[DBCoooper]

Hello.
I just picked up a seemingly very well kept 1999 SL500 that just started posting codes P0300 (random misfire), P0303 (cyl. 3 misfire) and P0305 (cyl. 5 misfire) after heavy or full throttle acceleration. The engine misfires only after heavy or full throttle application and will continue to misfire under all conditions until turned off. Once restarted the engine runs fine until the next very heavy or full throttle application.

The car has 73,000 miles. I assume original spark plugs, coils and wires. I have a fuel pressure gage and crank position sensor on the way.

I'm new to the MB world, so looking for a little guidance as to where to go with this?

Thanks for any information you may have.

Dan

 

[BoatWaco]

In my case when this happened and I got the P030x codes, the car ran rough all the time (not just after heavy acceleration) and CEL stayed lit. I replaced the coils on the affected cylinders, cleared codes and all was fine.


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[DBCoooper]

If it didn't run perfectly after shutting it off and restarting I'd be more inclined to think it's the coils. The fact that it runs perfectly after re-starting makes me think it's something else. That said, I've seen stranger things.

 

[sushipaste]

Swap coils to see if codes jump cylinders. New crank sensor wouldn’t hurt (hope you got Bosch)

 

[e-420]

Swap coils to see if codes jump cylinders. New crank sensor wouldn’t hurt (hope you got Bosch)

x2

 

[e-420]

Not sure if the v12 engine management is the same as the M119, but on the M119 if enough misfires are detected, the engine management shuts off fuel to the affected injectors, end result is rough running, Shut off the engine, turn it back on, runs smooth until enough misfires detected, then back to the rough running. Swap at least one coil pack to another cylinder. if P code follows, new coil packs. BTW on the M119 coil packs tend to start to fail around 100K miles, 15 years or so.

 

[DBCoooper]

Thanks all. I'll swap coils and see if it follows. Kind of strange that two coils started doing the same thing at the exact same time, but it does make sense to start with the easy stuff. Thanks again! Will report progress.

 

[RESTO SL]

Welcome,

IMO, I'd start with the MAF. If cleaning it makes a difference, then I'd replace it.

 

[ou1954]

I was lucky a while back to be able to buy 8 coils, with Wires after I had two fail and be replaced within a month or so.. I still have 6 good used coils and two new ones. I think I paid under $100 from some place in San Diego. I was surprised and happy to see they had wires,

 

[Photodude]

If you are only getting misfire codes, I would start with ignition components. In my experience bad MAF sensor (as an aging or dirty) would usually, in addition to misfire codes, trigger fuel trim codes (P0170-0173).


Given the milage and age of the car, I would replace spark plugs and wires. Beyond that I would not speculate without farther testing. Coils are either Bad or Good, so as mentioned above, you could isolate the bad one by swapping them and following the codes. After that I would really want to look at the diagnostic data rather than blindly throw money at the problem…


Here is a guide how to test both, fuel trim and misfire codes.

 

[ou1954]

I was trying to offer some coil packs for just the cost of shipping. I really doubt the MAF is involved.

 

[sushipaste]

If the coils you bought were for your ml, they will work for this guys m113 otherwise He can’t use the m119 coils

 

[ou1954]

Sorry, I wasn't specific. The 8 coil set was for the 1998 SL500 and the two unused brand new ones were just shipped to a fellow this morning. The reason for the number 6 in the set is that two had failed during the prior 4-5 months and had been replaced. I got the 8 for an incredible price but only needed 6.

I still have the 6 used ones which were performing well when I replaced them. I doubt anyone would want them but they are available.

The ML is still running fine, more or less, but I did replace all 12 plugs a while back. What a mess . . . the dealer wanted $500 labor but I was able to make some flex tools to deal with the problem.

 

[DBCoooper]

Ou1954, if I'm following correctly you have 6 used coils that are available? Please let me know what you want for them if I am following correctly. Having some known good coils would be helpful.
Thanks

 

[RESTO SL]

Your problem isn't coils.

 

[ou1954]

I suggest not MAF. If specific cylinders are consistently missing it's hard to see how the MAF can cause that.

Regarding the coils and wires, they are all related to my 1998 SL500-

In any case my memory is not as good as I thought. I never throw any old parts away if they might still work but here is what I found in my parts locker:

6 old plugs (Yes, I do that)
4 new ignition "wires:"
7 new coils with "wires" attached.
A couple of old "wires" which I just tossed today while searching my stash.

Not sure how I got here but I am pretty sure an indi (now closed) replaced all plugs and, over time, one or two coil assemblies, complete with "wires"

I remember later, when I was more familiar with the car, of having one cylinder fail when I was driving to a dealer and I just turned around and drove home on 7 cylinders.

In any case I live in Oklahoma and can sell a couple of coil assemblies for the SL if it will help anyone.

 

[ou1954]

Duplicate deleted

 

[ou1954]

How to be sure a spark plug is bad. (I keep this on my desk)

 

[DBCoooper]

This afternoon I swapped coils 2 and 3, and 5 and 6. The codes still show misfires on cylinders 3 and 5. Have also appropriately cleaned the MAF. It was extremely clean when removed but I cleaned it anyway. I also removed the elbow between the MAF and throttle body which was spotless on the outside of the throttle plate but had a bit of gunk on the inside which I carefully cleaned. No change to the misfire. I did verify that it is not throttle specific by leaving the car in a low gear and allowing the engine to reach almost red line. The misfire came in about 5200 RPM at light throttle.

The crank position sensor is on the way, as are a new set of Beru spark plug HT leads and OEM spark plugs.

Any other suggestions are welcome. Thoughts on fuel injectors?

Thanks

 

[ou1954]

If you swapped coils along with the attached wires I would suggest you pull the plugs on cylinders 3 and 5 and do a quick inspection and cleaning. A very quick test. If that doesn't fix the problem it has to be in the driver circuit or injectors.

I am trying to relate to heavy acceleration starting the problem and stopping the engine clears the problem.

I'll bet on the plugs at this point.


[I cheated a bit with my spark plug picture. [The plug does indeed have a bridge across the gap but it came from an engine that has two plugs per cylinder so the engine ran pretty well with the bridged plug.]