Engine running rich/gas slicks in water

[boater2065]

Hi all,

Before having the mechanic diagnose my engine, i'm curious if anyone on here has some ideas for me. Merc 5.7 EFI in a 2002 sundancer 260. Lately the engine is putting off a lot of exhaust/smoke at startup and beyond. It's not a black smoke, just excessive exhaust. It smells like exhaust and fuel and after turning off the engine, there are slight gas slicks in the water around the swim deck. What are some common things I can check for? What typically causes these symptoms? I dont mind getting the mechanic involved if needed, but i'd like to learn these things myself if possible.

Thanks
Jeff

 

[scoflaw]

Excessive fuel pressure and cylinders that aren't burning the fuel that's delivered to them. Pull your plugs and have a look, they will tell the story

 

[Jeff Delbuono]

I had something like that with my twin 5.0 efi in my 98 290 dancer. Was the water temperature sensor. It tells the ecm at start up to run richer when cold then lean out when warm. Easy swap out. On mine it was in thermostat housing on the left side. Other side is the guage temp sensor

 

[Jeff Delbuono]

Looks like this

 

[boater2065]

I had something like that with my twin 5.0 efi in my 98 290 dancer. Was the water temperature sensor. It tells the ecm at start up to run richer when cold then lean out when warm. Easy swap out. On mine it was in thermostat housing on the left side. Other side is the guage temp sensor

Interesting. I've been wondering about replacing this anyways due to my temp gauge reporting higher than the thermostat actually is. My gauge reads over 170 at idle and higher than that once over 3k rpms, but the IR gun says the thermostat is about 154. I was assuming either my gauge or the sensor was bad.

I have a closed cooling system. If i replace this sensor, will i lose a bunch of coolant?

 

[Jeff Delbuono]

I do not think so. I did not even take my housing off. I just put a 3/4 wrench on it. Unscrew and install. Solved all my smell, smoke and gas sheen. Easy fix and a good start

 

[Jeff Delbuono]

Just remember, guage temp sensor on right with just a push on or single lug wire, ecm temp sensor on left with a black plug like the pic

 

[boater2065]

Just remember, guage temp sensor on right with just a push on or single lug wire, ecm temp sensor on left with a black plug like the pic

I was down at the boat today looking at my thermostat and on mine it appears the sensor in the above picture is on the right and the a smaller single wire plug on the left (as you look at it). I will definitely start with this and see what happens.

 

[Jeff Delbuono]

Ok. My bad. Mine is on the left. Good luck

 

[boater2065]

Ok. My bad. Mine is on the left. Good luck

No worries, must be different on the 5.7. Much appreciated

 

[Jeff Delbuono]

Another thing i do with my efi's When i change my water fuel seperators, i take old out, add 3/4 seafoam and 1/4 gas. Start motors and bring to 2,000 rpm. Let run for about 5 - 10 minutes. This will clean out any crap in fuel pump, rail and injectors. Another easy maintenance trick

 

[Hey Ray]

I had the same 2 symptoms (excessive exhaust plus unburned fuel) and also decreased performance (had to give it more throttle to get to same RPM's as other engine) and it turns out it was 1 bad plug and 1 broken wire, so only running on 6 of 8 cylinders.

 

[boater2065]

I had the same 2 symptoms (excessive exhaust plus unburned fuel) and also decreased performance (had to give it more throttle to get to same RPM's as other engine) and it turns out it was 1 bad plug and 1 broken wire, so only running on 6 of 8 cylinders.

Thanks for the tip. It's never a bad idea to check the plugs, so i will do that for sure.

 

[boater2065]

UPDATE: I replaced both sensors on the thermostat housing and it did not fix the gassy exhaust or fuel slicks in the water. I'll have the mechanic take a look

 

[Tacoma290]

UPDATE: I replaced both sensors on the thermostat housing and it did not fix the gassy exhaust or fuel slicks in the water. I'll have the mechanic take a look

If you are looking for a suggestion on who to talk to, Aaron's marine would be my vote.

 

[boater2065]

If you are looking for a suggestion on who to talk to, Aaron's marine would be my vote.

Thanks, that is who will be looking at it next week.

 

[Tacoma290]

Thanks, that is who will be looking at it next week.

You are in good hands! Let us know what you find out.

 

[boater2065]

You are in good hands! Let us know what you find out.

I will update the thread when i have an update, thanks!

 

[glenncal1]

Jeff- I am sure the mech will check the fuel pressure. I had similar problems, fuel pressure regulator bad. Fuel pressure was around 90 should be 42-3.

 

[boater2065]

Update: Mechanic has not had time to check the boat yet, so I was going to make a trip down to check my spark plugs. Last winter I replaced the plugs with NGK plugs and gapped them according to my engines specs (.045).

Since i dont remember when the smoke/fuel smell/fuel slicks began happening i'm wondering if the wrong gap or something may be the culprit? Looking for anything easy while the mechanic is busy with other boats.

Thoughts?