Fuel/smoke out the exhaust of 7.4L MPI

[Capt.Will]

Engine: 1997 7.4L Mercury Bluewater MPI - MEFI 1

Symptom: Very noticeable amount of raw fuel and smoke from the exhaust at idle speed.
The engine starts, runs and planes without hesitation.

First attempts: New plugs, wires, cap and rotor changed. Tested for spark, tested good. Cylinder #8 spark plugs seems to have noticeable amount of fuel in plug when removed. Compression check on all cylinders: 125psi. Symptom remains.

Seconds attempts: install new injector for Cylinder #8. Symptom remains.

Third attempts: Local mechanic hooked up Mercury computer and after diagnosis computer recommended the issue is either MAP sensor, TPS or Fuel Pressure Regulator. Installed all three new parts. Symptom remains.

At my wits end with this issue. Please, any help with this will be greatly appreciated. Mechanics around here are either too busy to take on new clients or they just don't want the job. If anyone in the area have a mechanic they know that can work some magic on this boat please let me know.

 

[Wanderer 290]

Compression check would be a good diagnostic next step.

 

[JHornsby3]

Get you a can of carb cleaner. Spray around the intake manifold, the throttle body mount and around any vacuum ports and hoses.

The 97 block will have the reusable gaskets, From the GM factory, they weren't that "reusable" and are prone to failure. Start there and see if you have an air leak. Could be leaking and the puter is trying to compensate.

 

[Capt.Will]

Compression check would be a good diagnostic next step.

I forgot to add that information to my post. Compression check on all cylinders was 125psi.

 

[SeadawgVB]

Are you positive it's fuel? That was the path I was going down on my 8.1 motors, I was pretty sure I was seeing fuel (a very light sheen in the water) in my exhaust. Did plugs and wires, tested fuel pressure and was about to pull all my injectors and have them flowed / tested.

I turned out it was my oil cooler. Such little oil was leaking out, it took a very long time to notice the drop on the dipstick. Also the oil pressure is always higher than the water pressure, so you won't see water in the oil.

 

[Capt.Will]

Are you positive it's fuel? That was the path I was going down on my 8.1 motors, I was pretty sure I was seeing fuel (a very light sheen in the water) in my exhaust. Did plugs and wires, tested fuel pressure and was about to pull all my injectors and have them flowed / tested.

I turned out it was my oil cooler. Such little oil was leaking out, it took a very long time to notice the drop on the dipstick. Also the oil pressure is always higher than the water pressure, so you won't see water in the oil.

Pretty positive its fuel. Definitely a fuel smell and cylinder #8 is saturated with fuel as well.

 

[RollerCoastr]

I have the same engines and had those exact symptoms last year, except that I noticed excess fuel on the plugs in both cyl's 7 & 8.

I have fuel meters, so I could see that the port engine was consuming extra fuel and idle, but was spot-on at cruise.

The VST tank/high pressure fuel pump needed a gasket/seal kit. Easy fix, but getting the parts during COVID took two months!

 

[Capt.Will]

I have the same engines and had those exact symptoms last year, except that I noticed excess fuel on the plugs in both cyl's 7 & 8.

I have fuel meters, so I could see that the port engine was consuming extra fuel and idle, but was spot-on at cruise.

The CVT tank/high pressure fuel pump needed a gasket/seal kit. Easy fix, but getting the parts during COVID took two months!

I had thoughts about that pump as well. What is the CVT tank?

How did you diagnose the issue and how did you come to the conclusion it was that causing the issue? I looked at the diagram for that high pressure pump and see that there are O-rings in the assembly, not seals. Can you confirm this?

 

[RollerCoastr]

What is the CVT tank?

Oops. I wonder if I can blame that on auto-correct...?

VST! Sorry about that. I might even have the part number for the VST/Pump rebuild kit. It wasn't actually my diagnosis. I went down all the roads you did (except for replacing an injector, but I definitely suspected that).

My mechanic zeroed-in on fuel leaking from the VS tank/pump and flooding those cylinders.

 

[RollerCoastr]

looked at the diagram for that high pressure pump and see that there are O-rings in the assembly, not seals. Can you confirm this?

O-rings, but they called it a seal kit. I'm not sure if it's this one or not, but the price is about right, so it was at least a similar kit to: Mercury Mercruiser Marine - SEAL KIT (#808500)

 

[Capt.Will]

O-rings, but they called it a seal kit. I'm not sure if it's this one or not, but the price is about right, so it was at least a similar kit to: Mercury Mercruiser Marine - SEAL KIT (#808500)

SCORE! That is the issue. I switched VST/Pump between engines and the issue followed the pump. I am so excited and will be able to sleep tonight. I will be at the parts store in the morning to pick up the O-Ring kit.
Next time you make it up to Michigan I owe you a brew. Will

 

[RollerCoastr]

Awesome! If you haven't already, check to make sure it hasn't made oil. I went out of my way to avoid running the engine in that condition, yet a surprising amount of fuel made its way to the crankcase. (mine was probably dumping some fuel before I noticed the issue...)

 

[Capt.Will]

Awesome! If you haven't already, check to make sure it hasn't made oil. I went out of my way to avoid running the engine in that condition, yet a surprising amount of fuel made its way to the crankcase. (mine was probably dumping some fuel before I noticed the issue...)

My oil doesn't smell like fuel, but I intend on changing the oil before running the boat again. I need to anyways since it's last years oil. Been very happy with these engines as far as not consuming oil.

Thanks again for the help.