2003 Mercruiser 3.0L 135hp hesitates when I accelerate

[Boatcephus]

I have a 2003 Sun Tracker Party Deck 21 I/O, 3.0L 135hp, Alpha One drive.

I bought this boat new and it has been reliable and needed no repair until now.

She runs wide open just fine. Getting there though, has to be done slow or she'll often stall and die. It will sputter pretty bad when I accelerate. It feels exactly like it's running out of gas. Once I get up to a few thousand RPMs she does better. And at WOT you wouldn't know anything was wrong.
I take her out about every weekend on our OK lakes. I first noticed this 3 trips ago and it has gotten worse. I thought it was bad gas but I've run a lot of new 89 octane through since then.
I'm off Friday so I can open the cover then. Aside from the air cleaner and checking for foul spark plugs and maybe replace the fuel filter, what else can I check for? I wonder how I could test the fuel pump?

I hate to think it will be a major repair. This boat still feels new to me, but I've used it a lot.

It also has ALWAYS dieseled bad when I shut it off. The 89 octane made that somewhat better. And it's always been a b1tch to start. I'm told these motors are just cold natured.

Thanks!

 

[scooter4n]

Re: 2003 Mercruiser 3.0L 135hp hesitates when I accelerate

start with carb adjustment or rebuild.
Rebuild is around $250 (in shop)
This is very good start.
I had on my carb lose jet, and it was giving me all kinds of problem.

 

[Haut Medoc]

Re: 2003 Mercruiser 3.0L 135hp hesitates when I accelerate

It sure sounds like your carb's accelerator pump has gone south on ya.....
Pull off the flame arrestor & have someone pump the throttle (engine off)....
You should see two streams of fuel squirt into the carb throat....
If not you need to redo the pump, really not a big deal......
To start your engine, you should pump once or twice & set the throttle at 1/4 & turn the key, it is not like a fuel injected car that requires no help....
Again, the acc. pump will cause hard cold starts if malfunctioning......
The dieseling thing will be lees of a problem if you let the engine run @ idle for a few minutes before shutting down.....
Don S had a tech bulletin about the problem, if he catches this post, maybe he will post it.......​

 

[Haut Medoc]

Re: 2003 Mercruiser 3.0L 135hp hesitates when I accelerate

Here ya go!......
the engine to keep running once you have turned it off?​

Here is a copy and paste of the Merc service bulletin on dieseling.


Mercruiser Service Bulletin
No. 97-17
= Revised July 1999
Engine ?Dieseling? or ?Running-On?
Models
MCM 4 Cylinder, V6 and V8 with Carburetors.
Situation
Under certain conditions, carbureted engines may ?run-on? when the ignition key is turned off. Normally, it
will take a combination of several of the following conditions to make the engine ?run on?. These conditions
include but are not limited to:
1. Type and formulation of gasoline available in the area.
2. Type of boat and its engine compartment ventilation system.
3. Outside air, inside engine compartment air and engine coolant temperatures.
4. How quickly the engine is shut off after running at cruising or higher RPMs.
5. How the boat and engine are operated.
6. Engine idle rpm set too high.
7. Wrong heat range spark plugs or spark plugs with deposits on them.
8. Deposits in combustion chamber.
Carbureted engines, unlike EFI, do not shut off the fuel supply to the engine when the ignition key is turned
off. As the engine is coasting to a stop, air is still flowing through the carburetor into the manifold. This air
flow can still draw fuel from the carburetor, through the intake manifold and into the combustion chamber.
A hot spot in the combustion chamber will cause this fuel/air mixture to burn. The engine now ?diesels? or
?runs on? although the ignition system is not firing. EFI engines do not have this problem because the injectors
do not inject fuel once the ignition key is turned off.
Suggestion to Minimize ?Run On?
Items number 1 ,2 and 3 can correct or reduce most of these ?run on? problems. Letting any engine idle a
minute or two before shutting it off is desirable. This helps the engine to normalize at idle rpm before being
shut down.
1. Be sure engine idle mixture, idle rpm and ignition timing are correct. On 4 cylinder and V6 or V8 engines
with Thunderbolt IV ignition, setting idle rpm as low as possible can help stop ?run on?.
2. Adjust throttle cable barrel so that the carburetor?s throttle lever returns against the idle speed stop
screw every time the remote control throttle lever is returned to neutral, idle.
3. Idle engine 1 to 1-1/2 minutes before turning key off after running at cruise or higher engine rpm.
4. Try a different brand of regular fuel or try some premium fuel.
5. Increase idle mixture by 1/8 turn ?richer?.
6. Go to next colder heat range spark plugs.
7. V6 and V8 only: Install a 140 ?F(60 ?C) thermostat.
__________________

Don S.


Why is there never enough time to do it right the first time,
But always enough time to do it again? ​

 

[Boatcephus]

Re: 2003 Mercruiser 3.0L 135hp hesitates when I accelerate

Thanks for the info guys. I just can't believe the carb could be so short lived. I don't know that much about these engines and parts though. Any good resources where I can read about rebuilding the carb?

Also, the Don S post at the bottom of this link gave me some hope that it's possibly just a carb flow screw I need to adjust. Do you guys think this is likely my issue? I have all these symptoms except the backfiring.
http://forums.iboats.com/showthread.php?t=211371&highlight=3.0l+hesitates


Thanks!

 

[UTgws]

Re: 2003 Mercruiser 3.0L 135hp hesitates when I accelerate

Does OK still serve up that weak beer? Maybe the gas is like the beer. I just spent the weekend up in OK (Mother-in-law has surgery) and didn't think my truck would run long enough to get back to Texas and get good gas.

My bet is that the accelerator pump is not functioning just right or the carb needs cleaning. You might get some fuel treatment (many people use sea foam over in the vintage rice burning motorcycle world) and run a tank full of gas and see if that helps.