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Stuck at 3000rpm


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I have 2008 Malibu 23LSV with monsoon 340 motor has probably about 80 hrs give or take on it. I did not fog the motor this winter and changed all fluids beside the v drive and transmission which I need to do. Past two trips out to lake the boat will not go over 3000 rpm from time to time after running for little bit. But if you shut it off and restart it boom back to normal. Also did get a misfire cylinder #1 first time on the water this season. Hasn’t done it again but does idle little tougher than normal. Any help is appreciated. 

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i think that 3k rpm is a limp home mode.

if the ecm thinks it's running too hot it will limit rpm.

please back me up by doing a bit of research and consider swapping out one (of two) temp sending units that supports the ecm (as opposed to the one that feeds your gauges). 

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80 hours on original 2008 engine?  If so when was the last time you replaced cap/rotor?  If it is the stock one with aluminum contacts they are likely rotted out 

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14 hours ago, tvano said:

i think that 3k rpm is a limp home mode.

if the ecm thinks it's running too hot it will limit rpm.

please back me up by doing a bit of research and consider swapping out one (of two) temp sending units that supports the ecm (as opposed to the one that feeds your gauges). 

Yea I’ve kept eyes on the temp and it’s been running normal and are you referring to the temp sending units ontop of the exhaust manifold 

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14 hours ago, oldjeep said:

80 hours on original 2008 engine?  If so when was the last time you replaced cap/rotor?  If it is the stock one with aluminum contacts they are likely rotted out 

Sorry I should of clarified 80hrs on the new crate motor boat has 580 but new crate motor was installed 1yr and half ago 

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48 minutes ago, Eddy said:

temp sending units ontop of the exhaust manifold 

Those are probably O2 sensors.  Temperature senders should be in the intake manifold near the thermostat housing.  You can test them with a volt meter before you change them.

I would do the tune up first, and probably pull the plugs to check them out.  When you did the engine swap, did you use new plug wires, or just move the old ones over?

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52 minutes ago, Eddy said:

Yea I’ve kept eyes on the temp and it’s been running normal and are you referring to the temp sending units ontop of the exhaust manifold 

@Eddy  it's likely that there are two temperature sending units.  one provides an analog signal to the gauge on the dash.  the other is a switch providing an over-temp on/off signal to the ecm.  i'm suggesting that the switch it telling the ecm that engine is too hot and the ecm is responding by limiting rpm.

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17 minutes ago, justgary said:

Those are probably O2 sensors.  Temperature senders should be in the intake manifold near the thermostat housing.  You can test them with a volt meter before you change them.

I would do the tune up first, and probably pull the plugs to check them out.  When you did the engine swap, did you use new plug wires, or just move the old ones over?

Previous owner did I’m pretty sure they did new ones but I could be wrong. 

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On 7/14/2021 at 9:37 AM, justgary said:

Those are probably O2 sensors.  Temperature senders should be in the intake manifold near the thermostat housing.  You can test them with a volt meter before you change them.

I would do the tune up first, and probably pull the plugs to check them out.  When you did the engine swap, did you use new plug wires, or just move the old ones over?

Previous owner did I’m pretty sure they did new ones but I could be wrong. 
 

Changed spark plugs today and boat ran much better did restrict me 2x towards end of day of constant surfing would I need to check those sensors next?

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granddaddy55
10 hours ago, Eddy said:

Previous owner did I’m pretty sure they did new ones but I could be wrong. 
 

Changed spark plugs today and boat ran much better did restrict me 2x towards end of day of constant surfing would I need to check those sensors next?

if spark plugs made a diff in a new motor then the rest of ignition (wires and rotor) are suspect too! as @oldjeep said

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