PeachParts Mercedes-Benz Forum - 83 380SEC surges, runs rough

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Quote:

Originally Posted by Johnhef (Post 1760604)

turning the screw without pressing down at the same time wont do anything- is it possible thats what happened ?

It certainly is. That explains much.

Still can't figure the car out. regular troubleshooting is useless since it's inconsistent. This morning it surged all 16 miles to work. around lunch I drove it 8 miles and it purred like a kitten the whole way with nary a problem. 2 hours later, it died trying to go into a driveway. This evening, it drove home surging almost the whole way. After it had come up to temp, for almost exactly 1/2 mile, the surging stopped.

weird.

Quote:

Originally Posted by ctaylor738 (Post 1760824)

Please try a search on "lambda" or "duty cycle" or "on-off ratio" or closed loop." There are numerous posts on how to adjust the mixture. It's one of the most discussed problems on the 380's.

Basically, when the engine is warm and in closed loop, an analog computer is reading air-fuel mixture data in the exhaust stream from the oxygen sensor. The computer signals the frequency valve to open or close to lean or richen the mixture. The ideal mixture setting is when the computer sends a 50% "duty cycle" to the valve. That means that the mixture is set at a point where the computer can maintain ideal combustion by keeping the valve open 50% of the time. This is what you measure with your meter at pin 3 of the diagnostic socket. And you adjust the mixture to obtain the 50% reading.

But for this to work, you need a good oxygen sensor, a good fuel distributor, injectors working right, tight injector seals, and good vacuum. You cannot make up for any of these by twisting the mixture screw.

You cannot treat this car like it has a carburetor. The K-Jet injection is very straightforward, once you understand its principles. There is a reasonable explanation of how it works on Wikipedia and other web sites, as well as numerous books and manuals. If you don't have a basic understanding of how the system works, you should not be messing with it or asking this forum for advice.

Thank you, Chuck.
Perhaps I shouldn't be messing with it, but I'm the only one I've got to be messing with it, and it has to get me back and forth to work. I figured it shouldn't be more than an adjustment since it was running fine for a year before it had to get emissions checked.
I asked the forum because the sheer number of hits on "air/fuel mixture" was so great that I'd spent literally a week sifting through and learning nothing that really applied.
I appreciate the helpfulness from the people here, and your suggestions on better searches to work with. I will be searching through them to learn more. I hope you will not mind if I continue relaying information on this thread as well?

thanks again
John

We are hoping for some real data here!

Can you get your hands on a multi-meter of some sort? If you can:

1. Set function to DC Volts on a scale where you can read to nearest tenth of a volt.

2. After driving car (it needs to be good and warm), and with the car idling, measure the voltage across the + and - terminals of the battery. Remember the reading.

3. With the car idling, put the positive lead of the meter in Pin 3 of the diagnostic socket, and the negative lead in Pin 2. Remember the voltage reading.

4. Have someone bring the RPM up to 2500 and observe the voltage reading.

5. You can now compute your duty cycle (DC) like this:

DC in % = (Battery Voltage - Pin 3/2 Reading) / Battery Voltage * 100

Example:

Battery Voltage = 13.5
Pin 3/2 Reading = 7.5

DC = (13.5 - 7.5) / 13.5 * 100

Duty cycle = 44%

6. With engine at idle, oberve the Pin 3/2 voltage for a few minutes and see if it changes in a pattern corresponding to what the engine is doing.

Go for it! Let us know the results.

Note - if the meter has a Duty Cycle function, you can read the numbers directly. Every meter has a voltage function, though, so I decided to use this method.

I will have to see if I can find someone with a multimeter that good. the old analog one I have barely works under the best of circumstances. I have a buddy who has one if I can catch him over the weekend when I have daylight time to play with it.

I see from some of the other posts that you're somewhat of an expert on these FI systems.
Thank you again for your help.
John

Quote:

Originally Posted by ctaylor738 (Post 1762277)

Ok, I'm beginning to feel like a real idiot here (which may not be far from the truth). Where's the diagnostic socket? I looked on here for it, and my post is the only one that comes back with "diagnostic socket" for a 380.
Google wasn't much more help.
Thank you
John

Quote:

Originally Posted by ctaylor738 (Post 1762277)

its the round capped thing on the left side inner fender well.