Hard start when hot.
 

454 mpi with B&M blowers, Motors start well and now idle well with no stalling. But the starboard motor won't start after it sits for approximately 1/2 hr. without a lot of coaxing. When it does start you have to rev it until it settles in and then idles fine. The port motor starts fine everytime. If you turn motor off and then restart right away it starts fine. Any help out there?

check the carbs on that motor sounds like it may have a bad seat, float or vent and may be flooding the motor.

Good advise from Just Once. Also, check the fuel system to be sure the fuel isn't draining back through the pump. Check for hot blower, also. Good Luck!!!

Forgot to mention, these are efi motors, sorry. With electric pumps

I don't know much about blower motors, but I know that Merc. EFI motors have an occassional problem with vapor lock. Maybe you should try running the blowers and leave the hatch cracked open for a while after a run, to let out some hot air. I always do with mine.

ok are these the B&M 250 blowers that bolt to the factory intake[remove the upper intake and use's the lower part] if so they are used on the 454mag and 502mag mpi's only, what year are your engines? if I remember the first kits had the ECM's mapped byFlagship marine, they used the MEFI1 ecm,s which do not have a good map for boost???? some people are changing to MEFI3 ecm's and rewiring the factory harness to fit[ swap the pins in the connector] but you sound like a vapor lock problem, maybe, switch fuels, also run a higher octane fuel and see if that helps, some fuels out there have a low reed vapor which will boil easy, don't know if this helps any, maybe some more info from you will help out

Make sure that you've got clean fuel filters....

Mike

Do you smell raw fuel as your cranking it over, probably not, then it is not flooding. What is the temp on both motors when you stop and start?

Injector is leaking or motor is vapor locking. Do you have fuel pressure gauges? If so does fuel pressure come up as soon as you turn key on? If pressure comes right up then problem is a leaking injector.

Mine use to do that as well. For a couple years merc use to have the return line go back to the water/fuel seperator. I removed the line and had the return go directly to the tank. then I blocked of the return to the seperator with a plug. It never happened again:)
Good luck

OK, thanks for all the tips, I tried leaving the hatch open, it does seem to help a little but it still hard to start. Next , I already removed fuel return line to seperator and ran it back to tank, recommended by engine builder to eliminate vapor lock. Fuel filter is clean, and using 92 octane, marina fuel so don't know how good it is, but never had problem before these motors. As far as fuel pressure goes, I did install gauges, and they do come up as soon as I turn key, than go to 0 until you turn key to start. This is for prime, while they're cranking to start they build up slowly because they're pumping after they start they stay at 41 lbs. I couldn't get the mefi1's programmed right after numerous tries so was advised to install FAST formally Speedpro. ecu's and the tech says I need to tweek the AFTERSTART circuit and this should eleminate problem. He called the problem HEATSOAKING. I will try this also. Anyone ever here of this?

Before you start the motor move throttle off the idle position, as soon as motor starts then pull back to idle. See it that gets it to start quickly.

I think when a friend starts his efi motors the fuel pressure comes up as soon as he turns key on. Pressure stays up until motor starts. I will check next weekend when we are out.

J.V. Thats exactly what it's called, "Heatsoaking". I use to also run the blower for 5 min or so, AFTER shutting down. That also helped.
Your pump is running correctly. When you first turn the key on, the pump runs for about 5 sec. It stops because it doesn't get a signal that the engine is running. I also removed that plastic cover off the fuel/water separator, and the fuel pump to allow more air circulation around them. But the biggest thing I did was route the return FREE flowing to the tank... I havn't had that problem ever sence..

Here is the Dealer service Bulletin on your problem.

Mercruiser Vapor lock Bulletin

But shouldn.t the pressure stay up after the fuel pump runs for the first few seconds after turning the key on? If pressure goes down after pump shuts off before motor gets running there will not be enought pressure to inject fuel untll the motor sees oil pressure up, then the fuel pump will start again.

On my last 2 MPI engines I had a fuel pressure gauge, including this one.
The Pressure would go up when you first turn the key on, but if you are NOT cranking the engine. The fuel pump stops, and the pressure drops back down to "0". ;) If you are cranking the engine, the pump gets a signal from the distributor or what ever it gets the signal from, and keeps pumping. The problem he is having is very common, and it's not the pump.
Theres a trick to fooling the ECU into thinking the engine is flooded, and this gets the engine to start without waiting for the Vapor lock to go away.

Put the throttle to WOT, (in neutral of course) then the TPS will tell (fool) the ECU that the injectors need purging, or what ever you want to call it..;) The engine will crank over a few extra times and start right up with no waiting. Just be ready to lower the throttle so not to race the engine...

I'll give it a try, thanks

I use to have the 7.4 mpi in my challenger, no blower though. I made the mistake of putting in the 89 octane with 10% ethanol and it caused the boat to start hard especially after running for a while in hot humid weather. I had to pop the engine hatch and let it cool for a while before it would start again. Pain in the ass, especially when you are floating down the Missouri river @ 6-7 mph and it won't start! That was a night from hell. What ever you do don't use the 89 oct 10% ethanol... Good luck.

Todd