1991 300TE-24V Intermittent Misfire

[kevinnathanael]

I have a 1991 300 TE–24V M104 engine with the following symptoms:

• Use the vehicle up to warm (doesn’t have to reach 80 degrees C), sometimes only a 15 min drive
• Leave the vehicle for 30 mins – 1 hr (this is vital since if I restart the car immediately, there is no issue)
• Upon restart, sometimes the car would misfire, but not all the time
• Sometimes switching the car off then starting it back up immediately would make it normal
• Other times it may have difficulty starting, or misfire would continue and car would have no power with low idle rpm and rough running. (Some times I would restart 20+ times and misfire still continues, I would have to wait a longer time to let it sit till it goes normal again)

Parts that have been changed since I owned the car:

• 8 spark plugs (correct Bosch non-resistance)
• One of the two fuel pumps (since the other is normal)
• Fuel accumulator
• Idle control valve (Bosch)
• Air flow Potentiometer
• Distributor cap and rotor (both Bosch)
• Bosch coil
• Camshaft Position Sensor
• ECU switched to another used unit
• OVP Relay switched to another used unit
• MAS Relay switched to another used unit
• Crankshaft position sensor cable switched to another used unit
• HT Leads seem to be new (Beru)

Can anyone help guide me as to where else to look for? I’ve tried going to several mechanics without any results since they have difficulty in replicating the misfire issue, since sometimes just by restarting the car immediately it becomes normal.

 

[Doug1234]

Over the years on this forum most if not all misfire posts on the older Mercedes models like yours I have seen are caused by moisture in the distributor cap.
The 15 minute drive before it happens is the classic sign of this as it takes about that long for engine heat to warm the damp up for it to ‘steam’ and cause the misfire.

 

[malcolm E53 AMG]

^ +1 happened to me with my 124 300TE moisture causing ignition tracking in the distributor cap, I would source another cap and rotor arm before throwing anymore money at it and yes I would change the plug leads if they are old

 

[kevinnathanael]

I’ve recently changed the distributor cap and rotor with Bosch. Is there anything else I can do to mitigate this moisture issue?

 

[Doug1234]

Lots of posts on this forum to read through concerning the moisture.
One thing you can do is wait until it’s dark .. have a cold engine and put the bonnet up…pour a coffee and sit outside in the dark with no lights on so your eyes get accustomed to the dark and after about five minutes start it up and leave it running to warm up and just sit looking for shorts from the plug leads / anything like that.
Years ago plug leads that were bad would have an eerie green glow about them but I have not had a petrol car for around 25 years so technology may have changed

 

[rayhennig]

• Leave the vehicle for 30 mins – 1 hr (this is vital since if I restart the car immediately, there is no issue)

As said in another place, I had something similar and it was the injectors dribbling fuel while the warm engine sat for half an hour. On start-up it would misfire for a while and then clear. A morning's work, new injectors, guides and seals and normal service returned. Costly but satisfying.

As for HT, these engines demand the best. In 2020, I replaced the entire HT system, from coil to plugs, with MB original parts, leads included. I am more confident now than I have been for years.

Before the refit, I had a 9 month old Bosch distributor cap (fitted by cowboys in Toulouse) that was cracking around the centre electrode, but still working. That came out in favour of the new Bosch MB one and that, after 2 years still looks like new.

Is there really such a difference between Bosch MB and Bosch aftermarket??? They look similar when new.

The shield behind the rotor arm should also be in good nick so if it's old, get a new one.

Good hunting.

RayH

 

[Blobcat]

As said in another place, I had something similar and it was the injectors dribbling fuel while the warm engine sat for half an hour. On start-up it would misfire for a while and then clear. A morning's work, new injectors, guides and seals and normal service returned. Costly but satisfying.

As for HT, these engines demand the best. In 2020, I replaced the entire HT system, from coil to plugs, with MB original parts, leads included. I am more confident now than I have been for years.

Before the refit, I had a 9 month old Bosch distributor cap (fitted by cowboys in Toulouse) that was cracking around the centre electrode, but still working. That came out in favour of the new Bosch MB one and that, after 2 years still looks like new.

Is there really such a difference between Bosch MB and Bosch aftermarket??? They look similar when new.

The shield behind the rotor arm should also be in good nick so if it's old, get a new one.

Good hunting.

RayH

Were you getting fuel in the oil?

 

[rayhennig]

Were you getting fuel in the oil?

Not that I noticed. I don't think the dribbling was very severe. Just enough to cause a few splutters and then normal service.

RayH