97 cobalt, 454, wont acclerate over 3300 rpms

[evpierced]

I have a 1997 cobalt 22 ft. Has a 7.4 liter. Engine model number is MCM7.4LXMPI. It is multi port injected and has electronic ignition. Engine fires and runs fine. Runs great till 3000 to 3200 rpms. Then just flattens out and has a heavy bogging sound. It does not misfire so i dont believe its ignition. Sounds like its running out of fuel. Spark plugs and wires just changed. Maybe collapsing fuel line or fuel pump?

 

[Silly Seville]

Re: 97 cobalt, 454, wont acclerate over 3300 rpms

First of all...Welcome to iboats!

Is this your first boat? How long has it been showing these symptoms? We need more details about your maintenance history with this vessel to make an informed guess, but I would venture that it is definitely fuel related. Here's a list:

Bad fuel, bad fuel tank vent, bad fuel filter(s), failing fuel pump, clogged fuel lines, dirty injectors.

Oh, is the boat propped correctly? Air filter clean?

 

[evpierced]

Re: 97 cobalt, 454, wont acclerate over 3300 rpms

Not my first boat, but first inboard. Fuel filter is brand new, changed today. Vented fuel cap also and no change. 12 gallons of fresh fuel and no change, with 2 bottles of seafoam. Just cant seem to get info on these multi port engines. Do these engines have a mass air flow sensor?

 

[evpierced]

Re: 97 cobalt, 454, wont acclerate over 3300 rpms

it has a 21 pitch prop, but its reverse rotation, air filter is new

 

[Silly Seville]

Re: 97 cobalt, 454, wont acclerate over 3300 rpms

it has a 21 pitch prop, but its reverse rotation

That prop pitch is fine with that engine, (I'm assuming you have a Bravo 2 drive?) However, I do not understand what you mean by reverse rotation. That phrase applies to twin applications. Something is amiss. Your boat wouldn't behave properly with a "reverse rotation" prop (or drive) on it.

Did you dump 12 gallons of fuel on top of old fuel, or did you drain tank and start over with fresh fuel? And are you saying that you poured 32 oz. of SeaFoam on top of only 12 gallons of fuel? That is a questionable practice. What I'm saying is, if you were using ONLY fresh fuel, all that SeaFoam was an unnecessary waste of money.

 

[evpierced]

Re: 97 cobalt, 454, wont acclerate over 3300 rpms

I didnt even notice the reverse rotation until it was pointed out to me. Sure as franklins face on a hundred dollar bill the blades are curved opposite of a regular prop. The fresh fuel was added to get the level up high enough to handle 2 cans of sea foam. The fuel isnt stale or stinky, but i have know idea if there is varnish or dirt in the injectors. I used the high concentration on it to clean fuel system, not rehab fuel.

 

[evpierced]

Re: 97 cobalt, 454, wont acclerate over 3300 rpms

bump, sorry

 

[Fun Times]

Re: 97 cobalt, 454, wont acclerate over 3300 rpms

Spark plugs and wires just changed. Maybe collapsing fuel line or fuel pump?

Did you do new plugs and wires before or after you found out the engine won't go past 3300 rpm?

Just cant seem to get info on these multi port engines. Do these engines have a mass air flow sensor?

What kind of specific information are you looking for? Service Manual? if so, we will need your engine serial number to find you the correct Mercruiser part number for your engine.

No these engines do not have a mass air flow sensor.

air filter is new

You bought a new $168.62 flame arrestor? Item number 36 or 42, http://www.mercruiserparts.com/Show...br=14&bdesc=INTAKE+MANIFOLD+AND+INTAKE+PLENUM

Usually the flame arrestors clean up nicely.

it has a 21 pitch prop, but its reverse rotation was pointed out to me. the blades are curved opposite of a regular prop.

The fresh fuel was added to get the level up high enough to handle 2 cans of sea foam. The fuel isnt stale or stinky, but i have know idea if there is varnish or dirt in the injectors. I used the high concentration on it to clean fuel system, not rehab fuel.

The Bravo drives are bi-rotational and will handle left or right handed propellers as long as the shift cable travel from the shift control handle is set in the proper location. Your engine rotation should be left hand rotation viewed from the flywheel end no matter what way the propeller turns.

Some boats may work ok spinning left hand propellers but right hand propellers are much more common and In most cases, right hand propellers are used for single engine applications and one left hand propellers are used for twin engine applications to achieve better steering control.

You'll want to find out what your fuel pressure is to help diagnose what the next step going to be.

Good luck.

 

[bubbajckson]

Re: 97 cobalt, 454, wont acclerate over 3300 rpms

Sounds like maybe air is in the fuel rail, has it been bled? Put a guage on it but be careful because it is high pressure. The ECM might be bad too. Other.... bad injectors will cause this too. Like others said...get the manual!

 

[evpierced]

Re: 97 cobalt, 454, wont acclerate over 3300 rpms

This boat has been to the repair shop several times. I bought this boat at a good price because no one has been able to fix it. So no the engine has never been able to pass 3300 rpm since i have had it. Serial number on this engine is 0K151578. And i understand which way an engine runs, this is not an issue, just trying to understand my situation. Please understand im not trying to be a punk or have an attitude but i am an ASE certified auto technician with an advanced driveability education. I understand boats arent exactly the same but there close. What measures the air on a multi port injected boat engine.






p

 

[Fun Times]

Re: 97 cobalt, 454, wont acclerate over 3300 rpms

Likewise, we didn't know you or what you do or don't know about engines, so for sake of discussion on a forum till we get to know each other, it's better to put all the info out there whether needed or not.

To help you understand your engine better, here is the mercruiser service manual for your engine.
http://www.boatfix.com/merc/Servmanl/16/16cov2_1.pdf

The MAP sensor (manifold absolute pressure) measures the air on a multi port injected boat engine.

RUN MODE
When the engine is started and RPM is above 300,
the system operates in the run mode. The ECM will
calculate the desired air/fuel ratio based on these
ECM inputs: RPM, Manifold Absolute Pressure
(MAP) sensor, Intake Air Temperature (IAT) sensor
and Engine Coolant Temperature (ECT) sensor .
Higher engine load (from MAP) and colder engine
temperature (from ECT) requires more fuel, or a richer
air/fuel ratio.

Also an FYI for you, The reason there's no mass air flow sensor is due to this type of electronic fuel injection system is a speed and air density system.

You may want to start with checking fuel pressure at both idle & speed.

Hope this helps, Good luck.

 

[Scorpion210]

Re: 97 cobalt, 454, wont acclerate over 3300 rpms

I will put my level of expertise out there before making my statement. I am excellent at paying the marina tech to fix almost all issues on my boats

Had a similar issue at almost the same RPM's (then it started to get worse and happen at lower RPMs). Tried changing filters and fuel additives to clean the fuel. No luck

The final cause was 2 things
1) A previous owner had installed some secondary inline fuel filter
2) The fuel line had developed some pin holes and was sucking air when the engine was needing more fuel at the higher RPMs

Not sure if this helps, but it was my experience with "bogging down" around 3000rpm