350 mpi won't start hot

J

John Leis

New Member
Aug 19, 2018
5
Boat Info
2004 240SD
Engines
5.7 350 MPI Mercruiser
For more than two years our boat with a new 5.7l 350 mpi has experienced issues not starting under hot conditions. Over countless trips to the shop and thousands of additional dollars spent replacing most electrical parts, wiring, ECM we are still having the same issue. The closest we have been to normal is with one mechanic who discovered that if he unplugged the fuel pump and burned out the fuel when it was in this hot state and just cranking, it would then start when he plugged the fuel pump back in. So our work around was installing a kill switch for the fuel pump that I use to start the engine a couple of times to ensure it's burned the fuel and then toggle the switch back on for the fuel pump and it must be in neutral so I can quickly give the engine a little more throttle and then it will start. Once this engine is started and running she is perfect.

Sorry for the long list but I wanted to include the items addressed over the last two years:

Vacuum gauge in direct connection w map sensor - vacuum at 15-18 @idle
changed map sensor
changed cam sensor
changed coil and ignition module
changed distributor cap and rotor
changed fuel injectors and fuel rail
changed pressure regulator
cleaned and tested injectors
changed IAC
changed plugs
checked plug wires
changed ecm
changed coolant sensor
changed connection on fuel pump harness
changed throttle body and tps
installed new wire harness
changed ignition relay

Thanks in advance for any guidance that move us closer to resolving this very challenging fuel related issue.
 
Sure sounds like vapor lock. Merc suggests a boost pump or reroute the fuel return line, from the pump, directly to the gas tank.
 
Thank you. I found bulletin 99-7 from 2001 and will review and follow these steps. A local mechanic who shared a similar story on his 350 came out yesterday and he would push in on the small valve between the injector rails and since it was mostly fuel coming out (bubbles at first) and not gas pressure he said he didn't think I had the same vapor issue. He said that when he pushed that valve to release the pressure on his boat it would then start right up. That made no difference to mine. I still had to cheat and fire it a couple times with the fuel pump disengaged.


I've thought it was a vapor lock from day one although my marine mechanic skills are very limited I have only provided the details of the issue to the marine shops.

A few other details that may help isolate the issue:

- The boat was used for two seasons and the sat in a cave for many yeas due to a failed winterization and legal dispute. I bought it with 3 hours after they replaced the original 305 w a 350.

- It never did this for a few weeks after I got the boat, however this was my first boat and most of the early rides were very short and not during the peak heat of day because the Lake Ozark can be very busy during the summer weekends.

- I recall experiencing it more early on when I would have more fuel in the tank. When I ran it on 1/4 of a tank it happened less frequent but has now reached a point of happening every time.

- This may be unrelated but I also noticed significant pressure in the tank when filling it. It would take 20 minutes because it wasn't venting well and would force the fuel back out and shoot it in your face while fueling unless you filled it at a very slow pace.
 
Did you fix your vent problem? "bubbles at first" you should have no air in the fuel line. Maybe the cooling side of your CF3 is plugged up.
 
Shameful on mercruiser that they don’t know exactly what’s going on and can’t help.
 
When the 305 was replaced with the 350, was it a complete drop in? Or did they re-use some of the components from the 305.

I’m curious if the ECM was replaced, or reprogrammed, or reused from the smaller motor.
 
When the new 350 was put in the boat they used many of the components. The ECM wasn't changed and it worked fine for first few weeks. After the problem started to surface they replaced parts one at a time and at one point I sent the ECM to OBD Diagnostic in CA to reflash for the 350. They said the ECM is good and perfectly tuned for the new motor.

After a year with one mechanic I moved it to another mechanic who had it for a the next year and the second mechanic switched the ECM off a 350 he had in the shop and the issue was still present.

A third mechanic is questioning the cams used on the new motor. I know nothing about these systems but he is saying the wrong size cam or if the ECM doesnt know the wrong cam is in the motor it could create issues. He discussed pulling everything apart and checking/replacing the cams and reflashing the ECM again. I'm more than $6,000 and two years into messing with this so you can imagine my lack of enthusiasm to continue replacing parts. Also, when the boat starts (which it will each time with the fuel pump cutoff process) it runs perfect. The last mechanic had the computer on it many times while on the water and sent the files to Mercruiser and they stated everything was spot-on. So with my limited knowledge I question why cam/ecm issue wouldn't be reflected while running at any speed from idle to top-end.
 
Its a issue I've been chasing for a number of years. And to top it off. Not everyone has it. Go figure.
 
You have a fuel problem. Did the insulation sleeves get installed on the lines?
 
I have a 6.2/BIII with dual pickups (leg and thru-hull), it is very sensitive to vapor lock if I don't have the seacock valve open. The extra water from the thru hull reduces the block temp enough so it is not a problem, if I don't have the seacock open I have to run the blower and idle down slowly to keep the compartment temps in check. Besides the temp issue, I think mine is more sensitive to vapor lock because the fuel pump is slowly dying.

The least expensive solution is probably adding a thru-hull intake to get more water through the block/reduce the temps. If that doesn't get you all the way to where you want to be take a look at a new cool fuel unit with the merc helper pump.
 
The t-stat maintains the block temp at 160-170 so unless he's running higher temps than that it doesn't matter how the block gets its water.
 
I misspoke, I should have said more water through the exhaust manifold, elbows, and cool fuel 3. The exhaust manifolds and cool fuel 3 are teed off the line between the raw water pump and the thermostat so the exhaust manifolds get water before the t-stat opens up. Not sure how much it reduces the compartment temps, but its enough to prevent vapor locking on mine and I have the cool fuel 2 which is not raw water cooled. It will vapor lock on me when running on leg only, never vapor locks on me when the seacock is open.

Bravo Cooling System.PNG
 
Colo, you probably have some blockage in the pickup hose that routes thru the transom "Bravoitis" Your CF2 is indeed raw water cooled, but you make a very good point about overall temps effecting the issue the OP has.
OP, what is your running temp?
 
Its possible that the leg pickup has a blockage down low or a kink in the hose somewhere, but I had replaced my transom assembly so shouldn't be any bravoitis. My mechanic told me to run around with the cover open before we swap out any parts, that solved the issue as well. Mine runs at 156F with the thru-hull open.
 
scoflaw, thanks for the info on the cool fuel 2. Some folks told me I should "upgrade" to CF3 to get the raw water cooling.
 
Thanks for the input. I'm headed back to the lake tomorrow and will review with this input and respond with questions.
 
1. Verify your fuel tank vent is clear and working. It should not be slow to fill. Fix that first.
2. Verify your fuel pickup is not clogged and the check valves are working. Likely OK based on the fact that you said it runs fine once fires off.
3. Install a secondary pump in line if #1 and #2 didn't fix your problems. Reverify problem.
 
Thank you. Where would I find the details for where to add a secondary pump? I've read a number of threads where this helped in a similar situation. The boat has fuel line wrapped up into fuel filter on the wall in front of motor, that feeds into a valve or solenoid (Parker 3 psi), and then to the fuel pump.
 
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