'03 F115 Idle fluctuation -FIXED

[Casco Bay Outrage]

During my de-winterization and on my first run of this season, I noted the engine idle would cycle down to 7K then up to 9K and then go back to 8k. It occurred every 30-45 seconds at idle. Normal idle is 8k on digital Yamaha tach.

The idle fluctuation occurred on both the muffs and on the water when in neutral idle. No other problems at other RPM's.

Background:

Prior to starting the engine for the first time, I replaced the fuel/water separator cartridge and engine filter. I purged the fuel from the separator to the engine.

Fuel tank full of non-ethanol fuel treated with Stabil from October.

Engine run on muffs 20 minutes.
Engine run on the water for 3+ hrs without any other issue.

Last March, I had clogged VST, injectors that was resolved by dealer. See Issue with Fuel System thread

Boat ran ~ 60 hours last season without any signs of a problem.

Engine has 142 hrs.

Is this an early indicator of a fuel system issue?

 

[BaileysBoat]

Re: '03 F115 Idle fluctuation

Re: '03 F115 Idle fluctuation

You Do have a 10 micron filter don't you?

I would drain the VST and see whats in there.

If all is ok run out the fuel from last year and try a fresh tank of fuel.

Have 200 hours on my F150, no problem, but again I use a 10 Micron filter.

 

[cousinabe]

Re: '03 F115 Idle fluctuation

Re: '03 F115 Idle fluctuation

does pumping the primer ball while idling help? wondering if you could have a bad lp fuel pump not supply fuel fast enough to the vst.

can you test the pressure in the VST?

 

[Casco Bay Outrage]

Re: '03 F115 Idle fluctuation

Re: '03 F115 Idle fluctuation

Yes - the water separator is a Yamaha 10 micron mini.

I will review the service manual for draining the VST. With this model, the VST is buried inside of the intake manifold so getting to it is a PITA.

One additonal observation:

When running on the muffs, the engine cover was off. I did see the fuel level in the engine filter go down a 1" or so and then come back up.

 

[sdsaw]

Re: '03 F115 Idle fluctuation

Re: '03 F115 Idle fluctuation

Where's rodbolt?
I'd bet he'll know what's going on.
Have you ever had the idle air control valve out for cleaning?
I have the Mercury 115 four-stroke that has the Yamaha powerhead and I have cleaned mine 3 times since 2001 to help my motor idle correctly.
It's the round component that has the connector off.

Scott

 

[sdsaw]

Re: '03 F115 Idle fluctuation

Re: '03 F115 Idle fluctuation

I'm bringing this back up in hopes that Rodbolt will see it and give his expert opinion about it.

I have a feeling that this is a common problem.




Thanks

 

[rodbolt]

Re: '03 F115 Idle fluctuation

Re: '03 F115 Idle fluctuation

not really a common problem.
but now we have to go look at what creates the 700 RPM or so idle.
mostly its by the temp sensor,TPS and ISC.
its possible the t-stat is stuck its possible the ISC is sticking or dirty and its possible the TPS voltage is incorrect.
the ECU will adjust air,via the ISC and ign timing based on signals from the ECT and TPS to try to maintain the ECU mapped idle speed for the throttle angle and engine temp inputs.
now ya just gotta find out whats not quite right.
remember the ECU is rather stupid.
yall gotta remember I dont see alot of the small efi 4 strokes . but they all work mostly alike.

 

[Ro-Tator]

Re: '03 F115 Idle fluctuation

Re: '03 F115 Idle fluctuation

What Rodbolt said makes perfect sense to me; Ect is trying to compensate for something that is faulty and makes adjustments in the process.

I think the post will help a lot when it comes time to diagnose this

For the threadstarter; I understand the motor abbreviations but what does PITA stand for?

 

[Casco Bay Outrage]

Re: '03 F115 Idle fluctuation

Re: '03 F115 Idle fluctuation

Thanks to Rodbolt and sdsaw.

I appreciate the explanation. Makes sense. While the dealer can fit me in, my engine-savy (very experienced) nephew will be over in a few days. Despite spring arriving, have a number of distractions.

Ro- like the bread :~)

Will keep you updated.

 

[rodbolt]

Re: '03 F115 Idle fluctuation

Re: '03 F115 Idle fluctuation

PITA is short for Pain In The Rear
family forum ya know

 

[Casco Bay Outrage]

Re: '03 F115 Idle fluctuation

Re: '03 F115 Idle fluctuation

---- UPDATE----

My very mechanical-savy nephew came over today.

He removed the intake, drained and inspected the VST. Nice and clean.
He cleaned the ISC (idle control in photo above) but it was pretty clean.
He measured PSI at the fuel rail (on and running) and it is per Service Manual specs.

The thermostat is original (2003) and while we can't tell if it is sticking, it will be replaced.

It appears that one needs a test harness to measure the TPS so we will not be able to do this unless I buy the harness. Is this worth the investment?

Would it be plausible that the coolant temperature sensor (opposite side of the block) have a role?

When I first got the boat in 05, engine hours were 32 and the idle was a constant 7 on the digital tach. After last year's service (in prior post linked above) the engine idled at 8. Is this significant?

 

[jaymz101]

Re: '03 F115 Idle fluctuation

Re: '03 F115 Idle fluctuation

Its not impossible without the harness, but it is a PITA without it. Probably worth it, but you can make your own with a male and female connector of the same type, a few female bullet connectors and some wire.

I dont think that the change in idle from 700 to 800 after service is that significant. Its still within range (750+50 RPM), probably was adjusted during the service and it just drifted a little bit since.

 

[Casco Bay Outrage]

Resolution

Resolution

It appears the problem is fixed.

The thermostat was changed and the motor is now idling at 7 and there is no intermittent fluctuation. An in the water test will be done to confirm.

There was some salt globs that appear to be keeping the old thermostat from closing completely. I would hazard a guess this part was original (2003).

Thanks to Sdsaw, Rodbolt and Jay. I really appreciate your help.

 

[sdsaw]

Re: '03 F115 Idle fluctuation -FIXED

Casco Bay Outrage,

Glad you fixed it!
Thank you for getting back on what you did to remedy your fluctuating idle issue.
There are more of these 4 strokes motors every year and I'm sure there will be others that will benefit from this info.