7.4 mpi not running right

[alk]

My bowrider has a 1999 mercruiser 7.4 mpi. I put this motor in 4 years ago, it had about 500 hours on it, and has run perfectly. Until yesterday. When I returned to my slip, as soon as I throttled all the way down the motor stalled. When I started back up, in neutral the idle is not constant, almost stalls, surges up a few hundred rpms, drops back down, and repeat.

i dont have a merc scan tool, to see what is going on with the various sensors. would like to take it to a pro, but to find anyone and still have a season left is going to be nearly impossible. I suppose I could start throwing parts at the problem - but not sure where to start. Anybody have any expertise in these computerized motors?

next i will run from an auxiliary fuel tank, and put a pressure gauge on the fuel rail - to rule out bad fuel, pickup or pump. after that, not sure were to go.

 

[cwms]

Sounds like a fuel issue and running from a different tank is a good idea.
Could be water in the fuel, clogged fuel filter, clogged fuel tank vent or a clogged anti-siphon valve.

 

[sugilbert]

Don't rule out a loose fuel filter that is sucking a bit of air. Happened to a friend years ago.

 

[ddurand]

I am looking at buying the scan tool for my Volvo Penta. I assume your boat is an I/O. As time goes on it will be harder and harder to get marine mechanics to work on I/O engines. People who are able to may want to get the tools needed to do a lot of the work on their boat engines themselves.

 

[Phillbo]

Might need a new Idle Air Control valve... (IAC).... Or the filter cleaned.

 

[alk]

Running brand new fuel from another tank did not help. the parts store near my marina had the IAC valves in stock, and they were reasonably cheap - so I threw one on, in hope it would solve the problem. It did not. I checked the fuel pressure, was about 40 psi, so i don’t think it’s the fuel pump.

i finally gave up, pulled the boat out, and dropped it at a shop last week. They are pretty busy though, so season might be over for this boat.

 

[Bruce Herrington]

That sux. Carb? Throttle body? Assume electronic ignition.

I'm no mechanic by anyone's definition, but think it's gotta be air/fuel related as opposed to electrical.

 

[alk]

fuel injection. spark, idle speed, fuel pressure, mixture - all controlled by an ecm computer, with a handful of sensors. And a $900 fuel pump. I’m still on the original 23 year old injectors - never serviced at all. That was the mechanics first guess, before starting any diagnostics.

when it works, which it usually does - it’s awesome. Starts instantly, idles smoothly, relatively efficient. But when it gets wonky, tough for diy hacks like me to straighten it out.

 

[alk]

Those of you with fuel injection, have you ever sent your injectors out for cleaning/servicing? My mechanic was pretty horrified that my 23 year old injectors have never been out of the intake and cleaned. I rememebr thinking I should have it done, but that was like fifteen years ago - and then I guess I forgot.

 

[cwms]

Those of you with fuel injection, have you ever sent your injectors out for cleaning/servicing? My mechanic was pretty horrified that my 23 year old injectors have never been out of the intake and cleaned. I rememebr thinking I should have it done, but that was like fifteen years ago - and then I guess I forgot.

I have Crusader 8.1’s and a couple years ago, was having real problems with one engine. Mechanic said my injectors were the issue. Replaced with new and sent the old ones out for service. Now I have a set ready to go if I have any issues again.

 

[alk]

If you don’t mind me asking, what did a set of spares run you? I’m seeing Merc injectors at $410 EACH. Sierra around $300, and cheapo Chinese imports on Amazon for $15 each. Injector RX will service mine for $18 each - but that feels like an off-season project.

 

[cwms]

Seems to me, I paid around $200 ea, but it’s been a couple years now. Servicing the old ones, maybe in the $15-20 range. Engine ran great with the new ones