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Outstanding Contributor
1989 560SEC, 1989 560SEL, 1995 E420
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1990 Mercedes-Benz 300SE (W126) In Astral Silver with Dark Blue Leather Interior
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173 Posts
The only driveability issues I’ve had with my C126s have been intermittent hard start cold, misfiring at idle, and stumbling at times on acceleration.

In 2004 this was some corroded/loose spark plug suppressors found when I replaced the plugs & wires. Easy Peasy!

In 2017, my newly acquired 500SEC started having intermittent idle misfires and I found loose corroded plug suppressors. I changed out the plugs and for reason or other just swapped the plug wire ends one car to another. Both cars ran fine!

Until yesterday 2021, my 560 started the intermittent idling misfire and stumbled on acceleration at times

De ja vu all over again!

Yep I found some corroded and loosely attatched plug ends… again.

Today I learned to screw the connector onto the plug as I was pushing it home… sounds nuts yes, but in two cases, for whatever reason, the connector clicked down onto the plugs another quarter inch!

The car starts and once again starts & runs smoothly! Yay!

A simple fix.

It’s in the drive for it’s outside damp cold start tomorrow morning. I bet its

M
View attachment 2716800 View attachment 2716801


On both, that’s dielectric grease from the last time & I might have been more careful really.
Won’t happen again.
Dielectric grease is one of the best tips for ease of installation & removal of M/B HT Leads on both the distributor & the plug ends.
I reckon a good 25% of the "Misfire" issues posted about w126's on this forum could be solved by new leads... All it takes is trying to pull a boot off a plug from a little high up ONCE & the lead becomes disconnected between the boot & the cable subsequently causing intermittent misfire. Usually they'll seem fine while under load because the RPM's are fast enough to not notice the miss & the current from the coil is firing strong enough to jump the break in the line but becomes very noticeable at idle or at low RPM's.
First thing I do with every M/B I've owned is all new: HT Leads (with dialectic on both ends), Plugs, Coil, Distributor & Rotor Button. They invariably run substantially better for it.
 

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1985 MB 500sel (Euro)
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911 Posts
I appreciate all the good tips that are on the W126 forum like the above and usually print them out (like this one) or make notes of the names of recommended products (like the water pumps on today's Time for a new water pump thread. Thanks all, or as my new Swedish mechanic would say "Tak."
"
 
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1987 & 1991 C126
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5,820 Posts
Dielectric grease is one of the best tips for ease of installation & removal of M/B HT Leads on both the distributor & the plug ends.
I reckon a good 25% of the "Misfire" issues posted about w126's on this forum could be solved by new leads... All it takes is trying to pull a boot off a plug from a little high up ONCE & the lead becomes disconnected between the boot & the cable subsequently causing intermittent misfire. Usually they'll seem fine while under load because the RPM's are fast enough to not notice the miss & the current from the coil is firing strong enough to jump the break in the line but becomes very noticeable at idle or at low RPM's.
First thing I do with every M/B I've owned is all new: HT Leads (with dialectic on both ends), Plugs, Coil, Distributor & Rotor Button. They invariably run substantially better for it.
You are SO right there!

That is the very first thing I did to the all new to me 560 that spring.

Yesterday, I was waxing on about turning the connectors to the right as I pushed them into place.

Well today, I can tell everyone dielectric grease, a little, on the rubber sleeve inside the MB plug connector makes ALL the difference!

Literally night & day as to results generated.

Over & Out!

MBL
 

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1990 Mercedes-Benz 300SE (W126) In Astral Silver with Dark Blue Leather Interior
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173 Posts
You are SO right there!

That is the very first thing I did to the all new to me 560 that spring.

Yesterday, I was waxing on about turning the connectors to the right as I pushed them into place.

Well today, I can tell everyone dielectric grease, a little, on the rubber sleeve inside the MB plug connector makes ALL the difference!

Literally night & day as to results generated.

Over & Out!

