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5.0 MPI hard starts sometimes but runs great.

E

Egan317

New member
2003 Chaparral with 5.0 L MerCruiser MPI engines and Bravo II drives. Starboard Engine will not start at times and other times starts perfect. Either hot or cold same problem. Batteries are good. 2018 - New cap and rotors, OEM Merc ignition wires, fuel filters and IAC replaced in 2017. Engine runs great after starting but often will not start unless throttled. Has spark when not starting.

Need some help as my marine mechanic can’t figure this out.
Checked with computer scanner and has no codes or error messages.

Engine serial number is OM342923.

Any help is appreciated.​




 
On ALL fuel injected engines, opening the throttle has NOTHING to do with starting. In fact it makes it worse as you are introducing more air into the engine at a time when it needs a rich mixture. Keep the butterfly closed at startup. It should start instantly. If it doesn't something's wrong so don't just crank away you'll burn up your starter.
What is probably not happening is your high pressure pump is not cycling when you turn the key. In order t fire, there has to be pressure behind the injectors. When starting, there isn't any cz the pumps aren't pumping until the engine spins up. Your ECU will send a signal to the fuel pump to begin pumping when you turn the key. So you need to check that this is happening or not. My guess is either bad connection at high pressure pump, bad pump, or bad ecu. Just a guess tho!
 
Last edited:
o2batsea said:
On ALL fuel injected engines, opening the throttle has NOTHING to do with starting. In fact it makes it worse as you are introducing more air into the engine at a time when it needs a rich mixture. Keep the butterfly closed at startup. It should start instantly. If it doesn't something's wrong so don't just crank away you'll burn up your starter.
What is probably not happening is your high pressure pump is not cycling when you turn the key. In order t fire, there has to be pressure behind the injectors. When starting, there isn't any cz the pumps aren't pumping until the engine spins up. Your ECU will send a signal to the fuel pump to begin pumping when you turn the key. So you need to check that this is happening or not. My guess is either bad connection at high pressure pump, bad pump, or bad ecu. Just a guess tho!
Click to expand...

Thanks for the reply. I was there tonight and cranked engine and started immediately. My mechanic will be checking fuel pressures tomorrow. I’m troubled he can’t diagnose this. No codes or error messages. If the ECM was bad, would t the scanner tell this? The ECM is about $2400.

After running a few minutes tonight, I shut it down and restarted without any problem.

PUZZLED!
 
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