Mercury motor problem

I have a 2011 Mercury 90 hp 4 stroke ELPT4 model 1F90413HD

I think I had bad old in it last spring. It would run and then lose power and stall.
I drained the gas added fuel injector cleaner, stabil and 10 gallons of gas.
I ran it down 5 gallons and the issue seemed resolved.
Now it is back again.
yesterday I went out and it started right up.
I noticed the safety tether was not in and installed it, which cut the engine.
It was hard to get it restarted.
It ran good after restart and I ran 3 gallons of gas at high throttle settings (up to 6000, the max).
I stopped and it restarted right away.
After sitting about an hour, It would not restart until a long session of trying.
Once is started, it ran good, but after a few minutes is lost power again.
This time it would not restart.
I tried starting at higher throttle settings and it chugs, but does not restart now.
Any thoughts?

Have you tried a new fuel filter and spark plugs ? Is the tank 100 percent fresh fuel or you still blending it ? I had the tiniest bit of corrosion on a spark plug one time that threw a fit when warmed up .

Is the fuel line collapsing internally?

Both the low and high pressure filters are changed, as have been the plugs.
I had drained the tank down as low as it would. Then ran 5 gallons fresh through.
When it quit yesterday I added 5 more gallons fresh gas.
On the way home I topped the tank(19 gal capacity) up.
It took 9 gallons.

I am going to pull the cover today and check connections and gas line as suggested.

eyekatcher wrote:

I am going to pull the cover today and check connections and gas line as suggested.

Look for any possibility of air leaks that is allowing air into the fuel lines. Your symptoms give me the impression that somehow you are losing fuel pressure between the tank and the engine somewhere due to an air leak.

It would be nice to know in the situations where the motor does not start if you pull a spark plug, is it wet from fuel? That is the next step I would pursue is seeing if the engine is getting fuel and somehow you have an electrical issue that is cutting off the spark.

I’m kinda of confused. The safety tether was not installed, so you installed it and the engine quit? So the motor was running without the tether? Now it won’t start with it installed? It sounds like you have other issues, but have you checked into the kill switch to confirm it’s working properly?

Outside from what others have mentioned.

On the fuel side, this seems very similar to how a motor reacts when the fuel pressure regulator is failing. Which would lead me to the VST. The fuel pressure bleeds off quickly so the motor can be very hard starting even when warm. When you pump the bulb, make sure the arrow on the bulb is point up, does the bulb firm up? Or does it seem like you can keep pumping? If you get it started, normally a bad VST will show it’s ugly head in the telltale, you will smell raw fuel. I do not know specifics on checking a bad VST on your motor, but it’s worth looking into.

The kill tether was not in place. to get it in place the switch has to be moved to off which kills the engine. then I put the tether in and moved the switch back to on. then it was hard to start
but ran good after that for 3 gallons of gas and up to max rpms.
The boat does not use a pump bulb in the fuel line.
What’s a VST?

Also, I have vessel view app bluetooth to motor if that helps to diagnose.

eyekatcher wrote:

The kill tether was not in place. to get it in place the switch has to be moved to off which kills the engine. then I put the tether in and moved the switch back to on. then it was hard to start
but ran good after that for 3 gallons of gas and up to max rpms.
The boat does not use a pump bulb in the fuel line.
What’s a VST?

Having no pump bulb would be very strange, in fact I have never seen an outboard without one. Are you positive it’s not hidden in the mess of wires near the transom? That right there could be your issue. Your low pressure lift pump does not do a good job of pulling fuel, it’s pushes fuel. So it’s going to take a lot of work to get fuel to the lift pump if you have an air gap somewhere in the line.

VST is a vapor separator tank. It’s your high pressure fuel pump, at least that’s an easy way to explain it.

eyekatcher wrote:

What’s a VST?

It’s a vapor separation tank. Mercury, I believe, actually uses a different system the call a fuel supplier module or something like that.

We’re getting out in the weeds pretty far here from what the average guy can diagnose and repair without risking replacing a lot of parts unnecessarily. Check the basics and try to determine is this a fuel or spark issue.

So, I hook up the hose to the water muffler , turn it on.
Motor starts right up and is running like a top.
Going to let it come up to temp and do some restarting as well

TillrLife wrote:

Having no pump bulb would be very strange, in fact I have never seen an outboard without one.

Yeah that is very strange.

This part, looks like a member of the fuel delivery system,
Making a noise similar to a bilge pump pushing air, when the motor is surging.

