Rough Idle Update on 300TE....I'm totally out of ideas - PeachParts Mercedes-Benz Forum

ps2cho · #1 04-02-2009, 09:51 PM

How it drives:
Idles rough, but as soon as you touch the throttle it disappears. Performance is slower than should be (iWrock has driven my car). Can't feel any misfiring when accelerating...its all at idle.

Static Tests:
-Throttle Plate adjusted + centered correctly.
-Fuel Pressure perfect (5.4bar 4.9bar + holds 3bar after 30mins on specs)
-Airflow Potentiometer reads .7v, dead on specs.
-No error codes
-No vacuum leaks
-Mixture tried/set all different configurations. Settled with 50%
-Compression perfect.
-EZL reads 85% engine off ignition on. WOT reads 20%
-I get ~15MPG average so fuel economy lower than it should be. With 140miles Freeway, 30miles City I get 17MPG.
-Slower than the car should be performance-wise

Parts swapped/replaced:
New OVP
New Coolant Temperature Sensor
New Voltage Regulator
New Injector's+Seals
New Non-Resistor H9DCO's Spark Plugs
New Ignition Wires (checked for arching as well)
New Distributor Rotor + Cap
New Fuel Filter
New Breather Hoses
New Ignition Coil
New O2 sensor
New Motor Mounts
New Head gasket + valve job -- Compression 190psi across the board.
Swapped Jetronic Brain /w used one
Swapped Fuel Pump Relay /w used one
Swapped Cold Start Valve /w used one (in case it was leaking)
Swapped EZL /w used one

I don't understand why I still have this rough idle

Many of you have helped me through this and a lot of the things I have replaced were bad/old or improved the engine in some way or another so I don't feel bad spending the money...I just want my smooth idle!!

I don't know if there any more static tests I can do on this? Honestly what the hell is left that could be causing this??

Not sure where to go from here....

I don't know if I should just take it to the dealership? I'm afraid they wont find anything as I have gone over much of the stuff they will check for...?

Can I rule off the fuel distributor if the fuel pressure is okay?

ps2cho · #2 04-02-2009, 10:27 PM

A few pictures so you can see cleanliness and general condition of engine.

pawoSD · #3 04-03-2009, 12:17 AM

Personally I'd probably try swapping a different fuel distributor on just for the heck of it....who knows, maybe yours has some weird internal issue that's causing it to not meter the fuel properly.

I've noticed that my M103 idles "rough" sometimes, and by rough I mean you can tell its idling, but its not misfiring (I've experienced that).....but at other times (most of the time) its so smooth you can't tell its running. Mine seems to get about 18.5-20mpg even with me being a leadfoot.....

I am being picky when I say it feels "rough".....my sister's van and my uncles cavalier engines both vibrate "in general" when idling.....by comparison the M103 runs like a fine watch....with a slight vibration sometimes.

Is the vibration strong? Or just enough to be annoying?

Have you considered trying...dun dun dun....Resistor spark plugs? I ran mine on the H9DCO plugs, and it ran like crap....well, it ran great for 50 miles, then it lost about 2 cylinders and ran horrible....got 11mpg and had no power. I quickly replaced them with NGK resistor plugs, and it has run beautifully since....and that was quite a while ago. When I took out the non-resistors....they were totally coked.

I noticed you've not replaced the fuel pump. Is it possible that it might be tired?

deanyel · #4 04-03-2009, 12:20 AM

I've had one like that, tried everthing, never could rid of it. Some people told me that's just what 103 motors do - idle rough when they get old, built into its DNA. But you don't have that many miles. Cam wear is another theory, but that shouldn't be on that mileage.

ps2cho · #5 04-03-2009, 03:40 AM

Cam has zero wear on it at all along with the pistons and valves etc...Looks pretty brand new inside there.

I'll swap the fuel distributor...I have a couple in my random parts box.
Wouldn't it show as a fuel pressure problem though if it was bad?

I might try the NGK resistor plugs for the hell of it. Got a part # for them? Are they easy to find at a local pepboys/autozone or will I have to order online? I believe there is an NGK warehouse in Irvine, CA close to where I live...maybe they sell them there too...
I've pretty much used the H9DC0's since I got the car because I read they are the OEM recommended set.

