Help

T

tbontrager

Member
Jan 3, 2016
37
Seattle
Boat Info
2003 Sundancer 320
Engines
Twin 350
Hello All, I am need some help diagnosing RPM problem. After about 1.5 hours of cruising at different RPMs (3800, 3400, and 1800) the starboard engine RPM dropped off from 3400 to 2800 and fluctuated, noticeable power dropoff. Starboard ran about 100-150 higher per the sync gauge but engines sounded fine. No noticeable irregular sounds or vibrations. Once the problem occurred, would maintain RPM for about 15 seconds and then drop off at higher speeds, ran ok lower and slower (2200 and <). Engines tuned up with about 6 months ago, and fuel filters changed before event. Filled up fuel tank (76 gallons) and added Quickleen. Recent engine survey had compression at 170, Sea Ray checked starboard #6 at 150 without checking other cylinders. Any help will be appreciated.
 
start by checking /replacing all the fuel filters. when was the last complete tuneup?
 
Start with fuel filters and give it a complete tune up and see how it works after that. Sounds like fuel starvation to me.
 
But wouldn't the fuel filters and tune up condition have occurred early in the 1.5 hour run at normal conditions? Mercruiser plug wires are known to break down over time so one of the wires may be arcing to a ground after it gets hot.

But, what do I know.........I'm more of a diesel guy these days.
 
Sounds more electrical than fuel. Unless something broke loose in the tank to cause a sudden starvation, fuel filters clog slowly over time, right. It's amazing how long people go without changing plug wires. Put a performance set and quality spark plugs while you are at it. Your engines deserve the best so give it to them!
 
Thanks for the great info. Did not put in post but it is with 350 Mags. Ran the boat with 3 hours total (1 +2) on new fuel filters and seems the problem happens about 2 hrs while running while at higher RPMs. Tuneup was done in the last 6 months by previous owner. Will look at doing another complete tuneup. Can anyone recommend a good mechanic around Lake Union in Seattle?
 
tbontrager said:
Thanks for the great info. Did not put in post but it is with 350 Mags. Ran the boat with 3 hours total (1 +2) on new fuel filters and seems the problem happens about 2 hrs while running while at higher RPMs. Tuneup was done in the last 6 months by previous owner. Will look at doing another complete tuneup. Can anyone recommend a good mechanic around Lake Union in Seattle?
Click to expand...

Make certain the "tuneup" by the PO included new caps and rotors. I have found this frequently gets forgotten about when some people are talking about tune ups. The cap/rotor can cause issues such as this. It is easy enough to remove the caps and check for corrosion if you cant be sure.
 
Just found out plugs, caps and rotors replaced in August. Wires not replaced, no telling how old the wires are.
 
The reason for my suggestion was that we on repeated occasions had three plugs on our port motor drop out after 45 minutes of on plain operation. After much expensive diagnosis chasing ECM and fuel issues that did not exist, pulled plugs out in the lake when it occurred and three were wet. Replaced and never had the problem again, and the plugs were new at the beginning of the season.
 
DaFish thanks for the info. Found the starboard distributor cap a little loose. Needed some better light but flashlight was dead. I went to store to get batteries and car died at store and had to be towed to shop. Wait for another day to investigate problem distributor cap and wires. Also, heater is not producing heat. Good thing I'm having a cocktail after the Bronco win.
 
I don't know about the rest of you, but I change my plug wires when I do a ' full tune up'. I assumed that was SOP, guess I should never assume.
Anyway, if the problem occurs after filters, then those are not likely the problem. Wires and plugs would be top of list now.
As an asside, I have had terrible luck with Champion plugs in small engines like garden tractors, they short out with no warning. I don't trust them any more and won't use them in the boat.
 
Found starboard distributor cap loose. Still have to take it out and run up at speed. Hope this was the problem.
 
I had a very similar issue last fall!! Make sure your spark plug wires are good. Meaning, not lose or bad. I had to replace the #3 plug and the issue went away!!
 
An arcing plug wire was responsible for my loss of engine rpm on one motor about half an hour into a trip back in November. RPM didn't fluctuate though - just wouldn't pull over 3500 rpm.
 
Last edited:
Was your engine throwing codes and how did you know to change wire? My engine was put on an analyzer and no codes.
 
No codes, mercruiser tech said no probs when hooked up to the diagnostics, changed plugs based on the advice of this forum. Started changing plugs, even though not that old, when I came across the plug wire with cracks in the insulation and arcing marks.

i then replaced both sets of plug wires as preventative maintenance. Did a 140 Klm round trip last weekend and all good.
 
Last edited:
I agree with the wires as the likely suspects. Had identical problem on prior 454 Mercs. Removed the engine room hatches and brought the engines up to cruise speed at night and was treated to quite a lightening display. Lucky the fuel system was free of leaks.
 
I hope it is electrical. If it isn't I would look at fuel pressure next. Any fluctuation in fuel pressure is one of these: 1) filters 2) regulator 3) pump.

What you describe seems more like a fuel pressure problem at higher loads/speeds. The ECM won't pick it up as a fault and the next thing you know.....it won't start. EFI systems usually pressurize the injector rail between 38 to 42 psi. The regulator bleeds off pressure over that amount. Any less and the engine won't start. If the pressure is on the low side....that will present a problem under higher load conditions since the injectors expect constant stable pressure. Low pressure is usually due to one of the three things previously mentioned.

I'm a gas guy. The only thing I know about diesels is what Frank says: "they run forever if you take care of them..."
 
You must log in or register to reply here.

Forum statistics

Threads
113,253
Messages
1,429,309
Members
61,128
Latest member
greenworld
Back
Top