Hot Engine No Start - Temp Controllers

rinker270nb

For 3 seasons I have had a no-start when engine is warm scenario, I am finding more info on it as it is somewhat common, especially in warm water environments.  This means when engine is cold, it starts right up.  Turn it off and sit for a while, it won't start and I need to give it some throttle to start.  This is a 2015 Merc new repower w 383 Stroker.  I am reading that the temp read is to high by ECM and to change either the thermostat, Temp Sending Unit, or Therm Temp Sensor.  Does anybody have experience with this or have thoughts of the best path to pursue.  Each part runs about $30, easy fix if it does fix the problem.  (This isn't a vapor lock issue as it starts when given throttle.)  Thanks for your help here...!

212rowboat

before you throw parts at it, simply turn the key and wait approximately 15~20 seconds before throwing it over to engage starter.  the suspect item isn't your temperature sensors, but your idle air control instead.  the plunger/piston needs an opportunity to react to the built-in-test (BIT) after the computer calculates environmentals... what you're doing when you open the blades of the throttle body is the same thing the engine 'should' do by itself, which is allow more air (and resulting fuel delivery based on the mass of the air flow or manifold absolute pressure depending on how your set up). 

make sure the filter on the IAC is clean, too.  it should adjust even for that, but you may be at it's limits if it's dirty.  

rasbury

@212rowboat is that iac a good maintenance item to replace? My mpi 350 has around 320 hours on it and I'm sure never replaced or touched...

212rowboat

the filter on the thing is a good item to clean/replace... the IAC itself follows the wash basin failure rate- meaning, if graphed, there is a high rate of failure in the beginning and a high rate at the end of their life looking something like a bath tub on a graph... make it past the beginning? you've likely got a long life ahead.. 

they get dirty just like the throttle body.  they should be cleaned, but be careful not to move it around much when you do as it is possible it will lose its position map.  it 'should' find it again, but... many failures are reported right after 'cleaning' them and moving the valve.  

you'll know when you need to replace it. starts will become difficult without opening the throttle a little, and idling will become choppy low or too high ( <650rpm or >800rpm )

rasbury

If it ain't broke I ain't going to fix it in this case. I need to get on the water!

rinker270nb

I have good transition at idle, going from nuetral to forward / reverse is seamless. The IAC should present a problem here.  This is a "fuel not being present" problem, I feel.  The computer is denying fuel due to temp is what I think is happening. 

212rowboat

the same 'trick' applies to fuel (waiting 15 or so seconds before throwing over to engage starter).  a warm engine may determine it's in closed loop, polling the sensors for information, when in fact it's in open loop and operating off 'stale' information... temperature absolutely plays a role- and causes the engine to trim some fuel out or fatten it up, either or.  giving it a second more allows the sensors to report more accurate data. it doesn't even take the 15 seconds i'm suggesting, but it's a safe play to wait that long.  

this circumstance is so common with more-than-some that it may be something that merc should look into insofar as their tables hard scripted into the ECM. 

rinker270nb

I give it 10-30 seconds and outcome is the same. The ECM has bad info, in my scenario.  I am going to try and update the parts to shuffle the deck some...