Mercruiser 5.7 Mag MPI quits if it idles too long

[Mark72233]

I have a 2005 Searay 260 with a Mercruiser 350 Mag MPI 5.7L. Last year it started quitting if it idled too long. That time varies. It can idle or run at low rpms (600rpm up to 1500) for 15-30 minutes with no issues but sometimes without warning after low rpms it will drop down to less than 500rpms and won’t throttle back up and stalls out after about 10 seconds. Than it won’t start back up. Engine turns over but won’t start or stay running. It almost wants to start but won’t.

You have to let it sit for 30 minutes to an hour. Than it starts back and runs fine. The solution has been to not let it idle but that’s a pain.

Sometimes if I got to fuel rail and let air/fuel out of the air shrader valve it sometimes starts but that doesn’t always work.

Last year I replaced the Idle Control Valve and the problem seemed to go away. It came back last week leaving me stranded. Thank you Seatow. So I bought another ICAC valve and put it in today. Went out today and tried to get it to quit well it did after running at 1000rpms for 25minutes. So here I am in the bay anchored up dead in the water typing on my phone.

Its obviously not the Idle Control Valve. What else could it be.

 

[Mark72233]

It’s been sitting for 30 minutes and it cranked up. It’s running fine and may not do this again for a month or it may do it again in 30 minutes.

Since its nice and Im in the bay I am going to idle around and see if it does it again.

I love boats

 

[alldodge]

I would like to know what the fuel pressure reading is.

Real long idle might help with vapor lock, but normally wouldn't and as you said, you've bleed the shrader valve and it didn't help

Being an MPI it should have the flat distributor cap, has it been replaced?

 

[Mark72233]

24 minutes running at 1000rpms and it quit again. Turns over and tries to start. Pops on at 500rpms but won’t go over that and quits right away. Will sit here for 30 minutes or so and it should start right back up and run fine as long as I don’t let it run at around or under about 2000rpms. I can run it on plane all day with no issues.

 

[alldodge]

That does sound like VL and could very well be because of a weak fuel pump. Have a pressure gauge at home, or rent one from a parts store maybe?

 

[Mark72233]

Hey AllDodge, it’s Mark I also have the 1972 Thunderbird Formula 233 with the twin Merc 165hp.

Have no idea on fuel pressure. I will try and get something rigged up for my next outing.

Venting the Shrader valve has worked in the past I just don’t like doing it because of all the raw fuel that comes out. Haven’t tried it today but did try bleeding the shrader valve last week and it still wouldn’t start. Again I haven’t messed with it today.

Let me check the distributor cap and no it hasn’t been replaced.

 

[Mark72233]

Looks flat and have not messed with any ignition parts. Thought it was most likely a fuel problem.

 

[Mark72233]

Ok will find one this week and come back out and see if I can hook it up. Where do you plumb it into on this engine.

 

[alldodge]

Hey Mark, yes I remember your other boat, that took some nice working getting her going.

I don't think its the cap either, but those caps cause all kinds of issues, and they look great when inspected, but can still be bad. Folks with one of them need to keep a new or even used in the boat, because they can fail all together without warning

 

[Mark72233]

When I bleed that shrader valve should it vent just vapor or is it normal to have raw fuel come out as well.

 

[Mark72233]

It’s been over 30 minutes and this time it’s not starting back up so will give it another 15 minutes and try again. If not I will try bleeding the shrader valve. Last week when I tried bleeding it it was vapor at first than lots of raw gas. Which I so enjoyed down in the engine compartment.

 

[alldodge]

Last week when I tried bleeding it it was vapor at first than lots of raw gas

Yep that would be VL, there should be nothing but liquid gas come out

And what no rags, paper towels, toilet paper or other :noidea:

 

[Mark72233]

Gave it 45 minutes this time and it cranked right up and ran fine all the way back to marina. Worker at marina suggested ICAC valve. Told him just put new one in. Than suggested ECM.

 

[Mark72233]

Yes plenty of rags and paper towels just had family on the boat and don’t like messing with family and fuel

 

[alldodge]

Gave it 45 minutes this time and it cranked right up and ran fine all the way back to marina. Worker at marina suggested ICAC valve. Told him just put new one in. Than suggested ECM.

Suggest not asking his opinion again unless you want a laugh

 

[Mark72233]

So what causes the VL and or how do you fix it

 

[Mark72233]

Agree, haven’t had much luck with the mechanics at my marina

 

[alldodge]

Fuel heats up and starts to boil. When it boils pressure increases enough to displace liquid fuel for vapor. This normally happens when a motor has been run on plane or higher, heat increases and then the boat stops and folks swim for a time. A couple hours later, try to start and things are not good.

Wait long enough for things to cool down and pressures reduce, pump turns on and fills rail with liquid.

Merc has been trying to fix the issue without really doing what is needed. Indmar has the fix and many have fix them selves (including me). The fix is move the return line from going to the filter canister to the fuel tank. This is an easy enough fix, but does require so new hose routing

 

[Mark72233]

The engine has a fuel cooler maybe it’s not circulating water well enough.

 

[alldodge]

Understand the comment, but that wouldn't be a VL concern with your issue

It idle to slightly above there is very little fuel being used. So all the fuel in excess of reg pressure (Exp 40 psi) goes back to the filter. The pump will produce 60psi and everything over 40 goes back to the filter. Increasing pressure on fuel also increases heat, so a tad bit of extra heat goes back to the filter every millisecond.

Heat keeps increasing for long periods over time while idling. The way to reduce this is to return excess fuel back to the tank