M103 wires fall off spark plugs
 

I was wondering why my recently acquired [and recently tuned up] 1989 300E was idling poorly and had next to no power [driving around the block was an expedition].

Over the past few weekends, I've checked for vacuum leaks, electrical gremlins [the infamous OVP relay] and not come up with anything obvious.

I decided to look at the spark plugs - and I discovered the main reason the engine seemed to be running on two cylinders is that is was. Four of the leads had wiggled loose, and could be pushed in quite a distance before feeling the threads on the plugs engage in the sockets.

The car runs much better on six cylinders, but after a short test ride [30 miles or so - the odometer is strictly decorative] I noticed the idle was rougher, and there was hesitation starting off from a stop. After the engine cooled, I checked and the number 2 and 4 wires were loose again.

I think they have been abused
 

Ignition wires don't last forever and have even shorter life spans if they get abused. I would say that you are due for a new set of wires, there is no sense in trying to get more life out of them. After you install the new wires, always treat them very gently, (and try to fuss with them as little as possible) When you do have to pull them off a spark plug, do it very gently. There is a special tool available which makes removing the lead from the plug easy and is gentle on the lead at the same time.

The trouble here is that they are new wires [new plugs, wires, coil & rotor]. I don't know who the manufacturer was - need to check the invoice from the shop. [and next time do work like this myself].

OK, then how about this
 

If they are new, then I would look at them very carefully and see if perhaps with a needle nose pliers, see if you can get into that rubber cup that slips over the plug and try to pinch the metal end to fit the plug tighter. The connector at the end of the wire would have to be bent or broken.

dlevitt, what do you mean by "before feeling the threads on the plugs engage in the sockets"? The threaded caps on Bosch spark plugs should remain in place. If the previous owner removed the caps no wonder the ignition wires come loose. If that's the case I recommend replacing the spark plugs. Use dielectric grease on the rubber to facilitate removal from the spark plug porcelain.

The sockets on the wires should "click" firmly into the plug ends. If you don't get the click, they're not on securely. Put some dielectric grease on either the wire ends or the plugs, then push them firmly on. Then, just to check, pull them off and see how easily they come off. (Be careful not to torque them too much, that'll crack them too.)

Yes, the metal end-caps on the plugs will sometimes unscrew.
These are what the plug wires grip to.
Pull the offending plug and be sure the metal ends are still tightly screwed on (they may have come unscrewed and are in the plug wire boot).
Use pliers to tighten them firmly onto the plug.

DG

I don't believe we are talking about the same kink of plug ends for the wires.

The style of connector shown at this link has the high voltage connector buried deep inside [not like the simple rubber boot for normal wires]. There is no way to simply crimp them without extensive dissasembly - since this may be a manufacturing defect, I only want an exchange at this point [and since the mechanic was the 'customer' for the wires, I can't just take them back to AutoZone]

All the plugs on the engine just have the threads exposed on their tops - none of them have the adapter that 'normal' spark plug wires use.

dlevitt, obviously the threaded cap/top has loosened and come off each spark plug. Remove the cap from inside each ignition wire boot and either securely attach/tighten one to each existing spark plug, or replace all the spark plugs. As mentioned, each ignition wire boot should "click" onto its respective spark plug for a secure connection. The pliable rubber boot of each ignition wire fits snugly over the spark plug porcelain and won't wiggle loose on its own.

I think I'll need to take a closer look at any identifying numbers on the connectors when I get home.

I found at page 12 of this pdf on the Beru web site there two different parts for the connector - one for the bare thread [4 mm] and one for SAE style spark plug tops.

So the ZLE 121 [0 300 062 102] style would attach directly to the threads, and the ZLE 135 [0 300 152 106] style requires the SAE style knob on top of the plug. Probably the mechanic didn't notice the mismatch.

The number on the connecter is 0 356 321 028 - trying to find something online ....

Pebkac
 

There's an old saying in tech support 'Problem Exists Between Keyboard And Chair'

This seems to be a simple case of PEBKAC.

The mechanic and owner of the shop I [mistakenly] had the 'tuneup' performed are certain that the plugs and wires connect without the SAE style knobs on the plugs. [but they keep falling off].

So today, I finally put new plugs in _with_ the SAE knobs, and the wires click into place.

The lesson for Long Islander's is to _avoid_ Country Automotive [Eastport NY], since when they make a simple mistake, they will continually blame the customer for the problem.

Next step, new wires [I don't believe they ever installed the wires that I paid for], new cap, rotor, 'suppressor' housing, valve cover gasket [minor leaks] & upper timing chain seal [classic leak from the description]