MBL
....and right you were about the "twist" that makes the difference.
Both getting leads on and off but there's nothing more reassuring than hearing/feeling that "click" after the twist that ensures the lead is securely connected to the plug.
I think it's very easy to think "well it's very high voltage, surely if there's 1mm gap it won't make much difference" but Mercedes tuned these machines to the enth degree, working under the assumption of all components being properly installed. So you add a gap at the plug to a torn boot on the distributor end & an EHA that's contacts are "glazed" in petrol film... Of course she ain't gonna be running too well!!
 

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1987 & 1991 C126
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5,820 Posts
....and right you were about the "twist" that makes the difference.
Both getting leads on and off but there's nothing more reassuring than hearing/feeling that "click" after the twist that ensures the lead is securely connected to the plug.
I think it's very easy to think "well it's very high voltage, surely if there's 1mm gap it won't make much difference" but Mercedes tuned these machines to the enth degree, working under the assumption of all components being properly installed. So you add a gap at the plug to a torn boot on the distributor end & an EHA that's contacts are "glazed" in petrol film... Of course she ain't gonna be running too well!!
I did the cool & foggy morning road test yesterday and that 560 has not been so smooth in recent memory, at least 4 years.

Encouraged, I collected the 500SEC from its slumbers and went to do the same cleaning & tweakage to its plug leads as well.
Can't be too, too sure right?

All four of the leads I removed already had the bulb grease in the connectors, and all of those were bright yellow brass too!
I guess I had done that already four years ago when when I swapped out those plug connectors.

I ended up lubing the throttle linkages before reinstalling the air filter, quite pleased....

M
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1987 & 1991 C126
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5,820 Posts
Well, then there is today’s feature…..

For the last daze, I’ve been searching for my 500SEC key wallet. It’s the one I’ve always used and became missing. I’ve really looked everywhere here in the house, in three cars, under two of them etc…..

I went back to the warehouse to look again and install a battery cutoff as a theft deterrent.

I got my pants dirty looking under both 126s with the strong light….

I had forgotten the battery cable connector was a 12mm, not a 13……. Etc.

As I headed for home, I looked to make sure the warehouse security light was on….and look what I found on the switch array.

My key!

Someone found it & placed it where I would likely see it!

Thank you Thank you…..

It was found outside the building on the ground last night I’m told.

Can’t tell you how pleased & relieved I am.

MBL

Wood Grey Rectangle Door Art
 

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1990 Mercedes-Benz 300SE (W126) In Astral Silver with Dark Blue Leather Interior
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173 Posts
Well, then there is today’s feature…..

For the last daze, I’ve been searching for my 500SEC key wallet. It’s the one I’ve always used and became missing. I’ve really looked everywhere here in the house, in three cars, under two of them etc…..

I went back to the warehouse to look again and install a battery cutoff as a theft deterrent.

I got my pants dirty looking under both 126s with the strong light….

I had forgotten the battery cable connector was a 12mm, not a 13……. Etc.

As I headed for home, I looked to make sure the warehouse security light was on….and look what I found on the switch array.

My key!

Someone found it & placed it where I would likely see it!

Thank you Thank you…..

It was found outside the building on the ground last night I’m told.

Can’t tell you how pleased & relieved I am.

MBL

View attachment 2717391
Restores faith in humanity that kind of thing!
Far nicer than coming back to your car after shopping to find someone has run a trolley into it or carelessly opened their door into the side of it (of course leaving no note or information).
Whenever I park in parking lots, I park as far away as humanly possible from all other vehicles yet every time I come back to my car, there's always someone parked right along side!!
 

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'86 W123 200, OM617 non-turbo, bastard 5-speed; '95 W202 C250 Diesel, OM605 non-turbo, 5-spd man
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4,686 Posts
I installed a 2nd hand, working Hirschmann antenna yesterday, along with a new grommet. I will install the W124 antenna switch in place of the FAD button as well.

And all of this just to keep the rain out of my car.
 
