Twin engines won't go past 3000 RPM

[Mike from Bristol]

I bought a 1998 Sundancer 290 at the beginning of the summer. Both engines ran well and the boat planed nicely when I purchased it, but now neither engine will go past 3000 RPM. They get to that at about ½ or 2/3 throttle, but even if I bury both handles they go no faster. The boat won’t plane, and goes somewhere between 11 and 16 mph depending on wind and tide.

I have two Mercruiser MCM 5.OL EFI, stern drives. Hobbs meter for engines reads 605, 601 hours. The port engine was rebuilt in 2021 because it had been improperly winterized in 2020.

In the beginning of June we did the sea trial and survey, the only anomaly was that the tachometers showed about 200 rpm difference when the throttles were matched and the engines sounded the same. That problem seems to have corrected itself. When we pulled it out to inspect the bottom it was painted and the props were beautiful.

I took possession mid June and everything was great. There was about half a tank of gas, I filled it, used about half a tank, refilled, and used another half tank all in the first week I had it. I left it on a mooring in saltwater (it had previously been kept in brackish water) for about two weeks, went back up for an overnight and it did as I said: 3000 top, no speed, no plane. I didn’t have time to do anything then, but I went back a couple days later dove in and found marine growth on the outdrives; the hull seemed clean. I scraped and brushed and got everything shiny again but no change.

One suggestion I’ve had is a clogged fuel vent, I’m told that if the vent’s clogged but I run with the gas cap off it will clear up and confirm that’s the issue.

So where do I go next? Almost every suggestion online: timing, bad plugs, clogged fuel injectors, etc. makes sense for one engine, but it’s hard to see the same problem with both engines at the same time. What systems are common to both engines? What else can I try?

 

[bobeast]

I would set aside the entirely reasonable assumption that it must be something common to both engines. Maybe it is and maybe it isn't. Start by concentrating on one engine and find what's wrong with it. The same issue may or may not be the problem with the other engine, but you'll know that in due course.

As for commonality, the condition of your hull is something to consider. If it is completely fouled it can have a large affect on the boat's performance. Also do your engines surpass 3000 in neutral or is this a strictly under-load problem?

 

[Strecker25]

I bought a 1998 Sundancer 290 at the beginning of the summer. Both engines ran well and the boat planed nicely when I purchased it, but now neither engine will go past 3000 RPM. They get to that at about ½ or 2/3 throttle, but even if I bury both handles they go no faster. The boat won’t plane, and goes somewhere between 11 and 16 mph depending on wind and tide.

I have two Mercruiser MCM 5.OL EFI, stern drives. Hobbs meter for engines reads 605, 601 hours. The port engine was rebuilt in 2021 because it had been improperly winterized in 2020.

In the beginning of June we did the sea trial and survey, the only anomaly was that the tachometers showed about 200 rpm difference when the throttles were matched and the engines sounded the same. That problem seems to have corrected itself. When we pulled it out to inspect the bottom it was painted and the props were beautiful.

I took possession mid June and everything was great. There was about half a tank of gas, I filled it, used about half a tank, refilled, and used another half tank all in the first week I had it. I left it on a mooring in saltwater (it had previously been kept in brackish water) for about two weeks, went back up for an overnight and it did as I said: 3000 top, no speed, no plane. I didn’t have time to do anything then, but I went back a couple days later dove in and found marine growth on the outdrives; the hull seemed clean. I scraped and brushed and got everything shiny again but no change.

One suggestion I’ve had is a clogged fuel vent, I’m told that if the vent’s clogged but I run with the gas cap off it will clear up and confirm that’s the issue.

So where do I go next? Almost every suggestion online: timing, bad plugs, clogged fuel injectors, etc. makes sense for one engine, but it’s hard to see the same problem with both engines at the same time. What systems are common to both engines? What else can I try?

you only have one tank feeding both engines so something like a plugged vent, bad fuel, etc., will affect both engines alike. There are however individual pickups for each engine.

First check your spin off fuel filters for water and replace them while you’re at it. Easy job on that boat. The fuel cap test is also worth doing because it’s easy.

 

[Golfman25]

If you're sure you hull is clean, any chance you picked up some bad gas?

 

[Jeff Delbuono]

I had a same issue with my 98 290 dancer. My fix was a timing issues. If 1 motor is off it will drag the other motor. Check the timing. Easy enough. Start small and easy before you go big and wide. Also make sure your bottom is clean. I run my big bitch at 3900 rpm at 30 mph. If i dont hit that speed i get the gear out and clean the bottom. Works everytime

 

[Setimos]

I bought a 1998 Sundancer 290 at the beginning of the summer. Both engines ran well and the boat planed nicely when I purchased it, but now neither engine will go past 3000 RPM. They get to that at about ½ or 2/3 throttle, but even if I bury both handles they go no faster. The boat won’t plane, and goes somewhere between 11 and 16 mph depending on wind and tide.

I have two Mercruiser MCM 5.OL EFI, stern drives. Hobbs meter for engines reads 605, 601 hours. The port engine was rebuilt in 2021 because it had been improperly winterized in 2020.

In the beginning of June we did the sea trial and survey, the only anomaly was that the tachometers showed about 200 rpm difference when the throttles were matched and the engines sounded the same. That problem seems to have corrected itself. When we pulled it out to inspect the bottom it was painted and the props were beautiful.

