300E (W124) Running Rough / Missing - PeachParts Mercedes-Benz Forum

treiberg · #1 12-30-2010, 03:17 PM

My 92 300E (2.6L) starting running rough the other day, like it is missing. This was not a progressive problem, but started to do this one cold rainy morning a few days ago. The car shakes at idle and light acceleration, and seems to be an engine miss and not just bad motor mounts (although I am not ruling anything out).

I have replaced the plugs, the distributor cap and rotor, and had replaced the wires about a month ago, so they should be fine. Have looked around for any vaccum leaks and have not found any. I also replaced the OVP relay, with no change. I did a compression check and all cylinders are 130-160 psi.

What else? The miss is very consistent, and doesn't go away under any conditions (engine cold or hot). Car fires right up when cold with little hesitation. Could it be bad fuel? Bad injector?

I think this is unrelated because I believe it did this before the problem above, but when it idles, there is a looping as the RPM changes just slightly every 4-5 seconds, like something is cycling on and off. At the same time when this happens, I can hear a click coming from inside the driver's door, like a relay or switch, and it conicides with this looping. Very strange, as I am not aware of any relays in the door that would click intermittently! This may have nothing to do with the engine miss, but when you start really paying attention to details, you notice things like this.

Looking for help...thanks.

mbzman · #2 12-30-2010, 03:57 PM

What brand/type of plugs and wires are you using? Have you checked fuel pressure? That is always a good place to start. How many miles on the car? Your injectors may have a poor spray pattern if the miles are high.

treiberg · #3 12-30-2010, 05:17 PM

Car has only 100K on it, a very low mileage 300E. I put in Autolite 104's, which is a non-resistor plug that many other W124 owners are having good success with. (I realize the spec plug is a Bosch H9DC). The plugs I replaced were only in for about 10K, and they were HR9DC, the resistor version. The wires are Bosch wires that seem of very high qaulity and are custom fit for this car.

What is a good way to check fuel pressure? At the injectors? Any other details would be appreciated.

Just filled up and am getting 13 MPG and the motor's power is way down.

By the way, figured out the clicking in the door and looping at idle (the more minor of the two issues I reported). It turns out that the driver window switch was being pushed down because the wood paneling at the console was not put back in properly (by me) and was actually pushing one side of the switch down. So the clicking was coming from the motor or lifter inside the door. Reinstalled the trim properly and that problem is solved. The 5-7 second looping is gone.

stormyc88 · #4 12-30-2010, 06:04 PM

treiberg,

have you checked your oxygen sensor? Since your car is low mileage and 18 years old, I'm sure the O2 sensor is original, since they only last about 100,000 miles now might be a good time to change it anyway, even if thats not the problem. My 87 300e had a shot O2 sensor when I first got it. It ran rough and got poor mpgs. New sensor, problem solved! Otherwise, you can try running a few bottles of Seafoam through the gas tank, It worked REALLY well in my car (smooth performance, instant starts

). Also, I hope you're running 93 octane gas...

Anyways, other things to check are vacuum leaks (which you've already done), EHA valve on the fuel distributor can crack and leak with age (regardless of mileage), and make sure the airflow meter plate is clean, and of course the air filter. Also, do pull the plugs and take a look just in case, because I have seen them fail.
To see if you actually have a completely dead cylinder, you can test each one by removing a plug wire one at a time with the engine running. When you get one that makes no difference, that's your dead cylinder.

Hope this helps

treiberg · #5 12-30-2010, 07:00 PM

Yep, done almost everything mentioned, but still really appreciate all the input. Replaced the O2 sensor about 10K ago, when I first bought the car (I guess it could be bad again, but would that cause it to miss horribly and shaking?). Has good air filter, fuel filter replaced about 5K ago. Cleaned meter plate. I will check the EHA valve for leaks. Brand new spark plugs today. Pulled each boot too earlier today, and noticed an increase in roughness with each boot pulled, so don't dedect a dead cylinder.

When driving the car, it is really rough, like someone removed a couple of plugs! And it happened overnight, with perfectly running engine, to running very rough in one day. So more of a catastrophic condition versus a gradual failure. It will put a big smile on my face when I figure this one out. I have spent hours reading on this forum with lots of good ideas, but haven't hit the jackpot yet.

lsmalley · #6 12-31-2010, 02:38 AM

ok, so this may sound really dumb and I don't mean to insult you, but did you pull/unscrew the little metal caps that are at the tip of the new plugs? I had the same exact issue after a tune up I did and went back and checked all my work and realized that these caps were on all the plugs that I just installed and it was affecting the engine with rough idling almost to the point where I was afraid to drive because I thought the engine would stall. again, not trying to insult you just trying to help.

