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  #1  
Old 06-11-2002, 10:55 PM
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Location: Washington, D.C.
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Unhappy 560 SEL Rough Idle (Another one!)

OK. I really need help now. I don't want to take this thing into the shop to have this diagnosed, but I still have a rough idle problem on my 560 SEL. It's rougher when cold, and gets better when warm, but still feels like a cylinder is "missing" occasionally, even though no bad cylinder can be isoloated by pulling the plug wires.

So here are the exact symptoms:

Oil pressure gauge fluctuates with the roughness, though the change in RPM is not sensitive enough to be seen in the tachometer. The roughness feels like the engine is being hit with something occasionally.

It seems the engine is reving and slowing down in 5-second cycles, though it is too subtle to see on the tachometer. It can be heard, though, when under the hood.

It's driving me nuts!

Here are some background facts:

1. Vacuum is great - tested it with a gauge, and visually examined everything.

2. New injectors, seals, seats, etc.

3. New fuel filter

4. All secondary ignition parts new (wires, plugs, coil, cap, rotor)

5. Air filters new.

6. OVP valve new

7. Fuel pump relay new ('2000)

8. One of the fuel pumps looks new.

9. 98 k miles

10. All parts OEM.

Any way I can diagnose this without going to the dealer?

Thanks for any help....
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Henry Bofinger
1989 560 SEL (black/black)
2001 Audi TT Roadster (silver/grey)
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  #2  
Old 06-11-2002, 10:57 PM
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Correction

By "OVP valve" I mean OVP relay...
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1989 560 SEL (black/black)
2001 Audi TT Roadster (silver/grey)
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  #3  
Old 06-12-2002, 10:06 AM
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Location: Atlanta Ga
Posts: 281
Well, I don't know if you've checked this but have you considered your cold start valve? My 560 didn't need it, but when I got her I decided that I'd replace every valve, vac line, and all of the little odds and ends under the hood to get a squeaky clean look whether the part was working fine or not (I'd just put the old part in the new box and file it on my "560" part shelf

This was my first 126 (as I'm a 115 fan) and I would do a little research on the 126 parts as I replaced them... the cold start valve is one of the components that helps with the immediate startup and idle of the car when its cold... You may also want to check out your o2 sensor although usually it would run a bit rough after warmup because that's when the o2 really kicks in when hot and sends the info to your computer for it to tell your electronic fuel components what to do as far as the correct mixture,etc.

Mine was surging, and getting some crappy gas mileage as well as a rough idle once the car got up to operating temp.I could also smell a sulfur-like odor sometimes when at idle and noticed that my o2 sensor light went off a couple of times and although it didn't stay on, I figured it was either GOING to go or is gone. I replaced it and she's running fine cold or hot. How's the ride when you're on the accelerator (on the highway)? Does it also surge or use more gas than usual? Sometimes it may be sluggish when you hit the accelerator although the tach will show you using a crap load of gas to get going. Hope this helps.

also try a search on rough idle or similar and see if that will give you some ideas. Good luck!

Alan
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  #4  
Old 06-12-2002, 10:10 AM
moedip
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Had the same thing on my 88 560sel - did all your stuff same thing. What worked for me was replacing the fuel lines. They were so rusted inside that one broke in two from it's own weight when we removed it. The rust is not trapped in the fuel filter as the filter is in the back of the car and the fuel picks up the rust and takes it to the fuel distributor. Take the injector lines off the fuel distributor and carefully pull out the screen in each injector line hole. I think you will find them partially clogged with dirt. I cleaned my screens and replaced them - you can put new ones in - but check your fuel lines first. Before i changed my fuel lines - I cleaned the screens and the car idled perfectly for about 100 miles and started to act up again. Try cleaning or replacing your screens - they are cheap from Phil and if the problem goes away - then you know to change your fuel lines. The oil guage on mine fluctuated the same way - the tach barely showed the missing but you sure could hear it and feel it - it was annoying. Hope this helps you.
Just had a crazy thought - if you do have rust in your lines and want a cheap fix (I didn't think of this until now!) cut the fuel line in two just before it goes into the fuel distributor and cut off about 3" of line and put in an aftermarket - cheap inline fuel filter with rubber hoses. Clean your screens and that should work . If you get one of the see through fuel filters you should be able to see the rust in it after you drive for a while. Surprised Mercedes didn't use one up front for contaminants from the long fuel lines.
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  #5  
Old 06-12-2002, 10:48 AM
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Thanks for the replies

DJNEWK2 - I'll try your approach when all else fails. I thought that a bad cold start injector could be part of the problem. But so could be the idle control valve, the fuel pressure regulator, the old fuel pump in the back, the ignition control module, the idle control module, the fuel ditributor, and the O2 sensor (though taht's less likely, since the idle improves significantly as the car warms up, and the emissions are good) . I.e. I'd rather diagnose first at this stage rather than use new parts as a process of elimination. Eventually I might have to do what you suggest, however.

moedip - I've heard of the clogged screen problem before, and I am very eager to explore this. Since now I am dealing with a very costly part (the fuel distributor), I'd love to know how you got the screens out (would dental picks work?). Also, exactly what fuel line did you replace? The entire line from under the car?

