$2500+ in parts and still rough idle
 

Hi everyone!

I have a 93 500SEL (w140, m119) with about 90k miles. It developed a rough idle and was missing while accelerating a while back. Here is what I replaced to date (thinking I had to eliminated these things anyway before I went anywhere further):
1. New wiring harness
2. New air mass sensor
3. New spark plug wires
4. New Bosch spark plugs
5. New distributor caps and rotors
6. New coolant temp sensor

Had some improvement, but still rough idle (like its missing, but not all the time - rather occasional, once every 4-5 seconds) and it misses when accelerating in the first 5-10 seconds (until I reach 20+ mph). BUt - if I accelerate slowly, keep rpm under 2000 - it may go very smoothly and gain speed OK, but if I floor it - it doesn't respond very well, gains speed slowly and is very rough (shaking a bit). Also, during the winter time (when it was cold) it was less pronounced than when its warm (now). And wet weather (rain) seems to aggravate the condition.
The only thing I didn't replace is O2 sensor. Should I replace it also before I go any further? Or does anyone have other suggestions?

Thanks in advance!

I hope this does not come across as disrespectful.

It is real easy to fall into the trap of "shotgun troubleshooting." This is what I call it when you just start throwing parts at the problem.

In about 99 out of 100 situations like yours, you would be money and frustration level ahead by taking the car to a competent and properly equipped troubleshooter and PAY him to diagnose and repair.

This is especially true for a car with expensive parts such as this.

I realize that it is sometimes difficult to find a competent troubleshooter, but I expect that in your case you would have saved money even at dealership labor rates.

Good luck,

Larry Bible is right on the money here.
The symptoms you describe, would make it a good candidate for , at least, a scope test. ;)

Hi Arthur,

I had problems with the idle being rough on my 1984 500 SEL. That was significantly improved when i got the cam timing right. For me that meant replacing the timing chain, the tensioner and the timing "shoes".

Have You checked this possibility?

Best regards,

Mikael Westerberg
1984 500 SEL
Vanersborg, Sweden

Larrys right on take it to the dealer or good shop. It can be a hard lesson guessing
GoodLuck

Guys,

Thanks for the responses. Let me tell what led to my "shotgun approach" - I took the car to 2 dealers in my area (HBL, Inc and Euro Motorcars) - they've told me to replace the harness. After I did that I went back to them - the list of "needed replacement parts" grew. Good thing I was ordering and replacing myself - otherwise I would be looking at $5k+ bill by now. And couldn't find any competent independents in the area as well. So - wanna do something right, do it yourself. As for the chain and timing, I doubt its thats - about a year and a half ago I had the chain and guides replaced by a mechanic, the car was fine for about a year after that...

Maybe I got the wrong impression but it sounds as if you are expecting the shop to diagnose and then you want to buy and replace the part yourself. There are few shops that can operate in this mode.

If they are diagnosing and replacing parts themselves, then a part that does not fix the problem can somehow be adjusted in the bill.

The shops are not in business to hand out free or low priced diagnostic information. They are in business to repair your car and make enough profit to stay in business.

Had the responsibility to "fix the problem" been put in the hands of the shop, they would in all liklihood have it done and the final bill would have been less than what you have spent so far.

I am all for DIY repair for those who have the tools, equipment and time, but there are just some situations where you are better off "handing it over."

The difficulty for me, and it seems for you as well, is knowing WHEN that time is.

Not all shops operate in the same manner. I agree that it is difficult to find a competent and fair shop. If I only lived in Gainesville, Florida I'm sure my hands would be cleaner with many fewer cuts and scabs. But once you FIND a good competent shop that you can trust, you need to work with them so that you can get a fair price AND they can make a profit.

Good luck,