Idle Surge
 

Hi folks!
I have an '87 300e with a 103 engine. The engine surges when idling. It doesn't seem like it's miss-firing, it idles smoothly and every second or so it increases in speed by a hundred or two hundred rpm. In addition, my plugs have a lot of deposits on them. The deposits are black, fuzzy and very thick (like a very rich mixture). The surging continued with new plugs. Oil usage is about 1qt per 1000miles (I use Mobil 1 oil). Compressions run about 145 psi. I recently purchased the car with true mileage unknown.

It has been suggested that the injectors need replacing, I'm wondering if this could be a timing issue, fuel pressure issue, or an oxygen sensor issue. I'd feel better about replacing the O2 sensor if the light was on, but I'll replace it if the forum feels it would be wise.

Thanks in advance all!

Bob

One thing I'd like to add. My plugs look like the one's in the thread titled "Spark plug horror." Guess I'll be replacing my valve guide seals.

Thanks,

Bob

Does anyone think that the leaky valve guide seals would cause the idle surge. I would think that changing the plugs would have solved the problem if it was fouled plugs, so it must be caused by something else. Any suggestions would be appreciated since I'll be taking the time to replace the seals anyway.
Thanks,
Bob

Besides the valve seals (which need to be done if they are fouling the plugs), check for vacuum leaks around the breather hoses by lightly spraying them with carb or air cleaner while the engine is running.

Also might want to remove the idle control valve and clean it out (it can hold a lot of gunk and start to stick). Applying 12V to the leads (although a 9V battery works as well) will move the valve and help with the cleaning.

Can also try depressing the air flow plate and spraying air cleaning under/around the plate arm towards the engine, to try to clean gunk off the air flow meter.

Anthony,
Thanks for the advice! I'll give it a try when I get home from California.
Thanks again,
Bob Kopicki

1 qt. oil per 1000 miles oil consumption should not foul the plugs. This consumption indicates that replacement of seals would be a good idea but not abselutely necessary (it is considered more or less normal for this engine, I hear... even the users manual says so).

So you should look for rich mixture.

If it is on all plugs, it is common for all cylinders.

I had a faulty EHA (Electohydraulic Actuator), which caused way too rich mixture and eventually no starting (and fouling plugs as you say). You may have the same problem.

A rich mixture sounds likely to me, but how do you test the electrohydraulic actuator?
Thanks,
Bob

Please check the condition of the hoses that connect to your idle control valve. When they get old and hard, they crack and leak air into the system. I just replaced all of those hoses on my 92 300TE. Engine does idle more smoothly now, but I've also been chasing and fixing several vacuum leaks as well, which has also contributed nicely.

Good Luck

With ignition on and engine not running, the current to EHA should be 20 mA.

With engine running, the value should fluctuate +/- 3 mA according to feedback from oxygen sensor.

The resistance of EHA should be 19,5 ohms.

Otherwise, there is really no other way to test it.

It works so that it will provide a certain differential pressure when no current is supplied, and it will provide more or less differential pressure with current flow in one or the other direction.

Its center position is (as well as understand) springloaded.

I don't know what can go wrong inside it, but in my case, it provided too much pressure (too rich mixture) no matter what input it got.