'85 500 SEL idle probs... help! - PeachParts Mercedes-Benz Forum

Vaden85SEL · #1 01-26-2007, 08:19 PM

Hi everyone,
I have found many posts with people experiencing the same problem as I but it seems nobody over there has solved it -- not to mention the threads are almost 4 years old.

Anyway, here is my problem: The 1985 500 SEL starts up and runs fantastically until it gets warm. Once up to temperature, the idle becomes rough and choppy... like it's missing a cylinder or something. It remains choppy for the first few seconds of acceleration and then it smooths right out. As soon as it stops at a light or something, the rough idle and choppiness comes right back. I have changed the plugs, wires, dist. cap, rotor, O2 sensor, and given it a general tune up.

Some posts i have found suggest one of the injectors could be bad, but i just pulled the plugs looking for one that may be fouled and all of them look clean and proper like they are burning fuel correctly. I have also seen suggestions that it may be a vacuum leak, but all of my lines seem to be in order.

This is driving me nuts, and apparently it has driven others nuts too. I am just wondering if anybody may have come across something i missed and perhaps has a more narrow idea of the culprit and how to fix it.

Any help will be greatly appreciated.

Thank you all!

Kebowers · #2 01-26-2007, 11:53 PM

and the cylinder compression readings are? Have you checked for air leakage around the injector seals? -try running a little water on them while engine is running-very quickly finds tiny vacuum leaks.
Oil consumption is? Any exhaust smoke? Economy is? We need information

david s poole · #3 01-27-2007, 10:34 AM

get a smoke test done on the intake and you will see the vacuum leaks.you have all the correct symptoms.vac leaks will affect engine at idle but rev up and problem dissappears.

dpetryk · #4 01-28-2007, 07:18 AM

Very likely that the fuel distributor is bad. I have had three of them go bad recently and the symptoms you have follow exactly. Smooth idle when first started, rough idle when warm, and normal performance when driving. Because of the cost I tried everything else first before I finally changed the fuel distributor. That was the problem. I opened the bad ones and sure enuf there were holes in the diaphram.

I just recently completely overhauled the intake manifold on my 560 for that very problem of rough idle. After re-sealing the manifold the idle improved but it did not completely cure the problem. The fuel distributor solved the rough idle. I am sure I had several vacuum leaks in the manifold. In fact I was sure of it. And I was hoping the seals were all I needed. I was not that lucky.

Let us know when you get the problem solved and what the cure was.

Gerard · #5 01-29-2007, 09:55 AM

I had very similar trouble with my M103 motor recently. Turned out the vacuum line going to the EZL module was leaking badly. As a result the ign timing was off at idle and it ran terrible when it became warm. It ran OK at higher revs.......if you could get it there! New rubber tube solved the problem. Have a look at the vac lines, take off any airfilters, etc to get a good inspection!

gerard

Kebowers · #6 01-29-2007, 11:37 PM

Its the ethanol in the new reformulated gasoline that is attacking the 'rubber' diaphragm in the fuel distributors. Unless you can get them 'rebuilt' with an alcohol resistant material, expect frequent failures.

Vaden85SEL · #7 01-30-2007, 10:26 PM

Well, I had the vacuum system smoke tested. There were leaks in cylinders 1, 2, and 3, in the injector seals. The mechanic said it was the "injector cups" but i can't seem to find that part name. I think he is talking about the seals or the "injector holder" -- am i correct in asuming this?
Thanks for the help by the way guys.

david s poole · #8 01-31-2007, 01:30 PM

yes injector holders.replace them all and use plenty of grease on the new injector seals when you slip them in.then retest with smoke to find the rest of the leaks.to replace the holders you will need to also buy the two idle air balance rubber tubes as yours will be petrified and will --repeat will break when you take them off.

Mike Freeman · #9 01-31-2007, 07:51 PM

If you're going to change any injector holders change them all and don't forget injector seals. As long as you're that far into the engine replace the rubber fitting at cyls 2&3 intake runner and the nipple on the back of the manifold at #4.
Even with all those repairs you may still have a miss (while the injectors are out have them tested).
Mike

Vaden85SEL · #10 02-01-2007, 06:20 PM

Thanks for the help guys. By the way David, are those "two idle air balance rubber tubes" the fat hoses resting on top running along the tops of the cylinders that are plugged at each end and have 4 nipples coming out the bottom that go into the injector holders? If they are, then *********************** has them listed as "air distribution hose" -- perhaps a terminology difference? --- and by the looks of the ones I have on the car, you are probably right, they seem a bit hard and are probably really brittle. I guess I have a few more days to wait on shipping before I can get to work.

Okay, since I need to place another parts order, I shoud get it all in one try. So on that note, I currently only have the injector seals and holders. I just ordered shop manuals, I should receive them tomorrow. I will order the "Air distribution hoses" and plugs. But before I do, are there any more parts that anyone can think of that I should definitely have on hand while making this repair? -- I need to get it all done on one Saturday, as this is my only transportation at the moment.

Wieder, danke für die ganze Hilfe!

Vaden85SEL · #11 02-08-2007, 01:05 PM

I just got to say thank you to all of you that replied. I changed out the injector holders, seals, and the idle air balance tubes. She is now idling very smoothly at about 650 rpm and she no longer has a miss or hesitation while accelerating. In fact, I cant remember the last time she ran this well! -- so thank you all and I hope this thread can help some people with their idle/hesitation issues.

johnathan1 · #12 11-17-2008, 03:46 AM

Wow, an awesome ending! I am having the exact issues with my car...except I think I may have bad manifold sealing rings.

When I spray brake cleaner anywhere near the engine (literally anywhere) the idle jumps up quite a bit.

When I spray the injectors, some make the idle jump more than others... but when I spray down between the air mass meter and the distributor, the idle REALLY jumps, almost like I revved the engine using the throttle! Does this point to bad manifold seals?

bevanet · #13 11-18-2008, 06:45 PM

I think I have some of the same problems on a 88 300SE. I have ordered new rubber, but I would like to make sure all the leaks are fixed when I am done. Does anyone have a link or document that explains how to perform the above referenced smoke test?
Thanks!

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