W140 M104 (S280); rough running and potential misfire, but n
Discussion
Hi folks,
Hoping for some sage words of advice for a problem that my 1996 S280 has had for a week or so...
The issues started on Friday 5th Jan, when I started the car up in the afternoon to find that it was suddenly running rough, felt down on power and was guzzling fuel, according to the MPG-ometer.
Andrew Thompson (agtlaw) kindly sent me a Mass Air Flow meter and an O2/Lambda sensor in an attempt to troubleshoot this, having experienced similar problems with his S280. Both were fitted, and while the MAF change seemed to improve things marginally, I suspect this was the result of curing a separate issue. I took the car in to my local specialist to read the STAR codes, which strangely resulted in no fault codes popping up whatsoever. Compression was also spot on.
Following this, I have changed the spark plugs, and have drained the fuel and changed the fuel filter, on the indy's advice. I have also put in some injector cleaner and fresh fuel, and yet the problem persists. The engine seems to run more smoothly the higher in the rev range I go, and the oil pressure/temperature is reportedly perfect, although the car smells hot when parked (perhaps unburnt fuel is lunching the cat)? The fanbelt is being a touch vocal at idle too, but I suspect this is more to do with the vibration from the running issue than anything else.
I wonder if anyone has any idea as to what could be wrong?
Current suspects are a vac line leak, gummed up throttle body, or faulty HT leads/Coils.
I suspect, but cannot prove, that the problem relates to ignition rather than fuel supply.
Many thanks in advance for your assistance,
Alex.
Hoping for some sage words of advice for a problem that my 1996 S280 has had for a week or so...
The issues started on Friday 5th Jan, when I started the car up in the afternoon to find that it was suddenly running rough, felt down on power and was guzzling fuel, according to the MPG-ometer.
Andrew Thompson (agtlaw) kindly sent me a Mass Air Flow meter and an O2/Lambda sensor in an attempt to troubleshoot this, having experienced similar problems with his S280. Both were fitted, and while the MAF change seemed to improve things marginally, I suspect this was the result of curing a separate issue. I took the car in to my local specialist to read the STAR codes, which strangely resulted in no fault codes popping up whatsoever. Compression was also spot on.
Following this, I have changed the spark plugs, and have drained the fuel and changed the fuel filter, on the indy's advice. I have also put in some injector cleaner and fresh fuel, and yet the problem persists. The engine seems to run more smoothly the higher in the rev range I go, and the oil pressure/temperature is reportedly perfect, although the car smells hot when parked (perhaps unburnt fuel is lunching the cat)? The fanbelt is being a touch vocal at idle too, but I suspect this is more to do with the vibration from the running issue than anything else.
I wonder if anyone has any idea as to what could be wrong?
Current suspects are a vac line leak, gummed up throttle body, or faulty HT leads/Coils.
I suspect, but cannot prove, that the problem relates to ignition rather than fuel supply.
Many thanks in advance for your assistance,
Alex.
Check for spark at each plug. Ultimately a misfire is what it is says on the tin. A misfire will seem smoother at higher RPM's on bigger cylinder engines, more rhythmic hiccup as there is more cylinders available to run the engine. Unlike a misfire on a 3/4 cylinder engine where it's practically trying to jump out the bonnet.
Based on your description of the fault try the below:
You can buy some HT lead testers, which are the safer bet and inexpensive if you're not too mechanically savvy, these sit between the plug and the end of the HT lead and flash if HT is passing through when the engines running. Easy enough to see if one light is either particularly weaker than rest, or just not lighting up at all.
If you're a bit braver, pull off each lead individually whilst the engine is running and earth it out on the block. (I shouldn't need to remind that there are 10,000+ volts running through this) You should be able to have a look at the spark at this point as well and compare the strength to the others. If you pull off a lead and it makes no difference to the engine running that is at minimum your culprit cylinder. If you have no spark, test the coil/HT lead. If you do have spark, have a look at the injector (if compression has been checked & new plug fitted - should be no need to test these).
Based on your description of the fault try the below:
You can buy some HT lead testers, which are the safer bet and inexpensive if you're not too mechanically savvy, these sit between the plug and the end of the HT lead and flash if HT is passing through when the engines running. Easy enough to see if one light is either particularly weaker than rest, or just not lighting up at all.
If you're a bit braver, pull off each lead individually whilst the engine is running and earth it out on the block. (I shouldn't need to remind that there are 10,000+ volts running through this) You should be able to have a look at the spark at this point as well and compare the strength to the others. If you pull off a lead and it makes no difference to the engine running that is at minimum your culprit cylinder. If you have no spark, test the coil/HT lead. If you do have spark, have a look at the injector (if compression has been checked & new plug fitted - should be no need to test these).
Edited by Classy6 on Monday 15th January 12:19
Is this the HFM engine? If so the coils have a hard time under the cover, I'd be tempted to change the whole HT system (3 coils, leads, and spark plug extenders)
Or just one coil and move it about to see it made any difference. Electronic diagnostics are fairly rudimentary on these engines.
Or just one coil and move it about to see it made any difference. Electronic diagnostics are fairly rudimentary on these engines.
Thank-you both so much for your helpful advice, I'm afraid I didn't spot it at the time!
Acting on a hunch, after noticing (and subsequently changing) a fairly dirty spark plug on cylinder no1, and a rough looking HT lead feeding it, I went on to replace the coil pack and HT lead combo feeding cylinders 1 and 6.
This has completely cured the misfire, so you were both right!
The icing on the cake is that replacing the MAF meter also seems to have immeasurably improved torque at low revs as well.
I just hope that I didn't do any damage to the cat. by pumping in a fair bit of unburnt fuel...
Either way, all's well that ends well!
Acting on a hunch, after noticing (and subsequently changing) a fairly dirty spark plug on cylinder no1, and a rough looking HT lead feeding it, I went on to replace the coil pack and HT lead combo feeding cylinders 1 and 6.
This has completely cured the misfire, so you were both right!
The icing on the cake is that replacing the MAF meter also seems to have immeasurably improved torque at low revs as well.
I just hope that I didn't do any damage to the cat. by pumping in a fair bit of unburnt fuel...
Either way, all's well that ends well!
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