Problems with 350 Verado Engine Surge and running temperature.

[opaleski]

Mercury 350 Verado Serial Number 2B202272

First let me explain the problem:
When the engine idles the temperature is 135F. When I go on plane the engine surges back and forth while climbing in RPM. It feels like cylinders are dropping out then picking back up as the plugs clean up?? It continues surging up through 6000 rpm. As the engine RPM is picking up the temperature starts picking up going to 170F and back down to 135+, back up and back down. The temperature of the water coming in is 45F to 50F.

The engine has 107 hrs on it and just had a 100 hour maintenance performed as of September 2017. I continuously use all three of the Mercury Marine additives. The tanks were filled up in September and I just started the boat and running it today.

I am thinking that I might have a bad gas problem with regards to the surging, because if I stay at the higher RPM it seems to clean itself up. What do you think?? As far as the running temperatures--Is this a problem??

Any comments would be greatly appreciated!!

Thank you.

[Jfritz_drfritz]

Maybe if cylinders are dropping out the block temperatures fluctuate wildly and the thermostat is opening and closing trying to keep up. Strange though, I thought these motors would go into guardian if the water temperature exceeded 160 degrees. Need to see your dealer for an electrical checkup

[iron banks]

I had something similar with my brand new 350. They checked everything. Finally they unplugged the wire harness to check it and then reconnected it. The engine has worked flawlessly since then. Evidently a bad electrical connection. Simple thing to check

[Erie660]

My 300 Pro does the same thing with the fluctuating engine temperature. I have had it checked out and everything checks OK but the temp will climb to 170-174 and drop down to 145 before you can snap your fingers

[opaleski]

Thanks for all the reply's I will try all the above.

[Jasonrs]

I have the 4 cylinder model 200 hp, and my water temps fluctuate from 130ish to 180 and back down again very very quickly. Was also told it was fine. Pretty sure the temp to trigger gaurdian is much higher than 160, but have never really found what temp does..

[Erie660]

I have the 4 cylinder model 200 hp, and my water temps fluctuate from 130ish to 180 and back down again very very quickly. Was also told it was fine. Pretty sure the temp to trigger gaurdian is much higher than 160, but have never really found what temp does..

I’m just curious if you tried unplugging the harness that was recommended to try and fix the fluctuation in your temperature readings? My 300 does exactly what your motor is doing.

[Jasonrs]

No i have not

[EuropeanAM]

Andy- were the plugs REPLACED at the 100 hour service (or simply inspected)?

Temps are relatively normal... but you may have something as simple as a fuel restriction (inlet filter not removed) or a failing spark plug.

[Jfritz_drfritz]

My 300 pro (2013) was dropping a cylinder. Changing plugs seems to have fixed it. I am surprised that would be the problem as I only have 107 hours, never troll with it, use only quality premium gas, and all gas that goes into it has stabilizer, quickcare and quickleen. I can only assume this happens because of the cool temperatures these outboards run at??

[Erie660]

Andy- were the plugs REPLACED at the 100 hour service (or simply inspected)?

Temps are relatively normal... but you may have something as simple as a fuel restriction (inlet filter not removed) or a failing spark plug.

Don, my 300 Pro has been doing the same thing since it was new and I have 65 hours on it now.

[EuropeanAM]

Don, my 300 Pro has been doing the same thing since it was new and I have 65 hours on it now.

WHICH same thing (two concerns). Surging is not normal.

[opaleski]

Andy- were the plugs REPLACED at the 100 hour service (or simply inspected)?

Temps are relatively normal... but you may have something as simple as a fuel restriction (inlet filter not removed) or a failing spark plug.

Not changed just inspected. Inlet filter has been removed

[opaleski]

I will order a set from Rhonda today

[opaleski]

Thanks all it was the plugs!!

[EuropeanAM]

Love to hear a "simple fix".

[Seabee]

I had the surging issue with both my main motor and my kicker. Both are Mercury motors. Problem was the vinyl fuel line. The vinyl lining inside the Mercury fuel line will break down and separate. This issue has caused surging due to restrictions in the filters. Some of the vinyl liquified and made it through the filter and plugged my injectors on main motor and the carb on my kicker. With more fuel in America having ethanol in it this maybe be a cause for some issues especially with longer lay up periods between seasons.

Glad to hear it was just the plugs in the OPs posting.

[BigBadBass]

Seabee, is your boat/motor a older model? I heard new fuel lines don’t have this issue with diff materiAl but I don’t know for sure

[EuropeanAM]

ANY fuel line can be subject to degredation, given the right circumstances.