bobkat

Hello everyone! Just bought a 1988 300E with 124k miles on it today. It was bought as a replacement for my 1982 300SD which has 258k miles on it. I have a couple of issues with it, that i hope you guys can help me with. I've used the search function and haven't found a similar situation to mine.

My biggest concern is the idle. It is extremely rough, like running on 5 cylinders or less and is variable. I opened up the airfilter housing and it was all nice and clean in there. the throttle plate was a bit sticky, but no oil or grease inside. I replaced all 6 plugs and gapped them to 0.8mm as per spec. The wires all checked out with a voltmeter. When I rev the car it's nice and smooth, but when i let the car idle the engine is bucking around in the engine bay.

Any ideas on what the issue could be? I've heard of injector seals going bad (though the plugs were not fouled), i've heard of an acumulator going bad, perhaps the timing is off?

Thanks for your help

mike690003

Well here are a few W124 tips:

--Replace all the vacuum rubber tubes in the engine bay. They become very brittle over time. Vacuum leaks are a culprit for many rough-running problems. Rubber vacuum line costs .99 cents a foot, at AutoZone. You will use about 1 and 1/2 feet of vacuum rubber, its a simple remove and replace job.


--Plugs and wires are a good start, but you must also replace the cap and rotor.

--Spray off the throttle plate with a little "Gumout" carb spray, it removes all the deposits.

--The injector seals do go bad, but a way to test them is to idle the car, and spray the carb cleaner or starting fluid around the injector seals. If the car runs rougher or the idle increases, then you need new injector seals.


--lastly, the timing can not be adjusted.


hope this helps

bobkat

Thanks for all the great info!

I just took a good look at the engine bay (was quite dirty until i started cleaning it) and i notice that there is a small line from the manifold that goes under the air filter housing. It is the line that connects to the manifold very close to where the large vacuum line from the air filter housing connects. This smaller line is broken. I am not completely sure what it's function is...could this be one of the offending problems?

mike690003

Oh yeah!

Buy some vacuum tubing from autozone. That broken line is bound to be giving you the rough idle problems.

bobkat

So i repaired the vacuum line that was broken, and that didn't fix things. I then proceeded to check my plugs again...turns out one of them was broken (in the ceramic) and was loose. I replaced it and now she idles like a dream. 700 rpm quite smooth and very nice.

New problem though, and it is reproducible everytime. When i have my foot on the brake, and shift into any drive gear (R,D,2,3) the rpms drop to about 500 rpm and the engine starts to shake, probably because it can't sustain a 500 rpm idle. What is causing my idle drop? In N and P, it's flawless, but in gear, she gets mad at me.

Jackd

I would suspect tired engine mounts.
If they have never been replaced after 14 years, you'll be amazed at the improvement new engine supports will make.
Try the right side first. The right side is more exposed to high heat, being closer to the exhaust manifold.
If you feel an improvement, do the left side.

bobkat

Hrm, so it is normal for the revs to drop from 700 RPM to 500 RPM with my foot on the brake and a shift from P to D?

mhorn

Do your rpms drop that much with the engine warm and cold? I would think this is more a transmisission problem. When my car is cold in the morning, the RPM's drop more when I shift into gear than they do when the car is warm. Cold tranny fluid is harder to move than warm tranny fluid. Does the engine stay at 500 RPM as long as you have your foot on the brake or does it just drop there for a second or two? Your motor mounts could be toast. The engine shouldn't rock that much even when you're starting the car up.

bobkat

it stays at 500 RPM there is no dip, it just drops and stays there.

I have read some about motor mounts, and htis may be the problem. Also i have read about an 'idle control valve' ... any infor on this?

mhorn

I checked my tach last night and my engine runs just a fuzz above 500 rpm when it's in gear with the brake on. The idle control valve is an device that regulates the flow of air into the engine at idle according to an electronic signal. They can go bad but I think this is a part that is blamed for a lot of problems that are not it's fault. You can spend a ton of time and money chasing these problems. My car has the infamous hard starting problem where it konks out right after starting up some times and then runs fine. I took it to a competent german specialist and they could not find any sensors out of whack. I think your best bet is to find a reputable M-B specialist and get them to diagnose the problem.