The rough idle isn't a strong one anymore, but its enough to be annoying. It is pretty inconsistent...some days can't feel it AT ALL...other days I can. Either way I get horrible MPG and a lack of power.

Ivanerrol · #6 04-03-2009, 05:13 AM

Of course pulling the Fuel distributor on and off the AFM will require a new o ring between them and the levels and adjustments made to spec.

Tried an EHA or adjustment of yet?

Your pictures of the head valve train look impressive.

What does your vacuum gauge look like at idle when warm?

dbmoylan · #7 04-03-2009, 05:38 AM

Check your purge valve if it is leaking in outside air & might want to change out the camshaft position sensor.

Cal Learner · #8 04-03-2009, 06:42 AM

As long as you're down to the "I'll try anything" stage, take a look at the routing of the ignition wire between distributor and coil. Mine was laying up against the hot thermostat/waterpump. I rerouted it so it now runs across the intake manifold instead, and stays cooler. I have no idea what the factory routing was, but that improved my idle considerably. That, despite no outwardly visible signs of damage to the wire insulation. Go figure.

teezer · #9 04-03-2009, 08:11 AM

i've experienced BAD new injectors, bad spray patterns at idle

arno++ · #10 04-03-2009, 09:17 AM

motor mounts might need changing if you feel a distinct difference
betwee idle in park/drive/reverse

a'

ps2cho · #11 04-03-2009, 09:29 AM

I have tried 2 different EHA's from the junkyard.
Vacuum gauge at idle sits just slightly left of the red line. With AC on it sits dead on the first part of the red. I've replaced every vacuum hose and had the car smoke tested for leaks.
It moves slightly of course with the rough idle from the rpm's going up and down slightly/mixture adjusting itself.

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The 1988 model does not have a purge valve.

I tested the resistance on the CPS and it checked out...but I have read that on one hand its a "hit or miss", but other people have reported funky things...so I am not sure.

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Ignition coil wire is fine...I've tried two different brand new set of ignition wires.

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Brand new injectors as of 2 weeks ago.

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Its an actual engine rough idle, not bad mounts. I swapped the mounts either way.
Something is killing my power and my fuel economy slightly.

david s poole · #12 04-03-2009, 10:24 AM

try this idle engine in drive with emergency brake on.grab 12mm wrench and open the fuel lines at the fuel distrib one at a time to see which one makes the least difference to the running.it helps to have a vacuum gauge under the hood when doing this.what i suspect is that your fuel distr is not providing enough fuel at idle on one cylinder.my 420 with exact same setup only 8cyl is doing the same thing.i can mask it almost completely by running the a/c at idle and loading the motor.

pawoSD · #13 04-03-2009, 10:31 AM

My CPS tested in spec.....but it caused the car to stall in traffic and throw a code 7....(Engine RPM fault)

With a new one it has run perfectly.....never stalled since, not even once.

ps2cho · #14 04-03-2009, 02:53 PM

Quote:

Originally Posted by david s poole

try this idle engine in drive with emergency brake on.grab 12mm wrench and open the fuel lines at the fuel distrib one at a time to see which one makes the least difference to the running.it helps to have a vacuum gauge under the hood when doing this.what i suspect is that your fuel distr is not providing enough fuel at idle on one cylinder.my 420 with exact same setup only 8cyl is doing the same thing.i can mask it almost completely by running the a/c at idle and loading the motor.

I'll give it a try although I think it will be very difficult to tell as the engine is alot smoother these days. I have two other fuel distributors that I can test. Maybe I will start with that first and see if it makes any difference.

What do you mean by a vacuum gauge under the hood?

I read somewhere that these fuel distributors can be cleaned by yourself...Is this true? I'll shoot some carb cleaner all through the one I have...but what about taking it apart?

pawoSD · #15 04-03-2009, 04:12 PM

I don't know if the fuel distributor can be cleaned like that by a home user....its quite complex inside.

I clean mine by running lots of Redline SI-1 through it.

The NGK spark plugs I am using I found on europartsdirect.com...just look up your car and under the spark plugs section they have one type of NGK available....that is what I am using currently and they work really well.

Here's a pic of it: