Johnson GT 150 Fuel Requirements

[WillyT]

First time on here in a LONG time. Hope everyone is doing fine! My buddy bought a used rig with a 1988 Johnson GT 150 on it. The engine had just been rebuilt by the selling dealer when he bought it. The dealer said that the engine had to be rebuilt because the previous owner had used premium gasoline, and he said that such fuel could (and did) damage this model engine. He warned my buddy to only use regular. While I know not all engines benefit substantially (if at all) from premium gas, I have never heard of premium gas damaging an engine (except for some early model Mercruiser 502's, where the electronic "brain" did some bad things when it "sensed" premium gas; Mercury later fixed that). Is this dealer "right on" or is the damage from premium gas in this engine a fairy tale? I would think that the enhanced engine cleaners that are typically in premium versus regular would help, even if the increased octane was not needed. What's the truth here? Thanks!

[Rusrog]

ridiculous thing I have ever heard....but it is very close.
The old cross-flows are very sensitive to octane. They will detonate at the drop of a hat and like the higher octane fuels (up to a point). Sounds like most dealers I see...
Incompetence reigns....

Russ Rogers
Ft Worth TX

[Rickracer]

.....pump gas that's too good or too high octane for a crossflow V-6. Maybe you should offer to take the dealer "snipe hunting"? :D

[WillyT]

Thanks guys. That's what I thought, too! The strange thing about it is that the dealer is also the mechanic that actually did the work, and he has been in business for years (and has a background in high-performace bass boat applications). Not sure if he has fallen off the dock one time too many or what. Anyway, I will pass this on to my firend. Murphy's Law being what it is, however, I will probably regret it since his engine has run great for quite a while now on regular. As soon as he uses premium, based on my recommendation, you KNOW what's gonna' happen...LOL!

[delawarerick]

Whats the compression one two what do the plugs read three what do you get for head temp. I have been playing with crossflows and have always at least fresh 93 octane. one thing I found out the hard way is that double oil even in break-en can lean the motor down. Cleaners and the likes when used must hold down the rpms. I dont profess to be knowledgable but my little 110 v-4 will turn 7k plus. Rick

[WillyT]

The engine is completely stock, so the compression should be in the typical stock range (though we can certainly check it). Bottom line is, however, that he ran this engine ALL last summer on regular with no problems, and has run it several times this year already! He uses stock "non-synthetic" oil. I keep waiting for the engine to blow, but it just keeps purring along. My buddy is not a "go fast" guy, and he seldom runs above the mid-40 mph range (he's a fisherman who actually bought the boat just to fish, not to race people the honey hole). I think the most he has seen is low-mid-50s (on a 1988 Winner bass boat with a four-blade prop). Still, if he needed the higher octane, I think he would have found out the hard way by now, don't you? Maybe he is just "holding his mouth right"...LOL! Like I said before, I will show him these comments, but it will be up to him if he wants to follow the advice. It may not make any technical sense at all, but I just have this feeling in my gut that he could continue to use regular for the next 5 years, and the engine will still be running like it does today. Thanks!