280c engine miss - PeachParts Mercedes-Benz Forum

PaVarrieur · #1 01-10-2004, 03:54 PM

My 1974 280c has developed an engine miss. It is present at Idle and continues throughout the engines rev. range. The points, distributor cap, rotor, plug wires, plugs, and ignition coil have all been replaced about 4,000 miles ago. It has also become difficult to start when hot.
Could this be the transitorised ignition box?

Any suggestions

Thanks in advancer

jjl · #2 01-10-2004, 07:27 PM

I suggest removing each plug lead in turn while the engine is running to see of there is any change in running. Use very well insulated grips. If there is no change in engine turnover then there is an ignition problem with that cylinder/lead.

If this fails, a compression test to see if there are sticking valves etc. is in order. A compression gauge is cheap, probably about 20$.

The transistorised boxes very rarely fail.

good luck - jack

PaVarrieur · #3 01-11-2004, 05:22 AM

I will try that, however, as I have said, the ignition parts are fairly new. As the miss is intermittent, I'm not sure if this will reveal the guilty party. All of the spark plugs appear to be working properly with good color, no fuel/oil fouling. The car does burn a little oil, a puff after sitting at a stop light. More than likely dried/worn valve seals causing this. Valves were adjusted 10,000 miles ago. Very frustrating as after the tune up it ran like a watch, very quiet at tick over etc.

psfred · #4 01-12-2004, 02:12 PM

Transistor boxes usually either work or don't, they rarely go intermittant.

However, a bad coil can cause intermittant miss, as can corroded wires on the primary side. Check for carbon tracks on the coil, and rusty wires on the ignition resistors.

Peter

PaVarrieur · #5 01-13-2004, 10:14 AM

The coil was recently replaced (this summer). It was done by an independent shop. I seem to rememberthat a Bosch "Blue" coil is the recommended replacement. I wonder if they put an incorrect coil in and now it is failing. This is the same shop that recently odered 3 master cylinders before they got the correct one. Anyone know if I'm right about the coil?

ctaylor738 · #6 01-13-2004, 11:52 AM

Blue coil is definitely the one to use. But it's hard to see how the coil could cause an intermittent miss.

The hard hot start makes me think you have a carb problem or vacuum leak, and maybe a weak plug or plug wire.

BTW - is it a constant miss like a single cylinder, or that it runs rough on all cylinders?

Tomguy · #7 01-13-2004, 11:29 PM

You MUST bypass the ballast resistors!
Just connect a short line to the ends of the resistor. Just to make sure though, measure the coil's resistance. I have a "Bosch Super Blue" coil with ~3ohms resistance, stock is about ~0.8-1ohm on my coil. Not bypassing it will cause a weak spark, missing (especially under load), difficult starting, etc. If your coil is ~2ohms or more in resistance, bypass the ballast resistors! (stock is 2ohms - 1 for coil, 1 for resistors. One is bypassed during startup I believe, but bypassing both will solve the problems I believe).

PaVarrieur · #8 01-14-2004, 07:30 AM

Well I'm confused here.
If the problem is a coil providing 2 or more ohms resistence, wouldn't it have starting missing the first time the car was started when the new coil was installed? As I have said, the coil in the car is about 4 months old. The old coil was replaced when the car would suddenly go dead at stoplights and then not start until it sat for two or three hours. I finally found that it had a weak coil only putting out .07 ohms. After the new coil was installed the car has run like a watch, nice even idle, almost silent at tick over. I will measure the resistence. Are you saying this coil has gone bad because it was connected to the balast resisters?
If this is so, where are they and do I just jump them? I am going to replace all the spark plugs as well

ctaylor738 · #9 01-14-2004, 08:01 AM

WADR to Tomguy, you should find out what coil you have before you change the voltage to it. If you have something other than stock, you may need to bypass one of the resistors, but don't do it with a coil designed for low voltage.

Also, if you up the voltage, you may start frying points. The whole point of the low voltage arrangement and the transistor box was to lengthen point life.

You can pull the coil wire out of the distributor and hold it about an inch from a ground and watch the spark while someone cranks the engine. If it is a nice blue-white, your coil and low voltage supply are probably OK.

Did you ever determine if it is a dead constant miss on one cylinder or a general stumbling? This sounds like 4A1 mischief to me.

PaVarrieur · #10 01-14-2004, 09:51 AM

No I haven't determined if it is a bad / plug etc. the wires are brand new, the cap and rotor and points are brand new. I will change the plugs today and do the remove lead by lead first to determine if it is one specific plug or not. The dreaded 4a1 carb on this car has always worked great to the contrary of most. The miss is at idle as well as at speed, so I really think it is electrical.

PaVarrieur · #11 01-14-2004, 12:20 PM

Ok
Worked on the 280 this morning. Here is what I found. All Spark plugs, leads checked out fine, but I replaced the plugs anyway. Still had the miss. Removed and cleaned / filed the point set, reinstalled, still had the miss. I noticed that when I started the car, it acted like the timing was too far in advance, as if the plugs were firing way too soon before pistons wre at the top of the travel. Attached the timing light, and the timing was off the scale. way too far in advance. I have set it at 0/0 for now, it seems to run best there (I know that it not correct). Also, the coil is only putting out .004 ohms!
So now, How could the timing have changed so dramatically, and why is the car running at all with only.004 ohms available at output of the coil.
I'm Stumped.

psfred · #12 01-14-2004, 02:16 PM

You cannot measure coil output in ohms, only the internal resistance. Normal coil output will fry a volt/ohm meter, and maybe you, too!

I'd set the timing at about 7 BTDC with no vac, 0 is probably too late unless you have vac lines connected to the distributor fo the ide retard.

Toss the points -- if you have a transistor swtichbox, you've ruined them (they are platinum plated), and if not, it's not good to file and re-use points, they never seat together correctly. Chances are you have a transistor unit.

Timing could be way off because the vac lines are switched -- advance instead of retart at idle, or the mechanical advance (if you have one) got stuck.

Peter

ctaylor738 · #13 01-14-2004, 02:57 PM

Filing the points would have increased the gap and advanced the timing.

Tomguy · #14 01-14-2004, 08:13 PM

PaVarrieur: To measure coil resistance: With the engine/ignition off, place the red (+) terminal to terminal 15 (+) on the coil, and black (-) to termilal 1 (-) on the coil - not the output, but the input resistance, is important to decide whether or not to bypass resistors!

PaVarrieur · #15 01-14-2004, 08:14 PM

The Timing was set with the vacuum lines disconnected. The points were filed only to remove the "stalagtite" looking growth wich was on the base side. The point gap was reset. The timing was way off before any of the work was done to the points. They are mating together flat.
I know that the timing should be 8 btdc, but the engine does not run very well at this setting, it is not missing as bad, but it is still missing at that timing. At TDC the car runs great. Very smooth Idle and good power, no missing!. I will have to pull off the cam covers, this sounds like a jumped timing chain.
The switched vacuum lines also intrigues me.
Thanks for the input guys.