Engine wouldn't fire without throttle after overnight

[KD4UPL]

My 2004 Merc 350 Mag has always just started with a turn of the key, as it should. I've owned it for 8 seasons and never done anything to start it other than twist the key; it cranks and fires. This weekend after running the boat for a few hours I tied it to the dock and left it overnight. In the morning the boat would crank fine but not quite ever fire. I tried it 3 or 4 times. It would just almost want to fire but not quite. Finally I treated it like I used to start my old carbureted Merc 350. I pumped the throttle once and then left it at about 1/4 throttle and cranked it. The boat fired right up and ran and started fine the rest of the weekend.
So, what would have caused it to need some throttle? Is this a sign of something going bad? It was about 80 degrees ambient when I parked it the evening before, about 60 degrees overnight and about 70 or 75 when I tried to start it. Nothing unusual. I've done this exact same thing many times before. I replaced the IAC about 2 years ago because it wouldn't idle right and that cured that problem.
Any insight would be appreciated.
Thanks.

 

[JustJason]

So it could be anything, but where you would start is by checking for fuel pressure leakdown and throwing the scanner on it to check for codes.

Having to open the throttle to start the engine can be an indicator that the engine is flooded, which will happen if the injectors are leaking, but it doesn't mean that's what is actually happening until it's checked.

 

[cyclops222]

Post # 2 and a leaking injector might be at fault. Soounds correct. Old enough.

 

[KD4UPL]

After the boat sat for about 4 weeks I fired it up in my yard on the trailer before going to the lake. All I did was turn the key and it started. At the lake I launched it and it started the same way. Ran for about 15 minutes and shut the engine off. After about 20 minutes tried to start it and it wouldn't without giving some throttle. The rest of the day with probably 10 to 15 starts and stops it was fine and didn't need throttle.
I borrowed a fuel pressure gauge. After the boat sat on the trailer for 8 days I connected the gauge and had zero fuel pressure. Turning the key on made the fuel pump run and it went up to about 42 psi. The boat started fine with no throttle. At idle the fuel pressure was around 36 to 38 psi. I turned the engine off and let it sit overnight. It's been about 20 hours and the pressure has dropped to about 18 psi.
Is this too much drop too fast indicating a leaking injector or some other leak? Even if it is, since the boat started from 0 psi with just turning the key I'm still not sure why it requires throttle some times.

Thanks for you help.

 

[Scott06]

After the boat sat for about 4 weeks I fired it up in my yard on the trailer before going to the lake. All I did was turn the key and it started. At the lake I launched it and it started the same way. Ran for about 15 minutes and shut the engine off. After about 20 minutes tried to start it and it wouldn't without giving some throttle. The rest of the day with probably 10 to 15 starts and stops it was fine and didn't need throttle.
I borrowed a fuel pressure gauge. After the boat sat on the trailer for 8 days I connected the gauge and had zero fuel pressure. Turning the key on made the fuel pump run and it went up to about 42 psi. The boat started fine with no throttle. At idle the fuel pressure was around 36 to 38 psi. I turned the engine off and let it sit overnight. It's been about 20 hours and the pressure has dropped to about 18 psi.
Is this too much drop too fast indicating a leaking injector or some other leak? Even if it is, since the boat started from 0 psi with just turning the key I'm still not sure why it requires throttle some times.

Thanks for you help.

I would think needing to add throttle would be indicative of the IAC not working or screen being clogged.

I wouldn't chase the rabbit hole of fuel pressure based on what you outline

what kind of shape is the igntion system in ?

 

[KD4UPL]

IAC was just changed last year after having rough idle and the code for bad IAC. That cured the problem.
I've been wondering if it's time to change plugs, wires, cap, etc just based on time. I've owned the boat for 8 seasons and never changed any of that. I wouldn't think the problem starting is ignition related since throttle fixes it but maybe I'm missing the connection.

 

[Scott06]

IAC was just changed last year after having rough idle and the code for bad IAC. That cured the problem.
I've been wondering if it's time to change plugs, wires, cap, etc just based on time. I've owned the boat for 8 seasons and never changed any of that. I wouldn't think the problem starting is ignition related since throttle fixes it but maybe I'm missing the connection.

it at least bears looking into especially given the issues known with the crab style caps. in my experience if you have a good spark and any gas presence it will fire off easily.

 

[KD4UPL]

Correction, the IAC was replaced 2 years ago.

 

[Scott06]

Correction, the IAC was replaced 2 years ago.

I would make sure the filter is clean, it would be the first thought when more air gets it running, but that seems to not be required every time?

When you do have to add throttle is the exhaust black afterwards?

if its been 8 years would be interested to see what the plugs and inside of dist cap look like

 

[ESGWheel]

I've been wondering if it's time to change plugs, wires, cap, etc just based on time. I've owned the boat for 8 seasons and never changed any of that.

Your question regarding the replacement interval of ignition components was robustly answered in your recent thread (link).

