1xsculler

This car has run flawlessly for 100k but when I started it up this afternoon it would drive pretty well but it's running very roughly at all speeds. It seems to have a persistent miss. It acts like spark plug(s) fouling.

Max Blast

Check engine light on? All vacuum hoses connected and whole? Bad gas?
do you have a code reader? That’ll tell you something.

mefferso

Have you or a mechanic done anything recently? When i changed my air filters, I did not seat the air cleaner assembly completely onto the MAF. It resulted in a rough idle and engine miss.

1xsculler

Check engine light is on. I just drove it to the auto parts store to use their diagnostic tool and it ran perfectly there and back. Random misfire detected...cylinder 8 misfire detected...cylinder 5 misfire detected...turned check engine off and it stayed off all the way home (10 miles)...bought new plugs and one coil...will pick up another coil tomorrow.

No one has touched the car for 10.000 miles except when I changed the oil about 10,000 miles ago when I bought the car with 86,000 miles on it.

Max Blast

Swap coils 5 and 8 with cyls 1 and 3 to see if the problem follows the coil. 100k on the original plugs is a lot, if correct.

1xsculler

It seems to be running fine now but if (when) it happens again I will be ready with new plugs and two new coils to keep it simple although a little more expensive.

Is there much of a chance that the misfiring won't occur again?

Max Blast

depends on what the root cause was....if plugs or coils it will happen again.

1xsculler

I've put 1500 miles on this car with no recurrence of the problem so I returned the spark plugs and coils to the auto parts store.

thtguy

you should at least pull the plugs and have a look. Do you know when they were last changed? guessing not since you bought with 86K, it is likely if they were following MB service guidelines they were done around 50K which means it is time to do them again. MB recommends plugs and coils. It can be pricey but I usually do both for piece of mind, because it is only once every couple of years. I am not sure what your history with MB cars is, but if you keep up on maintenance, these cars can be very dependable. it is when you start delaying and slacking on maint, that they become troublesome.

BlownV8

The factory plugs can easily last to 150k miles. I took .mine to 200k miles. At that point, I changed all the plugs, coils, and plug wires. As I recall everything was pretty inexpensive since the coilpacks are shared across many different manufacturers.

mglsite

My 2012 GL450 with 105,000 miles does that every so often usually going uphill. You can floor it and nothing changes. Then it straightens it's self out. When I put a code reader on it, it shows no misfires. Can't figure out what it is.