bad coils
 

can a coil be bad and not show a code? 2 how can I test the coil to compare with another one?

Which car? When I had a bad coil I was running on 3 cylinders and it threw a misfire code immediately. What is the symptom?

My C280 had a bad coil pack 2 years ago. The symptom was a shaky engine at idle.
As if the motor mounts were bad.

It did set off a code.

No I'm wondering could a coil be fine when theres hardly no load on the engine (m104).Then fail with a load.

Things like a coil, electric motor, alternator, etc. are typically going to have a problem or not have a problem as they heat up / cool down. Thermal expansion / contraction.

I replaced all three of my coils caused one made a miss in cylinder number two at idle.So rather than having old coils and one new I replaced all three.I bought them new from Bosch in the box from a closed down auto parts store Western Auto.I do imagine them being on the shelf for years as the price was 3 for $157.Could they have rot internally.I've tested them and they look good on a machine,but I don't know if they can fail under load.

So this is the S320?

I also changed plug wires will test them if it ever gets to 40 degrees. yes 99 s320

As was said, electrical things are often temperature sensitive. As far as load, the coil really doesn't "perceive" the engine load. A lower voltage to the coil would cause a lower spark voltage out. However there is no suggestion that you have any battery voltage problem. A coil technically would "like" a slower RPM more than a higher RPM since it has more time to build up the magnetic field in the coil before it has to make a spark voltage out of it. However, it is designed to operate at the maximum RPM of the engine and I assume you are not running anywhere near that speed. Since you are not throwing a code I am wondering what the symptom is? The computer doesn't think there is a problem. If you were throwing a code then you could swap around the coils and see if the code moved. On my engine (M111) there are "dogbone" connectors under the coils which are often suspected of causing trouble. These constitute the wires in those locations. I don't know about the M104.

the car behaves like its hitting the limiter above 3000 rpms.other problem at certain temp engine bounces rpm going down the road with no load

Hi,
many components within the engine compartment are sensitive to temperature changes as mentioned before. However, the manufacturer designs, or have them designed and made, with these temperature changes in mind. There is always a temperature range involved.

I have a hard time to believe that a coil pack fouled on the shelf. The coils are sealed in a potting compound. Unless exposed to extreme heat, the potting compound will not deteriate and should be easy to inspect visually. If it is the same type of coil that was replaced, you can probably check them off.

Is the car acting the same way as it was with replaced coils?

A sparkplug can collaps under high pressure. This means that what used to be a strong spark, could now be a mere glowing. At this condition, it also changes its impedance to much lower values. That in turn can cause the coil to burn out, if not taken care of the problem.
Make sure your sparkplugs have been changed on schedule and that they are in fact the right type for your model.

With an OBDII vehicle it should show a code somewhere near the coil or ignition page.
However, the code could not be a direct result of a misfire, it could be triggered by something else.
It depends on how smart they made the firmware. For example: Several reading on an O2 sensor may indicate rapid changes in O2 content. If the firmware was smart, it would report a misfire.

The plug wires, if new or did they sit in the shelf for some time?

The rubber insulation can get porous by just lying around. The insulation factor would degrade, even though it may still have it's original resistance. In this case, you could have some of the energy meant to be at the spark plug electrode, lose before it reaches the plug.
This would typically happen at higher RPM / Load because the average voltage at the coil is higher.

To test the coils on the bench, you would need the electrical specification from Bosch and recreate a low voltage input pulse and measure the High voltage output pulse amplitude. WARNING! the output is at several thousand volts (15,000 - 35,000 Volt) with a good working coil.

Hope, I gave some new ideas.
Cheers,
Norbert

yes soon as it gets warm I will test wires,they are a Karlyn brand

That fact that it doesn't throw a code might be a clue that it isn't ignition related. Maybe it is fuel related?

maybe ?
 

Your car seems to be 2 years outside the danger zone...


http://www.peachparts.com/shopforum/general-information/128181-mb-wiring-harness-failure.html

300d mines a 99 no wiring issuses like the v8 model,with one wire problem.Can't be fuel related passed fuel pressure tests.I know that it is something smiple,that is being over looked.I went after spark,fuel,and air.Problem started when I double dosed car with Marvel Mystery Oil,for a miss,that turned out to be a coil.Replaced all coils.all wires 4 sets of plugs.As the MMO made my plugs foul.Drain fuel tank refueled.However I will go back over everything.Vac gauge at 19psi,was at 14,found leaks.Need to by pass charcoal canister too,might be problem