My 1979 280SE with 139k miles has developed a rather severe but intermittent miss, especially at idle but it's getting worse and happening at speed now. I suspect ignition because the missing cylinder(s) cut on and off quite abruptly. I have replaced cap, rotor, plug wires, plugs (and injectors last year).

I cast a suspicious eye on the magnetic pickup (it's the Euro model M110 with the breakerless ignition) since that's what always failed in my Fiat and I would have replaced it but I can't find a replacement for it. When it runs on all cylinders it runs great except for a touch of rough idle which it's always had (vastly improved though now) and I think is caused by a dirty idle bypass.

Anyway, it misses at idle on one, sometimes two cylinders that will suddenly start firing and then stop, and today on the freeway it suddenly became very sluggish and spat a cloud of black smoke and then was OK again. Something else - it started after I replaced the plug wires (old ones were hard and cracked) but they are the good Bosch wires and were rather expensive so I hope that's not it I didn't do anything unusual with the plug wires like yanking on them by the wire. The little plastic comb that holds them broke off the valve cover (still holds them all together though) and it vibrates against the air cleaner but I checked and don't see any chafing. I haven't tested the coil resistance.

Any thoughts? I will have to take it to the shop Monday if I can't fix it this weekend.

Thanks!
Josh

Your "cloud of black smoke" is a clue. Pull the new plugs and inspect them. Could be gas fouled - sooted up, which will short them out. If so, it might be possible to clean them, otherwise, you'll need another set. A temporary fix might be a set of hotter plugs. You'll probably have to figure out what's causing your over-rich condition.

Happy Motoring, Mark

__________________
DrDKW

I did that already. They were grayish tan, no fouling. I think the cloud of smoke was from a cylinder not firing at speed.

Second clue - from your description " it started after I replaced the plug wires". If the missing at idle is consistent - that is missing from the same one or two cylinders, it's probably not the electronic ignition pickup.
Test the resistance of the new wires. (It's possible you got some defective ones)
The readings should be pretty close to each other and close to what's marked on the wire's distributor and sparkplug connectors. IF you still have the old wires, it would be a good idea to compare the resistance markings with those on the new ones. (It might even be a good idea to reinstall the old wires just to see if the problems go away)
Sometimes, replacement wires will have a higher resistance than the originals. If your coil is marginal, that can cause arcing as the higher firing voltage created by the increased resistance, causes the spark to seek a path elsewhere.

__________________
DrDKW

OK that makes excellent sense. Too bad I tossed the old plugs. I'll go test.

Still have the old wires, or did you toss them too?

Happy Motoring, Mark

__________________
DrDKW

What I've figured out so far:

1. I have the TSZ 4 breakerless transistorized ignition (pre-resistors, yellow dot on "switching unit").
2. I found the relevant section in the 116 manual and went through the tests - voltage is good, but while testing the big resistors I touched the metal bracket on the lower one and it was blazing hot, far too hot to touch. i tested them but the values were a little high (.7 and .9 respectively for the 4. and .6 resistors). The connections are a bit corroded so maybe my scraping wasn't adequate or my cheap multimeter just isn't accurate, probably more likely.
3. At first the coil resistance was in spec but after I turned on the key for #2 above and then turned it off again, I tested the coil again and found that the primary winding was at 1.5 ohms, well above the spec of 0.33-0.46. It went down a bit to 1.1 but stayed there.

SO does this mean the coil is going? and if so, would that cause the pre-resistor to get so hot?

Thanks!

I dont know how the later switch units work, but if you leave the ignition on and engine off while the points are closed on the older transistorized units, it will send voltage to the coil the entire time the ignition is in the "Run" position. It WILL cause ballast and/or ignition box failure. NEVER test the ignition system with the engine off but the ignition in the run position!

__________________
Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k

The manual for the breakerless ignitions say to test voltage with the engine off but the ignition on. At least I think it does, I'll go check to make sure.

For testing voltage to the coil, yes, but not for testing resistance. Remember, the reisistance will vary with the voltage/current going through it. To test voltage to the coil it probably says to disconnect the lead from the coil as well? The coil is a variable, a bad one will have different resistance which will vary voltage readings.

__________________
Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k

I installed a new coil and also got a new plug wire clip - the old one had broken off the valve cover and the wires were getting crammed behind the air cleaner. I pulled them out before I did anything and the miss went away. I installed the clip and the coil and the miss did not return, and the car is running a little more smoothly and seems more responsive although that may just be wishful thinking. I think it's either running a bit lean now or there is a small vacuum leak as it is now consistently just a tad hard to start (actually sort of coughs to life, firing on a cylinder or two, then a couple more, then smooths out within a few seconds) when cold and there is still a little stumbling in the idle. Could be fuel pressure too maybe.

(There were a lot of bad adjustments made by idiot mechanics trying to fix rough running, no power etc. that I am still sorting out).

Now the radiator and AC have both sprung a leak, arghhhhh!

I have a '78 280E with the M110 US engine as my daily driver. I have been living with your symptoms for several months. Last Monday, after sitting idle over the weekend, she just struggled to start, then ran badly on 2-5 cylnders for 5-7 miles. I left her at a local indie for the day for some professional diagnostics.

Prior to this I had replaced all the ignition parts from plugs to coil, including the wire holders, and injectors and their related parts, timing is good. I even sprayed my new wires with a marine waterproofing agent.

The only thing I did before taking it to the indie, was give the mixture screw on the fuel distributor a quarter turn to lean out the mixture. At that moment, it effected no improvement.

When I returned to the indie after work on Monday, the car had not been looked at due to short notice. I drove the car home, hoping I'd make it and check on it myself.

The car has been running much better since its failure on Monday. It was running REALLY rough at idle, now less so. At speed it is running like a youth, smooth as silk. And now, only the occasional miss fire.

Tomorrow, I'm going back to the fuel distributor and the idle adjust screw to see if there is some fine tuning to be done.

I also have a hot start problem, which may or may not be related. I'll re-cap on Monday.

__________________
'78 W123 280E, 58,000 mi!
'97 Mazda Miata
'94 Isuzu Trooper, 4WD
'03 MB C240 4-matic Wgn - Wife's
From the Archives:
'73 2002
'68 TR-250
'67 Austin Cooper "S"
'59 Austin Seven (Mini)
various p'up trucks