How many miles are on that engine? As an engine wears, uneven compression between cylinders and slight misfires eventually roughens the idle.
Hmm good point! This is definitely the highest mile engine I've ever owned at close to 150K miles. The slight roughness is nothing worse than my 350Z did from literally brand new. It always felt like it had a "hiccup" but that was due to the relatively aggressive cams and stiff engine/trans mounts.
look for a vacuum leak
All new hoses under there. I haven't smoke checked, but next Cuban I bring home will be a good enough excuse to light one up and head to the garage
Originally Posted by 2000 E 320 White Wagon
Good questions !
I'd guess 100k for plugs; wires that or maybe more.
But if plugs over 100k, it's not to much to thrown new wires on 'just in case', right ?
I'm personally, avoiding the plug job as my M113 V-8 runs great so far.
If mpg tanks, or big misfire happens, I'll pull them.
Basically, rig was perfectly clean when I got it at 102k and 3 years ago, and I am going on the assumption plugs were done by the $$$ prior owner.
But at 150k now, I do wonder. Just have a busy life and yanking 16 plugs sounds both unnecessary & distracting.
My idle is slightly lumpy, not bad, and combined with my loud performance exhaust, sounds a bit menacing.
Every once in a while a Charger or Mustang or Vette wants to race. So, there are upsides to a lumpy idle, LOL !
But I digress, amen to checking for a vacuum leak. I'm sure those who know more can give direction.
I know there are molded vacuum hoses that are on top of engine - the ones that must remove if doing valve cover gaskets - are those the ones that are suspected for vacuum leaks ?
On the coils yes they could be contributory. I once had fine cracks in a couple of my coils and when humidity was high (rainy day) I'd get a bad miss on those 1 or 2 cylinders.
Replacing them was they key. Hasn't happened since, and 4 are original.
(This was wifes wagon with M112, but same engine as ML320)
Have you cleaned your MAF Sensor ? Or replaced ?
How many miles on your rig ?
PS: Off topic, but what kind of Jets do you pilot, if I may inquire ?
Vac hoses appear to be good. Also, coils have no cracking and all appear to be in good shape as well. I haven't cleaned/replaced the MAF because in my experience cleaning them was always no difference or short lived anyways. I could replace, but at $150 I don't think I really have any reason to suspect the MAF as a culprit yet.
I'm just shy of 150K on my 2001. What I did find however was a slight leak on the right bank valve cover which was pooling oil into one of the spark plugs. After cleaning out the oil in the wire with some deox/it, and cleaning the plug tube with some rifle swabs, the rough idle is almost completely gone.
I've got a new set of Vic Reinz' gaskets to throw in to stop the leak. Then I'll remove the spark plugs near the leak and inspect/clean them. I assume MAF cleaner will be OK to cut through whatever film of oil may be causing resistance.
Overall MPG on the highway is just shy of 22, which isn't bad for an 18y/o single cam V8.
Edit: I fly Embraer 190 regional jets for a US based airline. It's a 100 seat jet that looks like a B737.