Check Engine Light Misery

[B52]

First of all, thanks to the hosts for this great website and forum. It is a great help!

I have a '98 ML320 with 127,000 mi and an intermitent check engine light. I went to local independant shop and got stored trouble codes P0300, P0303, P0305, which indicate misfire on cylinders 3 and 5. His suggestion was to replace the ignition coils on 3 and 5, and new plug wires and new plugs. I have done so and it did not fix the problem.

Car runs fine except: when fully warmed up and left to sit at idle for extended period, such as at a long stoplight, it will begin to missfire. I can turn off the engine for a minute and start up again and it will be fine, untill the next long stoplight.

What else could be causing such an intermitent problem? Engine position sensor? If anyone has seen similar problem or any ideas it would be greatly appreciated.

[B52]

I read somewhere on this forum that the Bosch +2 platinum plugs were not a particularly good spark plug, designed for longevity and not for performance, and that copper core plugs are actually better performers.

So I tried a set of Champion copper core spark plugs and that cured the problem! So much for the bonehead mechanic that charged me $170 to simply read the code off of the computer and suggest replacing all of the coils for $1700.

There was some perverse joy in throwing out $6 spark plugs and replacing them with $1.19 spark plugs that really work.

I will never buy Bosch spark plugs again.

[Stan Pittman]

Originally Posted by B52

I read somewhere on this forum that the Bosch +2 platinum plugs were not a particularly good spark plug, designed for longevity and not for performance, and that copper core plugs are actually better performers.

So I tried a set of Champion copper core spark plugs and that cured the problem! So much for the bonehead mechanic that charged me $170 to simply read the code off of the computer and suggest replacing all of the coils for $1700.

There was some perverse joy in throwing out $6 spark plugs and replacing them with $1.19 spark plugs that really work.

I will never buy Bosch spark plugs again.

You didn't by chance have the engine steam cleaned or anything

[benzboy87]

Every time the check engine light comes on on my ML320 it is always the mass air sensor. I also have the intermittent miss at idle. It happens rarely and when it does I think the vehicle is going to die.

[B52]

Originally Posted by Stan Pittman

You didn't by chance have the engine steam cleaned or anything

No cleaning done. It would happen any time it was left to idle for an extended period, rain or shine. I irritates me that such headache and cost could be caused by a spark plug. And with 2 plugs per cylinder firing alternately, it is almost impossible to diagnose the problem without hit or miss replacing of expensive parts.