Mercruiser Horizon problem

[Skipper Dick]

My B28 has the above engines and they are just over 500 hours. About 6 months ago, I put in a new set of platinum NKG TR5IX plugs and both engines purred like kittens. I didn't have a fuel monitor then, but I could tell the engines liked those plugs.

Lately though, the starboard engine has lagged behind. It uses more fuel now than the port and blows a little smoke. You can see it slightly when cruising and it is on the side of the boat. I lags when incresing the throttle slow or fast.

I pulled the plugs on the starboard today and they were covered in black carbon. Not gas and not oil. So I wire brushed them off with a soft brass brush and checked the gap and put them back. And it ran the same. Hesitating on throttle up and lagging when you push the throttle up fast. No backfiring, but a faint smoke. I checked the injetors and the fuel pressure and they are right on, so I pulled a couple of plugs again and they were carboned up again. When I am running at the dock and the exhaust is blowing under the dock, a strong smell of rich exhaust can be smelled.

I have a Rinda scan tool and hooked it up to the MEFI 3 computer, but no error messages. I know it is running rich, but nothing in my manual covers a troubleshoot for carbon fouled plugs and getting the fuel rich mixture back to normal. These damn computers are too smart for me I guess and unless you do this type of stuff every day, you are just a dude with a manual. I've got a feeling it is one of the sensors, but I haven't got a clue where to start or how to get a fuel injected system to shoot the right amount of gas and air into the manifold.

Has anyone had this problem or know of a possible solution.

Thanks

Dick

[Ted_R]

Hey Dick>>>> don't know what sensors are on that engine, but your book should show you. My first guess would be a water or cyl head temp sensor, telling the computer it's cold (not yet to operating temp) and the comp is calling for more fuel. Good luck!!
Ted

[Bruce]

Dick,
One thing about Rinda diagnostic computers, they won't tell you if a sensor is off voltage.

Get yourself the Merc ( not clymer or other brands) shop manual. It will show you how to check voltage and resistance of the sensors.

To the computer it appears to be working but say if the temp sensor is telling the computer the engine is still cold after warming up because its not quite right, it will run rich cause the computer will be giving it more fuel as if cold.

Check temp sensor, map sensor. The Rinda scan tool will do an injector test, it will also show fuel consumption data.

My guess would be temp sensor since multiple plugs show fouling.

[Skipper Dick]

Well, mystery solved. I took the advice of the board and moved sensors from the port to starboard engine, but the damn thing still ran the same. Then a little light went off and I remembered that when I was checking the injectors out last week, every time I turned on the ignition to get the fuel pressure up again for the next injector to fire, the pressure was up 65 psi then dropped to 28 when the pump shut off after two seconds. So I took the cover off the fuel pump and cooler box with the engine running and the psi at 65 constant and I tapped the pressure regulator which is attached to the cooler. The pressure went up to 85 psi. So I removed the regulator and was getting ready to find a part number on line and call around to see if anyone had one. I was sucking up a cool one just looking at the parts breakdown in my Merc manual and noticed this little screen filter image that was just under the regulator, so I gingerly edged it out and it was crammed full of crap and there was a good 1/4 teaspoon of junk behind it. That was easy to blow out and once the screen was cleaned and put back, the engine ran fine. The gas could not get to the regulator to be regulated, so it just kept the whole manifol and fuel rails pressurized at 2 to 3 times the the specified pressure. No wonder it ran so rich. It only cost me a lot of sweat, blood from cut knuckles and an apology to my neighbors wife who probaby heard everything I was saying to the fuel pump/cooler taking it apart and putting it back together.

Dick

[Bruce]

With the higher pressure your right, the amonut of fuel injected would have been greater leading to fouling.

Fuel pressure readings are important to diagnosing FI systems.

That would have eliminated electric issues.