1E081297 300R I just changed the oil last week ran it for the first time yesterday. Developed an engine miss after a short run. #8 spark plug is fouled. Replaced plug better for a short time but now missing again. Seems like I read a comment from Don about not filling these engines to the top bead on the dipstick. That is where mine is at. I it a possibility that this is causing the misfire issues. Draining 1/2 to 1 quart out may fix it?
Last edited by hooken203; 04-14-2024 at 08:42 PM. Reason: Not a serial number.
That is overfull, though I'm not completely convinced that is the cause. However- let's start with the obvious (most of these engines are at halfway up the stick by 6 qts).
Note that if one plug was fouled, it's quite likely others will be directly behind it.
BTW... the number provided is a model number.
Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor
Correction made on serial number. Oil level being cause of miss was wishful thinking on my part. It was running ok and that was the only thing I did so that was my thinking on it. Today I drained about 24oz of oil out of it this put it mid way on the stick. Took it back out and ran it no change. So now have checked resistance on the coils and 4 injectors readings were the same on the coils and the injectors. The miss is intermittent so good readings didn’t really surprise me. By the way it feels to me it’s a coil or plug wire. Don when you hook up to these engines with what I call a scan tool can you see misfire codes and cylinder miss fire counts?
Very limited on that side. What you can see are O2 figures LIVE, or where fuel trims have exceeded their preset max parameters.
How many hours do you have on this engine at this point?
Checked plug wires and replaced ALL spark plugs?
There are scenarios that would cause fouling of #8 that have nothing to DO with #8 (at all). And we do occasionally see injector harness problems (usually bottom two cylinders, often on the STB side but can occur on Port side as well). Often simply that the harness became pinched under the plastic cover retaining boss/bolt.
Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor
174 hours on this engine. I could not get my hands on spark plugs today. So I have not replaced them or the plug wires.
Similar issue on mine… final diagnose and fix was a coil wire… dealer found by swapping out the wires …
Sterling 22XS / Mercury Pro XS V8
Service manual has resistance tests listed for the plug wires- very handy to be able to test them while still on the engine (have caught SEVERAL this way).
Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor
I did not have time for my guess work so I took it to the dealer. Dealer ran cylinder power balance this didn’t show a problem. Not surprising because it’s intermittent and needs to be under load for miss to happen. Dealer recommended spark plug change and because of what he also saw with the number 8 spark plug his thought was there was an injector problem. So he swapped 6 and 8 injector to see if it would move to #6 cylinder. I got it back took it to the river I put 2 hours of run time on it and no miss. I suspect a small amount of water in the fuel causing the injector to be blocked intermittently. When the injectors were swapped fuel and water both drained from rail. So I am going to check the rest of the fuel system for signs of water.
Hmmmnnn... make sure to remove the Fuel Filter (the one with the water in fuel sensor) and dump it's contents into a glass jar. If water is found there, the fuel system module (FSM) is likely also affected.
My bet is on the spark plugs. When there's one cylinder not firing correctly (or at all) the PCM will jack up the fuel delivery to that bank as it's seeing too much oxygen (making the remaining cylinders very rich). This is so common that I've seen it used as a training bug more than 4 times on different engines, in the SAME CLASSES' LAB EXERCISES.
Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor
No water found in fuel filter. Does this engine use more that one O2 sensor?
Only one.
Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor
Reply With Quote