2000 Yamaha F115 not firing #4 - tried everything

[Towboat26]

Good evening -

I have a 2000 Yamaha F115EFI that is running on 3 cylinders. Originally had plugged injectors and a contaminated fuel system so I disassembled everything - VST, rail, LP pump, etc and cleaned then flushed the fuel system. Pressure is 40# at the rail, spark is good at all 4.

Getting good fuel spray pattern on all 4 injectors, replaced the spark plugs and still no fire on #4. Can unplug the #4 injector or the plug wire and get no change in the rough idle, all others stall the engine when unplugged. Swapped coils and the problem stays with the #4 cylinder.

Compression is between 190 and 200 on all 4

What am I missing? Should I be looking at a stuck valve or lifter? Hate to jump to that conclusion but this is driving me crazy.

Thanks for any help - looks like a great forum for these motors.

[greasyshaft]

Compression test would be a start, leak-down test even better

[herb.wilson@uno.edu]

No expert here for sure, but is #4 plug wet? Swap around injectors
, although you said all had good pattern. Wonder if #4 is getting signal to spray? Not sure how you'd test that.

[boscoe99]

You got the three things you need for ignition - spark, fuel and compression.

How did you verify that the injectors are working?

You swapped coils with no change in performance. Have you swapped injectors to see if the problem moves or stays?

If you put a fuel pressure gauge on the motor and turn the key on and then off, does fuel pressure hold steady at 40psi for a bit of time? I am thinking that an injector might be sticking open. Too much fuel will cause an overly rich misfire.

It is difficult troubleshooting in person. Harder still via the internet.

[Towboat26]

Thanks for the replies - wanted to close the thread in case someone has a similar problem, however unlikely.

Yes, I had swapped injectors as well - the problem always stayed with the #4. Turns out when I was fishing around in the cowl for a bolt I dropped, I found the plug to the intake manifold at the #4 cylinder. How it fell out I have no idea as I didn't take them out for cleaning, but there is was laying at the bottom of the cowl. Strange, but at that point I didn't care.

For anyone else reading with a similar issue - don't forget an air leak. I was so busy concentrating on the fuel, spark and compression it never occurred to me there might be an intake leak.

Anyway - problem solved & it runs better than ever. At least I'm not crazy - this time.

Yes, internet troubleshooting is difficult, but isn't that the point of the forum?

Thanks again

[rodbolt17]

I cant believe you could not hear that one.
not only is it noisey but it tends to backfire and toss flames out.

[Towboat26]

I cant believe you could not hear that one.
not only is it noisey but it tends to backfire and toss flames out.

You and me both - completely baffled by how it happened and why it wasn't obvious. Only indication of a problem was a dead cylinder and rough running. I'm not exactly a novice with these things either, but certainly humbled . Yet another instance of 'just when you think you've seen it all'

Lesson learned.......I think?

[boscoe99]

Do you have a part number for the plug or a parts catalog reference number?

Only thing that I can see are the manifold pressure sense lines that run to the fuel pressure regulator. Did the hose for the number four cylinder come loose? Or, did the hose barb fitting pull loose from the intake manifold?

Glad you found the problem.

[rodbolt17]

what I read was the manifold plug had been removed for some stuoid reason.
then as there are screws and mysterious thangs.
we put our ****skinners on them and whacked away.
then my Yamaha wont run.
help.
\
this last week that's about all I have delt with.
untrained dicckskinners on a stupidly simple EFI setup that after the untrained internet based monkeys got done.
was expensive to fix.

trust NOTHING on the net without double checking.
DO NOT even TRUST anything I post without CHECKING.
some of us are simply behind a monitor assuming what the poster says is correct.

[99yam40]

I am curious of where that plug is also if someone will post it's location on a parts break down or a pic

[rodbolt17]

on the cast aluminum intake runners there is a 10mm hex bolt with a copper sealing washer, its used to attach a manometer to test and sync.
on the plastic intakes there is a nipple with a rubber cap.
some of the single throttle plate EFI motors have eliminated them.

on the techmate vacumate there is an avg function and a dynamic function.
most techs haven't a clue how to use the dynamic function but its a quick way to observe a cylinders performance.