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1988 300SEL 158K;1980 450SL 31K
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847 Posts
took down the headliner, gave it a good wash, installed W140 glass roof, dyed the vents and the roof handle, took off the vinyl from the old roof slider glued it to the W140 sunshade, and sewed the rear section of it as it shorter, till I find out a complete headliner (one piece) from the junk yard.
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Hood Vehicle Motor vehicle Automotive lighting Automotive design
 

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1988 300 SE 133K MILES
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44 Posts
This past weekend I replaced the fuel accumulator and hope that helps hot start issue. Changed trans fluid and filter, including draining torque converter. Attempted to change engine oil. but someone previous had rounded drain plug, and I could not remove even with vice grips. Have ordered an oil extractor pump, and will change oil that way for now. Yesterday I had exterior of car detailed and it looks great now. A month ago I buffed the car but I guess I just wasn't aggressive enough. The detail guy said he had to clean up a lot of oxidized paint.
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1987 Mercedes 560 SEL, 126.039, M117.968, 1986 Mercedes 560 SEC 126.045, M117.968
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175 Posts
Funny story with my 560SEC I recently acquired. I have been driving it and sorting issues as I enjoy the car. So far I have replaced the leaking cam cover gaskets, repaired the ABS issue for now (faulty control relay on pump but also discovered the front sensors are starting to fail), changed the oil, plugs, and air filter, diagnosed the cruise control issue (currently working on the faulty amp), ordered a C140 sunroof to repair my broken sunroof, changed over the A/C to R134a by removing and draining the mineral oil from the compressor and system, replacing the black O-rings with green, and installing a new dryer.
While driving the car I was smelling fuel at times. I was carrying a 5 gallon gas can I filled up the the station so I thought that was the issue so I dismissed it. Whenever I smelled it I would lift the hood and check the EHA and any visible signs. I also removed the vent line from the fuel pressure regulator nothing was ever found. So last week I really got a strong smell of gas while driving. Definitely from under the hood as the smell was coming from the HVAC. This time I removed the air filter assembly and verified the EHA decided it was time to die. Parts ordered and installed feeling a sense of relief I didn’t have a potential fire hazard I went on driving the car. The next day I started smelling gas again! So last night after getting home from a long drive off came the air filter assembly again. Fortunately the EHA was dry but what I did find was fuel bleeding out of the fuel pressure regulator into the breather hose. This stupid thing as been leaking intermittently all the while but it took that one time for me to discover it while it was misbehaving. Parts ordered and this is what I call the classic nickel and dime people go through with classic car ownership.
Now all I have to do is rebuild the sunroof, repair the idle issue (it’s not the control unit), replace the drivers seat base and see can repair the orthopedic seat function, and get some paint work done on the drivers side.
 

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'87 560 SEC, Pearl Grey/blue; 300,000+ mi; '07 CLS 550, Barolo/stone; 90,000+ mi
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1,126 Posts
I was able to successfully get the wiper fluid reservoir aside (I still can't get a good grip on the pumps to pull them off, I'm not sure what I'll do). I'm so far surprised and thankful to see no rust around the dreaded sway bar upper mount and bracket (I ordered new ones beforehand but these look suprisingly ok). It may be another story when I put on long rubber gloves up to my elbows to deal with what looks like a rat nest on the other side of it. The battery tray looks typical, but we've seen worse (some have chunks missing). The only body rust I see so far is where it developed under the battery tray "leg" (last photo). Automotive tire Motor vehicle Electrical wiring Automotive exterior Twig
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1988 300SEL 158K;1980 450SL 31K
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847 Posts
Did you take the matte wrap off (or was it matte gold paint), or is it someone else's car?
I took the mat wrap out, took apart the car from outside, send it to the body shop sanded to bare metal, installed Lorinser body kit (rear and front euro bumpers and side skirts) ,euro headlights, wipers and motors , Rial staggered mesh wheels, W140 glass roof. this project took 4 months. converting US W126 to euro, I even installed the headlights adjustable knob with all original tubes. euro front turning amber signals. I bought euro Lorinzer car (rusted out and bad engine) and swapped everything to my US car. the kit was in a very bad shape, the front lip broken into 3 separate pieces, the rear bumper was all cracked and you can see through it from the left and right corners. all fixed and repainted
 

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