F90 Instant Stalling during Cold Startup

[Angelo4730]

I have a 2005 Yamaha F90 TLRD with almost 500 hours.

During cold engine startup, my engine will frequently stall within a half second of starting. Once the engine is warm, there are no startup problems.

If I open the throttle a bit during cold startup, the engine will start without a problem. If I then drop the open throttle down to idle too soon, prior to letting the engine warm up a bit, it will also stall sometimes.

Once warm, the engine starts and runs great. Reaches almost 6000 rpm. No codes with yamaha diagnostic software (although some history). Compression tests of cold engine were good at 170, 175, 185, 175.

Fuel pressure is strong. I actually measured about 5 psi over the values in the service manual. The pressure gauge I used is old so I am going to confirm this with a different pressure gauge.

If I do the yamaha diagnostic stationary test on the IAC valve, I can hear the valve move, and then click at the end of the test. I also hear the IAC valve click when I shut off the engine. However, if I do the engine active test of "fully open IAC valve", the engine will stall every time.

What do you think the problem can be? Do you think it is the IAC valve? They cost $250 so I don't want buy one unless I am pretty confident that it is the problem.

[Jason2tpa]

Once the engine is warm, there are no startup problems.

If I open the throttle a bit during cold startup, the engine will start without a problem. If I then drop the open throttle down to idle too soon, prior to letting the engine warm up a bit, it will also stall sometimes.

To me, what you're describing is normal. Especially for a 12 yr/old engine. Are you using the choke for cold starts? If you're using choke during cold starts with the throttle lever in neutral, it's probably stalling because the mix is too rich (too much fuel). When you open the throttle, you open the throttle plates thus allowing more air into intake. And it sounds like your engine likes this.

I bolded your words above to indicate an important factor you've described....prior to letting engine warm up. Instead of completely pulling back the throttle to neutral while it warms up, do it in increments until your back in neutral (no throttle).

If you were experiencing stalling with a warm engine, this would be a problem to address. But in my lengthy experience cold starting outboards, I've always applied choke along with opening the throttle a tad. You'll know when the engine is warm because the idle rpms will begin increasing because of the added throttle applied during cold start. At which point I pull back throttle to neutral and a way I go.

[fairdeal]

well, the OP's D model has EFI - hence the IAC and no choke.

I don't see it worth the trouble to re-test the fuel rail pressure with a different gauge -
the symptoms don't really point there anyway

Hope Rodbolt weighs in on this one.


I can give you one bit of expert advice, though-

do NOT remove those tamper-resistant Torx screws on the IAC valve body

thinking to get a better look at it while you run the YDS test

[rodbolt17]

fix the motor.
it is an EFI model.
no chokes.

[Angelo4730]

fix the motor.
it is an EFI model.
no chokes.

Rodbolt, are you saying that you think the problem is the IAC valve and that I should go ahead and replace it?

Thanks

[boscoe99]

Carefully inspect the wires to the IAC valve connector. They are known to catch on the cowling and get chaffed, abraded, pulled loose. The signals from the ECU can't get to the connector as they should. That can then make the IAC valve do some crazy stuff.

[rodbolt17]

as there are no codes carefully test EVERYTHING else BEFORE springing for a new ISC.
sometimes they can be cleaned with combustion chamber cleaner.
it should not stall when running the active test.

[rejesterd]

When the engine is cold in the morning, how do your YDS intake air temp and intake air pressure values compare to your local weather station?

[Angelo4730]

When the engine is cold in the morning, how do your YDS intake air temp and intake air pressure values compare to your local weather station?

At 6:30 am I measured 70.4 degrees F and a pressure of 1021.4 with Yamaha diagnostics. The two closest weather stations measured 66.2 and 67.4 degrees and 1022.35 and 1023.02. I'm assuming this is close enough.

I did notice that if the engine is cold and I turn the key to the on position for about 5 mins (but do not start engine) the IAC valve will warm up somewhat. Is this normal? Is there a procedure to check if an IAC valve is going bad with an ohm meter? If so, I see 4 connection points on the plug, which ones would I check?

Thanks

[fairdeal]

turn the key to the on position for about 5 mins (but do not start engine) the IAC valve will warm up somewhat. Is this normal? Is there a procedure to check if an IAC valve is going bad with an ohm meter? If so, I see 4 connection points on the plug, which ones would I check?

in my experience, it is normal - although I don't understand why Yamaha does it that way

the IAC is a "stepping motor" -
it has two coils - so four connectors, one for each coil end

so obviously each pair with continuity, is a coil

theoretically, "going bad" could be
open coil - no continuity
shorted coil - continuity w/ little to no resistance
good coils, but locked rotor

Rodbolt can tell us which is the most common cause for replacement

[arny]

Had the same issue on a similar motor,changed the icv -problem solved.Dont know if yours will

[rodbolt17]

typically the rotor locks or gets sticky.
then the ECU keeps toggling the ground path and both the ISC and the ECU start overtemping and the system goes stupid.

[boscoe99]

Wondering if some LED's could be wired into the system so as to see the frequency of the ground being applied. If it exceeds X then that could indicate something is amiss. Of course we have no baseline to know what X should be.

Now if I had an F90 or other model that was susceptible to the valve damaging the ECU I would fabricate a driver as a buffer between the two. The ECU ground would be miniscule compared to the valve ground.

[Angelo4730]

as there are no codes carefully test EVERYTHING else BEFORE springing for a new ISC.
sometimes they can be cleaned with combustion chamber cleaner.
it should not stall when running the active test.

I have checked fuel rail pressure, fuel filters, injectors, cylinder compression, spark plugs, intake temp/pressure sensor, vaper locking, inspected wires. Anything else you recommend checking?

Also, you said that a bad ISC can cause the ECU to overheat since it keeps sending current. I'm trying to avoid using my boat until fixed, but I do need to make one trip. Would unplugging the ISC valve prevent the ECU from frying, or can this still happen with an unplugged ISC since the ECU will keep trying to send commands to an unplugged and therefore unresponsive ISC?

Thanks

[boscoe99]

Got me to thinking. Above idle RPM I presume that the ECU no longer sends a ground to the valve. Is this correct or not?

If the motor is started, warmed up and operated well above idle RPM I presume the likelihood of the valve damaging the ECU is nil since the valve is no longer in the equation. On the other hand, a motor being idled for long periods of time would be at greater risk of damaging the ECU. I suspect.

Now if I had some lights on the valve grounds I would be able to see this for myself.

Fairdeal, what say thee? Busted, plausible or confirmed?