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Mercruiser 350 Mag mpi starting issue?

oldschool

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I have an 05 Mercruiser 350 mag mpi that is sometimes tough to start without giving it some throttle. When the engine is dead cold it has no issues at all. But SOMETIMES, when starting it through the day it won’t start without going into neutral and giving it partial throttle and then it lights right off.
Ideas?
 

Shlbyntro

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I have an 05 Mercruiser 350 mag mpi that is sometimes tough to start without giving it some throttle. When the engine is dead cold it has no issues at all. But SOMETIMES, when starting it through the day it won’t start without going into neutral and giving it partial throttle and then it lights right off.
Ideas?

without knowing SN or diagnosing:

Needs Tuneup,
IAC and/or IAC Filter,
Fuel Pump and/or needs boost pump
Needs Valve Job



in order of most likely to least likely
 

LargeOrangeFont

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IAC going bad?

If yes buy the OEM gasket and Ford Windstar IAC valve. Save yourself hundreds. $60 for a made in Japan Hitachi valve instead of a couple hundred for unknown origin Mercury valve.

IAC Hitachi ABV0006
Mercruiser Gasket 27-863112
 

DaveH

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engines, when warm, need less air to start then when cold. this suggests your IAC is OK.

when an engine needs throttle to start, its a sign of the engine being worn out IE low compression. how many hours on it?

it could also be a faulty engine coolant temp sensor, where the enrichment during start is most critical. your gauge uses a different sensor then what the computer uses.

an ignition tune up may also be in order.
 

checkrdpast

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I know its not the same induction bout I had the same motor with the carter carb. cold starts were never a problem but stopping for 15 min or longer was hard starting and the cause was a little leaking of fuel from short setting. I guess the was an inherent problem with the marine carter. Good luck with the MPI.
 

Bigbore500r

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Does it sound a little "loaded up" when it finally starts, and takes a second to clear up?

If so - something to check -

pull the vacuum line off your fuel pressure regulator and see if any remnant or sign of fuel comes out. If so - your FPR diaphram has gone bad and when you shut it off, its getting a bit of fuel sucked in as it sits causing it to flood slightly and cause extended cranking on restart. The typical symptom of this, is that it starts fine cold, starts fine hot if you've just shut the motor off....but if you sit for 15 minutes or longer, it becomes difficult to start. Here is the FPR -

1690311608698.png
 

oldschool

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engines, when warm, need less air to start then when cold. this suggests your IAC is OK.

when an engine needs throttle to start, its a sign of the engine being worn out IE low compression. how many hours on it?

it could also be a faulty engine coolant temp sensor, where the enrichment during start is most critical. your gauge uses a different sensor then what the computer uses.

an ignition tune up may also be in order.
450 hours.
 

oldschool

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Does it sound a little "loaded up" when it finally starts, and takes a second to clear up?

If so - something to check -

pull the vacuum line off your fuel pressure regulator and see if any remnant or sign of fuel comes out. If so - your FPR diaphram has gone bad and when you shut it off, its getting a bit of fuel sucked in as it sits causing it to flood slightly and cause extended cranking on restart. The typical symptom of this, is that it starts fine cold, starts fine hot if you've just shut the motor off....but if you sit for 15 minutes or longer, it becomes difficult to start. Here is the FPR -

View attachment 1256764
I know less than shit about fuel injection, but my thought was a bad/leaking injector might cause this. Could that happen from a leaky regulator(cause an injector to leak)? And YES, it feels “loaded up” when this happens and it starts. It never ever loads up at any other time.
 

Bigbore500r

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I know less than shit about fuel injection, but my thought was a bad/leaking injector might cause this. Could that happen from a leaky regulator(cause an injector to leak)?
Wont cause injector to leak, but will act similar.....

When the rubber diaphragm in the fuel pressure regulator starts leaking, the motor sucks the fuel thru the vacuum tube connection and into the intake. They will sit there and siphon when the motor is hot and has been shut off. The fuel puddles in the intake and runners, causing a flooded hot-start condition once it's been sitting for 10-15 minutes after being shut down.
 

Shlbyntro

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when injectors leak it can cause a fat starting condition after sitting for short periods. I'd start with a compression/tuneup and take note of any spark plugs that look different from the others
 

lbhsbz

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engines, when warm, need less air to start then when cold. this suggests your IAC is OK.

when an engine needs throttle to start, its a sign of the engine being worn out IE low compression. how many hours on it?

it could also be a faulty engine coolant temp sensor, where the enrichment during start is most critical. your gauge uses a different sensor then what the computer uses.

an ignition tune up may also be in order.
engines need the same amount of air to start warm or cold....they need a richer mixture to start cold (choke, cold start injector, ECT input which commands enrichment...etc). The only reason an IAC exists is to control idle speed....in emission controlled engines to accelerate the warmup process and otherwise work with all the other parameters being adjusted in open loop to keep the thing running.

Lots of perfectly healthy carbureted engines need throttle to start...mostly because carbs are pieces of shit and should be replaced with fuel injection so it works right, but it doesn't mean you've got a bad engine....it just means that you've got a carburetor.
 

Deckin Around

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I have an 05 Mercruiser 350 mag mpi that is sometimes tough to start without giving it some throttle. When the engine is dead cold it has no issues at all. But SOMETIMES, when starting it through the day it won’t start without going into neutral and giving it partial throttle and then it lights right off.
Ideas?

