8.1s - RPM issue under load

Team PCBeach

Active Member
Oct 12, 2010
183
Boat Info
360 Sundancer 2003, SIMRAD 3G Radar, Raymarine Autopilot
Engines
Twin 8.1L Horizon
Long post here, but wanted to include all information.

2003 Sundancer 360 with Merc 8.1s. 465 hours salt water both engines. Back in April/May had the boat hauled due to a vibration when on plane. This was after having the boat trucked from the Fl Panhandle to Jacksonville. Ended up having the cutlass bearings replaced, props scanned and aligned, shafts pulled and machined and engines aligned. Vibration gone and ran 3200-3400rpm for 32 mile trip back to home marina. Boat ran better than it ever has since we bought her in Aug 18. Boat then sat until this weekend due to a surgery I had. We did get to the boat and start the engines once a month, but could not run her.

Went our for the first time Saturday and could initially neither engine would go above 2200rpm when under load. No alarms. Put transmission in neutral both engines easily rev to 3600+ and sound great. Drove around for about 30 minutes going a short way with the with the current and against the current. A little improvement, PORT will come up to 3000 and STBD 3600 at WOT. Boat is lightly loaded with two adults and 100gal of fuel. Diver cleaned the hull 3 weeks ago.

The only maintenance that has been done since the perfect 32 mile trip in May was to replace the PORT IAC and the IAC mufflers in both engines. Just to make sure the mufflers were not the cause, pulled them today and did a test run. No change 3000 PORT, 3600 STBD. Put the mufflers back in.

Any suggests on a possible cause or what to look for? I am going to have the diver dive the hull early to clean again and look for any possible fouling. The only time I have had an WOT RPM issue in the past was when I had a significant fouling issue due to me being new to wet slip boating and not having a diver when we first bought the boat.

Also I am due for plugs so am going to have them replaced (mechanic always does compression check when changing plugs). I am skeptical this is the problem since my issue is in both engines and they ran great back on the last trip in May. Also, last fuel fill-up was in April and stabilizer was added at the time to both tanks.
 
Long post here, but wanted to include all information.

2003 Sundancer 360 with Merc 8.1s. 465 hours salt water both engines. Back in April/May had the boat hauled due to a vibration when on plane. This was after having the boat trucked from the Fl Panhandle to Jacksonville. Ended up having the cutlass bearings replaced, props scanned and aligned, shafts pulled and machined and engines aligned. Vibration gone and ran 3200-3400rpm for 32 mile trip back to home marina. Boat ran better than it ever has since we bought her in Aug 18. Boat then sat until this weekend due to a surgery I had. We did get to the boat and start the engines once a month, but could not run her.

Went our for the first time Saturday and could initially neither engine would go above 2200rpm when under load. No alarms. Put transmission in neutral both engines easily rev to 3600+ and sound great. Drove around for about 30 minutes going a short way with the with the current and against the current. A little improvement, PORT will come up to 3000 and STBD 3600 at WOT. Boat is lightly loaded with two adults and 100gal of fuel. Diver cleaned the hull 3 weeks ago.

The only maintenance that has been done since the perfect 32 mile trip in May was to replace the PORT IAC and the IAC mufflers in both engines. Just to make sure the mufflers were not the cause, pulled them today and did a test run. No change 3000 PORT, 3600 STBD. Put the mufflers back in.

Any suggests on a possible cause or what to look for? I am going to have the diver dive the hull early to clean again and look for any possible fouling. The only time I have had an WOT RPM issue in the past was when I had a significant fouling issue due to me being new to wet slip boating and not having a diver when we first bought the boat.

Also I am due for plugs so am going to have them replaced (mechanic always does compression check when changing plugs). I am skeptical this is the problem since my issue is in both engines and they ran great back on the last trip in May. Also, last fuel fill-up was in April and stabilizer was added at the time to both tanks.
I would first pull the distributor cap and look for corrosion. If the boat sat for awhile that can happen. Also, I recently had a similar problem and it ended up being a distributor coil. It either has to be electrical or fuel related….my bet is electrical.
 
My bet is there is nothing wrong with the boat. Have the diver do a thorough bottom and running gear cleaning. I think things will be back to normal.
 
You could have a fuel pump issue. I had an issue this year where my port engine wouldn’t go over 2200 rpm. I would get her back to the slip check the usual things and fire her back up, while sitting at the dock and tied up she would rev up past 2200 and go all the way to 4K rpms with no issues. However, there was no load on the engine. So I thought I was good to go but as soon as I got her back out in open water and opened her up same issue of not getting over 2200 rpm. Ultimately had to replace the fuel pump and haven’t had an issue since.
 
My bet is there is nothing wrong with the boat. Have the diver do a thorough bottom and running gear cleaning. I think things will be back to normal.

Unfortunately the hull cleaning made no change. I kind of expected this as our last cleaning was just 3 weeks ago.
 
You could have a fuel pump issue. I had an issue this year where my port engine wouldn’t go over 2200 rpm. I would get her back to the slip check the usual things and fire her back up, while sitting at the dock and tied up she would rev up past 2200 and go all the way to 4K rpms with no issues. However, there was no load on the engine. So I thought I was good to go but as soon as I got her back out in open water and opened her up same issue of not getting over 2200 rpm. Ultimately had to replace the fuel pump and haven’t had an issue since.

Thanks for the suggestion. I will take a look at this. I am thinking my issue is just with the STBD. Since STBD won’t come up above 2900, I think that is causing the PORT to be limited to 3400 as it is working too hard to push the boat.
 
