Mercruiser 165 - missing and power loss

[brodie123]

Hi All;

Mercruiser 165 - inline 6 - chev 250

Hi all, I'm about to do all the standard tests, but wanted to post my problem and what I found o far.

We were out on the boat yesterday and she ran beautifully for the first hour - lots of power and an smooth.

When running at about 1500 RPM, she very mildly started to miss and slight power loss - felt like a single cylinder not firing. Over the next hour, the problem became worse (more frequent) - power loss and engine shake/miss most noticeablly at 1000-2000 rpm. 3000 rpm and above, you could feel a power loss - power would come back and forth.

When I pulled my plugs, I found 2 with a crack. One with a crack down either side from top to bottom along the ceramic. Another with a crack from the top about 1/3 the way down.

So my question is does this finding sound consistent with the problem? What causes a spark plug to crack so I can avoid this.

I will post my compression and other test shortly.

 

[brodie123]

Re: Mercruiser 165 - missing and power loss

Here are my test results:

Oil - good + clean - pressure is good
Fuel: fresh - lots of pressure (finger test)
Compression: 150 lbs on all 6 cylinders
Timing: 8 degrees
Plugs: very light tan - 2 had crack in ceramic
Distributor: brand new
wires: 4 years old - no damage evident


My next step is to replace all the plugs and take her out again next weekend. Please let me know if a crack in the ceramic is consistent with missing (kind of an easy assumption it is) and any other tests I should perform prior.

 

[cr2k]

Re: Mercruiser 165 - missing and power loss

That would make it miss for sure.