Still have a "bumpy" idle - PeachParts Mercedes-Benz Forum

psfred · #1 06-15-2003, 06:50 PM

I've had rough idle problems on the 300TE since I got it. This week I've started working on it again since Mom was complaining it was very rough -- I don't drive it very often.

Sure enough, it was VERY rough. I'd already bought engine mounts, so put those in (an adventure, I dropped one of the lower bolts into the crossmember, took a while to fish it out, don't ask!).

I also got a duty cycle meter and set the mixture to 50% at pin 3 of the X11 plug, this helped quite a bit. Runs very smooth off idle, has way too much power (I was test driving it and flew past the State Police post at 75 in a 55 -- forgot it has a 160 mph speedo instead of a 140, was looking at the wrong marks!).

However, I still have a very rhythmc thump and vibration, almost like I have a dead miss in one cylinder. I replaced the secondary ignition parts two years ago, everything looks fine, duty cycle is stable, so what gives?

I've put a bottle of Techron in the half tank of fuel, will run it though and see what happens in case I have a drity injector, but what's next? Injectors? Can't imagine how a bad fuel distributor could cause a regular idle vibration.

Thanks!

Peter

stevebfl · #2 06-15-2003, 07:16 PM

The next step should be to identify your problem. If the idle is rough chances are that the power derived from each cylinder is different. This can happen for many reasons but the first step is identifying the problem cylinders.

Probably the safest way is to remove the air filter and crack the injector lines one at a time while the engine is running (use a rag to sop up the extra gas, there won't be a lot if done quickly). Naturally the cylinder dies when this happens and should cause a noticable shake. The more it shakes the better that cylinder WAS running. if you have a weak cyl it will make very little difference when the line is cracked (be sure to wiggle the line as sometimes the line sticks and the fuel stays inside even with the nut loose).

If you can ID a cylinder try swapping injectors, plugs, plug wires etc. to see if the miss moves. Also try pushing down on the airflap to give more fuel.

If there is a moment where the engine runs good as you press the airflap down then the problem is that the car won't run on some cylinders at the lambda mixture. Remember that lambda is as lean as it can run with power and the system makes the richest cylinder that lean. All leaner cylinders are running at less than lambda mixture. To put numbers on it; idle CO is about .3 - 1.0% CO. Placing ones finger on the flap can easily run the mixture to 2-4% CO. When doing this if a injection circuit (STARTING ay the fuel distributor - of course it could be the fuel distributor) has less fuel it may get above 1% with some up to 4%. The car will run great but not in closed loop.

In other words if you have cylinders that are lean in closed loop they will misfire as the richest cylinders are the ones being controlled. Try again, if the injectors are measured with a differential flow meter they have to be within 10% on flow. The problem I am discussing above would happen if flow was much different than 10%.

psfred · #3 06-15-2003, 07:21 PM

Thanks Steve!

I REALLY don't want to have to buy a fuel distributor this month.....

Peter

J.HIDALGO · #4 06-15-2003, 09:48 PM

After trying fuel injector cleaners, spark plugs and wires, I took my car to Steve who diagnosed the problem! I did not wanted to buy a new fuel di$tributor. So, I bought a used one from e-bay and installed it myself. No idle problems now.
In other words, if you already tried to fix the rough idle with the "usual stuff", get a good diagnosis from a trusted mechanic and then look at your options.
Good luck!

psfred · #5 06-15-2003, 10:08 PM

JH:

I currently have Bosch Platinum +4s in it, and when I looked at them this evening the electrodes were black and slightly oily. They all looked pretty much the same, with #3 a tad darker.

I'm probably going to get a set of Supers tomorrow and see what that does -- the electrode base was the correct color, so I know the running mixture is OK -- duty cycle floats between 50 and 54% at idle, the roughness isn't connected to any one cylinder by leaking down the injection lines, secondary ignition stuff is fine (almost new) and the plugs are all the same.

Funny thing is that it idles smoothly just off the highway, only starts rumbling when it's driven on country roads at low speeds for a few miles. This makes me suspect the plugs -- I know now that resistor plugs aren't a good match for this car, didn't know that when I put them in.

The other possibility is that I need new valve guide seals. It uses about a quart in 1500 miles, part of if a front cover leak, part a valve cover gasket leak (needs a new gasket), but I think too much may be going down the valve guides.

I will also check the O2 sensor, since I ahve no idea when it was changed last.

Peter

#6 06-16-2003, 12:51 AM

Do a search on "air mass potentiometer". Does the car often stall when first started? Does it sometimes have a hesitation when accelerating? I have heard that it is caused by the air mass pot.
Tim