Misfiring R129 300SL 12v

[hexxor93]

I've been having on and off recently in the hot weather misfiring that won't go away.

When its cold, say in the late evening the car runs as it should. During the day when its much warmer the car once its up to temp is misfiring and stalling like crazy.

I've put new Bosch HT leads, ignition coil, Mercedes new spark plugs, I've had the duty cycle adjusted to arond 50% as it was set way way too rich before (80%+). And it still comes back with the misfire if outside is warm/hot weather.

I've cleaned inside the dizzy cap and the arm. Both are in good condition.

Need help badly at this stage as I don't like being stuck in the middle of London randomly when the car feels like giving up

 

[grober]

COULD BE ALL MANNER OF THINGS. It could be your EZL ignition unit is on the verge of failing - the heatsink paste between it and the wing dries out and causes it to overheat. It could be the CPS [ crank position sensor] that tells the EZL the engine is running it can also fail with age. Worst case scenario is a partially blown CHG [ cylinder head gasket] symptoms of this vary according to location but usually accompanied by loss of coolant/overheating. Here sometimes sparkplugs can be a good diagnostic. If one or two are more sooted up/black/ oily than the others that's a sign of incomplete combustion on that cylinder/s where the CHG has gone. CHG failure can also be detected by sniffing the headspace above the coolant with an exhaust gas analyser- any trace of combustion gases is positive indication- usUAlly followed by a compression test on all cylinders.

 

[DSB SL AMG]

Good advice from above...get the plugs out and look at condition? are they black, sooty? seems its ambient temp related if the car runs okay from cold...

Incidentally what fuel are you using? I would not recommend supermarket stuff for this engine as my 1992 R129 300SL which is also a 12v runs crap on it, different car when I put Shell V-Power in her...

The M104 / M104 engines can suffer with rough idle, and stalling due to a number of reasons, generally stuff like this is either an ignition or fuelling problem. Note on the ignition system I strongly recommending only changing stuff with MB OEM parts as the ignition systems are temperamental.

I had a recent similar issue, rough idle, on my R129, I chased it and had previously changed dist cap and rotar arm (MB OEM parts) with no change, HT leads appeared fine, new plugs, but finally found a crack in the suppressor cover (insulator disk) that sits behind the rotor arm, on inspection there appeared to be a scratch (attached pic), indicating the spark was passing through the disk to ground, imho worth also checking as well.

 

[DSB SL AMG]

Typo above, should read M103 / M104 Engines...

 

[hexxor93]

Thanks for all the suggestions, I've had someone on benzworld give me a huge list of what to look for and how to check.

I've checked the orange disc which is fine and undamaged and a Mercedes part as well! EZL I am going to take it off and add some thermal paste and see how that goes. I've taken out all the new Mercedes bocsh spark plugs and the two at the front of the engine are white ish and the rest are sotty but the tips are completely dry and clean :S

I may have a vacuum leak as my air vents don't change directions and the block in the engine bay next to the MAS relay has a crack in it! But the new part is coming in a day or so hopefully.

Posted a couple of pics of the dizzy cap and arm as I keep getting lots of blue deposit build up...

 

[DSB SL AMG]

Some info pal.

Do check that your O2 sensor is good, otherwise, you’re burning more fuel than you really need to so its another part that pays for itself over time. It’s connected under the passenger side carpet and gets fed out through a grommet on the trans tunnel. MB recommend change at 100k . Did you do this before you adjusted lambda? as you could now be running lean

I've had the duty cycle adjusted to arond 50% as it was set way way too rich before (80%+).

 

[hexxor93]

I'll try check under the carpet, I remember I couldn't get it all off last time I tried, unlike my w124

 

[DSB SL AMG]

If it looks dodgy get a new one they are not that expensive...

When your checking note the sensor wire might feed itself out of the grommet and give itself a lot of slack to be waving around as you drive your car. There’s a problem with this as it’s able to touch the spinning driveshaft. I’ve heard of sensors SHORT out on the driveshaft and make the car completely inoperable. It would buck and run extremely rich. Misfire like crazy, etc. A short in the O2 sensor sends a variety of signals to the engine computer and everything else just goes terrible. Crawl under the car and make sure the O2 sensor wire is nowhere near the driveshaft. Easy, preventable maintenance really...
Note in hindsight this should be done before lambda adjust...which is set at the factory...although if you found it set at 80% seems someone may have already tampered with it...

Have you also checked and verified the EHA (Electro hydro Actuator), its on the back of distributor, if this is faulty you normally get a strong petrol type smell....also worth looking at OVP as this can also cause stalling issues...problem you got is on these cars with no digital diagnostics its all fun and games

I'll try check under the carpet, I remember I couldn't get it all off last time I tried, unlike my w124

 

[hexxor93]

Thanks for the advice. The tamper ball was removed before I got the car so someone has played with it before thats for sure. I've checked the EHA before and that was fine and no fuel smells at all!
With regards to the adjustment I made, the car had a K&N air filter, and some aftermarket red ****ty ht leads. so I know the car has had some questionable work carried out. And I've sorted that out at least with the proper parts.
The OVP is the later style with the two fuses on top so that has been replaced it seems. It never has any issues restarting after it finally stalls when misfiring from what I've experienced.

 

[mazza]

I've been having on and off recently in the hot weather misfiring that won't go away.

When its cold, say in the late evening the car runs as it should. During the day when its much warmer the car once its up to temp is misfiring and stalling like crazy.

I've put new Bosch HT leads, ignition coil, Mercedes new spark plugs, I've had the duty cycle adjusted to arond 50% as it was set way way too rich before (80%+). And it still comes back with the misfire if outside is warm/hot weather.

Hi

80% seems extremely high in terms on mixture adjustment, engine would run rough as fcuk and would exhaust would also smell very rich. 80% on the fault codes is telling you you may have a problem with the air temp sensor or others problems.

do a search on how to test the coolant temperature sensor and test it out, its the most important sensor on the KE jet.

good luck