I've got a 91 Mercedes 420SEL about 111,000 miles on it. Great car. For past few months, I've noticed idle at a stop (when in Drive) is very rough, feels like a plug is "missing", but not always rythmic. It's erratic. Same with acceleration. Weak, and erratic and jerky at first, then picks up. Not strong and smooth acceleration like normal. Driving me nuts!

I've checked vacuum lines (as much as I could, and am reasonably sure I don't have a problem there).

I'm in process of replacing spark plug wires and leads (plugs are 1 yr. old), and the distributor and rotor (also going to check timing, etc.).

Question: Does this car have a "knock sensor" (say, like Volvos, etc.?). When those go bad, it produces similar symptoms....

Any ideas??

Bob Irving
Charlotte, NC

I would also look at the O2 sensor

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1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car
609 Certified

Fully checkout the ignition system and when you replace the plugs and wires, use the ORIGINAL plugs that were specified in the owners manual and not for example a large electrode version (a plug with a zero on the end of it) that Mercedes will give you at the dealership. Do not use platinum plugs unless you want to guess if those will work, your results will vary with those.

Use the CORRECT plug wires from Mercedes/Bosch, not aftermarket ones. The resistance in these is setup properly. Most aftermarket wires I've seen are crap anyway and have the wrong resistance.

If you don't do the above correctly, you'll spend time chasing those issues just to return to the original issue. Speaking from experience.

After you have gone through the ignition system, then you can go to the fuel system. Once you are confident the ignition is straight and the car isn't misfiring heavily on acceleration, you can put some Chevron Techron in the tank (the big bottle) when the tank is empty and then fill it. Then drive the car hard. I swear by Techron, it's for me the best of the bunch for fuel injector cleaners and yes, it really can solve fuel and intake dirt problems in there. You need to drive the car hard. But if the car isn't driving right, use your own judgement because driving it hard with something up in there may cause its own damage (such as heavy misfiring damaging your catalytic converter).

Yes of course check for vacuum leaks and the standard array of issues such as the air cleaner, etc. It's OK to check on the oxygen sensor also-- that's an easy fix/swap-out. Be sure to splice that wire very nice and tightly and make a VERY good connection. The O2 sensor gives a very low voltage, the connection has to be nice and tight.

If none of the above solves the problem, then onto the fuel system. First verify the kind of Bosch fuel system in your car. Mine (an 84) is K-Jetronic. I think yours is electrical, I don't know maybe KE-Jetronic. Then learn about that fuel injection system (injectors, fuel distributor, etc). The first and easiest/cheapest place may be to check your fuel injectors. Be sure to replace new seals/etc when you remove/replace them. You can introduce vacuum leaks there that will plague you later as well.

Thanks so much for the information. Really appreciate the guidance. I'll let you know how it turns out!

Bob

Check the idle control valve hoses, you have the classic symptoms of a vacuum leak.

This can also be a bad O2 sensor going way lean, too -- check the idle mixture.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

TO all those who kindly replied to my thread re: rough idle problem in my 91 420SEL --

Problem solved by replacing the spark plug wires (originals were 13 yrs. old) and the distributor cap and rotor (all Bosch parts -- from MercedesShop). Just purrs......RUNS GREAT!!

Of course, per your suggestions, I first checked out the O2 sensor, the vacuum lines, etc. the easy, obvious stuff before spending close to $ 300 bucks on parts!

Again,, thanks much!