Rough Idle $2600.00
 

I have a 97 E420 (102,000 mi) that just came down with a sympton of a rough idle and the check electrical engine light came on. Driving at normal speeds it is fine. A local shop says I need a new control unit or have it rebuilt, and a new wiring harness for $2600.00. Does this sound normal.
I told them to hold of for now.

Does any body have any ideas?

I don't know if either item is the cause of your problem, but wiring harnesses and the ECM are not typically problems on '97 E420's. Wiring harnesses were only bad from '92-'95 anyway, so yours would be the first I've ever heard of failing on a '97 model.

Who did the diagnosis on your car? It sounds like they are grasping in the dark.

A typical cause of a rough idle on your car would be a bad mass air meter or a throttle actuator.

How would you describe your rough idle? Is it like an engine miss? Maybe all you need is a new spark plug or a coil wire.

I'm with Paul - your shop is suspect. What codes came up? I get the codes read somewhere else and see what they say. Or get an OBDII code reader and see what you get - should get something with the CE light on.

OBDII is okay, but 90% of the on MBZ I get more than one code than just the P codes, and I have never gotten any C codes. You could have bad wiring, but I have never seen wiring loom going bad unless you have had the head off recently, or have been moving a bunch of wires very rough. Sometimes you can get away with just the loom too, if you catch it in time and are just shorting the low voltage signals. But I would go after the tune first, unless they see broken wires, but still, they have to be familiar with Benz. Coil grounds are known to go bad too. They will make a miss at idle.

I have seen lots of coil packs go bad on those cars, as well as the mass airflow sensors.

If you get misfire codes, you can access the spark plug coils by removing the coil covers along the top of the valve cover.

With the car running ,get a pair of pliers and unplug the electrical connector one at a time from each coil pack.

If the idle gets worse, you know your coil is good.

If the idle does not change, chances are you have a bad supressor end and or coil.

You may get away with just the bad supressor end, but if you do have a bad coil, replace the supressor end too.

Usually the air mass with give you multiple misfire codes.

Have a dealer hook it up to the proper machine and really get the codes. Then you will know what the problem is.

I think its called SDS, Star Diagnostic System.

Friends S430 showed similer symptoms, turns out it was a coil. But said coil took out the cats somehow, so don't ignore it!

Prime suspect...
 

... a shop seeing dollar signs. Unless you gave us too little info as to what they checked to make this diagnosis. Or they don't know the car.

Out of my league here (haven't paid much attention to anything newer than the Gen 2 126) but -

- that's my $.02 (invoice to follow)

Regards,
Kevin