Greetings all,
I apologize if this has been asked. I've been poking through the forums for about a week now, and haven't found anything that quite seems to fit.
I have an 83 380SEC that I got because it wouldn't run right. The previous owner had had it into the shop numerous times, and it would run good on some days, and then randomly, it would surge down the road as if someone had popped the clutch too fast (it is an automatic, no clutch. Just that's what it feels like). The engine would not hold a steady RPM.
I got it, went through with some starting fluid and found about a bazillion vacuum leaks and replaced the hoses. It behaved better, but still not right. replaced the idle control valve and relay, and it got a bit better, but still not right. replaced the distributor, and then had a different shop finally tweak it the rest of the way and it ran great for about a year.
Then it began to run REALLY rich. Of course, it did this about two months before it needed a smog check. The machine still ran steady and got me back and forth to work, it just had gone from 19MPG to 16MPG.
I took it in to get it smogged and figured they'd adjust the idle. There is only one import shop in the area. They replaced a bad hose and tweaked on it and got the mixture down to a reasonable level and it passes smog.
Unfortunately, now it won't hold a steady idle, and going down the road it surges. Basically, a similar problem to what it had when I got it.
Symptoms:
When I initially cold start it, it holds steady at about 1000RPM, and the "economy" vacuum gauge is about 3/4 to the good side with the needle steady. If I let it sit until it starts to register on the temp gauge, it drops to about 750RPM, and begins to run more rough. The vacuum gauge strays towards the center, but still holds a steady needle.
As it warms up further, the RPM drops to about 550RPM, and she runs really rough. The vacuum gauge slowly moves to the bad side and if I don't do anything, she'll die.
She generally fires right back up, holds steady at about 750RPM for a few moments and then creeps back down and dies again.
If I keep her running till she's about 60C on the temp gauge, then she'll idle rough at about 550 RPM and won't die.
If I crank it up to about 2000 RPM and hold the pedal steady, she holds for a few moments, and then decided that she wants to go to 1750RPM, or maybe 2700RPM. There doesn't seem to be any reason or rhyme to this. If I reset my pedal depth to 2000RPM again, then it'll hold there.
If I try to drive down the road, she just surges.
I don't know if it means anything or not, but when idling at 550RPM, and running rough, the vacuum gauge is just to the good side of center. If I raise the RPM to 2000, it goes almost all the way to the good side.

This time I can find no vacuum leaks.
distributor, idle control valve, idle control relay were replaced previously as noted above.
fluid levels are all good.
It ought to be some kind of adjustment, since it went in running steady, but rich, and now after being adjusted to pass smog, it's running poorly.
I don't have a manual (and can't afford one at the moment, even if I could find one), so I have no idea where the mixture adjustment is, which was my first thought.

If anyone has any idea what might have been tweaked and how to tweak it back, preferably without resuming the intense sucking of fuel, my suburban which has been pressed into daily driver duty would really appreciate it.

Thank you
John

As a rule of thumb on these, after they pass inspection you'll want to set the lambda back to about 40% or so- they seem to run the best around that point. It does sound like you have some other problems though than just that.

__________________


1980 500SE/AMG Euro
1981 500SEL Euro
1982 380SEL
1983 300TD
1983 500SEC/AMG Euro
1984 500SEC
1984 300TD Euro
1986 190E 2.3-16
1986 190E 2.3
1987 300D
1997 C36 AMG
2003 C320T 4matic

past: 1969 280SE 4.5 | 1978 240D | 1978 300D | 1981 300SD | 1981 300SD | 1982 300CD | 1983 300CD | 1983 300SD | 1983 380SEC | 1984 300D | 1984 300D | 1984 300TD | 1984 500SEL | 1984 300SD | 1985 300D | 1986 300E | 1986 560SEL | 1986 560SEL/Carat | 1987 560SEC | 1991 300D 2.5 | 2006 R350

It gives me a place to start.
er.
what is the lambda, and how do you set it?

Thank you
John

thats the 3mm allen screw smack dab in the center of the engine on top, thought the access hole in the air filter housing. it adjusts air/fuel mixture.

__________________


1980 500SE/AMG Euro
1981 500SEL Euro
1982 380SEL
1983 300TD
1983 500SEC/AMG Euro
1984 500SEC
1984 300TD Euro
1986 190E 2.3-16
1986 190E 2.3
1987 300D
1997 C36 AMG
2003 C320T 4matic

past: 1969 280SE 4.5 | 1978 240D | 1978 300D | 1981 300SD | 1981 300SD | 1982 300CD | 1983 300CD | 1983 300SD | 1983 380SEC | 1984 300D | 1984 300D | 1984 300TD | 1984 500SEL | 1984 300SD | 1985 300D | 1986 300E | 1986 560SEL | 1986 560SEL/Carat | 1987 560SEC | 1991 300D 2.5 | 2006 R350

Might want to search the mbcoupes.com archive for the, "surge from hell" threads. My experience (and also a friend with his 380) has been that the replacement parts are often bad (primarily the valve, but also the idle controller), or mismatched--can't get away with substituting a different PN/rev controller for a given valve. A theory would be the valve is a poor design to begin with and "new old stock" sitting on the shelves tend to freeze up. Also, the vacuum hose below is such a large diameter and odd shape, even if there are no cracks, it can be hard to get a tight seal. Just a few thoughts--wouldn't eliminate what you already replaced as suspects. Cheers, Andrew.

