2002 F115 bad idle issues Please help

[smelly fingers]

I have a 2002 F115 and it has this problem that is driving me nuts. The boat runs great until I run it for 30+/- minutes straight at cruse RPM (4500) and when I stop it will spit, surge, and then stall. It will also do this when it’s cold at idle sometimes. The boat hits WOT no problems. When it running like crap I can unplug the TPS and it will shoot up in RPM for a split second ( I can hear the IAC valve open up more) and then go back to idle and runs great. After I unplug the TPS and it goes back to idle I will plug it back in (no change) and I can run the boat all day long with no problems but as soon as I shut it off and then restart it, it runs like crap again. This is what I have done in the last week to try to fix this.

I put a reman set of injectors in (no difference)

It was plugged in to the yds and no codes. Also when it was running bad and But couldn’t see anything because the numbers were all over the place. When running ok everything seemed to be in check.

VST tank was cleaned last summer with the filter change (I didn’t have to do it but I did it).
Fuel pressure is at 37-38 running and at 44-45 with just the key on
Brand new Iac valve installed
Brand new T-stat installed
Tried a computer from another motor but didn’t work
I did a compression check and all cylinders with in 2-4psi
I changed the water sep (old filter didn’t have any water or dirt in it)
The boat has a full tank of new gas (high test)
Cleaned the little filter on the motor ( not dirty)
I’m thinking it could be a Intake Pressure sensor, or an Crank position sensor.

Can bad spark plugs do this? I don’t think so because I can make it run great by unplugging the TPS and the same goes for a valve getting hung up? If a valve was hung up it would never run good.

Thanks for any possible help.

[smelly fingers]

I forgot to say when its running bad on the YDS the injector pulse is something like 6.2 at idle. It does run real rich.

Thanks

[rodbolt17]

stop
quit getting yiggy and splatter gunning.
splatter gunning may work well at goose hunting.
aint worth a fart at trouble shooting.

intake pressure sensor is stupidly simple to test with YDS.
simply turn the key on,look at baro pressure, have a cup of joe and watch the weather channel.
if both baro pressures in your area agree, it aint your map sensor.

myself I would be looking at a malfunctioning/sticking ISC valve.

[rodbolt17]

did you notice your fuel pressure figgues suck ?

[boscoe99]

did you notice your fuel pressure figgues suck ?

How so?

SM says fuel pressure should be 44.1 psi with the key on and motor off. OP says he sees 44 - 45 with key on and motor off.

With the motor running at idle the pressure regulator will lower the fuel pressure to approximately 36 psi. OP says he is seeing 37 -38 which is about spot on. Assuming he is talking about the motor idling since the fuel pressure will vary as a function of intake manifold pressure. The gauge might be reading a bit on the high side.

[smelly fingers]

Rodbolt, thanks for looking at my post.

I didn't think the fuel pressure was problem, since the motor runs WOT with no problems. Also I looked in a Mercury book for a F115 and thats spec. When it's idling (like crap) it has an injector pulse at 6. something and is running real rich.

When it's idling bad I can hear the air moving through the IAC manifold at different speeds. It sounds like the IAC is trying to do it's job. When I unplug the TPS that air noise goes away and the motor idles great. I let the boat idle ( idle bad )for about 10-15 min and the IAC temp got to around 150 degree at a slow pace. Is that too hot? What is the chances that a brand new IAC is bad?

I never did that 20-30 min test for the IAC on the YDS. Will that tell me anything? What does that test do? Will it tell me if I have a malfunction with my IAC?

Thanks Ed

[greasyshaft]

Check the reference voltage to TPS. Should be 5v +/- .25, but better check the manual (the TPS is useless until throttle at 7° or more assuming it is set correctly)
Is the TPS voltage within spec?
Are the throttle valves synced & linked correctly?
Is the water temperature sensor showing warmed engine operating temperature?
IAC valve will get quite warm
30 min test is for syncing throttle bodies (fixes ISC) as far as I know (I'm just an idiot apprentice LOL)
What do the spark plugs look like?

[smelly fingers]

greasyshaft,

Voltage for the TPS checks out at 5v and is in the 7. range on the computer and volt meter.
the water temp sender is working. starts off cold and goes up to 120.
When I leave the motor idling bad for a few min the plugs will be rich / black.

I'm going to check the sync today. I have a buddy coming over with the correct tools to do it.

I think rodbolt17 is right about the IAC have issues. Something is not telling the IAC to be where it needs to be after start up. Thats why when I unplug the TPS and can hear the IAC adjust to the correct setting and it runs great.
I do hear it chatter at shut down. Can a motor being off sync do that? It's just weird that a motor can go out of sync for no reason.

Thanks

[rodbolt17]

fuel pressure running should be about the same as fuel pressure at key on.
when you unplug the TPS the ECU defaults ign timing and some other points.
the ECU on the F115 back then did not display much information.
what is the MAP sensor reading at key on at when idleing?

[greasyshaft]

Unlikely throttle has gone out of sync for no reason.
Fuel pressure is higher at key on as there is no manifold vacuum at the regulator

[boscoe99]

fuel pressure running should be about the same as fuel pressure at key on.
when you unplug the TPS the ECU defaults ign timing and some other points.
the ECU on the F115 back then did not display much information.
what is the MAP sensor reading at key on at when idleing?

What???

Remember when we talked about the fuel pressure rising when the throttle was opened and you said the pressure went up solely to provide additional fuel when the throttle was opened up? Now you are saying the pressures are the same?

Here is what a Yamaha service manual has to say about fuel pressures with the key just turned on and when the motor is running at an idle.

[boscoe99]

Unlikely throttle has gone out of sync for no reason.
Fuel pressure is higher at key on as there is no manifold vacuum at the regulator

At least someone understands fuel injection. However, I would have stated it as:

"Fuel pressure is higher at key on as the manifold pressure sensed by the pressure regulator is higher."

I hate the term vacuum. It is a whole lot easier to speak in terms of absolute pressure. High pressure in the intake manifold, high pressure from the fuel pump. Low pressure in the intake manifold, low pressure from the fuel pump.

Engine running and throttle closed, low manifold pressure and low fuel pressure. Open up the throttle, manifold pressure will rise and so too with the fuel pressure. This is the job of the pressure regulator. To keep a fuel pressure tied to intake manifold pressure.