Yamaha F40 EFI will not run once warmed up.

[Barney S]

Engine starts and runs great until
I stop to fish etc. After that it will start but not stay running until it has completely cooled off again (several hours). I have replaced
Fuel filter, tank and hose, drained the vapor separater tank, new
Spark plugs, checked fuel pump flow. Nothing has worked.
Anyone have any ideas?

[marchmadness]

when it will not start is there fuel in the vst tank... if its starts cold unless theres no fuel it should start

[99yam40]

Make sure the fuel pump cooling water is not plugged up, some of the early motors did not come with water cooled fuel pump.
need to make sure if there is spark or not when it will not start.
No spark or too much fuel can be a problem just as well as not enough fuel for the cylinders to light up

[Barney S]

It will always start but it will not stay running. It will idle for awhile before it dies sometimes but it will die as soon as throttle is opened any. It does not have a water cooled fuel pump.

[rodbolt17]

EVERY 4 stroke EFI Yamaha has a water cooled fuel pump.
some of the F40 motors had a recall on the pumps.
if an EFI 4 stroke wont run it is simply broke.
it is a stupidly simple speed density EFI system. it is even open loop.

[Barney S]

EVERY 4 stroke EFI Yamaha has a water cooled fuel pump.
some of the F40 motors had a recall on the pumps.
if an EFI 4 stroke wont run it is simply broke.
it is a stupidly simple speed density EFI system. it is even open loop.

The fuel pump in mounted to the valve cover and has no water hose or ports going to it. I know it's broke. How can I fix it ????

[pstephens46]

Not sure what Rodbolt was referring to other than water cooled motor overall. Since this is a "HOT" issue, I wonder if the VST vent back to airbox is dumping too much vapor back into engine. Creating a rich condition. I would run it with cowl off and get it hot. Duplicate whatever conditions that create the issue. Before you shut it down, pull the vent hose off airbox. Wait a few mins then try to restart.

[boscoe99]

Technically the electric fuel pump (not to be confused with the mechanical pump) is not cooled. What is cooled is the fuel that returns from the fuel rail and which flows back to the vapor separator tank. It goes through a device aptly named the "fuel cooler".

Barney, you might try some free stuff just to see if you can help find where the problem is. What do you want to do? Are you familiar enough with the motor so that if we make some suggestions that you will know what we are talking about?

For instance, try using the primer ball before making the first start attempt after having just down the hot motor. Does it make any difference?

As pstephens suggests, you can remove the vent line from the vapor separator tank to the air intake system to see if excessive fuel fumes are being supplied to the intake manifold.

Have you tried to let the motor run at idle for a minutes time before you turn it off to see if this changes the hot restart issue?

Do you have a fuel pressure gauge? Do you have a service manual so that you can refer to some diagrams that some of us might reference here?

Unfortunately, what you are experiencing can be caused by any one or more of a number of issues. Fixing a problem is relatively simple. Finding the problem can be quite difficult.

[99yam40]

I thought the early F40s had a fuel pump overheating problem and Yamaha came back with a replacement with a water connection to help keep the fuel from boiling away.
Is that not correct?

[boscoe99]

Only F40LA bulletin I am aware of is for the electric fuel pump replacement. Not related to a hot engine however.

[99yam40]

did the new pump have water to cool the fuel added?

[boscoe99]

did the new pump have water to cool the fuel added?

Nope. The fuel cooler has been in the motor since day one.

[Barney S]

I took the cowling and plastic covers off of it and took it for a good long drive and stopped just like always. When I was ready to move on I fired it up and it ran perfect. I ran awhile and stopped awhile again. It ran perfect ever time. I do not know if it fixed itself or if the problem is intermittent or if the cowling and covers are causing a heat/vapor problem. I will be trying to isolate it as time permits over the next few days.

[boscoe99]

Just some wild speculation on my part but it could be a valve that is not closing when the motor is turned off, thereby allowing excessive gasoline fumes to enter the air intake system. Having the cover removed could be allowing those fumes, if present to dissipate sooner than they might with the cowling installed.

If the old problem rears its ugly head when the cowling is left on, let us know. We can then tell you what valve to check to see if it is open or closed. Off or on in Yamaha's vernacular.

[pstephens46]

Those of us that are mentally ill enjoy these types of exercises.

Boat ownership....

Then we can tell the wife how much money we have saved.

I just spent $137 on diaphragms and orings for low pressure pumps that are working perfectly. But they are 12 years old.