Mercury 4.3 running rough - ideas?

[kbryant]

Hello
2011 Tahoe Q5i Mercury 4.3. Changed fuel filter, new plugs. Idles smooth, go to give it some gas and it runs rough ( like it’s missing) and then it smooths out. Back to slow speed and idles smooth give it gas and runs rough again and then smooths out. I put a can of seafoam marina in it.

oil gauge read 40 and temp gauge reads a hair under 160. So those seem fine.

 

[Scott Danforth]

Verify your plug wires

 

[kbryant]

Verify your plug wires

Yes
I did put new wires on as well. Forgot to add that. Sorry.
thank you for your suggestion. I will double check.

 

[Scott Danforth]

Make sure you didnt cross wires

Do a cylinder drop test

New doesnt mean good

Most likely you are running on 5 cylinders

 

[Bt Doctur]

cap and rotor

 

[Lou C]

Was it running rough before? If not go back to the last thing that was changed lol.

 

[kbryant]

Was it running rough before? If not go back to the last thing that was changed lol.

No, it just started to run rough. We were cruising and we stopped to let the kids swim and when I started it back up that’s when it stated to rough rough.

 

[kbryant]

Do you think I lost a cylinder?

 

[Lou C]

Possible but when you changed plugs did any of them look oil fouled or were the electrodes slightly rusty? First could be valve seals/piston rings second might be leaky exhaust manifolds/elbows. Based on your description it is possible that the engine sucked in water. Was the boat more heavily loaded in the stern? That can do it. Lowering the exhaust elbows to the point where water can back up into the exhaust when you stop.
I’d definitely go thru the rest of the ignition system first but after that fuel system and perhaps a compression test.

 

[kbryant]

Possible but when you changed plugs did any of them look oil fouled or were the electrodes slightly rusty? First could be valve seals/piston rings second might be leaky exhaust manifolds/elbows. Based on your description it is possible that the engine sucked in water. Was the boat more heavily loaded in the stern? That can do it. Lowering the exhaust elbows to the point where water can back up into the exhaust when you stop.
I’d definitely go thru the rest of the ignition system first but after that fuel system and perhaps a compression test.

Yes, all of them were very oily when I replaced them. I’ve never had a inboard.
it sounds like the maybe it sucked in water. How do I fix that issue or check on that.

 

[Lou C]

Well if they were very oily and not corroded you may not have a water problem but an oil burning problem. Might need new valve seals and/or rings. I’d do a compression test first. How long have you had it and for how long did it run well?

 

[kbryant]

Yes, all of them were very oily when I replaced them. I’ve never had a inboard.
it sounds like the maybe it sucked in water. How do I fix that issue or check on that.

Yes, all of them were very oily when I replaced them. I’ve never had a inboard.
it sounds like the maybe it sucked in water. How do I fix that issue or check on that.

Well if they were very oily and not corroded you may not have a water problem but an oil burning problem. Might need new valve seals and/or rings. I’d do a compression test first. How long have you had it and for how long did it run well?

I have had the boat for about 6 months.
It was running perfect and when I stopped to have the kids swim for a 2 couple hours I started it back up and it started to run very rough. That’s when I replaced the fuel filter and it was running a little better but still not better. So when I got it home I changed the spark plugs and wires and still runs rough.

 

[Lou C]

Double check the firing order & distributor cap, get an ohm meter and measure the resistance of each plug wire. You may find one with very high resistance that isn’t firing the plug

 

[kbryant]

Double check the firing order & distributor cap, get an ohm meter and measure the resistance of each plug wire. You may find one with very high resistance that isn’t firing the plug

Thank you for your help. I will dig in and try to fix it.

 

[Lou C]

PS many years ago my wife had a 1988 Toyota Camry. When it was about 3 years old it started running like a 2 stroke engine! Now I had 2 different Toyota twin cam 4 cyl engines in the family but did not really know much about them past doing oil changes and tune ups. First thing I did was ohm out the plug wires and found two with very high resistance. New set of plug wires and all back to normal.
BTW that 4.3 should run pretty smooth, I have an old pre-vortec, no balance shaft 4.3 with the Prestolite points distributor & Quadrajet 4bbl carb and I have it so it will idle smoothly even cold at 600 rpm. Yours with the balance shaft, electronic igniton and fuel injection should run really smooth!

 

[achris]

Sounds more like an elbow or manifold has finally corroded through and let water into one (or more) cylinders. Pull the elbows and check them and the manifolds...

Chris....

 

[kbryant]

PS many years ago my wife had a 1988 Toyota Camry. When it was about 3 years old it started running like a 2 stroke engine! Now I had 2 different Toyota twin cam 4 cyl engines in the family but did not really know much about them past doing oil changes and tune ups. First thing I did was ohm out the plug wires and found two with very high resistance. New set of plug wires and all back to normal.
BTW that 4.3 should run pretty smooth, I have an old pre-vortec, no balance shaft 4.3 with the Prestolite points distributor & Quadrajet 4bbl carb and I have it so it will idle smoothly even cold at 600 rpm. Yours with the balance shaft, electronic igniton and fuel injection should run really smooth!

So only thing left to replace is the distributor cap.

 

[kbryant]

Sounds more like an elbow or manifold has finally corroded through and let water into one (or more) cylinders. Pull the elbows and check them and the manifolds...

Chris....

How would you check if water in cylinder? Would it show in the oil?

 

[achris]

How would you check if water in cylinder? Would it show in the oil?

Pull the elbows off and check them and the manifolds.. If they're leaking it'll be obvious. If not, you tick another item off the ANNUAL MAINTENANCE list.

Chris.

 

[Lou C]

Like I said earlier when you remove the spark plugs look for rust on the electrodes. That, and disabling the ignition and EFI and crank it over with all the plugs removed. If water or even just a mist of water comes out it’s time to remove the exhaust. You must have the dry joint units so if it’s coming from the exhaust not likely a gasket but actual corrosion. Fresh or salt water?