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Why does Mercruiser 30 on Bayliner 175 2006 idle rough

daemon_555

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"Engine: Mercruiser 3.0 on Ba

"Engine: Mercruiser 3.0 on Bayliner 175 - 2006

Symptoms:

October 2008:
Idle was rough (sputtering and jumping, would sometimes calmly die). Did not notice the original idle RPM, but increased RPM – idling became “smoother” (but faster). When we were on the water, noticed idle (not in gear) was about 1210 RPM.

For reference, expected idle RPM in gear is 700 according to the manual.

Winter 2008-09:
Starting is hard (too many rotation before starting), even with “priming” the engine (pushing throttle forward into gear and back 3-4 times before starting / cranking the engine). Sometimes need to crank in 2-3 sessions (about 5 seconds each with 10 seconds between cranking) before it starts.

Each time, on the very first acceleration during the day, the boat would pick up RPM and the boat would start accelerating well, but about 15 seconds into acceleration, boat speed would drop dramatically (for the car, this would when the gears shift) for 2-3 seconds (with passengers often being thrown forward a bit). I would reduce the throttle to low and the re-accelerate – the boat picks up RPM and speed and runs solid through the end of the day.

April 2009:

When accelerating away from the boat launch after exiting the no-wake zone, boat lost acceleration for a couple of seconds as above, but then re-accelerated fine. I put the boat under some stress with towing a tube hard.

When the engine was turned to idle (RPM drops to idle = 1210 RPM), then stopped (even when the key is taken out of ignition!), the boat keep running – without the key or ignition – for another 10-12 seconds (sputtering). The boat is in idle, key is out, lanyard switch moved to OFF state, but the engine keeps “running” and jumping. Seems like gas keeps detonating even without a spark. After the engine stops completely, flame arrestor emits a single puff of smoke (smells like oil smoke).

As a separate symptom, during high acceleration, boat seemed to lose power. While the engine’s RPM is very high, the boat seems to lack power – as if it is stuck in the low gear (using a car analogy).

Diagnosed the first symptom as dieseling, reduced RPM back to about 700 idle (measured when not in gear). Was testing with flushing assembly, could not put boat in gear (outside of water) or accelerate hard, but would guess the dieseling problem went away.

Problem:

Now idle is rough again – sputtering and coughing (not too bad, but clearly not a completely smooth idle).

What could be causing this problem?"
 
"All you have there is a bunch

"All you have there is a bunch of well documented common symptoms of a poorly maintained engine. Sounds like you need to do a tune up.

Fuel filters, cap, rotor, spark plugs....

While the engine’s RPM is very high, the boat seems to lack power – as if it is stuck in the low gear (using a car analogy).

But ^^^ that one sounds like your propeller hub is slipping."
 
"The boat is very well maintai

"The boat is very well maintained - I have receipts of tune-ups from the previous years from the previous owner.

The engine is only a couple years old and visually is in a great shape - just idles roughly.

The boat always uses fresh fuel (we almost always burn through the entire tank of gas over the day of boating), is trailered and cleaned after each trip, and always garaged. It is used regularly (therefore not winterized).

I am planning to take it to the dealer next week, but am trying to understand how much $$ I should prepare to spend and how bad it can be..."
 
"2006, Sounds like carb issue,

"2006, Sounds like carb issue, and not stabilized good enough causing the carburator to get gummed up.

Do you have an engine hour meter? how many hours does it show ?

The dieseling issue is definately Carb cleaning, but Hy Stat hit it of course, the high RPM and lack of power sounds like prop hub slipping or the drive was trimmed way to high.

I'd expect to pay about $100 to rebuild/clean the carb and another $100 for labor, $200 for a tuneup, prop could be less than $100 depending on the prop."
 
sounds like carb and tune up b

sounds like carb and tune up but id bet the accelerater pump is shot.also if you have any vacume hoses check them all real good
 
"Nope, no hour meter on the en

"Nope, no hour meter on the engine. Dieseling seemed to go away when I lowered the idle RPM - will check out the prop hub!"
 
the accelerator pump is on you

the accelerator pump is on your carbarater usaly a small rod from your main throttle that hooks to it it has a diaphram in it along with a few other parts spring and a ball but when the rubber diaphram goes bad it will cause alot of your symptoms plus thats a good place for gunk to start forming
 
"[b]"All you have there is

""All you have there is a bunch of well documented common symptoms of a poorly maintained engine. Sounds like you need to do a tune up."

Ayuh,... That pretty much sums it up...
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