I took possession mid June and everything was great. There was about half a tank of gas, I filled it, used about half a tank, refilled, and used another half tank all in the first week I had it. I left it on a mooring in saltwater (it had previously been kept in brackish water) for about two weeks, went back up for an overnight and it did as I said: 3000 top, no speed, no plane. I didn’t have time to do anything then, but I went back a couple days later dove in and found marine growth on the outdrives; the hull seemed clean. I scraped and brushed and got everything shiny again but no change.

One suggestion I’ve had is a clogged fuel vent, I’m told that if the vent’s clogged but I run with the gas cap off it will clear up and confirm that’s the issue.

So where do I go next? Almost every suggestion online: timing, bad plugs, clogged fuel injectors, etc. makes sense for one engine, but it’s hard to see the same problem with both engines at the same time. What systems are common to both engines? What else can I try?

Check the high speed sensors, my next door neighbor has the same single engine and experience the same issue, his mechanic said it was the high speed sensor for the gas.

 

[techmitch]

Check the high speed sensors, my next door neighbor has the same single engine and experience the same issue, his mechanic said it was the high speed sensor for the gas.

Mike from Bristol was last seen: Aug 11, 2022

But I'll listen to you all night long telling me about those high speed sensors.
Who is this "mechanic" and where is he at?

 

[scoflaw]

Funny stuff

 

[PlayDate]

You guys are so negative. Unfortunately we never learned why both of the OPs engines "refused" to go over 3000 rpm.

In fact we never got to the point of asking him to check for any stored codes.

But we did get to the point of framastats and dohickeys which have been known to cause engines to do all sorts of strange things.

 

[Mike from Bristol]

Sigh,

Checking back in with nothing to really report.

Went back in the water and really scraped everything off the outdrives. Got a little more out of the starboard engine, but nothing more out of the port and still couldn't get out of her own way.

Checked the fuel vent --- no problem and no help. Ran the gas tank to near zero, filled it back up, added detergent/stabilizer, and changed the fuels filters --- no luck.

The twin challenges I have are 1) I'm in Connecticut and the boat is in Maine, so every attempt at a fix is a two -- three day sojourn, and 2) I can't even find anybody to work on it. Big boat yard I called said they didn't work on anything this old, first smaller yard I went to said they have too many boats and not enough people to get everything done in the short Maine season and so they prioritize their slip and previous customers. Of the two independent mechanics I was referred to one didn't return my call, and the second one said they would call back in a week and didn't. Finally did get it into a second small marina, they said their mechanic thought he saw metal filings in both engines' oil and they weren't interested in going further than that. I was skeptical, and went back up and checked both dip sticks myself, and I don't see anything, but that doesn't change the basic position.

At this point I've thrown the towel in for the season. It's sitting in marina number two rather than back up river on its mooring. It came with a trailer, so I'm going to see if the marina will load it up for me and I'll bring it down to CT and see if I can find someone down here that will work on it --- maybe before the weather closes out, maybe over the winter, maybe on a promise to put me first in line in the spring.

I will say, despite it not running right I did spend some beautiful nights on the mooring: feeling the tide change swing her around, watching the ospreys hunt, having coffee in the middle of a glassy harbor. So not a completely lost summer.

Next post: any recommendations or connections for boat yards in the Niantic/Groton area, and any tips for towing the big B. She's too big for my SUV so I need to rent a truck. I need to figure out how best to tie it down besides just the winch in the front.

 

[Golfman25]

Sigh,

Checking back in with nothing to really report.

Went back in the water and really scraped everything off the outdrives. Got a little more out of the starboard engine, but nothing more out of the port and still couldn't get out of her own way.

Checked the fuel vent --- no problem and no help. Ran the gas tank to near zero, filled it back up, added detergent/stabilizer, and changed the fuels filters --- no luck.

The twin challenges I have are 1) I'm in Connecticut and the boat is in Maine, so every attempt at a fix is a two -- three day sojourn, and 2) I can't even find anybody to work on it. Big boat yard I called said they didn't work on anything this old, first smaller yard I went to said they have too many boats and not enough people to get everything done in the short Maine season and so they prioritize their slip and previous customers. Of the two independent mechanics I was referred to one didn't return my call, and the second one said they would call back in a week and didn't. Finally did get it into a second small marina, they said their mechanic thought he saw metal filings in both engines' oil and they weren't interested in going further than that. I was skeptical, and went back up and checked both dip sticks myself, and I don't see anything, but that doesn't change the basic position.

At this point I've thrown the towel in for the season. It's sitting in marina number two rather than back up river on its mooring. It came with a trailer, so I'm going to see if the marina will load it up for me and I'll bring it down to CT and see if I can find someone down here that will work on it --- maybe before the weather closes out, maybe over the winter, maybe on a promise to put me first in line in the spring.

I will say, despite it not running right I did spend some beautiful nights on the mooring: feeling the tide change swing her around, watching the ospreys hunt, having coffee in the middle of a glassy harbor. So not a completely lost summer.

Next post: any recommendations or connections for boat yards in the Niantic/Groton area, and any tips for towing the big B. She's too big for my SUV so I need to rent a truck. I need to figure out how best to tie it down besides just the winch in the front.

Certainly sucks. An 1998 isn't fkg old. Just wait a few months when the recession really hits and these bastards will be working for food.

As for tie downs, get youself some good ratched straps. You should have u-bolts on the transome to hook into. Good luck.

 

[Jeff Delbuono]

The big girl is 13,000 lbs dry so more like 15,000 lbs. Make sure your truck is big enough for the tow. Good luck