W124 hesitation after engine is hot and then stalling (M103)

[kobold]

Car starts up great. Drives normally until engine is hot. Then becomes hesitant on acceleration. The hesitation quickly worsens over about 5 to 10 minutes. The car will then stall when accelerating from idle. It can be restarted and will idle rough with the car shaking. At this time, if put into Drive it will stall. If I leave the car idling in Park, the rough idle/shaking will suddenly disappear after about 20-40 minutes of idling. It will then run normally for the rest of the day. A few days later, the problem reoccurs again. It has been following this pattern now for many weeks.

Car is a 1991 W124-026, M103 engine. No change after replacing the spark plugs.

I was thinking it may be a moisture related problem because of the way it always recovers after idling for a while. Maybe something is wet, then dries up and resumes working. But I cannot find any water on the electronics. Anyone recognise the symptoms?

 

[GP801]

Perhaps distributor cap or plug leads? Distributor cap is a very common problem and they like only top quality items.

 

[kobold]

I took off the distributor cap to have a look and all the contacts are covered in deposits. I don't see any damage to the cap. The rotor arm has some peeling plastic at the bottom, and the well in the centre is filled with deposits, but looks undamaged. Both are Bosch parts with Mercedes markings on them. Is it just a case of cleaning them up or is it necessary to replace.

 

[mannikon]

Had the same problem a few times also on a 1990 W124 300e-24 (M104 engine).

AA and RAC on different occasions and a general garage all suspected fuel issues - pump or tank.

However, each time the fix was electrical, when I managed to get to indy Merc garages. Spark plugs, distributor cap and rotor arm replacements have done the trick each time. I suspect the cap seal is far from perfect and allows dampness to get in which starts tracking and then everything gets worse. Only started a few years ago. Cars from this era should probably be garaged (I don't have a garage) to keep out dampness? I've also had to replace every fuse in recent years, all fuses are now decent copper fuses (imported from the US).

The dist cap can be a pattern part but the rotor arm needs to be OEM (what last indy merc garage told me).

 

[GP801]

Unless you can be sure they are perfect its well worth replacing them. We used to have a 300e-24 and it took a bit of effort to work out why it was running rough. There was a very tiny crack in the cap. They are very fussy engines in this regard and only like top notch caps and rotor arms.

 

[grober]

Quite often the condensation/moisture is down lower in the body of the distributor. The car will start OK but once the engine heats up the water volatilises up into the distributor cap/rotor arm area where it shorts the HT to earth causing a misfire. Cure is to dry the rotor arm cap and body out completely with a hair drier.
the distributor cap is toast you can see the green corrosion/burning on the cap contacts- these should be bright metal -- there are also distinct arcing paths on the inner surface of the cap like spiders webs from the central post to the spark lead contacts- these are alternative little carbon tracks for the spark to travel along. The rotor arm end contact should be periodically cleaned with fine emery as it always gets burned away.

 

[snovovic]

Mb 260e w124 1990 hesitation

Car starts up great. Drives normally until engine is hot. Then becomes hesitant on acceleration. The hesitation quickly worsens over about 5 to 10 minutes. The car will then stall when accelerating from idle. It can be restarted and will idle rough with the car shaking. At this time, if put into Drive it will stall. If I leave the car idling in Park, the rough idle/shaking will suddenly disappear after about 20-40 minutes of idling. It will then run normally for the rest of the day. A few days later, the problem reoccurs again. It has been following this pattern now for many weeks.

Car is a 1991 W124-026, M103 engine. No change after replacing the spark plugs.

I was thinking it may be a moisture related problem because of the way it always recovers after idling for a while. Maybe something is wet, then dries up and resumes working. But I cannot find any water on the electronics. Anyone recognise the symptoms?

Hi

I have the same problem with my 260e. I have it serviced and checked with specialist mechanic and electrician separately and they have found nothing as the problem disappears once the cap is dry I presume.
Did You change the distributor cap and did the problem disappear?
I live in Scotland and don't have a garage so the dampness could be a problem adding a fact that I don't use a car every day.

Thanks

Slobodan, Glasgow

 

[Sauternes]

Exactly the same problem on a W124 - M103 (idle rough & stall after 10mn).
It was moisture/condensation in the distributor cap.
Was skeptical at first, but it was exactly that: I cleaned it. Runs ok since then.
Cap & rotor need to be change though.

 

[GaryLockyer]

Below the Rotor is a Suppression Disk - made of the same material and of the same colour as the Dissy Cap. If the Disk develops "Burn Tracks" or pinholes, the spark will short to earth through it. Most often this shows itself in random misfires, but the damage could be serious enough to shut things down totally. The disc should be taken out and closely inspected every time the Dissy Cap is replaced. To get the Disc out, the bracket that the Rotor attaches to must be removed. It's held on by a T30 bolt - on my 1990 300E 2.6, but I believe that earlier versions had a different bolt. If you Distributor has moisture problems, it would be worth spraying the bolt with some penetrative oil and letting it rest a bit before undoing it.

 

[optimusprime]

Never had a problem with damp on my M103 engine, and she is outside day and night . This distributor internal cover is a part i would only look at , as a last resort . Distributor cap , and rotor is first on my list , along with the ht leads , and plugs these are none resistor type . And these parts are the first port of call . So untill these parts are replaced i would change nothing .One thing is the original Bosch distributor caps have an extra cover on them . And after market distributor caps come without the extra cover, and this can cause damp in the distributor cap .

 

[Geewizzpip]

Hi. I had this problem with my 300CE. It would start perfectly and then just cough and splutter after 15 minutes and then refuse to start, as if there was a spark problem. After I tried everything from new dizzy and leads to new coil with no change. Then I took the air cleaner off to look for the crank sensor. I couldn't reach it so I just started the car without the air cleaner. It kept running!! So I put the air cleaner back on and disconnected the rocker breather. It now runs perfectly and hasn't stalled since. My only thought is that after 250,000 miles maybe the air from the rocker cover is too oily when the engine is warm and that was either confusing the fuel injection (the little tube from the rocker cover pipe) or it was just bad air going straight into the fuel injection system. Whatever it was it seems to work. So before you spend any money just disconnect the breather!

 

[ray_hennig]

So before you spend any money just disconnect the breather!

With respect, this sounds like fixing a symptom rather than fixing the fault. I would urge further investigation.

Good hunting.

RayH

 

[optimusprime]

Geewizzpip Another item that gets forgot are the two 1 small 1 large event holes in the rocker box .Both , or one of them, can get blocked up with grease type crap in there. So get a pipe cleaner and run it through. Or if it is blocked, you can say the rocker box if full of oil crap type of crud so it will be best to pull the rocker box off and clean it out .This job wants doing if you have never done so yet .I have seen some rocker boxes full and had to scrap the crap out with a plastic scraper