Mercruiser 350 (300hp) Difficult to start when warm?

S

Scuttsy

New Member
Jan 3, 2016
5
Sydney
Boat Info
Sea ray 240 Sundeck 2006
Engines
Merc 350
I have a Searay 240 Sundeck witha Mercruiser 350. It starts beautifully and runs beautifully but is difficult to re-start when warm, any ideas?

Thanks, Scuttsy
 
What year? Fuel injection or carburetor?
 
What is going on? Will it start and not run or just won't start at all?
 
My 04 300DA had 350mags in it. The starboard engine exhibited the same problem. After much trial and error I found it would start every time when hot if I first advanced the throttle a bit, then started it up. As soon as it started I would cut the throttle all the way back to idle. Worked ever time. Also, nothing we tried fixed it. We figured it was just a big hot engine in a tight place not having enough cool air around it. So we just stuck with the workaround.
 
hottoddie said:
What year? Fuel injection or carburetor?
Click to expand...

2006 Fuel Injection. I have replaced the IAC Value, replaced the distributor cap and more recently replaced the cooler housing, pump kit and fuel housing module (due to a fuel leak).
Thank you!
 
It will start happily cold, but when warm I have to give it a little throttle. It used to run perfectly...Thanks
 
Mine will start the same way, but its painful when skiing as I have to turn it off fairly regularly when people are boarding the boat...Thanks
 
2006 Fuel Injection. I have replaced the IAC Value, replaced the distributor cap and more recently replaced the cooler housing, pump kit and fuel housing module (due to a fuel leak).
 
I had a similar problem with our 300DA and it was solved by replacing the low pressure boost pump. After lots of trouble shooting we discovered the boost pump would generally work on startup when the engine was cold, but it did not operate when the engine was warm.

We initially used the workaround of starting the warm engine with a bit of throttle, but that always annoyed me with an MPI engine
 
Compression issues can also exhibit this behavior.


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Last edited:
Check for the fuel pump relay. Could be getting hot due to a short. Try getting a spare and replacing it next time it won't start while hot.


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Are you having to open the throttle all the way to start it?
If you are, during start up, the ECU will shut the injectors off to keep the engine from flooding.
A couple of things:
Leaky injectors.
A bad throttle Position Sensor.
Clogged Fuel Filters.
You say warm. How warm? Hot?
When it won't start, can you smell gas? Does it try to start, and then just turn over?
 
Greetings - new here :)

Has anyone come up with a defininate answer to this issue? With me, its a very minor inconvenience, but the geek in me wants to figure it out. My mechanic replaced:

plugs, wires, IAC, fuel filter, distributor and associated assembly.

We then went out, ran the boat for about 10 minutes, came back in and powered off for ~10 minutes. Tried to start and it wouldn't catch. I do note in the 2005 SD manual the following, which works every time for me:

WARM ENGINE - PRESS THROTTLE ONLY button and move shift/throttle level about 1/4 open throttle position. Do not pump lever.

Tom

2005 Sun Deck 220
5.7 Merc Mag MPI w/Alpha 1
 
Changed the thermostat to a 140. No improvement.
 
El Capitan said:
Vapor lock due to high fuel temps.
Click to expand...

Im x2ing this. I know this is a known issue especially with EFI Mercruisers, in fact the 7.4 blocks have a fuel cooling attachment on the lower port side of the block. The temps cause vapor lock in the lines so what happens is upon hot shut down, that is turning off engines soon after coming off plane, the gasoline vaporizes due to increased temps in the actual fuel lines.

What I have read is one way to relieve this is when you throttle down, do so slowly, allow engine to idle for a few minutes to cool, then turn off engine.

Hope this helps.
 
Was having the same issue with my 5.7 TBI - after researching I changed the Temperature Sending Unit - there are two, one for the gauge and one for the ECM - replaced the one for the ECM. Works like a champ.

You can get the Mercruiser one on amazon: https://www.amazon.com/Engine-Tempe...pID=41DpLSxLRnL&preST=_SX300_QL70_&dpSrc=srch

However, I was impatient and went to Advance Auto (plus had discount code - google for one) and picked one up there:
https://shop.advanceautoparts.com/p...36403/5899370-P?searchTerm=Temperature+sensor

Works like a champ and figured it was worth a shot before trying an ECM.

Also, on my engine (per the Mercruiser manual) flooded engine start is 50-75% throttle and not wide open.

Don’t know if this helps you guys (or someone new searching) or not, but if it does...happy boating.
 
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