I have the same issue which hopefully is fixed this week. My boat which is almost the same year and same # of hours as yours, would only do it randomly when first launching or starting first thing in the am. A few times it has started but with a lower than normal idle and then it catches itself and revs up then quickly back to normal idle. Overall, most recently it happened 2 times in 9 day trip so I'm guessing 1 out of every 40 times the boat was started. . I changed the fuel pumps and filter and it still happened so now I have a new IAC to install.
 
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PlanB

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I know its not the same induction bout I had the same motor with the carter carb. cold starts were never a problem but stopping for 15 min or longer was hard starting and the cause was a little leaking of fuel from short setting. I guess the was an inherent problem with the marine carter. Good luck with the MPI.
My Holley does that as well. Full throttle while cranking and it fires with no issues.
 

Shlbyntro

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engines need the same amount of air to start warm or cold....they need a richer mixture to start cold (choke, cold start injector, ECT input which commands enrichment...etc). The only reason an IAC exists is to control idle speed....in emission controlled engines to accelerate the warmup process and otherwise work with all the other parameters being adjusted in open loop to keep the thing running.

Lots of perfectly healthy carbureted engines need throttle to start...mostly because carbs are pieces of shit and should be replaced with fuel injection so it works right, but it doesn't mean you've got a bad engine....it just means that you've got a carburetor.

by your same logic, hydraulic brakes are pieces of shit and should be replaced with fully electric braking systems.😈
 
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OCMerrill

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I don't know what kind of boat were talking about but perhaps is baking under there. Crack the lid when you stop and also get an infrared thermometer and shoot the manifolds under power to make sure they are not introducing excessive heat. Your going to need a scanner more than likely but my Essex when run hard and then shut down will crank a bit before starting if I don't crack open the engine cover. My injection system is from 1997 though.

If that helps then replace the brakes with electric. 😁
 
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DaveH

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engines need the same amount of air to start warm or cold....they need a richer mixture to start cold (choke, cold start injector, ECT input which commands enrichment...etc). The only reason an IAC exists is to control idle speed....in emission controlled engines to accelerate the warmup process and otherwise work with all the other parameters being adjusted in open loop to keep the thing running.

Lots of perfectly healthy carbureted engines need throttle to start...mostly because carbs are pieces of shit and should be replaced with fuel injection so it works right, but it doesn't mean you've got a bad engine....it just means that you've got a carburetor.
sorry not buying this.

just about every ECU goes full open with the IAC to aid with starting, and drive by wire does the same thing. With the aftermarket stuff, there are very elaborate tables, based on engine temp, where you can fine tune the air delivery at crank, and then post start. not saying the mixture doesnt need to be richer, it does, but this is becasue the fuel poorly atomizes when cold, again, another factor of temperature being a huge deal in starting.

hell my 4runner when cold starts at 2000 rpm and slowly winds down with temp. and this is in a 2023 vehicle. that aint being done because it richer. only airflow will make that happen. sure they are also trying to warm the engine up faster likely with the high initial idle.

this is especially true in high performance engine with large cams where static compression is very low at cranking speeds.
 

oldschool

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I don't know what kind of boat were talking about but perhaps is baking under there. Crack the lid when you stop and also get an infrared thermometer and shoot the manifolds under power to make sure they are not introducing excessive heat. Your going to need a scanner more than likely but my Essex when run hard and then shut down will crank a bit before starting if I don't crack open the engine cover. My injection system is from 1997 though.

If that helps then replace the brakes with electric. 😁
Sanger V215 ski boat.
 

GR8WHITE

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I had a 2002 Supreme V208/350mag with a similar issue. Took it back to the boat dealer (was on consignment) twice and they couldn't figure out the issue. I replaced the IAC from the Windstar, still didn't change anything. While running it on the modified shitter plunger in the garage I had my buddy giving it some gas to keep it alive and noticed that somehow the damn tensioner was so loose that the belt was slipping on the alternator. Not enough electrical current coming out the alternator to keep everything happy and had missing/idling issues. Cranked the shit out the tensioner and tightened it up and ran like a champ until I sold it last year lol.
 

GR8WHITE

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I don't know what kind of boat were talking about but perhaps is baking under there. Crack the lid when you stop and also get an infrared thermometer and shoot the manifolds under power to make sure they are not introducing excessive heat. Your going to need a scanner more than likely but my Essex when run hard and then shut down will crank a bit before starting if I don't crack open the engine cover. My injection system is from 1997 though.

If that helps then replace the brakes with electric. 😁
I don't know what kind of boat were talking about but perhaps is baking under there. Crack the lid when you stop and also get an infrared thermometer and shoot the manifolds under power to make sure they are not introducing excessive heat. Your going to need a scanner more than likely but my Essex when run hard and then shut down will crank a bit before starting if I don't crack open the engine cover. My injection system is from 1997 though.

If that helps then replace the brakes with electric. 😁
My 98 Shockwave with a 454 Magnum would do that. Shut her down for a dip in the river, jump back in and the turd randomly wouldn't fire. Would take quite a while to get her fired back up.
 

Blackmagic94

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Bad fuel pump that when hot is not supplying volume and or presssure



Test pressure go from there
 
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