1994 SL320 M104 misfire

[RichardF]

Fine one day, then idle went awol and misfire started, felt like 2 cylinders but may have been one. Total mileage driven with miss about 4 miles.

Took car home and left it, whilst errands run in spare car.

Several hours later I started it miss still there, I ran a diagnostic blink code, none found. Whipped the plugs out, all looked okay, fitted an old set of Bosch resistor plugs (320 should use non resistor plugs) found in the bottom of my tool box. All fine engine runs smoothly again.

The plugs I took out were Bosch 18k miles 3 yrs. Ordered some NGK, which I'll fit next week when I service the car.

Questions
I've never had a plug fail suddenly before (except old 2 strokes!!) just a gradual drop off, if this is going to happen I need to carry spares?

A chap on a USA site reckons the coils, leads and extenders are consumables and should be changed periodically, is this so? I've notice a huge disparity in coil prices from around £17-£120 (the 320 uses 3). What would be recommended? Would a failing coil or lead show up on a blink code reader?

I use the car for touring and continental trips, breakdowns would be a right faff (fortunately not many problems in the last 4 yrs) and not endear me to Mrs F. So while happy to carry out preventative maintenance, I don't want to chuck money @ stuff that's perfectly okay.

The upper engine/TB/lower engine, wiring harness was replaced 2 yrs ago (a problem on this year) due to failing insulation.

 

[brucemillar]

Distributor caps on these can be very temperamental and I mean very temperamental. The jury appears to be out on the best ones to use. Some swear by Beru, others swear at Beru and use Bosch etc. But the cap, rotor and leads are well worth replacing, if you have not done so?

I have read stories of owners replacing the cap to have same issue reappear very shortly afterwards. Replacing the cap AGAIN then cures it. I would suspect this before thinking two plugs would fail together - but that is possible. However in disturbing the leads to replace the plugs you have may have cured the root cause (for a while)

Pull the cap off, first thing in the morning, and have a look for any evidence of moisture. Then run the engine for a few minutes and check for moisture again. It may well just be condensation?

 

[Charles Morgan]

There's no distributor on an M104 Bruce.

 

[RichardF]

Distributor caps on these can be very temperamental and I mean very temperamental. The jury appears to be out on the best ones to use. Some swear by Beru, others swear at Beru and use Bosch etc. But the cap, rotor and leads are well worth replacing, if you have not done so?

I have read stories of owners replacing the cap to have same issue reappear very shortly afterwards. Replacing the cap AGAIN then cures it. I would suspect this before thinking two plugs would fail together - but that is possible. However in disturbing the leads to replace the plugs you have may have cured the root cause (for a while)

Pull the cap off, first thing in the morning, and have a look for any evidence of moisture. Then run the engine for a few minutes and check for moisture again. It may well just be condensation?

HFM, so no dissy caps to worry about 3 x coils on plugs + extender & leads. Wasted spark system.

 

[Charles Morgan]

Plugs fail. Ignition leads and coil packs are consumables, but I only do them when they need replacing.

If travelling take a spare coilpack and plug if you're worried about it.

 

[grober]

As a rule of thumb anything on the HT side is going to be more demanding in terms of insulation breakdown- usually because of moisture ingress to places it shouldn't be. The simple acting of changing the plugs may have been enough to dry an HT lead/extender/sparkplug out during the replacement process? On the low tension side the first port of call is always the wiring but if this a has been renewed recently then the other alternative is a failing power transistor in the engine ECU - there's one for every coil. That's a considerably more expensive repair- its usually, but not always, caused by LT wiring insulation failure so a misfire on these systems should always be investigated.

 

[Druk]

Do the old elimination process. Pull one pluglead at a time (OK, I know three of them are coils) and see if the misfire gets worse or stays the same. Then swop over coil packs and extenders one at a time till you have narrowed it down to which cyl is playing up. If you think it's two cyls missing then it may well be a coilpack since each one fires two plugs simultaneously. The 'wasted spark' system.

BTW. Treat the coilpack wiring loom with care. Where it passes through the narrow gap in the head is where it gives most trouble.

 

[RichardF]

Do the old elimination process. Pull one pluglead at a time (OK, I know three of them are coils) and see if the misfire gets worse or stays the same. Then swop over coil packs and extenders one at a time till you have narrowed it down to which cyl is playing up. If you think it's two cyls missing then it may well be a coilpack since each one fires two plugs simultaneously. The 'wasted spark' system.

BTW. Treat the coilpack wiring loom with care. Where it passes through the narrow gap in the head is where it gives most trouble.

Thanks, no missing now, re-fitting some old(er) plugs seemed to fix it, new plugs on there way.

I'd be wary of pulling one wire at a time, I'd heard you can damage the electronics!

My question was more should I replace the HT system (coils/leads/extenders) as a matter of course, if so which brand?

 

[grober]

Thanks, no missing now, re-fitting some old(er) plugs seemed to fix it, new plugs on there way.

I'd be wary of pulling one wire at a time, I'd heard you can damage the electronics!

My question was more should I replace the HT system (coils/leads/extenders) as a matter of course, if so which brand?

If not going with orginal MB parts I would go Beru now owned by federalmogul but used to be OEM for mercs

 

[RichardF]

If not going with orginal MB parts I would go Beru now owned by federalmogul but used to be OEM for mercs

Thanks, if I shop around I can get Beru for a reasonable price.

 

[horatio]

I have 2 Bosch coils I used for about 100 miles before having to scrap my W202. I think they are the same. Shameless plug but som one might as well use them.

The original were Bosch btw.

 

[RichardF]

I have 2 Bosch coils I used for about 100 miles before having to scrap my W202. I think they are the same. Shameless plug but som one might as well use them.

The original were Bosch btw.

That may be very helpful, thank you. Do you have a part number for them and how much are you looking for?

 

[horatio]

PM sent. I couldn't work out how to attach a photo in the message so added one here...

 

[RichardF]

PM sent. I couldn't work out how to attach a photo in the message so added one here...

Do they have the extenders with them?

 

[grober]

the equivalent MB part no is A 000 158 7503---- 0221506002 is the Bosch No

 

[GP801]

Oh dear. Sounds just like what happened to our 280. Intermittent misfire which was thought to be due to failing coil packs/leads. Eventually it was traced to a fried ECU. Fried by the dodgy coil packs apparently. If changing the leads and coil packs does not solve it then look closely at the ECU.

 

[horatio]

Do they have the extenders with them?

The spark plug boots? Fraid not. GSF sell Beru's which fit nicely for less than a tenner each IIRC.