Thanks for the suggestion. I will take a look at this. I am thinking my issue is just with the STBD. Since STBD won’t come up above 2900, I think that is causing the PORT to be limited to 3400 as it is working too hard to push the boat.
In your first post, you said you were due for spark plug replacement. I don’t have your engine(s) but I’ve heard that they go through plugs pretty quickly.

If this was me, I’d do the plug replacement and maybe even the compression test you mentioned.
 
I would first pull the distributor cap and look for corrosion. If the boat sat for awhile that can happen. Also, I recently had a similar problem and it ended up being a distributor coil. It either has to be electrical or fuel related….my bet is electrical.
No distributor on the 8.1 motors.
 
Unfortunately the hull cleaning made no change. I kind of expected this as our last cleaning was just 3 weeks ago.

How do the transmissions sound at idle? (I'm asking because these engines don't tell you much when it is down one cylinder due a plug or wire.) If a wire comes loose or a plug goes bad, you will generally hear what folks describe as rattling or marbles in the transmission. But since the boat is relatively new to you, you may not have a good feel for proper sounds versus improper sounds. I would first change out plugs and wires, which can be pretty easy.

I used Volvo 8.1 wires because they are WAY less expensive than Mercruiser but still "marine" parts so folks don't whine about non marine parts. I also use iridium tipped plugs (straight auto plugs, proceed to whine).

Volvo Penta 3861295 plug wires
NGK TR55IX plugs
 
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You could have a fuel pump issue. I had an issue this year where my port engine wouldn’t go over 2200 rpm. I would get her back to the slip check the usual things and fire her back up, while sitting at the dock and tied up she would rev up past 2200 and go all the way to 4K rpms with no issues. However, there was no load on the engine. So I thought I was good to go but as soon as I got her back out in open water and opened her up same issue of not getting over 2200 rpm. Ultimately had to replace the fuel pump and haven’t had an issue since.

Luckily the '03 360 didn't have the cool fuel Gen III issue, so you don't need to go down that rabbit hole. But, I if I recall correctly, there are two fuel pumps. It is relatively easy to check the fuel pressure by connecting a gauge to the Schrader valve just under the plastic shroud.
 
Check the fluid levels and for clean filters in the transmissions. A neighbor experienced deterioration of RPM following several 25 mile runs and found that their transmission fluid levels were low and filters gummed up. They flushed the transmissions and replaced the filters which took care of it.
 
Check the fluid levels and for clean filters in the transmissions. A neighbor experienced deterioration of RPM following several 25 mile runs and found that their transmission fluid levels were low and filters gummed up. They flushed the transmissions and replaced the filters which took care of it.

Thanks, transmission fluid was just changed before our last good run (32 miles). I will check levels just to make sure.
 
How do the transmissions sound at idle? (I'm asking because these engines don't tell you much when it is down one cylinder due a plug or wire.) If a wire comes loose or a plug goes bad, you will generally hear what folks describe as rattling or marbles in the transmission. But since the boat is relatively new to you, you may not have a good feel for proper sounds versus improper sounds. I would first change out plugs and wires, which can be pretty easy.

I used Volvo 8.1 wires because they are WAY less expensive than Mercruiser but still "marine" parts so folks don't whine about non marine parts. I also use iridium tipped plugs (straight auto plugs, proceed to whine).

Volvo Penta 3861295 plug wires
NGK TR55IX plugs

Hopefully replacing the plugs this week. I did hear the rattling you describe when first pulling away from the slip. The noise went away once things got warmed up and was not there when we pulled back into the slip.
 
Bad plug, bad wire, or failed injector. Almost all poorly running 8.1 issues are one of these 3 things. Two friends boats this year with 8.1 and low rpm issue. One was a bad wire, one was a clogged injector…..
 
I wanted to follow-up to close this out. Finally was able to get the plugs changed. Also topped off the fuel since we were down to 1/3 tank since the last fill-up in April (with stabilizer added). Boat ran perfect! I am guessing it was the plugs as the fuel should not have gone bad in 6 months with stabilizer. Also compression check was good with 165-180 in all cylinders. Thanks for everyone's suggestions!
 
I would first pull the distributor cap and look for corrosion. If the boat sat for awhile that can happen. Also, I recently had a similar problem and it ended up being a distributor coil. It either has to be electrical or fuel related….my bet is electrical.
No distributor in 8.1 engines.
 
I wanted to follow-up to close this out. Finally was able to get the plugs changed. Also topped off the fuel since we were down to 1/3 tank since the last fill-up in April (with stabilizer added). Boat ran perfect! I am guessing it was the plugs as the fuel should not have gone bad in 6 months with stabilizer. Also compression check was good with 165-180 in all cylinders. Thanks for everyone's suggestions!

Thanks for following up ... it's always great to see that your issue is resolved, and hear what the resolution was.
 
I am surprised plugs affected both engines, at the same time. But assuming they were last replaced at the same time I guess it makes sense.
 
We used platinum tipped plugs in our Crusader 8.1s and the engines ran wonderfully well even when the plugs started to go. We used to change them at the recommended intervals but could never notice any difference in how the boat ran before or after the change out so we stopped doing that. Then, at around 250 hours the boat would start to miss a bit but but would quickly recover. We learned this was the heads up that new plugs were needed and we would change them. That stumble was about the only clue that work was needed. Our range never seemed to suffer. Loved those 8.1s. Great engines. Sorry GM stopped building them.
 

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