240EFI stall/bog off idle after sitting

[lilngineer]

Hi Guys-

I am looking for some help with a stalling/bog problem I cant seem to solve on our 2000 Speedstee with the 240EFI.

Basically it goes like this. We launch the boat and get underway just fine with no issues. We drive the boat to a anchor spot and shut it down for a while.

After it has been sitting off for a while (I'll say a half hour or greater), I start the boat and it cranks a little longer but still fires right up. It will the either idle or stall. If it stalls, I start it and it fires right back up.

Now on to the real issue. Once it is idling steady, if I give it any gas below half throttle it sounds like it just bogs down and dies. I can restart again but will have the same issue. The only way I can clear it up and get underway isby quickly shifting into gear and throwing the throttle up past halfway momentarily and then it clears up.

Other than this consistent issue, the boat runs and drives great. It will come back down to idle with no stalling and performs great up to WOT.

Last summer I changed the port temp sensor, cleaned the fuel canister, reset the idles, calibrated the oil delivery and throttle position sensors. I also changed the plugs this year. I cannot get it to go away!

I am thinking it may be over oiling as it does seem to smoke a lot at idle. Or maybe a fuel pump? I am stumped! Please help!

 

[WAJetboating]

Once it is idling steady, if I give it any gas below half throttle it sounds like it just bogs down and dies. I can restart again but will have the same issue. The only way I can clear it up and get underway isby quickly shifting into gear and throwing the throttle up past halfway momentarily and then it clears up.

My first thought it the Throttle Position Sensor (TPS). The TPS is really nothing more than a potentiometer (variable resistor) that senses the position of the butterfly valves so the ECU can modulate the injector pulse width to maintain the fuel/air ratio as more air is let in. If the TPS starts to give weird readings, that would confuse the ECU into giving too much/too little fuel for the airflow. That could explain why the engine starts fine (TPS is all the way at one end of its element, less likely to fail there) but dies as you ramp it up (TPS element is dirty or failing, so erratic readings). It's possible the "higher" end of the element is still in good shape, since the "lower" end sees more use (you probably spend most of your time in the lower half of the throttle range). This would explain why quickly getting the TPS into its higher range permits the engine to keep running.

The service manual has a test procedure for the TPS. As I recall from memory, it wants a special cable - but you can test it yourself with a low voltage battery (say, a 9V) and an oscilloscope or analog voltmeter. What you're looking for is a smooth, clean ramp in voltage as you rotate the TPS shaft. What I bet you'll find is jumpy, erratic readings particularly in its lower range.

Report back!