QSB’s, stb engine limited to 1350 RPM, Port engine fine until stb engine on

[Brian Rapp]

2006 Tiara Open with twin engine Cummins QSB 380’s, straight shafts. New to me in July 2020. Was fresh water boat until last summer. Boat ran flawlessly all last summer, never skipped a beat. Just under 1,000 hours. Full history of service, seemingly well maintained.

Symptoms: Port engine starts, revs, runs fine UNTIL stb. ignition is switched on (even without stb. engine running). Then Port engine is limited to ~1350 RPM. Stb. engine will never run past 1,350 RPM, with or without Port engine ignition on or Port engine started/running. Seems to be an issue with Stb. side, but unclear why turning on Stb. ignition immediately limits Port engine.

Stepping through the events:

Both engines off, ignitions off.

Turn on Port engine ignition
Start Port engine, starts promptly, no smoke, no stumbles
Idles fine, rev’s fine, full RPM range (in neutral or in gear)

Turn on Stb engine ignition, but without Stb engine started (and instantaneously the Port engine will not rev past ~1350RPM in neutral or forward gear)
Turn OFF Stb. engine ignition (and now Port engine is fine, revs full range in neutral or gear)
Turn on Stb engine ignition, but without Stb engine started (and again Port engine becomes limited to ~1350RPM)
Start Stb. engine, starts promptly (neither engine will rev past ~1350 RPM)

Turn off Port engine and ignition. (Stb. engine still limited to ~1,350 RPM).

Drove boat from launch ramp for 25 minutes at 6mph (no wake zone) to mooring, attempted speeding up, but engines got to ~1350 RPM but nothing more. Tried with and without engine sync. Without engine sync, I could bring either engine up in RPM to 1350 independently from each other, but neither engine would exceed ~1,350 RPM. With engine sync on, they rev’d up fine, but limited to still the ~1,350 RPM.
Never could hear the turbos spool up. Boat runs at these RPMs just like normal. Temps, pressures, fuel consumption all seem fine. No engine alarms.

Repeated process above with each engine on/off, etc., same results. Never any fluctuation.

No smoke, no stumbles, engines sound great, low idle at 550 or normal idle 650, rev evenly with similar fuel consumption up to 1350 RPM, but nothing more on both engines. Shift in/out of gear fine.

Boat ran beautiful all last summer season. Removed boat from water in October, winterized, and had started and run engines before launch, but didn’t rev them up higher than probably 1,200 RPM or so.

Both engines services as follows over the winter:
Port and Stb. engine aftercoolers removed and serviced with SBMar kit, aftercools were in good shape and had come apart easily (was a fresh water boat until last summer). Serviced and reassembled, reinstalled.
Port and Stb. replaced all Cummins engine hoses on both engines using SBMar hose kits.
STB engine only – Replace thermostat on Stb. engine, and coolant change (Fleetguard ES Complete).
STB engine only – Serviced heat exchanger, replaced gaskets at end on STB engine only. Showed no signs of debris or build up, so did not service Port side.
Port and Stb. replaced raw water hoses. Did NOT touch large ~6″ exhaust hoses.
Port and Stb. water impellers were replaced last July, and then removed in October 2020. Still appeared like new (only ~30 hours on them), left out of pumps for winter, then reinstalled with new o-rings.
Replaced both Engine start batteries (old ones were ~10 years old) in May 2021 just before launch, with new Crown batteries. Engine batteries were out of the boat for ~1 week. Battery switches were left off during this.
Port engine replaced belt tensioner (had replaced Stb. tensioner July 2020, and all belts replaced in 2020).
Replaced 10 micron Racor filter for each engine, both filled with diesel. Secondary filters at engines NOT changed (were changed last year).
Oil/filter changes on both engines done in October 2020.

Also installed new Garmin MFDs, autopilot, etc. and ran a lot of new wires into helm area (and removed old ones). Can’t rule out having impacted a wire connection at helm.

Could have impacted wire harness or connector somewhere around engines during hose replacements, aftercool removal/install. Look for disconnect plugs, but didn’t notice any.

I took pictures of engine displays while running, which I can post if value add, alone with engines, etc (engines look like new).

Thoughts?

Topic

[Tony Athens]

Brian,

All is GOOD!

Thanks for sharing..

Tony

[Brian Rapp]

Cruise switch was the culprit

I can’t believe that I was fowled up by the cruise switch. I had never had a boat with this feature, and once I got on board I could see it was in the down position of a 3-way toggle switch (appears almost the same as all the other 2-position switches also in the off position). Once I flipped it to the middle position, the engines ran flawlessly with full rpm range.

So for anyone else that may fail to realize the function of these switches, I will share my generic understanding here. The cruise switch is an electronic RPM limiter. It is connected to the master throttle which in my boat is the starboard side (side used when throttles are synced), and thus it does not effect the port engine until the starboard ignition is switched on. The Cruise switch is a 3-way toggle switch, with cruise 1, off, and cruise 2 positions. Both cruise 1 and 2 can be adjusted independently in 25 rpm increments using an RPM +/- toggle switch nearby the Cruise switch. These are then remembered/saved. Once you are running, you can switch on the cruise, and push the throttles all the way forward but the RPM will never exceed the setpoint. You can then also use the RPM +/- switch as a means of increasing or decreasing your engine RPMs as a form of “cruise control”. You do not need to have the throttle sync “on”, and if left “off” then you can control throttles separately, but neither can exceed the setpoint rpm.

Certainly glad it was an easy fix, but mostly I’m just embarrassed. Being convinced it was a true “problem” I was going to pull my hair out trying to find root cause. Thanks a lot for your responses and guidance.

[Brian Rapp]

Cruise switch? Will have to check

I did not try anything with cruise switch, but I’ll put it on my list to check. It’s there on the switch panel, but honestly never used it in the past. I think it may be a 3-way switch opposed to a 2-way, so I’ll definitely take a look.

[Brian Rapp]

Battery replacement

Whether it adds anything or not, the clocks on each Smartcraft display had to be reset. They were blinking to set them when I launched the boat Saturday. I set the time for each one, but hadn’t thought much about it. When I replaced the two starting batteries a couple weeks before launch, I noted these also had smaller orange wires connected directly to the battery. Reading in other postings today, these may be ECM constant power leads for the engines. If the batteries were out for a week, I’m not sure if anything else in the Smartcraft displays/system is also reset. Not sure if this is a factor. I will not be able to get back on the boat again until Friday.

[Dave Carlson]

Is there any chance the cruise switch is in the wrong position?

2 users thanked author for this post.

Tony Athens, Brian Rapp

[Brian Rapp]

Throttle show linear growth

Throttle at 0% at idle and goes up seemingly linear. I can experiment with it more.

[Brian Rapp]

Additional pictures

Thinking when you said control head, not sure if were inferring to the displays alone or the throttle and shifters, so attaching pictures of the latter (one from my boat that had available and another that shows the shifters and throttles more clearly but not mine). Apologizes if this was not what you were seeking, I can certainly obtain something different.

[Tony Athens]

Does the throttle position show 100% when at idle or ????

As you move the throttle, does it move from ZERO to 100% is lineal manner?

1 user thanked author for this post.

Brian Rapp

[Tony Athens]

Control Head ??

1 user thanked author for this post.

Brian Rapp

[Brian Rapp]

Smart craft displays, one for each engine.

Stb side attached here. The voltage is low only because the port engine was off when I took these, and the port side only charges the starter batteries.

[Tony Athens]

1) Let me SEE your control head to SEE what you are controlling the engines with.

2) Clear picture of your digital displays in the on position

1 user thanked author for this post.

Brian Rapp

[Author]

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