350 Mag Mpi hard to start when warm or hot

[nigb]

350 Mag Mpi year 2006 serial No OW633284 approximately 400 hours.
Problem:
Engine starts fine when cold but won't start when warm/hot without opening throttle.
If I put throttle to WOT & turn key it starts immediately.
If I put throttle about 30 - 50% & turn key it cranks but doesn't start immediately & occasionally blows back but then fires as I progress throttle forward.
Apart from that boat goes like a scalded cat performance wise.
Idles perfectly once started

work carried out recently:
New Distributor cap & rotor which addressed a rough low rpm running & poor overall performance.
2 year old (very few hours) Fuel cooler cleaned.
Seawater pump refurbished. This had been leaking unnoticed for some time & had caused rust to some of the brackets & pulleys etc.
IAC replaced.
Checked ECT resistance to corresponding temperature ok.
Cleaned engine negative connection.
FPR socket replaced due to melted contact that caused intermittent engine cutting out.
Fuel pressure holds steady at about 38 psi when engine off for at least 20 minutes.
Appreciate any suggestions on what else I can check.

 

[alldodge]

Fuel pressure holds steady at about 38 psi when engine off for at least 20 minutes.

What is the pressure when cranking and when running?

Opening throttle 30 to 50% increases the amount of air but does not shut off injectors. Opening to WOT shuts off the injectors which is flood mode

Pressure is to high or something is keeping injectors turned ON

 

[Fun Times]

Just thinking out loud but I’m starting to wonder if we’re starting to see a pattern with the Bravo engine models that stopped using water check valves to stop hot water from getting to any one of the designed cool fuel systems… The Gen 2 uses one at either side of the cool fuel depending on 3 or 7 point drain design… Wonder if maybe one should be incorporated into the Gen 3 If this poster is onto something here , https://forums.iboats.com/threads/350-mag-no-hot-start.765534/

 

[Fun Times]

May want to check and maybe replace your IAC muffler too.

 

[stresspoint]

if you have the means ? , unplug and check voltage to an injector when cranking with throttle in idle position. then do the same with throttle advanced .
Any injector will be fine.
report back.

? , have you had the motor checked with diagnostic tool , if not , that is where i would start.

 

[nigb]

Thanks for the replies guys that gives me something to try. Engine has a new iac muffler. My mechanic doesn't have diag tool i will find someone who has one. It will be a few days before I report back as boat is temporarily in a marina up the coast.

 

[QBhoy]

Hi. By far and away…the most common cause of this I’ve seen, with the mpi…is as a result of a bad battery. Or compromised battery cables, fittings or related items. Especially rang true, when you said she will start if you throttle. I’d go so far as 100%…if you told me additionally, that it pops and coughs as you let go of the key (from cranking), and will start at this very moment if you have throttle open. Further again..if you might have been sitting with engine off for a while, and a few items switched on.
A bad or even slightly weak battery, is where I’m going with all this. I go on loads about this on the mpi. Less than a tip top battery…(despite being good a healthy sounding when turning the engine over) just won’t play at all on these. Unless it’s a healthy healthy battery and high cca rated…of about 750+.

 

[nigb]

Hi. By far and away…the most common cause of this I’ve seen, with the mpi…is as a result of a bad battery. Or compromised battery cables, fittings or related items. Especially rang true, when you said she will start if you throttle. I’d go so far as 100%…if you told me additionally, that it pops and coughs as you let go of the key (from cranking), and will start at this very moment if you have throttle open. Further again..if you might have been sitting with engine off for a while, and a few items switched on.
A bad or even slightly weak battery, is where I’m going with all this. I go on loads about this on the mpi. Less than a tip top battery…(despite being good a healthy sounding when turning the engine over) just won’t play at all on these. Unless it’s a healthy healthy battery and high cca rated…of about 750+.

Thanks for reply I think you might be onto something as the batteries are fully charged & less than 2 years old both 780 cca. I should have mentioned in my original post that the engine would only crank & start with both batteries selected. If I select one battery it won't crank. Should be easy to check with multimeter whilst cranking. Also seawater pump spraying Saltwater over engine wouldn't help. Yes it does pop & cough & I get wisp of smoke from flame arrestor. I will let you know how I go next week.

 

[Fun Times]

You get nothing when one of the batteries is selected? Loose battery cable perhaps at the switch should battery be truly good.

Might also be time for new spark plugs and check the plug wires to make sure they're functioning correctly too.

 

[nigb]

Not enough to crank motor. It was the last thing I tried before leaving the boat & although I thought I need to look at that too I hadn't associated it with the hot start issue mentioned in the other post. Thanks again for reply I will report back during the week.

 

[alldodge]

I should have mentioned in my original post that the engine would only crank & start with both batteries selected. If I select one battery it won't crank.

Yes, this should have been in first post

 

[nigb]

Yes, this should have been in first post

Yep apologies for that

 

[QBhoy]

Good luck. Sure sounds like it’s a battery issue. Even if you have both batteries selected. That bad one, is now pulling the health of the better one.

 

[nigb]

Good luck. Sure sounds like it’s a battery issue. Even if you have both batteries selected. That bad one, is now pulling the health of

 

[nigb]

Update:
Replaced bad battery negative lead. Both batteries tested 100% health & charge. Motor cranks easily on either battery. Motor still not starting without throttle when hot. Sometimes it just catches & starts at a slow idle. What I have noticed is Motor will start if I put throttle to about 30% especially if I leave it a few minutes.
Fuel pressure about 42psi when cranking, 38psi when idling. Drops to about 30 psi & holds after turning off for 5-10 minutes
Will try measuring injector voltages tomorrow.

 

[alldodge]

Injectors have 12V on one side and the ECM grounds the other side to turn on/off

 

[dubs283]

Best to use a diagnostic tool and check for faults and monitor data list items.

WAG without diagnostic info would be overheating iac or issue with map sensor

 

[nigb]

Best to use a diagnostic tool and check for faults and monitor data list items.

WAG without diagnostic info would be overheating iac or issue with map sensor

Thanks for reply. Yes trying to find someone locally with diag tool. I have replaced iac & I think if the map sensor was faulty it would be playing up in other ways than the hot start issue. I have 12v at injector with ignition on. I will go up to boat again in the week & use a test lamp to see if the injector is pulsing while cranking

 

[nigb]

Update 2:
I tested the injector wiring with a noid tester light & that is OK so I am getting fuel.
I tested for a spark with a timing light & that is good as I have a spark.
What I have discovered is if I take the PCV valve hose off engine starts when warm/hot. This indicates I am not getting enough air to enable combustion which as I understand it a PCV is closed on idle so that can't be problem. It figures that it starts ok when cold as it needs less air. I have already replaced IAC valve with aftermarket one. Not sure where to look next. Is there a passageway in the venturi that the IAC air goes through possibly blocked? The only other scenario is that the ecm is allowing too much fuel when warm. I have checked ECT with multimeter ok.

 

[dubs283]

A faulty map sensor will cause over fueling especially if the signal to ecm is the same reading as baro pressure. Ecm will think throttle is wide open at all times