Rapidly dropping RPM cause engine to stall

[Cheapie408]

2005 F200, runs up to speed fine, easing off the throttle is fine but if I rev it up and drop the throttle fast the engine dies. Particularly when you're docking and needs to get the throttle up quickly and briefly to pull away from banging onto the dock. It cranks right back up. What could be causing this?

New to me boat and motor and only took it out once.

I'm able to duplicate this on muffs as well. It also seems the screw on the front the throttle friction adjust knob seems to do something. I can tighten it then it gets loose again.

[fairdeal]

well, the specific function of the ISC -
idle speed control valve -
is to manage that transition when throttle suddenly closes.

Do yo hear the ISC valve clicking normally when you shut the engine off?

here is what it should sound like

https://cdn.sndup.net/3qqx/F225+IAC+...res=1615648426

[99yam40]

fairdeal,
just letting you know that link did not work for me
gives 403 forbidden message

[fairdeal]

fairdeal,
just letting you know that link did not work for me
gives 403 forbidden message

interesting - does for me also.

Lets try again

https://sndup.net/3qqx

[fairdeal]

that one seems to work -
once the page loads,
there's a small "Download audio" link at the bottom

[Cheapie408]

I heard that yesterday when running the engine on muffs. I thought it was my trim taps going back to position.

[Cheapie408]

any advice on what to look for?

[DennisG01]

Possibly the fuel pump isn't able to keep up with the increased demand? Could be a weak pump, could be a clog somewhere in the supply side from the tank to the pump. Put a pressure gauge on the pump and observe while doing your thing. You can run on a portable tank to eliminate everything on the boat side. What happens with the fuel bulb when the issue happens? If it goes flat, there's a restriction upstream of it (or bad bulb). If it's a problem with a clogged vent, try with the fuel cap off.

[99yam40]

I really do not think a pump or lack of fuel is causing the motor to die when backing off the throttle quickly.

And I have no idea if this F200 would be like the F150 that Rodbolt and others wrote about readjusting things to fool the ECU a little.

Might look for some of the old posts about the F150 shutting off like that to see how they readjusted things

[DennisG01]

Hmmm. I read the post differently. When he said "Particularly when you're docking and needs to get the throttle up quickly and briefly to pull away from banging onto the dock" that sounds like advancing the throttle?

[Cheapie408]

lets take the dock out of the equation. i rev the throttle up to a high RPM and I pull the throttle back into neutral the engine stalls. it does this maybe 3 out of 4 times.

I have not tried this when I'm at speeds yet but I imagine the same. I can duplicate this using the little lever forget. It's a yamaha 703 so there's this high idle level I think that's what it's called. if I lift it to rev up the engine and drop it down quickly it stalls.

[99yam40]

I went back in time and found this thread for you to read to see if that might help.
let us know. it is a long read but at least you can read how Rodbolt said to adjust things

Stalling Yamaha F150 - Yamaha Outboard Parts Forum

[DennisG01]

OK, that makes more sense. The "dock" wasn't the confusing part - it was the way it was worded. But as 99 mentioned, because it's stalling when you BACK-OFF throttle, disregard most of what I said. Wouldn't hurt to still check fuel pressure, I suppose.

[fairdeal]

when you abruptly close the throttle,
it causes the fuel mixture to "go rich" -
which would make the engine stall
if something doesn't allow more air.

that's why carburetors had a "dashpot"
to momentarily slow the throttle from completely closing

and that's what the IAC valve is supposed to do.

I suspect the problem is either -
- failure of the IAC valve to respond properly to the ECM
or
- a bad input to the ECM, from the TPS or MAP sensor