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2003 Mercruiser 5.7 MPI starts very hard but runs great

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CampionPB

New member
Hi, My mercruiser use to start very easy without any throttle. Last year I found myself having to give it some throttle while cranking to start it. This year it will not start (even with new batteries) unless I hold the throttle on full while cranking and even then it may not start. Once it starts it seems to run very smoothly both at idle and at acceleration. Any thoughts on this condition? I get the boat serviced every year.
 
Ayuh,..... Sounds like a lack of fuel,.....
Pull the filter, 'n examine it's contents in a clear container,.....
 
Thanks Bondo... I pay nearly $2000 per year to have my boat stored and serviced before the season. I also noticed the spark arrestor was covered in a moist soot like substance. Same with the air intake value under the arrestor. Should the service guy not be cleaning the arrestor etc. for the annual service?
 
Bring it back and demand the issues be addressed as you paid for it.

Yes the flame arrestor should have been removed and everything thoroughly cleaned.

We did so on every boat serviced.
 
Is this during hot starts, cold starts, or both? If it's only during hot starts and it fires up with the throttle cracked open, check your ECT (Engine Coolant Temperature sensor). If it happens all of the time, make sure the fuel filter is clean and the vent tube for the fuel tank isn't clogged. Also, the vent fitting is a favorite place for spiders to spin their web and if that's clogged, it won't necessarily start easily.

ALWAYS check your fuel pressure if you have a problem starting or running the engine.

As a test, turn the key to ON, leave it for a few seconds, shut it OFF and repeat. If it starts easily, the pressure has bled down and turning the key to ON primed the fuel line.
 
My 2002 5.0 MPI did this on cold starts. The fix was changing the IAC muffler. This happened before I learned to use the internet and forums like this. Remove the flame arrestor and look where the passageway goes toward the IAC. It stuffs down into a small opening. It's a fibrous foamy little thing normally white. If your is old and mucked up it will look black. Remove it with needle nose pliers or pick tool. Don't drop it down the throttle body. You can run the boat without it. All it does is prevent a whistling sound at idle. I guess when it gets mucked up it wreaks havoc with the IAC. Pray this is it. It's a very cheap fix.
 
Mark S. said:
My 2002 5.0 MPI did this on cold starts. The fix was changing the IAC muffler. This happened before I learned to use the internet and forums like this. Remove the flame arrestor and look where the passageway goes toward the IAC. It stuffs down into a small opening. It's a fibrous foamy little thing normally white. If your is old and mucked up it will look black. Remove it with needle nose pliers or pick tool. Don't drop it down the throttle body. You can run the boat without it. All it does is prevent a whistling sound at idle. I guess when it gets mucked up it wreaks havoc with the IAC. Pray this is it. It's a very cheap fix.
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That "muffler" must be a Mercruiser thing but I never saw it on any of the boats I serviced, although MasterCraft boats definitely whistled and people hated it. There's no way I would consider stuffing anything into the IAC port- first, it could easily affect the idle quality and speed, then, it could be sucked into the engine with who knows what kind of consequences.

The only Whistling with Mastercraft boats happened during hard acceleration and even then, it was only when the rectangular fame arrestor on their LTR engine was used- have you EVER heard a GM vehicle whistle during idle? I know how many clicks the MC IAC was set to open during idle and never heard one whistle at 600RPM.
 
jimn said:
Is this during hot starts, cold starts, or both? If it's only during hot starts and it fires up with the throttle cracked open, check your ECT (Engine Coolant Temperature sensor). If it happens all of the time, make sure the fuel filter is clean and the vent tube for the fuel tank isn't clogged. Also, the vent fitting is a favorite place for spiders to spin their web and if that's clogged, it won't necessarily start easily.

ALWAYS check your fuel pressure if you have a problem starting or running the engine.

As a test, turn the key to ON, leave it for a few seconds, shut it OFF and repeat. If it starts easily, the pressure has bled down and turning the key to ON primed the fuel line.
Click to expand...
Hi Jimn, once I get the boat started it runs as good as it ever… zero hesitation, idles smoothly, good smooth acceleration and gives zero indication of anything wrong. But next time I try to start it acts like it will never start. By the way, my spark arrestor has a thick layer of black oily soot on the outside of it. Will this cause starting issues
 
CampionPB said:
Hi Jimn, once I get the boat started it runs as good as it ever… zero hesitation, idles smoothly, good smooth acceleration and gives zero indication of anything wrong. But next time I try to start it acts like it will never start. By the way, my spark arrestor has a thick layer of black oily soot on the outside of it. Will this cause starting issues
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Put some duct tape over your mouth and run a marathon. How do you think that will go? Yes, the clogged flame arrestor makes a difference- an engine needs to breathe. remove it and look through from the inside- if you can't see through it, it won't pass air well (assuming it's the Aluminum type with thin sheets, rather than a K&N).

So, it's a hot start issue- open the throttle and if it starts, do what I suggested in my previous post.

Does the exhaust smell like gasoline and does the transom have black soot on it near the exhaust ports? That can be another indication that the ECT is bad.
 
