300SL Misfiring after 30 minutes of driving

[daz10dazza]

Hi all, I've just registered with this site so may take a while getting around it !
I would really appreciate any comments / suggestions for the problem I'm experiencing with my 1991 300SL. I bought it in December 06 and all was well. After a month or so I started experiencing misfiring / stalling after a period of driving (around 20-30 minutes) generally once slowing down to stop. It would cut out and not start again. This would then take about 15-20 minutes before it would start. A good friend whos a mechanic suggested it was the distributor cap (he was correct because there was a build up of mositure inside and corroded the points). He changed this for me and all was ok. About a month ago I started experincing the same again but the car doesn't stall (yet). Distributor cap bone dry, HT leads all ok etc. I've spoken to a few technicians at various Merc dealers who don't have any idear. The car will start first time every time wether cold or hot. I will drive for around 20-30 minutes at a relative constant speed and then approach traffic or traffic lights, sit in traffic and after about 10-15 seconds the car will misfire. Pull away and misfire is quite bad until past around 2500rpm or so. Start to slow down again and it carries on misfiring. I beleive there isn't a fuel pump relay on this model ? Checked behind the glove box, not there or under the bonnet. I've changed the OVP, no difference. Can anybody shed some light on this subject please - this is driving me mad now and can't find what is wrong!!! Thank you very much and would appreciate any comments / suggestions. Regards, Darran.

 

[jibcl500]

Ignition module failure or EZL as its referred to. Rough MB price about £1000, can I have the first 6 digits of vin number please will look up the part number.

jib

 

[BlackC55]

Wiring loom getting old and shorting is popular also.

 

[television]

I do not think it is the EZL as the car runs again past 2.5k OK , there is also less loading on it at Idle, by all means take the unit off, clean all of the dry heat sink compound off, apply new compound and tighten down again

 

[jamesmc]

I do not think it is the EZL as the car runs again past 2.5k OK , there is also less loading on it at Idle, by all means take the unit off, clean all of the dry heat sink compound off, apply new compound and tighten down again

Easiest place to obtain heat sink compound is your local computer shop, PC world, etc. It's used when attaching CPU's on the mother board.

 

[daz10dazza]

Thank you very much for your replies. I was hoping I wouldn't have to spend that kind of money ! Where abouts can I find the EZL on the car? Is it worth replacing the fuel pump (I've heard they can break down at temperature) ? I've also been told it could be one of the valve seals but the engine is clean, oil is like new, no leaks, spark plugs all clean with no deposits of oil staining on them. I travel the same way every day and its around 18 miles so its a good run for the car. The fault doesn't always happen but as time goes by the fault is becoming more often. Thanks again, Regards Darran.

 

[mattkh]

Hi
Clean the idle stabilisation valve if you have one on your car.

 

[Legoman]

ignition coil , try that first before anything else

 

[television]

ignition coil , try that first before anything else

Brilliant, I forgot that they can go down after a certain running time, and cheap to buy

 

[daz10dazza]

Hi everyone, Thank you all so much for your suggestions. I'll try the ignition coil first. I'll let you know how I get on. Thanks again, Regards Darran.

 

[battwell]

if it was ok for a while after fitting new dis caps? did your friend fit genuine mb caps or after market?
the reason i ask is on every 500 sel and sl500 ive had the same problems. tried cheap caps and the fault came back in a month or two.

i have a bodge mod on mine now. a tiny hole drilled through the top of the caps which i can use a wd40 straw to squirt some magic into. i hear you all moan and groan but it works and proves its the caps and nothing more.

the fault always arose after a short drive eg to get petrol or a pack of fags and when restarted it will misfire until revs are brought up, not good fun with a powerfull auto. it usually clears after approx another 10 miles but wouldnt be doing the cat much good if left.

 

[daz10dazza]

Thanks for your message. The cap isn't a merc unit but I have taken it off a few times to check for moisture and its bone dry every time. All the contacts are clean too. How often do you have to squirt wd40 into your cap ?

 

[mjtray]

Just my personal opinion.....I have found distributor caps from the likes of Euro Car Part to be variable in terms of quality!!!...not sure where you got yours from, but it may be worth investing in a Merc item.

I found the quality of the moulding to be the biggest problem, ie did not have such a tight fit as a Merc cap.

 

[daz10dazza]

Thanks, I've got the car booked into Medway Mercedes where they will be fitting new HT leads and a new ignition coil. I'll see if this makes any difference and maybe get them to supply a new cap. I've been told they are very expensive.

 

[Legoman]

easy to fit all of them yourself tbh , gsf do the dizzy cap , rotor arm , leads and ignition coil , ( i know this as im getting all of them on friday ) i worked out it totals £200 + free delivery (give or take a few quid)
yours might be alot cheaper too as mines shares the same parts as the 300ce-24 which is notoriously expensive!

 

[daz10dazza]

As mentioned earlier I've got the car booked for Wednesday next week to have the HT leads replaced and the ignition coil. I'll give that a go first and maybe replace teh dis cap with a MB one although I can't see any problems with this cap? Thanks again for your replies. Regards, Darran.

 

[daz10dazza]

I do not think it is the EZL as the car runs again past 2.5k OK , there is also less loading on it at Idle, by all means take the unit off, clean all of the dry heat sink compound off, apply new compound and tighten down again

Hi, Where do I find this on the car, I'm trying all that I can to source this problem. I'm confused as to why the car will start every time first time and run fine for 20 mins or so. Pull up to traffic and she starts missing, pull away missing quite bad but then clears until you slow down or pull up to traffic again!!

 

[daz10dazza]

Wiring loom getting old and shorting is popular also.

Hi, when you talk of wiring loom getting old surly this would effect the car when starting all the time? what wiring loom do you mention ? Thanks

 

[BlackC55]

The whole engine loom degrades and can cause all sorts of strange running faults at all diferent times.

 

[mjtray]

Stick with the easy stuff first before you start thinking about wiring looms......the loom problem tends to effect cars from 1993 to 1996 when they were manufactured with a bio-degradable insulation which bio-degraded rather prematurely!!