Bad coil causing hesitation? - PeachParts Mercedes-Benz Forum

Vaden85SEL · #1 05-22-2007, 06:50 PM

Hi,
I have an '85 500 SEL. It has always had a somewhat rough idle, and it used to have hesitation upon take off when the engine had come up to temp. After replacing plugs, wires, distributor cap, and all the stuff in a usual tune up, it still hesitated. So I had the vacuum system smoke tested and found that I needed to replace the injector cups and seals. I did, and the car ran great for about 3 months.
Just recently, I drove a 1200 mile cross country drive and it performed great the whole way. But about 2 days after the long drive, the hesitation and rough -er idle has popped up. I cleaned out and tested the actuator on the idle control valve and replaced the hoses connecting it as they were brittle and cracked. It didnt help. So I thought maybe it had to do with the O2 sensor. Replaced it and the car still hesitates. I checked the mixture and found it was a little rich and leaned it out a smidge.
I don't know what to do next! I have read on a few posts that sometimes the coil being bad has been found to be the culprit. Does this make sense? I mean, if the coil were bad, wouldn't it hesitate and idle roughly while warming up as well as when hot? My 500 SEL runs great while warming up, but when it is hot, the hesitation and rough idle come out.

Any help will be very appreciated!

brewtoo · #2 05-22-2007, 09:24 PM

For the performance of a coil to change from cold to hot is not unheard of.

Not saying that is your problem, of course.

raymr · #3 05-23-2007, 09:58 AM

Have you ever checked the fuel system pressure and control pressure?

david s poole · #4 05-23-2007, 11:30 AM

sounds like you need a rebuilt fuel distributor/and or a rebuilt warm up regulator.

Vaden85SEL · #5 05-23-2007, 02:33 PM

Ok. So I decided to remove the plugs and see if there is anything going on with them that I should know about. I had replaced them all a few months back when I replaced the injector cups and seals. Thank god I decided to check these. Cylinder #3's plug was covered in black soot. This leads to me to believe that the injector itself is leaking and dripping fuel into the cylyinder, which in turn, makes that cylinder rich thus setting off the O2 sensor which signals the fuel dist to lean up the whole mixture. I think this is what is causing my hesitation. I ordered a new injector and if the problem goes away on cylinder #3, then I will go ahead and replace all the injectors for preventative purposes.

What do you guys think?

raymr · #6 05-23-2007, 06:39 PM

Makes sense, especially if it runs OK cold before the O2 sensor kicks in.