Regapped plugs to .039 to try to smooth out rough idle.
Now the car runs rougher and has less power. Does anyone
know why this is???? Any ideas???

Gary

Gary,

I think your ignition systems is not strong enough to jump that large of a gap. This may be due to a number of things, worn wires, cap/rotor, coil.

Try regapping them to .9mm (.035in??) and see if that is any better, if not, then back down to .8mm.

What year/model is the car? How many miles? What are the age/condition of the other secondary ignition parts? From other posts, 1mm is possible and is recommended for smoother idle, but that is taking into consideration that the other components are new or in good working order.

Hope this helps.

-David

Oooops!

I guess I should read the sigs better. I asked questions with the answers right in front of me.

So verify that the components are in good working order. If the coil is original, it may be worn enough that it is having a problem getting a spark to jump the gap or it is a weak spark.

-David

It has original coil. Regapping back to .032 makes it
run better. It will not accellerate smoothly after
3800 rpm. It hits a flat spot at 4000 rpm. Does this sound like a failing coil under load?? Cap and wires and rotor
have less than 30,000 miles on them. No really bad pitting
on cap and rotor and no carbon arcing present. Plugs less
than 2000 miles on them.

Gary

Gary,

Since it runs better with the .8mm gap, it does point to an ignition issue with the larger gapping. But there is more to look at.

The poor performance above 3800 RPM maybe ignition, fuel, or air intake related. It could be the coil failing under load, but it may also be the fuel delivery system not able to supply enough fuel. It may be restricted air flow into the engine.

I'd look at the easy/cheap stuff first. When is the last time you replaced your fuel filter? How about the air filter? See, easy and cheap. :-)

I just replaced my wife's 89 300e coil with an Accel Super coil. Ran much better. See my post regarding that. So if the fuel filter and air filter have been recently changed, the coil would be my next stop. Simple R&R proceedure.

If you still have problems, new cap/rotor..... then fuel pump.

Hope this helps. You may want to see what the "professional" have to say.

Good luck!

-David

David,
What kind of wires are you using with your Accel Coil?
I was under the impression that any coil other than the
OEM would ruin the ignition system. Is this untrue??? Can you use other wires than stock??
I thought the stock wires were required due
to the resistors in the tips.

Gary

P.S. Air cleaner and gas filter has been changed less
than 1,000 miles ago.

Install a new O/E coil and you should be fine...I would never replace the wires since they are solid core. I only see the resistor ends fail from time to time. You may need a new cap and rotor while there.

We carry all of these in fast lane.

__________________
Benzmac:
Donnie Drummonds
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS

Gary,

I am running OE wires. I modified the existing coil wire to accept the "tower" connector on the Accel coil. Easy modification.

The Accel coil is designed to be used with computer controlled ignitions. I have had it in for about a month or so and no problems what so ever.

If there are any technical reasons why a person should stick with the Bosch OE coil, then I would like someone to explaine why.

Good Luck!

-David

Well,
Any takers on the answer to this from David.

"If there are any technical reasons why a person should stick with the Bosch OE coil, then I would like someone to explaine why."

I too would like to know??????

Gary

Gary,

Another member took my advice and just upgraded his coil to the Accel coil.

Check out his post:
http://www.peachparts.com/shopforum/showthread.php?threadid=15216

Might be worth a shot.

-David

Well there are plenty of reasons for using what the system was designed to use. Saying that any one coil can work in numerous installations is sort of like saying we should make all shoes size 12 cause they would fit me and just about everyone else, even if they might be a little loose.

Specifically the design of a coil ignition circuit depends upon the inductance and resistance of the coil to allow the control circuit to do a good job at all conditions for a long time. The current that flows through a coil changes from coil to coil and in many systems is either limited by the control unit or is anticipated by the control unit. If you use a coil that needs 8 amps to make full output secondary and your control unit limits at 6 amps then that hot coil doesn't even warm up. If the control unit is an early unit without limitation and the coil draws 50% more current, the control unit won't last long carrying the extra load.

Control units have specific circuitry that handles voltage spikes caused in open circuit conditions. This spike voltage is also dependent on coil construction. If you were an engineer you would express the RLC circuit with Laplace Tranforms a variation of a differential equation. Do you suppose the variables in that equation don't matter.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Steve,

Thanks for the excellent technical reply.

So what are the specs for the ignition control unit for a 89 300e? Does is only allow output 6 amps? If it is older unlimited unit, what is the max current draw it can handle without being overloaded?

I guess I could have the ignition scoped out to see what the output voltage to each of the plugs are. If they are at or above the 35kV level, then the coil is getting charged correctly. I just need to make sure that it is not drawing too much current IF my control unit is the unlimited type.

From the results I got along with what the other member experienced, (smoother idle, pulls stronger, etc) there is something different with the spark being generated at idle, bigger and stronger. Since the engine does not miss at the higher RPMs, then the coil is also delivering a spark with more current and energy in the upper range as well.

So as long as I am not drawing too much current from the control module, then I should be OK.... right?

-David

David,
If you get it scoped out, please post the results.

Thanks,

Gary

The real problem with getting into re-engineering systems is that many of the design criteria are not published (often trade secrets).

To give an example: I think the 300e is not current limited and I think they use about 6-7 amps. I have "current ramped" a number of MB coils but don't remember which are which. Current ramping is the act of using an inductive current probe with a high resolution DSO (digital storage oscilliscope). Each event can be captured and analyzed. The current starts at zero and either peaks and holds (current limitation - looks like a ramp to a maximum and then a horizonatal to the point of ignition where an almost vertical drop to zero occurs) or rises unlimited to a value regulated by the components of the system.

One thing that can't be determined is which components are doing the regulating in the last senario. Modern controllers are so good that instead of limiting current they plan on starting the event at the appropriate time so that the coil gets saturated at the precise moment of ignition (controller opens the circuit) In this case the ramp gets to 6-7 amps because the ramp slope and time used were part of the calculation. If you change the design of the coil the slope of the ramp changes throwing these precise calculations for a loop.

Basically I am saying that I don't know the specs. I know where to find them if I suspect a deficiency in operation. I, like most driveability techs, look at the effects with quality testing eqiptment, but very seldom get into the redesign of the system. I know the guys that designed it know way more than I do (especially in MBs case). They also have more time than I do. Real design changes that work take lots of engineering. Figuring how the system works exactly, is probably the first step.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician