1995 Mercruiser 5.7L TBI fuel injection idling high when coming off plane

[seahow1]

When I come off plane and at the lowest idle coming into dock the engine idles around 800 rpm in drive and when I put it into neutral the rpm is now at 1000 rpm. to get the idle lower I have to turn the ignition off and restart the engine and the idle will be around 800 in neutral which allows me to travel much slower to trailer the boat or approach the dock. I have checked and replaced the Idle Air Control valve and no change. The only way to lower the idle after plane is turning off the ignition and then restarting the engine. Does anyone have a solution to this issue?

 

[CaptainMak]

Even at 800 , the RPM are too high at idle. 600 to 650 is more to spec.
Maybe worth checking out the Throttle Position Sensor, or timing ? What ignition system do you have on this motor ?

 

[thumpar]

You could try readying for codes with the simple LED light and paperclip trick. If that doesn't show anything then you could try having someone with a real scanner that shows live data out with you and see what is different from before and after you restart it. It sounds like a sensor but hard to say which one.

 

[achris]

First check the throttle cable adjustment. If the cable is not adjusted correctly is may be holding the throttle open slightly. (When you shut off and restart, that can be enough to set the throttle plate back to closed).... The cable should be pushing the throttle closed with a very slight load.

Chris.....

 

[seahow1]

Hi Captain Mark, the ignition system is a Digital Ignition and my rpm tach says its 800 rpm and that may be incorrect. The engine may be at around 600 to 650 when the engine is first started since the boat seems to propel in the water quite nicely at low idle in gear when docking or trailering.

Achris, I double checked the throttle plate and they are closed and when you restart the engine the plates don't move.

I think thumpar is on the right track with a problem with a sensor. It's not a mechanical issue with the throttle plates and it's more of an electrical solenoid issue and a proper diagnostic scan tool will help.

TPS and MAP sensors are inputs to the ECM which in turn controls the IAC which is the output. IAC only works at idle. Turning off engine then restarting resets the position of the IAC. and then it gives me the lowest idle

I have booked this into the Mercury dealership to do a diagnostic scan and hopefully they will give me the answer to the issue.

Thanks to all for your input and solutions.

I will post findings to the issue.