Octane question

[huckle]

First of all thanks for taking the time to look at the question. I appreciate it.

I have a 3.0 liter Mercruiser engine that's a 4 banger. I've had the boat for 3 years and treat her like my baby. I've kept up on all the maintenance. The carb was rebuilt last year, oils changed yearly, and even had the impeller changed last year. I am struggling this year with either dieseling, or hard starting after the engine is warm, and sits on the lake for a while. I was always taught the higher the octane the better for the engine in boats. I like to run 93 octane, when I can find it, and that seems to take care of the dieseling, but I still have to choke a lot when the engine is warm. If I run 87 octane I get the dieseling, but not the choking problems. Thoughts, suggestions? I also let the engine run at idle for around 15-30 seconds before I turn it off to help prevent diesling.

My boat is a 2003 Crownline 180.

Thanks for any advice.

 

[mpdive]

Re: Octane question

I have the same engine. Running high octane is a waste of money and the engine is not designed for it. It is for engines that run different compression ratios. As far as the dieseling goes, you are probably looking at carbon hot spots in the engine. This results in dieseling. Another culprit is the idle speed. The dash gauge is not very acurate, and as little as 20 rpm's is enough to make it diesel. Same as mine. Run a can of top end cleaner through the tank and lower the idle by 25 rpm's. As far as the hard start, look at the choke angle and see where it's sitting at after you have sat for a while. Dependant on engine temp and length of time with the engine off, the choke setting may be off. There are other possibilitys too such as leaking down, but start with the obvious first. Let us know the progress.

 

[Brentathon]

Re: Octane question

I agree, octane is primarily the fuel's rating for its resistance to knock (diesel, auto-ignition, etc.). Unless the mfr calls for high octane, the engine should run perfectly on regular.
Don't look to the fuel octane rating as the culprit.

 

[RogersJetboat454]

Re: Octane question

I agree with the other posters. Your engine should be perfectly happy running on regular gas. The "higher octane is better gas" myth is a fallacy started by fuel companies (who want to sell you more expensive fuel), and people who don't know what their talking about. Octane is a measure of the burn rate of the fuel. Not the quality of the fuel. Higher octane fuel has a slower burn rate than regular gas. The slower burn rate is needed by high compression engines to prevent detonation.

Your engine on the other hand has an issue with pre-ignition. Basically, the fuel entering the engine is being burnt with out being set off by the spark plug (dieseling). Adding the higher octane fuel is a band-aid repair. You need to make sure that your idle speed is set correctly, and check your spark plugs to be sure you have the correct heat range plug installed.

 

[mpdive]

Re: Octane question

I'm glad I didn't get "chastised" talking about fuel agin. It's a touchy subject around here........Thanks to the other posters for there input.

 

[Volphin]

Re: Octane question

Your engine timing sounds out of adjustment. I would check your base timing THEN adjust the idle. This will help the hard starting and dieseling.

V

 

[HT32BSX115]

Re: Octane question

Yeah. It could be a combination of timing, running hot and carbon deposits in the combustion chambers or on the spark plugsor even heat range....

MCSB 97-17 outlines some of the above conditions.

 

[huckle]

Re: Octane question

Thanks everyone for the input. Looks like another trip to the boat doctor to take a deeper look at the engine. The spark plugs are still factory, so it sound like maybe a timing issue, or an idle issue.