300E 3.0L low, uneven, sputtering idle
 

90 300E 3.0 (well maintained 229k miles) with low, erratic idle. New fuel filter, cap and rotor. Plugs and wires good. Fuel pumps sound good (no buzzing, noise, etc.). Very low idle that "cycles" up to a little higher rpms, then backs down again. When given partial throttle from idle, it wants to sputter (almost backfire). When more throttle applied, it "smooths out" a little, but never fully. At takeoff it will only take partial throttle and picks up speed, but very slowly. When throttle pressed more fully, it doesn't seem to want to increase speed. Once up to speed (50-60 mph), when given full throttle, it kicks down to passing gear, pulls hard and everything is "almost" where it should with the exception of a small, almost undetectable "seat of the pants" miss under full acceleration. I'm stumped and don't want to throw parts at it to figure things out. I'm thinking possible O2 sensor, vacuum issue, etc., but I'm certainly no tech. Pro input is greatly appreciated. Thanks in advance for any help you may provide...

You say "plugs and wires good". Can you explain? Do you know how to check your duty cycle? Did you?

Maybe I should say plugs and wires most likely good. Both replaced w/in the last 6 months. No duty cycle check...and don't know how to do it.

What brand where the wires? I found in my many years of m103 ownership that genuine Mercedes wires last the longest; about 5 years. Bosch is a runner up with about 2-3 years, and the rest are garbage, as in they last 6 months or less.

Checking your duty cycle is easy. Make sure you have a voltage tester that measures duty cycle, usually it has a switch with a % sign. Find the round test port that is near your abs unit. It is mounted on the fender.

Set your tester to duty and put the RED lead in pin 2, and the BLACK lead in pin 3. Turn the car on and watch it run. Once the car warms up it should cycle between 50-60%. If it doesn't move, there is a fault somewhere.

Yes, but not 5 years, mine lasted 15.

Never tried Bosch.

No, I installed Beru 5 years ago and still going strong. However, people are beginning Beru's more recent quality - Chinese?

Cap and rotor - best to go for MB and make sure the rear shield and seal are good - MB only for those.

Also make sure you have no 'exotic' plugs - platinum, iridium, anythingelse-ium - forget them all and install basic, non-resistor, copper-cored plugs. I've used Bosch, Beru and NGK in the last 10 years with no problems.

With these engines, make sure the HT is 100% before suspecting other, more costly problems.

Good luck.

RayH

Thanks for the input guys. Don't know the brand of the wires, but cap and rotor (new) are Bosch and plugs are copper NGKs. Assuming the wires are ok, do you have any other ideas what could be causing the issues described?

Check your duty cycle. That is the first step to figuring what is wrong with your car.

Bosch for the cap and rotor are fine, as far as I am concerned. The MB brand is very expensive comparatively.

Once you check your duty cycle, we can progress further with diagnosis.

Ray...pardon my ignorance, but what does "make sure the HT is 100%" mean?

Sorry, HT is High Tension. That's your high voltage ignition parts. Coil, cap, rotor, leads, plugs and all that. These engines demand the best in "HT".

Your ignition control module is very important too but they are very reliable but, in my case, hugely expensive if needed. Usually, they are dead or alive so your problems are, hopefully, less threatening.

Bon courage.

R

Again...thanks for the input guys. Much appreciated. One thing I didn't mention is that the car is also running quite rich. I've done a pretty extensive search here on threads describing similar issues and I've decided on a tentative "game plan". After checking and verifying no electrical issues (coil included), my first move is going to be adding some BG fuel injection cleaner (cheap and beneficial). Next, replacing the O2 sensor. I think it's the original sensor and even if it's not the issue now, it certainly won't hurt to replace it anyway. Next, checking / cleaning the idle control valve and the attached vacuum hoses and replace if necessary. If no improvement / fix, I'll check the duty cycle...and dig a little deeper from there...

You should do that list backwards. Check your duty cycle first. If the % does not fluctuate, then you know you have a bad part, if it is running rich, you know you have a vacuum leak, or worn ignition parts.

Redline fuel additive is the best system cleaner I have ever used. I still use a bottle every few months. Don't waste your money on the others.

Update...
 

As opposed to throwing parts at the problem, I decided to go with my gut instinct and try a couple of cans of fuel injector cleaner before I went any further in the process. I'm glad I did. It eventually took care of the problem. As a precautionary measure before hitting the road on a 600 mile trip, I (well,
my mechanic) then removed and cleaned the idle control valve and replaced the attached vacuum tubes. Good move...when removing them, they came apart in pieces (dry rot). Replaced a few other vacuum hoses as well. I guess I was lucky. Btw, I used an "off the shelf" FI cleaner called Berryman B-12 Chemtool Fuel Treatment @$3.17 per can @Wallyworld for 15 ozs. I used 3 cans total...about the same total cost as just one can of the major brands with only 6-8 ozs. I'm sure the name brands are good, maybe better, but this stuff worked as advertised for me. If anyone is experiencing the same issues that I've described, try the simple fixes first. You might just get lucky like I did...