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86 560SEC
85 500SEC Lorinser
83 300D Turbo
75 240D

Thank you thank you thank you!
A simple thing, but if you don't know where it is, it's impossible to find!

I'll see if it works soon.

Thank you!
I went out and messed with it. I adjusted it 4 turns counterclockwise, and then drove it for 6 miles without any surges. The vacuum gauge is reading less vacuum than I used to get, but if it's driveable, that's a start!

Thanks again
John

If your economy (vacuum) gauge is not fully to the left at idle, you still have a vac leak somewhere. An often overlooked source is a small vacuum hose attachment at the intake manifold, rear between engine and firewall. The flexible attachment to that vacuum source lives in a usually oil-rich environment and rot easily. That attachment is hard to see and harder to access. You need tiny hands and a lot of patience.

The method that I know of for hunting down vacuum leaks is a can of starting fluid. If the engine speeds up (or slows down like when it was running super rich), then there's a leak. I found a hose at the back of the manifold between the engine and the firewall. It did not leak. I did find a vacuum leak right at the idle control valve that I'd fixed once before, and I fixed it again. No real change though. At idle, the economy gauge is right at the center. if I rev to 2000RPM in park, it goes most of the way to the left. Sometimes, it sits at 2000RPM, sometimes it decides that what I've set the pedal at isn't what it wants to be at.
I went over the whole engine and used half a can of starting fluid, but couldn't find any more leaks. I adjusted the mixture 2 more turns counterclockwise (for 6 total since I got it back from the shop) but it made no difference to the vacuum gauge. I'll take it out on the road here in a few moments and see if the surge is still fixed. it was intermittent when I first bought the car, so I'd always think I had it fixed and then it would come back.

Thanks again
John

I found another clue in driving it today. The surges are temperature related. when it's cold, it idles smoothly at about 1000RPM. As it warms up, it drops to 700RPM when it reaches 40 on the temp gauge. there's another mark above 40 that is unmarked. Between that and the 80 mark, it will surge. After it reaches 80, it stops surging.

Any ideas?
Thanks
john

John,
Do you have a spare idle controller that you could use as a swap experiment? If the problem was thermal/mechanical, e.g. cracked vacuum line, I would not expect it to get better above 80C. Also, if the temperature sensors/wiring/inputs were bad, would not expect the behaviour to change across temperature. So, I was wondering if the controller is making bad decisions between 40 and 80C...

You could mimic the idle controller (don't remember if it is a constant voltage or a constant frequency) to fix it at 750RPM with the engine warm but not hot. Then, drive it and see if it still surges. This would verify the surge is not coming from sources other than the idle-stabilization circuit, also would eliminate the valve itself and related vacuum lines from suspicion.

Just some more ideas, Andrew Seidel.

__________________
86 560SEC
85 500SEC Lorinser
83 300D Turbo
75 240D

This is not making any sense. If you went FOUR turns CCW on the mixture you would have completely leaned out the system. A half-turn in either direction is a HUGE adjustment.

Please do some research on the Forum on how to set the mixture correctly with the duty cycle reading on a multimeter, or at least get a CO reading at the tail pipe.

I think what is probably happening is that you have a one or more bad injectors, or a bad fuel distributor, and that you are not getting enough fuel at idle to one or two cylinders. But it's hard to tell until you can give us some baseline data.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

I will look some more on doing that. Initially, I couldn't find anything that told me even what screw to adjust to set the mixture.
There's no way I know to get a CO reading at the tailpipe for a home guy. It smells rich. Vacuum is low, which indicates rich. At least on my regular carbureted cars. These new-fangled fuel injected machines with computer controls are weird :-)

Thank you again
John

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

__________________


1980 500SE/AMG Euro
1981 500SEL Euro
1982 380SEL
1983 300TD
1983 500SEC/AMG Euro
1984 500SEC
1984 300TD Euro
1986 190E 2.3-16
1986 190E 2.3
1987 300D
1997 C36 AMG
2003 C320T 4matic

past: 1969 280SE 4.5 | 1978 240D | 1978 300D | 1981 300SD | 1981 300SD | 1982 300CD | 1983 300CD | 1983 300SD | 1983 380SEC | 1984 300D | 1984 300D | 1984 300TD | 1984 500SEL | 1984 300SD | 1985 300D | 1986 300E | 1986 560SEL | 1986 560SEL/Carat | 1987 560SEC | 1991 300D 2.5 | 2006 R350

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.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe