'01 250HP OX66 Surging at 3700RPM

[DennisG01]

Hello all. I'm relatively new to the big outboard world (tons of experience with inboards/sterndrives). But, I've been extremely impressed with the way this 250HP OX66 is performing on the back (bracket mounted) of a Grady 24' Offshore that I rebuilt (floor, bulkheads, etc) over the winter. Here's the one issue I'm having that I can't quite figure out...

At approx 3700RPM (running in very calm seas and a light load), after a few minutes, the RPM's will drop to 3500 for about 20 seconds, then back up to 3700 for about 20 seconds, and then continue to repeat like that. It's almost like clockwork the way it happens.

I had a long run at about 3800 - 4200 RPM's and this never happened. It only seems to be happening at 3700... but I'm still new to the engine so I don't want to say with "certainty" that this is the case. But, none the less, it does seem to be happening consistently at 3700.

The fuel is clean (both engine mounted filter and inline fuel/water sep where changed). I also dumped fuel out of the f/w sep and it was good. The previous owner (and now myself) always use Startron and RingFree.

Thanks for any guidance!

[99yam40]

just how old is the fuel?
since you have been working on rebuilding the boat the fuel may be fairly old,
even treated fuel will not last forever.

Rig up a portable tank with fresh fuel to make sure your fuel and or tank/lines are not a problem.

monitoring fuel pressures while problem is showing should direct you if there are problems with the fuel delivery

[DennisG01]

Fuel is good, I'm sure of that. The rebuild was only over the winter. The boat was used last season (treated) and a few days ago I put in another 70 gallons (92g tank). When I had dumped the f/w sep a little while ago, the fuel looked and smelled fine. I'm confident is saying the fuel is good, but I appreciate the thought - I would probably recommend the same thing to someone else!

I left my pressure gauge (had planned on bringing it "just in case", too!) back in PA - I'm in ME right now with the boat (where it stays). But... if there was a problem with the fuel delivery, wouldn't this problem show up at other RPM's?

[DennisG01]

Sure, I can do all that. But don't you guys think that I would then be having problems at other RPM's, too?

[TownsendsFJR1300]

Sure, I can do all that. But don't you guys think that I would then be having problems at other RPM's, too?

Possibly or probably, but rule out the basics first, then you can move on.

Just for S&G's, try draining the VST (with the fuel currently in the tank) into a nice clean container and let the fuel settle.. See if it stays clear or clouds up..
You can squeeze the primer bulb and get more fuel out to inspect if needed.


Please post what you find..

[DennisG01]

Didn't yet get a chance to run it on a portable tank, but I ran for about 1-1/2 hours today but had trouble keeping it at 3700RPM with the load that was in the boat today and the conditions (still pleasant, though). Ran great. First thing this morning, though, I drained some fuel. This is what it looked like after sitting about 3 hours. It looks even better in person - there is not a speck of dirt or water anywhere.

[IMG][/IMG]

[TownsendsFJR1300]

Agreed, looks good..

Even thou the fuel looks good, it doesn't eliminate the fuel pick up. As posted above, running off a separate container wouldn't hurt...(and rule out the fuel pick up)

[DennisG01]

Well, the engine is not being agreeable to figure this out -- but I suppose it's not a bad problem to have. I have not been able to get the engine to start acting up again so I have not yet put a portable tank on it. If I put the portable tank on now, we wouldn't know if it was solving the problem or not. I need to wait till the surging is more consistent.

But... let's assume for a minute that the engine is acting up even when it's on a portable tank (I've only got a couple more days up here with the boat and then it'll be a while till I'm back). If for nothing else, to give me some ammo for the future in case it starts up again.

What would be a few more things to check? Fuel pressure is one of them... what else? The key here is that it is (was) only happening at that specific RPM - other RPM's and WOT has always been good.

[TownsendsFJR1300]

Well, if its intermediate and is considerably better, IMO, (and I'll swear by it), is Yamaha "Ringfree". Its way better than Seafoam, Techtron, etc. It cleans the fuel system as well as the top end. Put some in there and let it do its job.

I've used it on small engines as well and could literally hear the difference as the carb cleared up(8hp chipper /shredder, wouldn't idle, misfire @WOT), Misfire went away, idles once I got done tossing a boat load of tree limbs in it..(about an hours run time, put in a very heavy dose)

I use it and K100 ONLY on my F150, and the engine runs like a top. You can put that Ringfree on the heavy side and it won't hurt anything. A small container, nce you mix it at 1oz per 10 gallons isn't bad.

The K100, IME, in almost 3 years, has removed every drop of water in my built in fuel tank (1997 vintage) as I check both filters, (transom water/fuel separator) and the Yammy filter plus drain the VST occassionally checking for debris water. The K100 is an excellent stabilzer as well. Use something to stabilize the fuel, "Gas shok" also works extremly well.
The tank has NEVER been drained or removed, never pulled the PU tube...

The engine is an 06, still have the original VST filter in it...


A short video and it does indeed do as is shown;

Demonstration Videos | Kinetic Fuel Treatment

[DennisG01]

Thanks, Scott. Yes, I'll think I'll just have to wait this one out and see what happens. Thanks for the tip on the K100, too.

Yes, the gas that's in the boat now (same as the picture I posted) has a heavy dose of Ring Free. I also use Startron in all my gas engines - from weedwhackers to all my boats. I've been using that stuff for years and even with ethanol gas, I have never had a fuel issue.

Boscoe or Rodbolt, if you're happening to read through this... for "future" reference, should this issue become more prevalent, is there anything else you would recommend I put on my checklist (besides the fuel pressure as mentioned above)? Have you ever seen a problem like this before? With Mercruisers, I have not seen this type of thing - it seems like a "strange one".

[DennisG01]

Reviving and old thread...

Still having the issue (albeit sporadically). To sum up, the engine pretty much runs like a top - throttles up great and really, other than this little issue, it's perfect. The only issue is that when cruising around 3,700RPM's, the RPM's will start to fluctuate down a few hundred RPM's and then back - not rapidly - more like a smooth, con*****ed fluctuation that can takes about 20-30 seconds to happen. It "feels" more like someone is gently throttling back and then after a short time gently throttling back up. The RPM's "roll" down and up - they don't "jump".

Could weak LP lift pumps cause this? I would think that if that was the case that I would have MORE issues at higher RPM's as they can't keep the HP tank filled in relation to demand?

I will get my hands on a portable tank, none-the-less, and give that a try (as I had "meant" to last season). If for nothing else, as mentioned above it helps to eliminate variables. I'll look/feel the primer bulb as well.

[zenoahphobic]

I presume it is doing this regular cycling again. And I presume you are still sceptical about fuel and flow issues.
Could your engine be adjusting from input from sensor somewhere - because it sounds like it is detecting something, making adjustment (ignition timing as an example) waits for sometime to see if it is getting the required result, thinks it is, then goes back to original setting.
Don't know much about this engine, but I hazard that temperature has something to do with it.
Sorry not much help, but throwing out an idea.

[rodbolt17]

fix your fuel system.
leave the sensors alone.
preferably you will fix the fuel system BEFORE you burn a piston, not after.
your symptoms are classic, I do a few every week.
when I am not fixing F4.2L or F v8 stuff.

[DennisG01]

While I'm doing the portable tank thing and checking the primer bulb, is there anything else to do, as well?

For what it's worth, in the off season I had my injectors cleaned and rebuilt by a local, reputable injection shop. They turned out to be pretty good to begin with, but I had no idea when they were last done (if ever), so I thought it would be good PM to have it done.

EDIT: Oh, Rodbolt, what type of things have you done to fix the ones that come into your shop. Thank you for the help!

[DennisG01]

Well, I ran for a solid 30 minutes at 3700/3800 RPM's and it didn't act up and I was still connected to the main tank. I was hoping to get it to act up and then switch over to the portable. I understand that this can be a difficult thing to diagnose - for one, none of you have the engine right there in front of you and two, it's tough to do when the problem isn't replicating. If anyone has any other thoughts or words of advice - maybe other things to check (Along with the portable) - much appreciated!