Optimax idle issues and overheat alarm

[Savage]

Glycerin must be used to stabilize the needle for accurate measurement. You should really purchase a Hard copy Service Manual from Mercury according to your motors serial number.

[Bigbassfish]

Oh, I have one! Picked it up at the beginning of this ordeal a few years back, I didn’t realize that procedure is in there, I will take a look tonight.

[JOE54]

report back with results of on the water fuel psi test

[Bigbassfish]

So I’ve just read through the manual as it describes testing the fuel pumps (low at the vst, high at the fuel rail) and that seems pretty straight forward along with the section describing testing the fuel injectors. It does say the engine can either be started or cranked for 15 seconds, is one of these ways preferred? I also read through the section for air compressor and fuel pressure testing, is it really necessary to have the dual gauge or can I just use the one and swap back and forth between fuel and air? If I really needed to, could I just get another single gauge and hook both up at once? I’d rather not buy another if I don’t need too.

[Savage]

Must have TWO gauges.

Get pressure Both air and fuel pressure readings:

At idle
Under Acceleration
5/10/15 seconds after shutdown (you can restart for each).

[Bigbassfish]

Okay, I will begrudgingly get another gauge tomorrow, I guess an extra $40 is better than $700 for the merc version. Why is it that this has to be done concurrently? I will most likely be doing this solo, what is going to be the easiest way to measure under acceleration? Do I have to launch the boat with the cowling removed and just leave the gauges hooked up and dangling while I get in plane? Can this be done on muff by just hooking up to a hose and revving the engine in gear?

[Savage]

Concurrently because the delta between the two gauges tells the story.
Cowling must be off.
Long hose connections to the console is the only way this can be safely done solo on the water.
Never rev a motor in gear on the hose.

[Bigbassfish]

Good to know, sounds like it’ll be interesting… do I need to get on plane or will simply accelerating say from idle to 6mph do the trick? I could see it getting tricky keeping the motor dry with no cowling while coming on and off plane. I’ll have to try and find some npt hose or something to extend my gauges.

[EuropeanAM]

Good to know, sounds like it’ll be interesting… do I need to get on plane or will simply accelerating say from idle to 6mph do the trick? I could see it getting tricky keeping the motor dry with no cowling while coming on and off plane. I’ll have to try and find some npt hose or something to extend my gauges.

"Under Acceleration". NOT "Wide Open Throttle".

Strap the gauges to the side of the engine, grab a buddy and set his cell to "video". Aim him and his phone at the gauges while you are at the helm. Safest bet.

[Bigbassfish]

Okay, i will try and find a time I can have an extra set of hands, especially considering I’m struggling to figure out exactly what it would take to extend the gauges I have more than the 18 inches of hose it came with. Hopefully I’ll have some results later this week.

[Bigbassfish]

Tested pressure today, hooking up to the boost pump showed 30psi, started the motor with both gauges hooked up and fuel shows 110psi, and air shows 95 psi, motor still ran rough and died several times. Pressure would rise immediately upon startup, but when the motor would stutter and rpm dropped to below 500, the fuel pressure would drop to 90 and air would drop to 80, and then bounce back to 110 and 95. Under acceleration, it the needles stabilized at 110 and 95, and the motor runs much smoother. Pinching off the fuel hose caused the gauge to jump to 200, but when I pinched the air hose, the pressure slowly builds to around 110 before the engine stalls out, this happened both times I tried. I have a new air filter on the way, but last time I replaced the air filter, the symptoms were the same. Hopefully this helps!

[EuropeanAM]

So... what were the pressures:

-Under Acceleration

-5 seconds after shutdown

-10 seconds after shutdown

-15 seconds after shutdown?

The symptoms you described with the "jumping" air and fuel pressures reminds me of a loose rivet in the reed plate of the compressor.

[Bigbassfish]

Checked again today, pressure upon startup was 110 at fuel, 100 at air, 5 seconds after shut off they both had dropped 5psi, and stayed there atleast a minute after shutdown.

[Bigbassfish]

I’m sorry, if it wasn’t clear in my first post, under acceleration neither pressure seemed to increase or decrease, stayed at the 110 and 95 the whole time it was running, except for when the rpm’s dropped and the motor stuttered as described.

[Bigbassfish]

Would a loose rivet still make sense considering the motor ran significantly better last season, all of a sudden it’s acting up again and running worse than ever. It’s to the point where the motor does not idle at all unless I get on plane first, after which it will idle without dying, but it still fairly rough and drops rpm occasionally. Is there anything else I can do to help diagnose this?

[EuropeanAM]

Checked again today, pressure upon startup was 110 at fuel, 100 at air, 5 seconds after shut off they both had dropped 5psi, and stayed there atleast a minute after shutdown....

I’m sorry, if it wasn’t clear in my first post, under acceleration neither pressure seemed to increase or decrease, stayed at the 110 and 95 the whole time it was running, except for when the rpm’s dropped and the motor stuttered as described.

VERY important that we get the most accurate information you can provide if we're to be of much help. There's a big difference between 100/110 and 95/110. Which was accurate?

A loose rivet on the compressor reed plate CAN be intermittent (at least in it's symptoms). So can spark plugs or plug wires (have you performed resistance tests on the plug wires)?

[Bigbassfish]

Just reviewed the videos, the first day I tested it, it was 95 on air, 110 on fuel. Under acceleration the pressure did nothing. I checked it the next day to reconfirm and it seemed like the air was definitely higher, maybe 98 air and 110 fuel. My gauges only have 5 psi increments so it can be hard to be precise. I have not checked plug wires or anything else involving ignition, outside of replacing the spark plugs. Could this also maybe be a coil issue? I think it’s so odd how without fail, it runs like crap until it’s gotten on plane and then coming down, it’s significantly better idling, every time.

[EuropeanAM]

If you've not ohms tested the plug wires (including lightly flexing them around while reading resistance), I would do so now. Seldom thought of but a very common item to give problems.

[Bigbassfish]

Just tested the plug wires, top wire was at 1250, the other two were both at 1240, no change when wires moved around. Dang it, a plug wire would’ve been an easy fix.

[EuropeanAM]

Any rust or flaky/rusty paint on the LOWER set of flywheel teeth?

Reeds inspected?