Mercruiser MPI Won’t Stay Running

[alldodge]

Does your gas smell like gas or something else?

 

[Bt Doctur]

click to enlarge

 

[BradsBoat]

There are two fuel pumps, the one I replaced was the high pressure one in the Cool Fuel box that’s hard to get at the bottom of the engine on the port side. The other is the low pressure circulating pump at the top on the starboard side. I didn’t replace that one. Definitely smells like gas.

 

[alldodge]

What your calling a circulating pump is called a boost pump and is above the fuel filter,

Was wondering if the gas was going bad but since it smells ok then its probably ok

 

[BradsBoat]

So should I replace the boost pump and possibly the injectors? The scan tool exercised the injectors.

 

[alldodge]

Note the IF

If the pressure stays as listed in your first post, then its not fuel pressure

If pressure is constant I would start with the injector balance test, or just send them out for flow testing. They would need to go to a marine test place not a auto place which will flow test to 6000 rpm

 

[QBhoy]

Run it off another fuel source to rule out the strong possibility that your fuel is still bad in the tank. Diluting bad fuel in this quantity with fresh fuel is always a hit or miss at best.
Good luck.

 

[BradsBoat]

Just ran it off a fresh supply of marine fuel. Fired it up about a dozen times to make sure the new fuel made it to the injectors. No change. The engine runs smooth for a few seconds then quits. The fuel pressure stays between 38 and 43 psi, just like it should. If I apply throttle, it seems to be starving for fuel - popping and an occasional backfire with white smoke out the throttle body. And the fuel pressure still stays between 38 and 43 psi.

 

[alldodge]

White smoke is raw fuel so there is to much fuel

 

[BradsBoat]

Seems we’re getting close. How might it be getting too much fuel?

 

[QBhoy]

Seems we’re getting close. How might it be getting too much fuel?

If it wasn’t firing right...like the usual diz cap failure for example.
Or your idle air control valve was playing up.

il aware you’ve thought of these, but honestly, just double check that diz cap again. The internet is packed full of reports of these things failing time and time again. Not sure if I said, but I’ve went through 5 of them in almost as many years. One right out the box was bad too. Easily to instal them just slightly catching the raised edge where they seat and seal onto the Diz assembly itself.
if you’re sure that is good...have a fiddle a few mm either way and see if you can get it to stop popping.

Check those plugs again too. If nothing else just to make sure they are iridium NGK.

 

[alldodge]

Seems we’re getting close. How might it be getting too much fuel?

I'm thinking the injectors are leaking, so they may not be shutting off after each fire. Your techmate can do a balance test

 

[QBhoy]

I'm thinking the injectors are leaking, so they may not be shutting off after each fire. Your techmate can do a balance test

Could be AD...would the pump run longer at key on and priming if that was the case ?

 

[alldodge]

Could be AD...would the pump run longer at key on and priming if that was the case ?

Initial FP run is a timed thing, there isn't a pressure sensor. The injectors should not allow the fuel to leak out even if it did run longer

 

[BradsBoat]

The new Sierra cap and rotor may be an issue.

 

[alldodge]

The aha moment

 

[QBhoy]

The new Sierra cap and rotor may be an issue.

Ah....always a chance if it. Honestly....despite what I’ve said earlier in this post....its the number one thing to check first, just to rule it out. Unbelievable how much grief these can cause...can send you down all sorts of stray paths too. Bought the t shirt on this one over the years.

 

[alldodge]

Guess we should also note its a Sierra cap and was replaced as said in first post. So a brand new cap and rotor burned that quickly

 

[QBhoy]

Guess we should also note its a Sierra cap and was replaced as said in first post. So a brand new cap and rotor burned that quickly

Yup. Happened to me too. Sierra cap. Didn’t look as bad as that though. It’s just didn’t work well. Mine looked very clean, but was still a dud.

 

[BradsBoat]

Just installed my new AC Delco cap and rotor. And, still the issue. I just won’t stay running. It idles for maybe 3 seconds, then stalls. If I try to give it some throttle while turning it over, it backfires and stalls. It consistently has 38-43 psi of fuel pressure, tried starting it with fresh fuel from an external source, there’s spark from the coil wire, we set the timing per the bulletin with the LED light via the cam sensor pickup on the distributor. Any ideas?