The idea of installing a clear aftermarket in-line fuel filter strikes me as particularily smart. An added safeguard plus a diagnostic tool.
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1989 560 SEL (black/black)
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  #6  
Old 06-12-2002, 11:01 AM
moedip
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An old German mechanic gave me the tip for removing the screens - get a long thin very sharp screw and gently turn it into the screen just so it grabs and lift the screen out. the sharp point will not leave a big enough hole to cause problems. Yes I replaced the two fuel lines (feed and return) They were a pig to get out and in around the rear axle area by the gas tank - but 4 hours and a little cussing and a couple of beers later it was done. It really helps if you can get the car on a hoist to manouver the lines when replacing - we didn't have one and just jacked up the the driver's side as high as we could. When we picked up the feed line from one end to take it to the bench to inspect - it broke in half from it's own weight - it was so rusted. If you are worried about making too big a hole in the screens - order a set from Phil and keep them as standbys - they are cheap. I'm kicking myself for not thinking of the fuel filter idea until now!!!
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  #7  
Old 06-12-2002, 11:19 AM
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moedip - I just talked to Phil

moedip - I just talked to Phil about those screens. He did not know anything about them, and wondered where you meant they could be found. I'd rather order them first before I start futzing with this, because I want to know what the part looks like before I try to extract it...
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1989 560 SEL (black/black)
2001 Audi TT Roadster (silver/grey)
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  #8  
Old 06-12-2002, 11:23 AM
moedip
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On your fuel distributor there are 8 metal lines that go to your injectors. When you remove the metal lines from the distributor - they were inside the hole you took the metal line off of, at least they were in mine.
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  #9  
Old 06-12-2002, 11:36 AM
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moedip - Yup, that's what I thought. I tried getting one of these screens out when I did my injectors (I removed the injector lines for the job). Used a wood screw. I guess it was not thin enough, because it grabbed the side of the wall, and I did not want to ruin anything on the distributor, so I stopped. I'll try it again.

I bought some dental picks the other day - one of them looks like it would be perfect for the job - sharp point, yet a small curve at the end, so it could grab below the screen.

Whish these things could be ordered as spares. Does not seem so, unfortunately.

I think you are most likely correct about this being the problem. I can't wait to go in there again this afternoon and checking for those screens...
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1989 560 SEL (black/black)
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  #10  
Old 06-12-2002, 11:54 AM
moedip
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When in doubt - improvise!
1) I cannot believe that they are not available - how about right from Mercedes?
2) If a fuel filter is put in just before the distributor -and the screens are not available- can they be eliminated with the added protection of the filter?
3)If Mercedes doesn't have them - can a similar screen be used from another car manufacturer?
Just a few ideas to do work arounds. It seems like I have to do work arounds a lot lately.
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  #11  
Old 06-12-2002, 10:48 PM
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moedip -

Did not get around to it today - will look at those screens late tomorrow afternoon. Somehow I think this is it. I've read about the same problem with the same symptoms on another site. I'll keep the thread informed...
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Henry Bofinger
1989 560 SEL (black/black)
2001 Audi TT Roadster (silver/grey)
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  #12  
Old 06-12-2002, 11:44 PM
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If the idle is fluctuating up and down, perhaps your fuel mixture should be adjusted. As far as the rough idle, that might take care of it. If not, when the engine is running and the aircleaner off, get a 12mm wrench and one by one, loosen each fuel line in the fuel distributer, see if the idle gets worse or doesnt change, then retighten. If you come across one where it doesnt affect the idle, then its that cylinder that is causing the problems. This is how my mechanic diagnosed my rough idle (worse when cold) on my 420. Turns out I have a bad exhaust valve in the no. 8 cylinder and is not sealing properly. For the money to fix it, I will live with the rough idle
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  #13  
Old 06-13-2002, 09:20 AM
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amg280 - I tried a cylinder isolation approach by disconnecting the spark plug cable cylinder by cylinder. I could not isolate a bad cylinder.

The mixture is something I'll look into. The problem there is that this is not DIY - you need something to read your exhaust emissions for this. Before I take it to the shop, I'd like to narrow it down. I'd hate to take it in, have a mechanic do the mixture, it not work, he tears the whole thing down and tells me it need a new fuel distributor, when all I really needed to do was clean the screens and replace a fuel line.

I wish I had my own shop, lifts, scopes, everything...
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1989 560 SEL (black/black)
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  #14  
Old 06-13-2002, 10:10 AM
moedip
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Don't think fuel mixture will help. I paid Mercedes to do mine with absolutely no change. As I said previously I did all the things you did with no change. For others please note- the miss is not rythymic it is a a constant random type of miss - not like one cylinder missing - sort of hard to explain until you see it. Pick up, mileage and power are great the only thing is the slight missing at random at idle - everything else is good.It's not really bad- just enough to get your attention. The main thing that gets your attention is the oil guage fluctuating in time to the miss and the SOUND of the motor changing in revs at idle. When a Flagship like a 560sel has a slight miss it really bugs you! :p
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  #15  
Old 06-13-2002, 10:31 AM
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Thumbs up

Could not have said it better (or described it better), moedip!

This afternoon I'm going to look for those screens...
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