jcimprover

I understand the 496 is a 9:1 to motor and with all the electronics, 87 octane is the fuel of choice. I am on my second season with this boat, the first season I ran premium, had a clean transom all summer. After listening to the oso board, I ran 87 octane all summer up until last week when the marina I was at only had premium. Before this weekend, I spent every sunday night cleaning black soot off the transom. One weekend of premium and the back of the boat is clean. Is there more detergents in the premium?
I don't get it.
Any insight would be helpful.
thanks
JC

hillbilly24

the detergent packages are the same for any brand of fuel throughout their unleaded product range. Diffrent octane fels have a diffrent heptane content, that is what determines the octane rating, as far as soot on the transom it would make more sense to have more with the higher octane then the lower. Higher octane fule does not burn more thouroughly or with more heat it is just harder to ignite and slightly slower burning therefore ussually you will see more soot when you run high octane in an engine desgined to run regular becuase it does not burn completley. I would question your marina as to what they are actually selling you, many are known to do alot of mix and matching.

Hydrocruiser

ARe you buying gas at stations on land or at a marine? Does the gas have Valvetech in it.

The reason I ask is because Valvetech is an oil based distillate and it can put out some unburned carbon as can the products you use to stabilizer gas for storage.

Just an FYI..

Randy Nevers

I have a 1995 MerCrusier 502 Mag EFI in my Baja Outlaw, recently I have experienced a stalling problem after I make a decent run. When I slow to idle after running, the engine will stumble and eventually stall out completely. After a while it will re-crank and run a while longer. I have recently changed my high Pressure pump in the VST tank and the Idle air control valve as well. Neither of these changes have made a difference!! Can anyone help shed some light on this problem? I am going to miss the Emerald Coast Poker Run if I can't get it straitened out by Friday!!

Dave_N

Randy,
You may want to start a new thread with your problem. You will get more responses.

(for the new thread) Are you seeing any evidence of a overly rich condition, like gas vapors/water sheen when this occurs? Have you checked fuel pressure?

Dave

vandeano

What is water sheen?

Dave_N

I was referring to unburned gas sheen on the water. Sorry if I was unclear.

Dave

saubouin

This whole soot on the transom thing is so weird. When I first got the boat it didn't do it. Then it did it pretty bad. Now it's stopped again. I can't figure out if it's because I switched Octane 87 to 92 back to 87, or because of extended idling. However I just spent a week straight boating everyday with extended idling out of our harbour and thought for sure I would have a sooty transom by weeks end. I pulled the boat out and the transome was clean. Between talk of detergents, octane differences, pig-rich factory A/F ratios and extened idling I give up trying to figure this one out.

GOODT

FYI Depending on when you purchased your boat merc did have a computer upgrade for that problem. merc calls it trailering affect and it is very common on 496s with integrated swim platforms and occurs mostly when idling though no wake zones

tcelano

Back to the original question, I'd be inclined to think that when running 87 octane, the motor probably begins to knock just a bit, and the computer richens up the mixture to make it stop. When running higher octane fuels, the ECU will find a happy spot with no knocking at leaner A/F mixtures, resulting in a cleaner transom.