Ferdman · #7 12-31-2010, 07:53 AM

treiberg, I would suspect a bad/failing coil.

The coil in our daughter's 1991 300CE failed while driving and the engine wouldn't run.

lee polowczuk · #8 12-31-2010, 08:47 AM

sounds like a bad o2 sensor with that type of mileage

treiberg · #9 12-31-2010, 09:34 AM

Quote:

Originally Posted by lsmalley

ok, so this may sound really dumb and I don't mean to insult you, but did you pull/unscrew the little metal caps that are at the tip of the new plugs?

The autolite plugs I installed yesterday do not appear to have a removable cap, so I left them on. But I know I have contact between the wires, because when I pull a boot off, it makes a difference (I dedect a change in the running of the engine). Also, the old plugs I pulled had caps on them. Also, keep in ind that the car ran really well with plugs in it with caps for thousands of miles since I bought the car. I would like to hear from others regarding the caps.

treiberg · #10 12-31-2010, 09:43 AM

Quote:

Originally Posted by lee polowczuk

sounds like a bad o2 sensor with that type of mileage

I don't think this is it for a couple of reasons. One is that mine was replaced about one year and 10K miles ago, as noted above. Second, I think a bad O2 sensor would not cause the type of miss that I am seeing, very severe. The O2 sensor controls the air/fuel mixture, which would cause car to run too lean or rich, but not cause this severe miss. This seems ignition related, or possibly injector related to me.

treiberg · #11 12-31-2010, 09:46 AM

Quote:

Originally Posted by Ferdman

treiberg, I would suspect a bad/failing coil.

The coil in our daughter's 1991 300CE failed while driving and the engine wouldn't run.

Yes Fred, the car runs exactly like my friend's 97 S320 when he had a failing coil pack. However, his coil configuration was quite different (one coil pack for every 2 cylinders), so for some reason I was not thinking in that direction. Of course, my 300E has one coil that serves all cylinders. Looks like I can buy a new one for about $80, but is there any test I can do to confirm this before I go throw another part at it? If the coil was bad, would it run at all? Keep in mind it runs, but with a severe miss or engine vibration.

Ferdman · #12 12-31-2010, 01:53 PM

treiberg, not sure how to test the coil. If possible temporarily install a known good one to see if that resolves the severe miss/vibration problem.

You might want to confirm that the spark plug caps are tight and make good contact with the threaded center electrode.

lee polowczuk · #13 12-31-2010, 05:07 PM

Quote:

Originally Posted by treiberg

I don't think this is it for a couple of reasons. One is that mine was replaced about one year and 10K miles ago, as noted above. Second, I think a bad O2 sensor would not cause the type of miss that I am seeing, very severe. The O2 sensor controls the air/fuel mixture, which would cause car to run too lean or rich, but not cause this severe miss. This seems ignition related, or possibly injector related to me.

ok..... i am not an expert....but i know a new o2 sensor in our 190e. 2.6 got the mileage back to normal and smoothed out a rougher running engine....especially at take-off....

treiberg · #14 12-31-2010, 07:49 PM

I was hoping that the resistance checks on the coil would show a problem, but they look good (8.8K on the secondary, and 0.5 on the primary). I read somewhere that a coil might be bad even if the resistance test passes, so not sure if I should spring for the $80 for a new one.

Any experience with single coil systems out there - if the car runs and the resistance test is good, can I assume a good coil? From my reading, most of the time a failed coil will result in the car not running at all (except for coil pack systems where the car has multiple coils).

sptt · #15 12-31-2010, 11:07 PM

This might sound dumb as well, but did you do the Spark Plug change after it started running bad or before? I have the Autolites and after I installed them, correctly, the car has ran so good! However, when I first changed them, I accidentally switched the around two of the middle plug wires giving me the same result you describe. I realized this when I took them back out to check for fouled plugs and smacked myself for being an idiot!
I also second the notion for the O2 sensor but you said you already changed that. EHA Valve was the second biggest improvement in starting and idling. I had 187K on my '91 W124 when I did this. I'm sure it will be something simple. Maybe rotor or distributor cap as well...

Good luck.