7.4 MPI Burning Rich?

[V.Marine]

Good day to all,

I have a 1999 260 Sundancer with a single Mercruiser 7.4L MPI/ Bravo 3 that is fouling up four spark plugs. The cylinders that are showing fouled spark plugs are #s 2,4,6 and 8.

The fouled spark plugs all look the same (matte black with very minimal oil residue).

The odd numbered cylinders show normal firing wear.

I have installed new spark plugs, new distributor cap and new fuel injectors.

The attached image is from about one hour of run time from the newly changed spark plugs.

Does anyone know what could be causing this issue?

 

[hottoddie]

That may be from oil not fuel. If compression is good the valve seals may need replacing if never done on a 23 year old engine.

 

[V.Marine]

That may be from oil not fuel. If compression is good the valve seals may need replacing if never done on a 23 year old engine.

You may be onto something, the valve seals have never been done before.

I performed a compression test on the engine and every cylinder fired at approximately 135psi.

I also hooked up a Rinda scan tool and did not find any codes.

 

[mrsrobinson]

I would think you'd smell the "richness" fuel with the engine idling as well if it was fuel related.

 

[V.Marine]

I would think you'd smell the "richness" fuel with the engine idling as well if it was fuel related.

Now that you mention it, I do not smell fuel.

 

[techmitch]

Seems odd that you are only running rich on the right bank.

 

[Bill Curtis]

Only rich on port bank would lead me to fuel problem / carb jet / choke and calibration in a carb'd engine, but an MPI?

What is common on the one side of the engine, but separate from the other side? The intake layout maybe? But a vacuum leak / bad gasket would be a lean condition. The fuel rail is common to both. Where does the breather enter the intake?

If it was oil (bad intake seals only on port side) burning, then there would be blue smoke, especially at start up.

Maybe there is a vacuum leak on Stbd and ECM is trying to get it right by richening up mixture. Were the injectors changed on port side at any time? Perhaps different injectors were used..

Just spit balling here.

 

[V.Marine]

Only rich on port bank would lead me to fuel problem / carb jet / choke and calibration in a carb'd engine, but an MPI?

What is common on the one side of the engine, but separate from the other side? The intake layout maybe? But a vacuum leak / bad gasket would be a lean condition. The fuel rail is common to both. Where does the breather enter the intake?

If it was oil (bad intake seals only on port side) burning, then there would be blue smoke, especially at start up.

Maybe there is a vacuum leak on Stbd and ECM is trying to get it right by richening up mixture. Were the injectors changed on port side at any time? Perhaps different injectors were used..

Just spit balling here.

The breather enters the intake on the stbd bank (bank with issues).

The boat is currently on the water and it is an I/O (exhaust is below waterline; won’t be able to see exhaust color while on the water).

The fuel injectors were replaced about three years ago by a local marina, I would have to remove the plenum and inspect the fuel injectors for differences.

I have a vacuum gauge and I can check vacuum.

 

[Bill Curtis]

The breather enters the intake on the stbd bank (bank with issues).

The boat is currently on the water and it is an I/O (exhaust is below waterline; won’t be able to see exhaust color while on the water).

The fuel injectors were replaced about three years ago by a local marina, I would have to remove the plenum and inspect the fuel injectors for differences.

I have a vacuum gauge and I can check vacuum.

Even when exhaust is below water line, you should get a blue smoke puff when starting (if using excess oil) or black puff (running rich on fuel.) , as the exhaust gasses rise to the surface pretty quick.

Mine smoked black when the choke was kaput. One thing you can check is under your swim platform, if you have one. The gas or oil smoke will collect under there and leave a film, if there is a problem.

Can you pull that breather tube off the intake and see if you have excessive blow by (oil smoke / mist)?

If it is excessive, that would be a possible culprit and an indication of bad valve seals, excess play between the valve stems and guides (normally intakes), or blow by on the rings or a bad oil scraper ring on the piston (s).

Compression and leak down tests may be in order, especially if breather smoke is excessive.