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2004 5.0l merc start up issue

charred1

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Boat starts fine first thing in the morning, however it has issues after warm up. It just cranks unless I give 1/2 throttle or more, then it will fire up. Other than that, the boat runs great. Any ideas on what the issue is and how to rectify? Out in Havasu now and would like to hit the lake today, if easy fix. Thanks in advance.
Mike

Also, boat was just professionally serviced.
 

charred1

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Thank you for the early morning replies. That helps me with a starting point!! Just so I am clear, the two items recommended to check/replace typically are problematic after running? Last two days, boat fires up normally when launching, but after getting to our destination and sitting for a few hours, she doesn’t want fire.
 

4Waters

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Thank you for the early morning replies. That helps me with a starting point!! Just so I am clear, the two items recommended to check/replace typically are problematic after running? Last two days, boat fires up normally when launching, but after getting to our destination and sitting for a few hours, she doesn’t want fire.
Yes a warm IAC (Idle air control) may work fine cold but once heat soaked may have problems, the mufflers are known to get plugged, it's probably not the issue but it's an easy diag by just pulling it out and seeing if it starts correctly, you can run it for the weekend with out it as well, they are problematic on the 496's and most guys just take them out.
 

charred1

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790E547F-E0CD-4034-9F7C-69F69A7D77B7.jpeg
A5126958-EBAF-482F-B420-592842BA7A8F.jpeg
9BD7A7A7-269E-4BB3-8636-5B4474035B35.jpeg

I don’t see the muffler in the carb. Also, the the first picture, is this the idle air valve?
 
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DaveH

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idle air valves ADD air when cold or other heavy load conditions at idle (going in gear). the fact it starts cold leads me to believe your IAC is working.i would be looking at fuel pressure and making sure the pressure doesn't bleed off after it warms up and you shut it off.
 

lbhsbz

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Still carb’d in 2004? Wow.

When it does start with 1/2 throttle...does it puke a bunch of smoke? If so...it’s flooding and your 1/2 throttle is giving it enough air to clear the flooding...could be the carb is leaking fuel into in the intake while it’s sitting there heat soaking.

Gotta figure out if it won’t start due to too much or not enough fuel. After you get to beach, pull the arrestor and peak down the carb...see if any fuel is dripping/puddling on the throttle plates. When you give it your 1/2 throttle, (before cranking)...watch to see if you’re getting a squirt from the accelerator pump nozzles down the throats...if you see a squirt, then you know at least the bowl has fuel in it and it all hasn’t leaked/boiled out.

When it does start, watch for black (rich) smoke...that’ll tell you what’s going on.

I don’t think the IAC valve has anything to do with this...it can’t open anywhere near the equivalent of 1/2 throttle, and if that’s what’s required, it’s something else.
 

charred1

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Still carb’d in 2004? Wow.

When it does start with 1/2 throttle...does it puke a bunch of smoke? If so...it’s flooding and your 1/2 throttle is giving it enough air to clear the flooding...could be the carb is leaking fuel into in the intake while it’s sitting there heat soaking.

Gotta figure out if it won’t start due to too much or not enough fuel. After you get to beach, pull the arrestor and peak down the carb...see if any fuel is dripping/puddling on the throttle plates. When you give it your 1/2 throttle, (before cranking)...watch to see if you’re getting a squirt from the accelerator pump nozzles down the throats...if you see a squirt, then you know at least the bowl has fuel in it and it all hasn’t leaked/boiled out.

When it does start, watch for black (rich) smoke...that’ll tell you what’s going on.

I don’t think the IAC valve has anything to do with this...it can’t open anywhere near the equivalent of 1/2 throttle, and if that’s what’s required, it’s something else.
A897708B-7728-43A6-9DF3-DB8B52159613.jpeg

Yup...May 2004. Someone else said same...lol.
No black smoke, but I will do what you recommended by peaking down the carb.

Thank you to all who have posted up...it’s much appreciated!

Mike
 

lbhsbz

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After you shut it down, just go back there and watch it for a few minutes...do the same periodically and right before you fire it to leave.

Not a damn thing wrong with carbs, just a bit more finicky sometimes.
 

oldschool

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Not to thread jack, BUT, I’m having a similar issue with a 2004 Mercruiser 350 MAG MPI tow sports engine. Fires first thing, then throughout the day it’s hit and miss. I can get it to start by giving it throttle, in neutral. Could this be a fuel pump issue?
 

lbhsbz

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Not to thread jack, BUT, I’m having a similar issue with a 2004 Mercruiser 350 MAG MPI tow sports engine. Fires first thing, then throughout the day it’s hit and miss. I can get it to start by giving it throttle, in neutral. Could this be a fuel pump issue?
Doubt it...the fuel pump only knows on or off. I’d try disconnecting the fuel pump and use your same half throttle start routine...if it starts or fires and dies or at least pops, it’s flooded...likely leaky injector(s)
 

Marios Metalworks

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@oldschool I had a similar issue with a Merc 5.0MPI. Ended up being the fuel line from the pump feeding the separator that’s routed under the oil pan. It failed internally and collapsed on itself which made it very difficult to identify with the naked eye. Had 560hrs at the time.
 

oldschool

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@oldschool I had a similar issue with a Merc 5.0MPI. Ended up being the fuel line from the pump feeding the separator that’s routed under the oil pan. It failed internally and collapsed on itself which made it very difficult to identify with the naked eye. Had 560hrs at the time.
I’ll take a look at that before I start throwing parts at it. Thanks.
 
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