Cleaned the spark arrestor. Boat is still extremely hard to start. Had to pour some gas down the air intake and it finally started with a sputter. Once it ran for a few seconds it idled great and as in the past the motor ran like a top… zero issues… smooth acceleration and smooth idle.
I just tried to start the boat today and no way would it start without gas… turned the key on twice and gave it full throttle during the starting but it would still not start. It is a 350 MPI Mag.
Once it starts there is zero hesitation…. And works hreatv
 
CampionPB said:
Cleaned the spark arrestor. Boat is still extremely hard to start. Had to pour some gas down the air intake and it finally started with a sputter. Once it ran for a few seconds it idled great and as in the past the motor ran like a top… zero issues… smooth acceleration and smooth idle.
I just tried to start the boat today and no way would it start without gas… turned the key on twice and gave it full throttle during the starting but it would still not start. It is a 350 MPI Mag.
Once it starts there is zero hesitation…. And works hreatv
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Did you crack the throttle? Try that.
 
jimn said:
Did you crack the throttle? Try that.
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Jimn I tried turning the key on and off a couple of times first (to turn the fuel pump on and off to get some fuel into the cylinders?)... that did not work. I cracked the throttle some and no starting... then cracked it all the way and still would not start. With it fully cracked the engine made a bloop bloop bloop bloop sound after I let off the starter. Like I said, once it starts it works like a new engine. If I put gas down the breather it will start a bit rough then smooth right out and works super.
 
CampionPB said:
Jimn I tried turning the key on and off a couple of times first (to turn the fuel pump on and off to get some fuel into the cylinders?)... that did not work. I cracked the throttle some and no starting... then cracked it all the way and still would not start. With it fully cracked the engine made a bloop bloop bloop bloop sound after I let off the starter. Like I said, once it starts it works like a new engine. If I put gas down the breather it will start a bit rough then smooth right out and works super.
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Opening the throttle completely with fuel injection is the way you would clear a flooded engine- at any position other than 100% throttle, fuel will be delivered.

You need to use real diagnostic equipment to see the data, not guess or see blinking lights.

How long has it been doing this? If it has happened all season, pull the dip stick and take a whiff of your oil- if it smells like gasoline, change it before you run it again.

I didn't see an answer to my "Does your exhaust smell like gasoline?" question- that could indicate a bad ECT (Engine Coolant Temperature) sensor, as I mentioned before. Measure its resistance and if it's outside of the normal range (could be higher or lower), replace it.

Did you check the fuel pressure? If not, do it.

If you're paying someone to service it, they should at least make sure it runs. Why aren't you going to them to get this fixed?
 
jimn said:
Opening the throttle completely with fuel injection is the way you would clear a flooded engine- at any position other than 100% throttle, fuel will be delivered.

You need to use real diagnostic equipment to see the data, not guess or see blinking lights.

How long has it been doing this? If it has happened all season, pull the dip stick and take a whiff of your oil- if it smells like gasoline, change it before you run it again.

I didn't see an answer to my "Does your exhaust smell like gasoline?" question- that could indicate a bad ECT (Engine Coolant Temperature) sensor, as I mentioned before. Measure its resistance and if it's outside of the normal range (could be higher or lower), replace it.

Did you check the fuel pressure? If not, do it.

If you're paying someone to service it, they should at least make sure it runs. Why aren't you going to them to get this fixed?
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You are a good man Jim!... thanks! I hate pulling the boat out so I have been nursing it along doing what I have to do to get it running when I want to use it because it works like a charm once is starts! Season ends soon at which time the service guy will be given the qualitive info I have to assist him to diagnose the issue. I do not think I smell gas in the fumes but there definitely seems to be an issue with fuel getting into the cylinders at start up but if I prime it with gas down the breather it starts and runs super. I am a handy guy but a little timid with marine mechanics. I could be the ECT or a vacumn issue in the fuel line etc. I will let the service fella fix it... or else!
 
CampionPB said:
You are a good man Jim!... thanks! I hate pulling the boat out so I have been nursing it along doing what I have to do to get it running when I want to use it because it works like a charm once is starts! Season ends soon at which time the service guy will be given the qualitive info I have to assist him to diagnose the issue. I do not think I smell gas in the fumes but there definitely seems to be an issue with fuel getting into the cylinders at start up but if I prime it with gas down the breather it starts and runs super. I am a handy guy but a little timid with marine mechanics. I could be the ECT or a vacumn issue in the fuel line etc. I will let the service fella fix it... or else!
Click to expand...
If you can work on a car engine, you can work on this- it is a car engine, after all. Has some different parts, but the basic system is the same- it needs air, fuel, spark (preferably at the correct time) and compression in order to run. If you squirt some gas into the throttle body and it starts, it eliminates spark, air and compression as culprits. If it doesn't start, it could be one of the sensors that can prevent firing if the signal isn't what the ECM wants. Since it CAN be started, I doubt it's one of those. Fuel pressure can be the cause, but you would need to check the pressure over a longer time than just Key On, with the engine not running.

The tests we were taught and did for Mastercraft boats, which applies to all:

Key On, Engine Off
Idle
2000 RPM
WOT (Wide Open Throttle)

If possible, run the boat on the water and have someone watch the pressure. If they see pressure drops that don't coincide with some throttle change something is wrong. The engine speed should be constant at any throttle position (unless you're fighting the current).
 
Thanks for the advice. I will try checking the the fuel pressure as you recommended. I did notice when I had the boat out the last two times it started on the second or third short starter engagement without any fuel prepping. However I did notice a little hesitation at full throttle around 4200 rpm. I never keep it there long as I don’t want to over stress the engine with 620 hours.
 
CampionPB said:
Thanks for the advice. I will try checking the the fuel pressure as you recommended. I did notice when I had the boat out the last two times it started on the second or third short starter engagement without any fuel prepping. However I did notice a little hesitation at full throttle around 4200 rpm. I never keep it there long as I don’t want to over stress the engine with 620 hours.
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620 original hours? That's really not a lot- it should handle anything you throw at it.

If it won't start after priming (turn the key to ON and wait a few seconds), something isn't right. The pressure should reach the normal running pressure during that cycle and it should start in about a second, assuming everything is able to deliver the fuel.
 
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