Rough Running 300ce when hot

[DavidJM]

Hi all. Please be gentle, I'm a newbie with some experience in Classic Mini A+ series engines.
I've recently bought a C124 300ce, I've been craving one for years.
It runs like a dream when cold, but as it reaches normal temperature, it runs rough...hesitant, poor acceleration, sounds like it's running rich. Also starts on the button when cold but a bugger to start when warm/hot. I haven't checked the plugs or anything yet, work getting in the way if important stuff like this! It doesn't overheat, coolant isn't like mayo so don't think it's a head gasket issue. Does smell strongly of petrol after a run. Someone at auction had to jump start it, which I've heard is very bad. Any pointers please would be amazing, TIA

 

[ray_hennig]

Jump start shouldn't be a problem. HT system needs to be 100%, ideally with genuine MB parts, from coil to plugs.

And then there's the fuel system. Lots of diagnostic information on the 'net. Think about buying a decent multimeter with Duty Cycle function. And a blink test diagnostic box to read any error codes.

I'm assuming you have the 12 valve M103 engine but the same will apply if you have the 24 valve M104.980 engine.

Good hunting.

RayH

 

[optimusprime]

First i would replace engine temperature sensor ..its on the side of cylinder head on the top right hand side The sensor sends important information to the ECU Like when engine is hot , it leans out the mixture .And when engine is cold it will enrich the mixture. ..If it is stuck on enriching the mixture then engine will run rich ...Check this free manual Model 124 Maintenance Manual Index

 

[DavidJM]

Jump start shouldn't be a problem. HT system needs to be 100%, ideally with genuine MB parts, from coil to plugs.

And then there's the fuel system. Lots of diagnostic information on the 'net. Think about buying a decent multimeter with Duty Cycle function. And a blink test diagnostic box to read any error codes.

I'm assuming you have the 12 valve M103 engine but the same will apply if you have the 24 valve M104.980 engine.

Good hunting.

RayH

Thanks for this....I've ordered a code reader from ebay. A little more complicated than my austin mini!

 

[DavidJM]

First i would replace engine temperature sensor ..its on the side of cylinder head on the top right hand side The sensor sends important information to the ECU Like when engine is hot , it leans out the mixture .And when engine is cold it will enrich the mixture. ..If it is stuck on enriching the mixture then engine will run rich ...Check this free manual Model 124 Maintenance Manual Index

Awesome, thanks, will do. And thanks for the link, really useful

 

[Pontoneer]

Hi all. Please be gentle, I'm a newbie with some experience in Classic Mini A+ series engines.
I've recently bought a C124 300ce, I've been craving one for years.
It runs like a dream when cold, but as it reaches normal temperature, it runs rough...hesitant, poor acceleration, sounds like it's running rich. Also starts on the button when cold but a bugger to start when warm/hot. I haven't checked the plugs or anything yet, work getting in the way if important stuff like this! It doesn't overheat, coolant isn't like mayo so don't think it's a head gasket issue. Does smell strongly of petrol after a run. Someone at auction had to jump start it, which I've heard is very bad. Any pointers please would be amazing, TIA

The M103/104(300-24) can be quite sensitive to spark plugs being the correct spec ( temperature ) as well as the mixture being right and not causing sooting up .

Also , on these cars , the distributor cap can be quite short lived ( 30.000 miles ) and again only fit Bosch as some of the other makes are noticeably inferior . Similarly the HT leads need to be in top order .

When running properly , these are great cars .

 

[ray_hennig]

the distributor cap can be quite short lived

In the case of the Bosch cap fitted to my 300-24 by some cowboys (about 150€), I binned it after 9 months. Cracking around the centre electrode.

I fitted an MB original one (closer to 250€), also Bosch, and it's fine 4 years later. My original 1991 MB cap was replaced at over 200K Kms and 19 years. For some reason I installed a Bremi cap and rotor. The rotor coughed off after a few months so I cleaned up the 19 year old examples and all was well for quite a while.

Sadly, much aftermarket stuff these days is not what it once was, even if it carries a respected name.

RayH

 

[optimusprime]

Best to buy original Mercedes parts ..But if on a budget go good used parts . As i did i purchased original seconhand used a few times . Doing this is far better than any aftermarket chinesium parts

 

[ALFAitalia]

Sadly, much aftermarket stuff these days is not what it once was, even if it carries a respected name.

....because a lot of that stuff with a "respected name" on it are Chinese fakes.....and not really Bosch, or whoever, at all. Loads of this stuff on eBay and the Chinese sites. Been caught out like that myself with "Brembo" pads......that were clearly not when they arrived.....but I fitted them to get through the MOT....and they stopped fine....so I left them!!!!

 

[optimusprime]

Yes i had Bosch clones few years ago .They were plug leads, and thay fell to bits on pushing the lead on to the spark plug , the round springs fell off . Found out they were not original .But i had a full refund at the time .I messaged Bosch ,,they in turn askled me for pictures and who the company was i purchased them from . After this the large company in Birmingham is no longer trading . I use a compay now , that deal direct from Bosch in Germany .

 

[Pontoneer]

....because a lot of that stuff with a "respected name" on it are Chinese fakes.....and not really Bosch, or whoever, at all. Loads of this stuff on eBay and the Chinese sites. Been caught out like that myself with "Brembo" pads......that were clearly not when they arrived.....but I fitted them to get through the MOT....and they stopped fine....so I left them!!!!

I got caught out that way once with a set of 'Pagid' discs which I suspect were fakes as they warped after just a few months .

 

[Pontoneer]

....because a lot of that stuff with a "respected name" on it are Chinese fakes.....and not really Bosch, or whoever, at all. Loads of this stuff on eBay and the Chinese sites. Been caught out like that myself with "Brembo" pads......that were clearly not when they arrived.....but I fitted them to get through the MOT....and they stopped fine....so I left them!!!!

The other one , with dizzy caps was Bremi , bought one from a well known UK discounter and it lasted only a few months . It is about who you buy from as much as what you buy these days .

 

[optimusprime]

David the code reader dont think there is a code reade for your car. without the one under the hood ..thast blinks like this

 

[Pontoneer]

David the code reader dont think there is a code reade for your car. without the one under the hood ..thast blinks like this View attachment 168709

I just use my trusty AVO meter 8 and count the number of times the needle kicks .

 

[optimusprime]

Good idea if it wirks /

 

[alabbasi]

Check the condition of the cap , rotor and insulator that sits under the rotor. The coil can act out when it gets hot. Also check for vacuum leaks.

 

[ray_hennig]

The other one , with dizzy caps was Bremi , bought one from a well known UK discounter

The rubbish Bremi cap and rotor I bought came from ECP or EPC or somthing in Ashford, Kent. Good for 6 months.

R

 

[Pontoneer]

The rubbish Bremi cap and rotor I bought came from ECP or EPC or somthing in Ashford, Kent. Good for 6 months.

R

Yep , mine came from the same company , it was the only brand they had at the time .