Hi all,
I'm suffering from occasional/intermittent rough running on my 1995 S420, particularly at lower "start-off" revs. The check engine warning light stays off. It's been happening on and off for a few months now, mainly after an initial short - say 5 minute - run, and it seems to happen more often when it has been parked outside on a hot day. It lasts for maybe 10 minutes or so of (staggering) driving, and then disappears.
I've finally started to try and sort it out. (I try to work on one thing at a time, and I've only just finished having the rear seatbelts replaced with brand new MB ones - NZ$1,666.38 for that job!)
Anyway, I've spent a few hours browsing through the experiences that some of the members here have reported with rough idle etc. I found some poeple have had pretty much the exact same problem as I have now e.g. kroglieri posted a similar story on 8/3/2005.
Anyway, I had my local garage check and change the plugs yesterday (we put in standard copper NGK plugs). I watched and we saw that some of the plugs were fine, but some were fairly badly carboned up. We think that is probably because some of the spark plug leads were not properly attached to the plugs. Some previous owner has got rid of the original MB spark plug leads, and put fairly good quality new generic leads in. They are a bit fatter than the originals, and don't fit under the covers quite so well.
Today I find that the rough running is back!
From my reading, people have suggested the following could be causing this problem:
Air mass sensor; (mine looked OK - but what can you tell by just looking at it!)
Low octane fuel; (I run 95 or higher, so that should be OK)
Bio-degrading wire harness; (I could see no evidence of that - peered at the lead to the AMR - but in truth, I couldn't see much beyond a perfectly good outer sheathing)
Distributor/rotor; (yet to check)
Fuel filter; (I'm going to check / change that next I think - hopefully it won't be too expensive)
Does anyone have any hints/warnings on checking and changing the fuel filter?
Relays:
Ignition coils
etc
bass1175 suggested spaying electrical cleaner in the EGR valve. I'll give that a go too.
If anyone has any suggestions on what is most likely to fix my intermittent rough running, please let me know.
I'll follow up as I try things and let you know how I get on.
Most definitely do your distributor caps and rotors next.
Check your engine compartment wires by cutting away the tape wrapping rather than just looking at the small exposed bits at connectors. Especially check the fuel injector wires.
Good luck with it, and please report your results for our benefit.
Update:
I just found that my local generic auto-parts supplier (Repco) doesn't stock or list fuel filters for my 95 S420. They only had parts listed for the W126 models!
I'm going to ring my local MB stealer and see what they quote for it.
I've just ordered a fuel filter from my local MB stealer for NZ$88. (That's about US$61 for our American friends). They are going to courier it to me today.
They also quoted NZ$420 for a distributor cap and NZ$204 for a rotor.
I haven't looked at the ones in the car yet - so I don't know whether they need replacing.
If I'm lucky the fuel filter will do the trick.
I've done a couple of short drives today - and it (my beloved S420) behaved perfectly. Mind you the outside temp is mild, and the problems occur more often on hot days.
I also received and fitted a second hand replacement rear interior light. The old one fell out (dangling by the wires) a day or so after the dealer fitted my replacement rear seat belts. The clips had broken off the unit. Luckily for me a local wrecker had the part, and it was fairly cheap - NZ$39. So I've had some success today!
The problem occurs more on hot days because the resistance increases even more as temperature rises. Mind you, you'll need 2 sets of dist caps and rotors.
Quote:
andrew TD - 1/19/2006 4:47 PM
I've just ordered a fuel filter from my local MB stealer for NZ$88. (That's about US$61 for our American friends). They are going to courier it to me today.
They also quoted NZ$420 for a distributor cap and NZ$204 for a rotor.
I haven't looked at the ones in the car yet - so I don't know whether they need replacing.
If I'm lucky the fuel filter will do the trick.
I've done a couple of short drives today - and it (my beloved S420) behaved perfectly. Mind you the outside temp is mild, and the problems occur more often on hot days.
I also received and fitted a second hand replacement rear interior light. The old one fell out (dangling by the wires) a day or so after the dealer fitted my replacement rear seat belts. The clips had broken off the unit. Luckily for me a local wrecker had the part, and it was fairly cheap - NZ$39. So I've had some success today!
Progress: I had my local mechanics fit the new MB fuel filter today (MB Part number 002 477 27 01). It cost me NZ$93.89.
I was armed with the excellent write up from Rainmaker on how to replace the filter. It went exactly as he described. Thanks Rainmaker.
We didn't see anything much wrong with the original filter - just the usual amount of dissolved black "stuff" came out of it. The mechanics said that it looked pretty normal. One less thing to worry about now though.
We also checked the distributor caps and rotors.
We found that one cap was in perfect condition, and the other was a little corroded.
Both rotors were a little corroded/worn. We cleaned them up and put them back.
One thing we found was that the non-standard spark plug leads had real botch jobs to connect at the distributor end. They connect, but not that well.
I just qot quotes from the local MB delaer for replacement parts as follows:
distributor rotors (each) NZ$190
distributor caps (each) NZ$392
sprk plug wire set - didn't sell them as a kit, but would sell me component parts (leads, suppressors, cable ends etc) that would add up to more than NZ$1,000 !!!
Whilst the rough running problem did recurr once after I changed the plugs, it hasn't recurred since then. I'll wait to see if it comes back at all before dishing out money on the rotors/caps etc.
We also sprayed contact cleaner into the Air mass valve. My mechanic didn't see much point in spraying the contact cleaner onto the EGR diaphragm (which is all we could get at under the connecting hose.)
Some months ago I recall someone on this forum offerring to sell me genuine MB parts - can anyone recommend a good MB part supplier.
Also, do you think it's worth trying to replace the spark plug lead set?
How many miles/kilometers on the distributor caps and rotors?
At some point, they simply should be replaced. They're like spark plugs. They wear, and the gap between the rotor and cap contacts gets bigger and bigger, which degrades performance. Distributor rotors and caps are routine maintenance items.
Do you want your car to run as best as it possibly can? Replace these routine maintenance wear items, and be done with it. Do it now, and you're probably good for another 30,000-40,000 km.
RE: How many miles/kilometers on the distributor caps and rotors?
The car has done about 98,800 km. I dont know whether the distibutor caps or rotors have been changed before.
One of the caps looked very clean - so perhaps it has been changed.
I lucked out trying to get replacement OEM parts from the local auto parts supplier - Repco. They spoke to Bosch who didn't recognise the Bosh part numbers I had found from a US online auto parts supplier. Bosch simply told Repco that I would have to buy genuine MB parts.
I figure that it may be much cheaper for me to import the parts from the US myself.
Linear Mode