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So as it warms up, I position the throttle about 1500 RPMs,
and is starts surging between 1000 and 2000 and starts chugging
and if I don’t throttle up it stalls. If I do throttle up it just takes a little longer to stall.
It restarts right up and runs for a few minutes till it chugs again.

be sure to check the coils.
Mercury did have a service bulletin out and they actually ended up paying/replacing a lot of coils for people.
But I believe that has now since expired.

You could even Google mercury coils and your symptoms are very familiar, at first they likely had reduced performance and then the problem becomes intermittent and it’s usually a slight crack or other in the coil that is also affected by ambient temperature with thermal expansion contraction

I am thinking it is part of the fuel system between the gas tank and whatever that part is in the picture???

Because it restarts readily.

CaptainMusky wrote:

<div class=”d4p-bbt-quote-title”>TillrLife wrote:</div>
Having no pump bulb would be very strange, in fact I have never seen an outboard without one.

Yeah that is very strange.

I’ve had two new 115 Merc 4 Strokes since 2011 and neither has had a primer bulb on them. I’ve never had a problem. According to Mercury, if your Mercury 4 Stroke is 75hp or greater, the onboard electric fuel pump will prime the fuel system. When you turn on the key, you can hear the pump.

My 9.9 Merc 4 stroke kicker on the other hand has a primer bulb.

eyekatcher wrote:

This part, looks like a member of the fuel delivery system,
Making a noise similar to a bilge pump pushing air, when the motor is surging.

That to me looks like the VST, or whatever Merc may call it.

It’s basically a fuel pump so it will make some noise. Do you smell raw fuel in the telltale?

Have you pulled the plugs to see what they look like? Then you can determine if it’s running lean or rich.

It’s tough to diagnose anything over the internet. You may want to bring it in to get checked if you’ve crossed off all things you can check. I would also put a new fuel line from the tank to the motor with a primer bulb. That’s so strange. How do you prime the fuel system when you replace a water/fuel separator?

My 4str mercury which is a bit older 2004 acted very similar as well and it wasn’t even part of that service bulletin and it would start right up and idle for a while as soon as you put the coils under a rpm load she would start chugging , and then finally die. Some days it would run fine until all of a sudden it didn’t then I finally figured it out. It’s definitely on my experience turned out being a hairline crack in one of the coil packs.

If you haven’t done so yet definitely do a quick compression check to make sure a cylinder or truth and drop out.
Unlikely but still worth a check

Pulled plug when stalled
Just a touch of moisture on one side of plug

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Here are the plugs I just replaced even though they are not very old.
Also tried swapping the pencil coils with a new one, one at a time with a single new one. Didn’t make a difference.
also replaced some fuel lines

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TillrLife wrote:

How do you prime the fuel system when you replace a water/fuel separator?

This was already answered.

Karry Kyllo wrote:

According to Mercury, if your Mercury 4 Stroke is 75hp or greater, the onboard electric fuel pump will prime the fuel system. When you turn on the key, you can hear the pump.

There is something in the fuel delivery system module that does that.

The symptoms you are describing sound exactly like what my motor did until I brought it to Iowaboy1 and he traced and found a wire to the injectors that had worn through it’s insulation under a clamp that was ironically designed to prevent the wire from vibration.
I find the fact that your motor runs well then does not run at all to be an indication that an electrical issue is more likely than a fuel issue.
Perhaps Iowaboy1 will respond.
I wish you good luck in resolving your issue. Keep us posted.

I think I am going to work on the Fuel supply module (VST).
When the motor starts chugging, one of the motors in the FSM starts making a sound similar to when a bilge pump runs dry.
I am thinking it might have a dirty screen on the lift pump.

Like Mojo suggested I would try and get ahold of Sheldon/Iowaby1. He would probably be able to talk with you over the phone and make some suggestions based off the symptoms. I cleaned my VST on my Yamaha a few years back and like you mentioned cleaned the pickup screen. Ended up sending my fuel injectors to Sheldon to have them cleaned and my motor has been running way better ever since.

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It’s tough to see what’s going on with those plugs. To do a proper plug chop, you need to get a motor up to temp, swap plugs, do a full throttle run and kill the engine. Pull the plugs and then inspect.

But, they do look ok. The first one you uploaded does look wet, which would lead me to believe there’s either too much fuel or weak spark. Obviously weak spark will be more evident under load. This is likely not the issue, but are you sure they are gapped correctly?

There should be a filter in the VST as you mentioned, I would pull that and inspect. You can normally reuse them, just clean it with carb cleaner.

Have you tried running the motor on a remote fuel tank?

There are two filter screens, one on each pump.
The rebuild kit was backordered locally. not taking it apart without that.
Ordered on line, should be here the 14th.