Help! rough idle and spark plug detonation

[lucky7's]

I have a 2002 mercruiser 6.2 mpi with 700 hrs. and have some problems.. Motor runs rough at idle and smooths out about 1500-200o rpm but it gets worse the hotter it gets. Pulled out all spark plugs and cylinders 1 and 3, the spark plugs were gone i mean the electrode was gone. All other plugs looked good. So i put new plugs in it and ran for about 20 mins ran ok 4 a while but then the idle got worse and worse. I pulled the plugs out to see how they were burning and all looked ok. What could be causing this... Weak spark? not enough gas??? Crap I cant figure it out. I pulled the distrubuter cap off and cleaned out everything but still didnt fix it. Help plz

 

[HT32BSX115]

Re: Help! rough idle and spark plug detonation

Couple things to think about,

How old was the gas in the tank? did you allow detonation to continue at high power settings?

If you had plugs that were severely burned due to detonation, you may have burned valves and/or pistons too.

You might want to do a compression check along with a check with a scan tool....to check timing, knock sensor etc...

 

[lucky7's]

Re: Help! rough idle and spark plug detonation

Think i figured it out. the IAC "idle air control valve does not work.. I pulled it out and the thing is like stuck barely open. I pulled it off and left the wires hooked up to it and it does not move at all. Its supposed to close when you give it throttle and its supposed to be open at idle. Also I have replace three of these things and I know it is supposed to be shut totally when there is no power going to it. I pulled it off and its like half way open. I ran the engine with it hooked up to see if it closed and it did not.... Sooo???? Usually the audio warning alarm goes off when these things go bad... but This ones different?

 

[lucky7's]

Re: Help! rough idle and spark plug detonation

No compression on two cylinders because of burned up intake valve was the problem.