Mercruiser 350 MAG MPI

[AusFox]

Gday to all the brains trusts here,

I have been assisting with a mates Mercury 350 MAG MPI that has had some running and hot starting issues.
The engine would run ok but after running for an hour or two if we shut it down to fish then tried to re start it 30 mins later the only way we could get it to start was to use the throttle and crank it for a bit. So we replaced all the spark plugs, set the timing correctly, changed the IAC, replaced the distributor cap and still had the same issues.
We then pulled down the GEN 3 Fuel cooler to look for the paint de laminating however that all looks fine. While we had it apart we changed the fuel filter and the screen and also sent the injectors off for testing and servicing. The injectors came back from a reputable Bosch agent fully serviced and told they couldn't fault them.

When we water tested it on the weekend after installing the injectors it started great but it was a bit slow to respond to throttle in the mid to top end so we slightly adjusted the timing and it ran better. However we noticed the fuel burn rate was allot higher than previous and we could smell fuel, knowing it was running rich we headed back to the ramp.

Once we returned to the ramp and shut it down it wouldn't re start no matter what we did so we took it back to the shed and tried again only to have it lock up! We then pulled the plugs out and it was obvious how rich it was running by the state of them and the port side plugs all wet so we turned the engine over and the rear port side cylinder threw out allot of fuel!!!

I am waiting on a fuel pressure gauge to be able to accurately check fuel pressure, but does anyone have any suggestion's or good diagnosing methods to help us out and find what the problem is? It seems the less restriction we have in the fuel system by changing filters the worse the problems are getting!

 

[Fun Times]

Hi, you'll definitely need to check your fuel pressure and it would be helpful to know your engine serai number so you (/we) can confirm your correct engine parts used/possibly needed on your end.

If it's running really rich then it could be other things too such as the fuel regulator is either bad internally or clogged underneath it/or the gen 3 top cover area under it too...You could try pulling off the vacuum line of the fuel regulator while engine running to see if it's flowing fuel out the regulator vent line and into the engine as it should not be doing so.

Other things could be bad to cause that much fuel in the cylinder may be a faulty MAP/T sensor, bad fuel damper up on the fuel rail, engine running way to cold, engine cylinder misfire conditions say bad cap and rotor, plug wire/s or boot ends, wrong spark plugs, internal valve issues, still misadjusted distributor system, crankshaft sensor or if installed on your engine model as you're right at/under the year of change would be the CAM shaft sensor if it's plugged into the back of the sensor sticking out of the HVS distributor assembly just to name few of many other possibilities still.

If the engine turns out to be hard starting when really hot (outside air too)
then sometimes you may experience a restricted water flow to the Gen 3 fuel cooler housing or inner water lines that flow through it all so bear that in mind as you go.

Sounds like you might not be in the USA? Might want to check your local ebay store to see if anyone may rent a Marine/Mercruiser scan tool or diacom setup to plug into a laptop computer system such as these two places that offer rental at $50.00 for 7 day, https://www.ebay.com/sch/i.html?_nk...2334524.m570.l1313&_odkw=rinda+tech&_osacat=0

That or buy the diacom system and start renting it out to get some $ back in your area and help your locals out too.

The following is not your exact engine, ECM or fault code numbers as it mostly covers newer engine models but if desired on your end to study up a bit of different ways of thinking to help solve your issue, it might be worth checking it out.

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[Lpgc]

I am no expert in these particular engines but I do know about fuel injection in general. Behind the scenes most fuel injection systems will give extra fuel for a cold start to give the same effect as pulling the choke on a carb engine. If you try to start a hot engine with that choke effect active the mixture can be too rich for it to start. Is there a temp sensor for the fuel injection ECU that could be faulty or disconnected?

 

[AusFox]

Thanks for your help so far! We are in Australia and don't have anyone we can hire a scan tool off that will work with Mercruiser!! I have 3 other high end scan tools such as G-Scan, Jaltest and Autel but none are able to work with this engine.
I hope to have the serial number later today but what I have also found out is that this engine was a "new" base engine 2 years ago after the original got drowned due to failed exhaust manifolds.
The previous owners purchased the brand new engine and intake manifold then had a marine dealership transfer all the electronics, fuel system and ignition system onto the new motor and installed it. For the first year all ran well and never missed a beat then the hard to start once warm begun and now since changing filters and inspecting the fuel cooler the over fueling issue has occurred.
Below is a picture of the ECM label

 

[Lpgc]

The lock up and rear port side cylinder pumping out a lot of fuel suggests hydraulic lock due to the injector on that cylinder being stuck open (or held open by constant power to the injector).

Injectors can get clogged / damaged if any debris gets into them through broken filters or if someone has the lines open and allows dirt to get in. I've known a few people do engine swaps but not plug the lines to prevent dirt getting in, replacement engine runs OK until dirt affects injectors.

 

[alldodge]

The injectors came back from a reputable Bosch agent fully serviced and told they couldn't fault them.

We then pulled the plugs out and it was obvious how rich it was running by the state of them and the port side plugs all wet so we turned the engine over and the rear port side cylinder threw out allot of fuel!!!

Original problem was a hot start issue and now after injectors came back from testing the motor is flooding.

While the hot start issue may have been vapor lock, this issue is most likely the injectors