 

[KD4UPL]

Needing the throttle to start has only happened maybe 4 times total in the 8 years I've owned the boat. Twice this year and I think once last year. I've probably started it 100 or more times this year and all but 2 times have not needed to give throttle.
I can't really tell on the black smoke as it's only happened when the boat is in the water.
I pulled a plug from the rear port cylinder. A photo is attached.
I went out this evening while the fuel pressure gauge was still hooked up. It was down to 18 psi. The boat started fine with no throttle but the fuel pressure never increased about about 30 psi this time. I turned the engine off and the pressure stayed at 30 psi. I cycled the key and could hear the fuel pump run. Each key cycle bumped the gauge by about 2 psi. I did it 3 times to 36 psi. I started it again and it remained at 36.
That doesn't seem right.
ESGWheel is correct. An earlier thread did determine I should be replacing the ignition components. I intend to do that over the winter. I guess I don't understand how adding throttle will fix an ignition problem.

 

[ESGWheel]

My intent was to highlight a previous answer to the question about 8 years, but I will also offer my thoughts on the issue you are experiencing.

This is how I view the world of motors: an internal combustion engine is a marvel of engineering and is akin to a well-rehearsed and tuned orchestra. There are so many components, actions and timing all operating in concert with one another to make it run. From the ratio of atomized fuel to air needing to be just so, to the effectiveness of the rings so have proper compression, to the need for a properly timed hot spark to create the bang at just the right moment. And with modern engines a host of monitors all feeding into the conductor so it can probably manage its players.

While I know I am waxing poetic if you think about what an engine does it really is poetry in motion. And proper maintenance and care is needed to keep it so. I can understand your frustration at needing to occasionally have a different start routine but IMHO I am surprised you have been as lucky as you have been.

What is the relationship between sometimes needing throttle and 8-year-old ign system? Unsure but without a properly tuned up engine cannot find out if there is some other underlying cause.

 

[alldodge]

Turning the key on made the fuel pump run and it went up to about 42 psi. The boat started fine with no throttle. At idle the fuel pressure was around 36 to 38 psi.

The boat started fine with no throttle but the fuel pressure never increased about about 30 psi this time. I turned the engine off and the pressure stayed at 30 psi. I cycled the key and could hear the fuel pump run. Each key cycle bumped the gauge by about 2 psi. I did it 3 times to 36 psi. I started it again and it remained at 36.

There is an issue with the fuel system, just not totally sure that is causing the intermittent start issue. Fuel pressure should be 38 to 42 with key ON, and being around 38 is not an issue at idle.

What is the battery voltage prior to start and what is it during cranking?

 

[Scott06]

I guess I don't understand how adding throttle will fix an ignition problem.

its a matter of ruling out low hanging fruit and multiple things could influence it as AD lays out.

What is the gap on the plugs looks fairly wide but hard to tell from the screen?

 

[JustJason]

After the boat sat for about 4 weeks I fired it up in my yard on the trailer before going to the lake. All I did was turn the key and it started. At the lake I launched it and it started the same way. Ran for about 15 minutes and shut the engine off. After about 20 minutes tried to start it and it wouldn't without giving some throttle. The rest of the day with probably 10 to 15 starts and stops it was fine and didn't need throttle.
I borrowed a fuel pressure gauge. After the boat sat on the trailer for 8 days I connected the gauge and had zero fuel pressure. Turning the key on made the fuel pump run and it went up to about 42 psi. The boat started fine with no throttle. At idle the fuel pressure was around 36 to 38 psi. I turned the engine off and let it sit overnight. It's been about 20 hours and the pressure has dropped to about 18 psi.
Is this too much drop too fast indicating a leaking injector or some other leak? Even if it is, since the boat started from 0 psi with just turning the key I'm still not sure why it requires throttle some times.

Thanks for you help.

The injectors will always leak down. The rule of thumb as to whether the leakdown rate is to much is to use 2 PSI per 10 minutes on a hot engine, not a cold engine. What I would do is go use the boat, get the engine hot. Shut it off, hook up the fuel pressure gauge. Start the engine again and let it run for just a minute, then shut it off again and watch the gauge.

If your leakdown rate is greater than 2 PSI per 10 minutes, there is a leaking injector problem. If it's less than 2 PSI per 10 minutes, then it's not a problem.

That said, it's always possible the engine has more than 1 problem, but this is 1 item that can be easily checked.

 

[sdowney717]

Fuel injected car engines seem to hold fuel rail pressure a long time after running.
I wonder if the injectors are dripping fuel after shutdown flooding the engine
That is why you need to crack the throttle, it needs more air to fire up

 

[Lou C]

That spark plug suggests an underperforming ignition system or too rich fuel mix….

 

[KD4UPL]

That spark plug suggests an underperforming ignition system or too rich fuel mix….

Thanks for that analysis. I've got new plugs and wires on order.