CEL on ml320 cylinder #5 misfire. Any advice?

[zmanzbo]

Hello,
My car idles rough at a stop light and the check engine light will come on then run rough. It has 110K miles on it.

I took it to Autozone to get the codes checked and it came back with P0305 cylinder misfire detected on cylinder #5.

I replaced all the plugs around 60K miles. Do you think it could be the wires? Can you just buy a single wire or so you have to buy the whole set?

Any other ideas?

Thanks!

[43sqd]

Yes you can buy individual wires at any dealer. Just tell them the year and model and what cylinder.

[zmanzbo]

Cool thanks. Would that be the best place to start?

[43sqd]

Originally Posted by zmanzbo

Cool thanks. Would that be the best place to start?

Without a scanner you will be making numerous trips to AutoZone just to see where the problem lies.

What you will have to do is remove the spark plug wires from the spark plugs on cylinder 5. Remove the elec. plug to the coil and then the one bolt holding the coil. Do the same to cylinder #2, and swap them both because both cylinders have the same length wires (see diagram below).

Then you will have to remove the negative battery cable (hopefully you have the radio code) for 1 min. to clear the codes, replace it and then start the engine. Then you will have to make another trip to AutoZone to see where the misfire is. Then post back with the results.

[zmanzbo]

Or should I just buy a single wire from the dealer and install that? Any idea what those go for? I know a full set is ~$200.

Thanks alot for all the info!

[zmanzbo]

Also, do you have any PDF's on a how-to change out the wires?

Thanks.

[43sqd]

There are two wires per cylinder, therefore you will have to buy two. And there is no pdf on changing wires.

[Michael Pillay]

Originally Posted by zmanzbo

Also, do you have any PDF's on a how-to change out the wires?

Thanks.

It is pretty straight forward. Just pop the old one of and replace with a new one. If the old one will not come of, use an open end 17 mm wrench to get some leverage.

[zmanzbo]

Just called the dealer they want $60 for each wire haha. I will need 2 so $120 for a pair. I can get a whole set online for $199.

Looks like I will be just swapping #5 with #2 tonight.

Thanks to all for the info.

[FAR888]

Originally Posted by zmanzbo

.....Looks like I will be just swapping #5 with #2 tonight....

Hello zmanzbo,

As I understand it, swapping wire #2 with wire #5 is NOT the solution to the problem - It is just a means of ascertaining the root of the problem.

I believe 43sqd advised as he did so as to help you ascertain whether indeed wire #5 was faulty and causing the missfire at cylinder #5.

IF on swapping wire #5 with wire #2 then erasing the existing codes and then having the codes read afresh you obtain a code P0302 (Missfire cylinder 2) you will have established beyond reasonable doubt that it is that particular wire (previously at #5) that is now causing the missfire at cylinder 2.

THEN you must simply purchase ONE wire from the dealer and replace it with the old faulty one.

Good luck!