350 MAG no hot start

[saaristo]

Just this weekend boat started to behave like never before. Starts up nice when cold, you drive around like half an hour with decent revs, shut it down and tried to start it up after like 10...15 mins and it doesnt do anything other than just turning and turning and nothing. Seventh or eight time with rather long tries and it finally started. Experienced same thing once more on that day. Now, here's what I have tried so far:
Starts nice when cold, also repeated starts are ok when cold.
Heated it up on idle, still starts up ok.
When started cold, idle revs are ~1000, then falls to about 600, so I recon idle air control is ok.
Cool fuel unit stays cold all the time, so again vapor lock shouldn't be an issue? Though, since the pumps are inside the unit that might not be the final truth.
Hooked up the computer today and the tech confirmed all running data is spot on perfect, thus no error codes. He's booked until July so didn't have more time to dig around.
So shat am I facing here, any ideas what to test or try on my own?

Edit.
Engine is manufactured 2004, boat from 2005. Coupled with B3 drive. Engine serial 0W068970

Ps. How to detach the fuel out line from the cool fuel unit if I still would like to clean water passages?

 

[Fun Times]

Engine serial number?
Year of boat?
What stern drive model?

 

[alldodge]

Hey @Fun Times he edited his post and posted SN

Measure fuel pressure at the rail item 18
https://www.mercruiserparts.com/bam/subassembly/31749/11652/90

 

[dubs283]

Could still have an iac valve issue. I know it works cold but when it heats up, especially after shutdown/heat soak it may be failing

Iac valves ime have failed without setting a fault.

Test is to advance throttle slightly to allow air past the throttle plate, if engine starts it could very well be the iac

 

[Fun Times]

Just a few other things to consider trying too for you below too.

To remove the hose you’d remove the bolt to the right in your photo going into the Gen 3 cool fuel module, see item number 38, https://www.mercruiserparts.com/bam/subassembly/31749/11652/70

Make sure there’s no fuel inside the smaller rubber hose going to the fuel regulator as that could cause odd issues.

Is the fuel in the fuel tank older or mostly new when last filled up? or mixed new and old?… older fuel could cause symptoms.

Has the 2 fuel filters inside been replaced.

Some of these Gen 3 modules have been known to allow paint to flow through the systems up to the injectors, causing issues… so be aware of that when you open up the module and check under the fuel regulator too.

350 MAG MPI ALPHA/BRAVO - 0W060000 THRU 0W309999

350 MAG MPI ALPHA/BRAVO - 0W060000 THRU 0W309999

www.mercruiserparts.com

Mercruiser Service Manual 40 Gen III Cool Fuel Supplement | PDF | Pump | Electrical Connector

Mercruiser Gen III Cool Fuel Supplement to service manual #30 & 31

www.scribd.com

Physically, clean the whole throttlebody plates & behind/back side of the plates or carbon buildup…

Also change the IAC muffler, item number six, https://www.mercruiserparts.com/bam/subassembly/31749/11652/80

Engine temperature staying around 160 degrees to maybe see at most 175 on the gauge?

 

[saaristo]

Fuel in the fuel tank is new, we have here 98 octane premium without biocomponent so moisture should not be the issue.
Engine temp stays around 160 no matter idle or planing.
The rubber hose from regulator stays empty, no gas there. And the fuel fklter is changed a week ago.
Did some digging and testing again and found the following. IAC muffler looks like crap. I have no clue is it actually a part or just some piece of sponge stuck in there? So probably need changing?



Another thing, the cool fuel unit is cold as ice during idle and high revs. However, after longer run when you shut down the motor it seems to me that hot water is coming down from exhaust manifold and through the upper hose back to the cool fuel unit and starts to heat up the unit. That hose is really hot. Isn't that one encircled on the picture (under the manifold) suppose to be some one way valve or something? Because if this heats up the content of the cooling unit that could be very well what I'm after? Or...?

 

[alldodge]

First pic is the IAC muffler, while a clean one is better, all it does is keep the sucking sound lowered

The second pic the the exhaust manifold check valve. It can get hotter with the motor running, and the check valve part is only used to drain Mans during winterization

 

[saaristo]

The second pic the the exhaust manifold check valve. It can get hotter with the motor running, and the check valve part is only used to drain Mans during winterization

ok, so it isn't a backflow valve, but hot water flowing back to fuel cooling unit - is that something that suppose to happen or is that something odd? I mean, not only the check valve body gets hot but it flows down to the cooling unit, you can feel how the upper hose being cold after shutdown gets hotter and hotter until the body of the unit gets warm.

 

[Fun Times]

Exhaust Manifold Drain Check Valve Operation:
Engines with 3‑point drain, single‑point drain, and air‑actuated drain systems are equipped with drain check valves on the exhaust manifolds.
The check ball within the valve is forced upward by water pressure from the seawater pump. This seals the drain fitting.
When there is no water pressure from the seawater pump, such as when the engine is off, the check ball falls down into the grooved drain fitting. This allows water to drain from the exhaust manifold and elbow.

If you're able to open and see section 4-A of the manual it shows the water flow diagrams including to the fuel module. See Coolant and Water Flow Diagrams.

I'd replace the IAC muffler as it's time.