2007 Mastercraft Prostar Idling Problem

[Dkeezer]

2007 Mastercraft Prostar 197

6.2L engine

PerfectPass installed

 

A few weeks ago I noticed my engine starting idling at higher RPM. It normally idles at 600, but would start idling around 1000, then 1200, 1500 and one time it hit 1900. Obviously, this isn't great for the engine as it jolts when I put it into gear.

 

This also coincides with another symptom which may help solve this. My regular speedometer always reads 0mph (driving or not). I bought the boat this way, and it's not an issue for me as I use perfectpass to measure speed. But whenever I have the idling problem, my speedometer starts giving a false reading at the exact same time.

 

For example, when I'm idling properly, my speedo = 0. Then my speedo reads 10mph (when I'm just sitting) and the idle goes up to 1000rpm. It seems like these two items are connected, which could help solve this.

 

Any thoughts?

[DW]

Mechanical or DBW linkage? Start by checking the throttle return spring, any possible items that would keep throttle from returning to idle.

[BraceMaker]

Does the Tach Idle seem to match what you're hearing? Like when the tach goes up the speed goes up and you can hear the engine going up? Does the RPM reading on the tach match the RPM reading in the perfect pass?

 

Any other gauges reading odd? Voltage for instance?

 

2007 is a truly rare bird, with the MMDC M2 set up that only lasted a few years.

 

By itself this could all be a manifestation of an issue with the gauge display or the computer for the gauges if it is actually idling that high that could be an issue, but it might also just be reading high or might be having a voltage issue.

 

[Dkeezer]

@DW It's a DBW

@BraceMaker

1. No, it doesn't match exactly. But when the idle starts messing up, the speedo has a reading of some sort.

2. No other guages are reading odd.

 

Thanks for your comments

[BraceMaker]

But it definitely sounds revved up?

 

Its going 1900 rpm and you pop it in gear the boat takes off? Trying to get at is the engine actually going as fast as the gauge says.

[BTheis]

@BraceMaker I drive his boat a lot and it’s definitely idling super high the PP tach closely matches the factory tach.

[BraceMaker]

Perfect thanks - suspect two separate issues - the Idle is probably the idle control valve and maybe also a vacuum leak.

 

The speedo issue would need more evaluation.

[UWSkier]

I'd pull and clean the IAC with some good carb and choke cleaner. I don't recall if that cycles through its range on power-on or not so I'm not sure how to test, but I'm sure there's a way.