elusive misfire sl320

langlois

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Hello all,

I have a 1998 V6 SL320, which suddenly drops to firing on what feels like three cylinders only. Cutting the engine off, then restarting immediately, always cures the problem. The problem usually, but not always, occurs after a run of 200 km or more, but after the first occurrence, it gets more frequent.
I recently managed to reach a Mercedes garage while the engine was still running in this sad manner, and after about an hour of electronic analysis, the garage changed coils N°s 3 and 5. (plus attendant plug leads )
I thought the possibility of two coils breaking down and then curing themselves simmultaneously was unlikely, but agreed to change them for want of another solution.
After 240 km, the problem occurred again, so I stopped the car for the night. The next day it occurred once after 200 km, then again 40 km later.
Oil & water temperatures are perfectly normal & everything else works fine on the car. The next-to-last occurrence happened after I had been stuck for nearly three minutes at roadworks traffic lights, so I thought it might be under-bonnet heat build-up, but 40 km later it happened after a 6-kilometer downhill run, foot almost completely off the accelerator !
Clearly I can't drive 200 kilometers every day just to provoke the incident, and the garages can't analyse the incident data on the inboard electronics because this model does not retain memory of incidents.
Does anybody have any idea which component or phenomenon could cause coils 3 & 5 to shut down in this manner ?

Any help or advice appreciated

Many thanks

Langlois
 
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langlois

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Elusive misfire - update

Hello all,

I've just spoken to an "oldtimer specialist" from Mercedes, who confirms this is a simple coil problem. Apparently, the coils are cross-linked in pairs, so if one goes into short circuit mode, two cylinders get kicked out. Switching the ignition off & on erases the defect from the car's "memory", so away we go until next time. Youpee !

Question : new coils are € 234 each + fitting from Mercedes France, or I can buy Bosch replacements/equivalents for € 57 each. ( and some lesser brands even cheaper !)
The Mercedes part N° is a 000 158 78 03. Can anyone recommend (or decommend) any particular brand of equivalent coil ?

thanks again for your help & hoping this info will be of use to the 129 community.
 

grober

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Bosch should be fine but source them from a bona fide Bosch Outlet to avoid cheap far east counterfiet versions
 

RichardF

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If your's is the V6 motor might be worth asking Chrysler!

Answered my own question I6 if cylinders are sharing coils.
 
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Vintage Racer

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+1 for Bosch, but as 'Grober' says, make sure they are genuine!
 

kth286

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IF inline 6 it is usually the coil extenders down to the plugs that go open circuit when hot. Inside the rubber is a 2k ohm wire which gets brittle and really should be replaced when plugs changed to avoid future problems. These extenders are pulled hard from plugs when changing and starts the damage to the internal resistance wire within the rubber casing.

IF V6 engine I do not know if same situation as nil experience.
 
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langlois

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If your's is the V6 motor might be worth asking Chrysler!

Answered my own question I6 if cylinders are sharing coils.
Hello there !

Sorry about delay in replying

As I explained, MB france changed the coils, then when the problem started again, promised to take them back & reimburse me.
My wife didn't believe them, & she was right - once the new coils had been taken off & the old ones put back, they refused to reimburse me, on a perfectly spurious excuse
Furious with MB, I changed ALL the coils & HT leads for BREMI coils, ref 11873, but the problem persists, and I think we have established it's statistically imposible for coils to be the cause.
Why do you sugggest I ask Chrysler ? Is this V6 common to one or more of their models ?
thanks for any ideas or advice you may have to give...
Cheers
 

grober

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The M112 V6 is slightly unusual in that each cylinder has 3 valves one coil and two spark plugs. One simple but laborious check after experiencing a period of misfiring is to remove and examine the spark plug electrodes for signs of misfire - this may help pin down which cylinders are effected. the
 

GP801

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Might be worth looking at the ECU. Not sure about the V6 but on the straight 6 running with dodgy coil packs will eventually fry the ECU - we found out the hard way!
 

markmifsud

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Try to swap the coils/leads from 3/5 with 2/4 and see if the problem moves. If its still on 3/5 then I would suspect either the ecu or actual mechanics. have you had a cylinder pressure test?
 

grober

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Try to swap the coils/leads from 3/5 with 2/4 and see if the problem moves. If its still on 3/5 then I would suspect either the ecu or actual mechanics. have you had a cylinder pressure test?
A misfire may be down to a sticky or burnt valve/s
 

RichardF

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Hello there !

Sorry about delay in replying

As I explained, MB france changed the coils, then when the problem started again, promised to take them back & reimburse me.
My wife didn't believe them, & she was right - once the new coils had been taken off & the old ones put back, they refused to reimburse me, on a perfectly spurious excuse
Furious with MB, I changed ALL the coils & HT leads for BREMI coils, ref 11873, but the problem persists, and I think we have established it's statistically imposible for coils to be the cause.
Why do you sugggest I ask Chrysler ? Is this V6 common to one or more of their models ?
thanks for any ideas or advice you may have to give...
Cheers


Chrysler used this engine in the Crossfire, maybe others. Sounds like you need a good Indy.
 

alabbasi

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No distributor on the V6 M112 ??

Well maybe I should have read more. I think that you're on the right track with the coils. You might also want to peak under the car and take a look at the cats on the same bank. They might have gotten damaged under the same condition due to unburned fuel and be compounding the issue. They will be red hot.
 

Pcn1

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IF inline 6 it is usually the coil extenders down to the plugs that go open circuit when hot. Inside the rubber is a 2k ohm wire which gets brittle and really should be replaced when plugs changed to avoid future problems. These extenders are pulled hard from plugs when changing and starts the damage to the internal resistance wire within the rubber casing.

IF V6 engine I do not know if same situation as nil experience.

I may have this problem with my I6 SL 320, could you point me towards a supplier of the parts apart from the main dealer ?

Thanks
 

rajinder_1

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another thought is the MAF could be starting to go and hence the slight misfire,
dirty fuel can also cause misfires, but would be random cylinders on errors

Try and get it on STAR to see if you can get an error code it will save you in time and money

Raj
 

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