Plug fouling ox66/250

I've got a 2001 ox66/250 with almost 1,000 hours that has run rough and fouled plugs since the day it was new. I've had every tech in town, (Jupiter Fl.) work on the boat without success and am just about ready to unbolt the damn thing and bury it at sea. At this point, I'm convinced the techs have no clue or are to lazy to take the time to properly diagnose the problem. Any ideas what to look for would be appreciated.

The engine usually fouls the bottom plug on each bank at the same time. I can be running along nicely and will begin to hear an occasional miss that soon turns the engine into a five cylinder and shortly after a four banger. Mostly it's the two bottom cylinders but can also be the two top cylinders. A quick change of plugs and I'm off again for a few more hours and it happens again. It also won't hold a steady rpm., ever. Jumps around 500 to 600 rpm without touching the throttle. Any ideas before this engine goes to the bottom?

Re: Plug fouling ox66/250

What is an ox66/250? Anyway, try the next hotter heat range plug, that should help, without being too much of a change.

Re: Plug fouling ox66/250

I recommend checking the following at a minimum...

Throttle position sensor range & adjustment
Oil pump adjustment
Check thermostats

Almost every time I've experienced plug fouling it's been the thermostats.

You could also have a problem somewhere else in the electrics that causes it to misfire occassionally which then leads to the plug fouling. Get a manual and check all the electrical stuff.. plug cap resistance, coil resistance, etc.

Re: Plug fouling ox66/250

vvreddy said:

I recommend checking the following at a minimum...

Throttle position sensor range & adjustment
Oil pump adjustment
Check thermostats

Almost every time I've experienced plug fouling it's been the thermostats.

You could also have a problem somewhere else in the electrics that causes it to misfire occassionally which then leads to the plug fouling. Get a manual and check all the electrical stuff.. plug cap resistance, coil resistance, etc.

Click to expand...

I'm not a mechanic and have only basic mechanical skills and have to rely on the local tech's for anything more complicated than changing out parts. So far they have replaced the O2 sensor, both thermostats, and fuel pumps without a change. Nobody has ever mentioned the TPS sensor or oil pump adjustment. I swear they just want to get the boat out of there as quickly as possible and I can't get them to take the boat on an extended test run to experience the problem themselves.

I'd like to try one more time before moving on to another engine as this one runs very well...for awhile, despite it's high hours. Got anything else I can suggest they check on my next visit to the stealer?

I've got my eye on a Suzuki 4-stroke that I hear is a pretty good engine.

Re: Plug fouling ox66/250

sounds like they have poked and hoped.
works well for some I guess.
test the TPS,test the oil consumption ,this test is really not in the manual but can be extrapolated.
test the plug caps and very carefully check the oil pump rod setting. most the 3.1L motors I have to cut about 5 threads off the oil link rod to actually place the arm in the correct position. DO NOT bend the arm or rod.

Re: Plug fouling ox66/250

MikDee, in case you are still wondering, the ox stands for oxygen sensor, 66 is six cylinder, six injectors. At least that's how I always thought it should be.

Re: Plug fouling ox66/250

UPDATE: First, let me thank all the kind folks here that offered suggestions on what to do with the plug fouling problem.

I did manage to locate a mobile mechanic with good recomendations and had him come over and check all of the systems suggested. Everything seemed to be within spec with the exception of a round piece of metal stuck behind the O2 sensor. Appears to be a piece of the previous O2 sensor changed out two years ago. Removed it, replaced the sensor and have run the boat about four or five hours since without problem. Hopefully this will cure the problem after 900 hours and five years.

Re: Plug fouling ox66/250

dunno, but is it me or is something wrong with the math ?