the inside. Mine was 4 layers, going coarse-finer-really fine - coarse (maybe to hold it?), looking at the stack of filters from the top of the regulator. Only my really fine filter was plugged maybe 5%, and still is because I couldn't get it completely clean. I reinstalled the filters in this order, using the end of a drill bit slightly less OD than the ID of the threaded hole. Screwing in the fitting seats everything.

PS: there was also residue in the "chamber" of this area, so I cleaned it.
 

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1985 380SL; 2004 E320; 2007 GL450
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296 Posts
Thanks Tiger for the quick response. I found the toggle button in my meter and now it displays %. It displays 75% if the positive lead of the meter connects to the #3 pin and the negative lead to #6. If I reverse the positive lead to #6 pin and negative to #3 pin, I get a 25% meter reading. So which one is the correct reading?

When I turn the mixture control screw clockwise or counter clockwise, it does not change the duty cycle reading. And, if I turn the mixture control screw more than 360 degrees in either directions, the engine will stop. Seems like something is not working properly. Any ideas? Thanks in advance.
Did more testing when the engine is at operating temperature. Connect the positive lead to #3 pin and negative to #6, the duty cycle is 58%, which is within factory spec (40-60%).
 

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82 380 SL ; 78 450 SLC ; 84 BMW 533i
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263 Posts
Did more testing when the engine is at operating temperature. Connect the positive lead to #3 pin and negative to #6, the duty cycle is 58%, which is within factory spec (40-60%).
That duty cycle comes from the reading of the O2 sensor to the lambda control module which then adjusts the " duty cycle " of the frequency valve so the O2 sensor needs to be good and hot. Some feel that you need to run the car a few miles to get the engine temps and exhaust gasses stable. I have also seen where they tell you to run the engine at 2000RPM for 3 minutes and then blip the throttle each time you make very small adjustments. I used to do this adjustment using the voltage readout from the O2 sensor itself but that's a pain on the 107's.
 

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1980 450SL, 2003 BMW 530i
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35 Posts
Discussion Starter #18 (Edited)
Do you have any idea on the O-ring size? I can tell you that Harbor Freight's is too thick - 14mm I.D., OK, but 2.5 mm wide. The best I've been able to find is 1.78mm wide. With my O-ring smashed I was not able to get truly accurate measurements but I think that this is very close. The Porsche people, who also have to deal with WUR's, warn of using too thick of an O-ring. The unit has to seal AND the plate has to lay right on top of the plug when assembled. Too thick of an O-ring will prevent this.

On removing the filters, I have found that once you get the thin metal plate off, it's pretty easy to get a pick to poke it out from the inside. Mine was 4 layers, going coarse-finer-really fine - coarse (maybe to hold it?), looking at the stack of filters from the top of the regulator. Only my really fine filter was plugged maybe 5%, and still is because I couldn't get it completely clean. I reinstalled the filters in this order, using the end of a drill bit slightly less OD than the ID of the threaded hole. Screwing in the fitting seats everything.

PS: there was also residue in the "chamber" of this area, so I cleaned it.
I tried everything on my filter but it wouldn't budge. The car has run well without it for several months, so I will keep my fingers crossed.

Sorry, but I don't know the size of the o-ring. I just took my WUR and old o-ring to the supply house and matched it up there. I have some spares. If you want one, just let me know where to mail it.
 

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Premium Member
1986 560SL with M120 V12 Engine, 1988 560SL Stock
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9,732 Posts
I tried everything on my filter but it wouldn't budge. The car has run well without it for several months, so I will keep my fingers crossed.

Sorry, but I don't know the size of the o-ring. I just took my WUR and old o-ring to the supply house and matched it up there. I have some spares. If you want one, just let me know where to mail it.
Best place I found fo "O" rings is Mcmaster-Carr. They clearly state the dimensions and material of everything they sell and there selection for O rings is wide.
 

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Tom sent me a few O-rings (thanks!) and I found it to be 1.8mm thick. I've read on Porsche sites that the thickness is crucial but they never say what it is. So, now I should have a rebuilt unit, but I don't see any way to "test" it without actually installing it.
 
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