Mercruiser 4.5l low vacuum, WOT 4350 and higher fuel consumption

[damo79]

Hi everyone. I'm trying to figure out what's wrong with my engine. Noticed it won't rev above 4300 and fuel consumption is 30% higher than usual at 3000rpm and 50% at 4000rpm. It's 4.5l 250HP Mercruiser Bayliner VR5. Compression 180psi each cylinder, new TMAP and O2 sensors. See attached diagnostic checks on Diacom Marine for idle, 3000, 4000 and WOT(4350). Any ideas what's wrong with it?

 

[alldodge]

Welcome
What is your motor serial number?

I'm not seeing any misfires but have you replaced your distributor cap and rotor? The caps are well know to look good but be bad

 

[damo79]

Welcome
What is your motor serial number?

I'm not seeing any misfires but have you replaced your distributor cap and rotor? The caps are well know to look good but be bad

 

[damo79]

It's 2016 MPI engine. No distributor. Ignition coils controlled by PCM112 module

 

[Scott06]

Hi everyone. I'm trying to figure out what's wrong with my engine. Noticed it won't rev above 4300 and fuel consumption is 30% higher than usual at 3000rpm and 50% at 4000rpm. It's 4.5l 250HP Mercruiser Bayliner VR5. Compression 180psi each cylinder, new TMAP and O2 sensors. See attached diagnostic checks on Diacom Marine for idle, 3000, 4000 and WOT(4350). Any ideas what's wrong with it?

Not sure if @muc may be able to weigh in.

 

[Scott Danforth]

did you change props?

this says the motor is giving it nearly all it has.

 

[damo79]

No and tried two different props didn't see much difference. I noticed that intake manifold pressure is too low at idle. It's only 12.9inHg at idle as you can see on the screen and 12inHg showing on gauge. Not sure how much it should under load when on plane but seems very low.

 

[Chris1956]

Gee, the data shows some misfires. Is that normal?

 

[alldodge]

It's 2016 MPI engine. No distributor. Ignition coils controlled by PCM112 module

Check fuel pressure at the rail.

Ive found most marine motors manifold pressures are a tad lower than autos

Gee, the data shows some misfires. Is that normal?

Its list each cylinder but no actual numbers so no misfires

 

[nola mike]

Your manifold pressure is going the wrong way. At idle was the vacuum steady? You said you replaced TMAP, is that the MAP sensor? That would be my initial thought, or intake restriction somewhere. Is your throttle opening all the way? Fuel trim is good, which means that whatever air it thinks it's getting is correct.

 

[Scott06]

Your manifold pressure is going the wrong way. At idle was the vacuum steady? You said you replaced TMAP, is that the MAP sensor? That would be my initial thought, or intake restriction somewhere. Is your throttle opening all the way? Fuel trim is good, which means that whatever air it thinks it's getting is correct.

good catch yes vacuum at idle should be higher than throttle open near WOT should be less than 3" would think

 

[damo79]

Gee, the data shows some misfires. Is that normal?

It was during startup. If diacom connected after warmup no misfires.

 

[damo79]

Your manifold pressure is going the wrong way. At idle was the vacuum steady? You said you replaced TMAP, is that the MAP sensor? That would be my initial thought, or intake restriction somewhere. Is your throttle opening all the way? Fuel trim is good, which means that whatever air it thinks it's getting is correct.

What do you mean wrong way? TMAP it's integrated MAP with temperature sensor. New sensor readings in Diacom verified with gauge and shows the same. Needle steady at 12inHg on idle. But should be 17-22 not 12. Checked throttle body works full range to 90deg.

 

[damo79]

Check fuel pressure at the rail.

Ive found most marine motors manifold pressures are a tad lower than autos


Its list each cylinder but no actual numbers so no misfires

Fuel pressure steady at 45psi - checked under load while on plane.

 

[Fun Times]

With 600+ hours, is this a fresh or salt water boat mostly?

If you haven’t, pull the small vacuum line off the fuel regulator and see if it’s dripping fuel..

Pull and check your Positive Crankcase Ventilation Valve for rattling sounds, etc..

Another area that warrants opening up for inspection would be the entire catalyst exhaust system for blockage including the back flow water stopper flappers not allowing air to escape should they have broke apart.

Does the engine sound normal or is there a change in pitch throughout full RPM range?

 

[nola mike]

What do you mean wrong way? TMAP it's integrated MAP with temperature sensor. New sensor readings in Diacom verified with gauge and shows the same. Needle steady at 12inHg on idle. But should be 17-22 not 12. Checked throttle body works full range to 90deg.

Vacuum should decrease with throttle. At WOT it should approach zero.

 

[jimmbo]

How long have you owned this Boat and Motor? Before this low rpm issue came about, what Rpm was it getting at WOT?

 

[damo79]

Vacuum should decrease with throttle. At WOT it should approach zero.

It does but too much imo. Vacuum is not shown directly. Vacuum is a difference between barometric pressure and manifold pressure. At WOT they both read nearly the same as you would expect but it's very low at 3000rpm when throttle demand is at 58.2% but arbitratted throttle position only 21.4.%. Got a feeling that engine is unable to create enough vacuum and PCM closing throttle to keep RPM steady.

 

[damo79]

With 600+ hours, is this a fresh or salt water boat mostly?

If you haven’t, pull the small vacuum line off the fuel regulator and see if it’s dripping fuel..

Pull and check your Positive Crankcase Ventilation Valve for rattling sounds, etc..

Another area that warrants opening up for inspection would be the entire catalyst exhaust system for blockage including the back flow water stopper flappers not allowing air to escape should they have broke apart.

Does the engine sound normal or is there a change in pitch throughout full RPM range?

That vacuum line from fuel supply module was first i checked. No fuel. Tried to suck on that hose and seems that pressure regulator holds well. Can I leave it of for test on water plugging intake manifold side? Maybe under load it does something different.
It's no catalyst version. Also checked exhaust flappers and they look ok. Didn't notice any unusual noise.
Not sure where was the boat use before I got it

 

[Fun Times]

No and tried two different props didn't see much difference. I noticed that intake manifold pressure is too low at idle. It's only 12.9inHg at idle as you can see on the screen and 12inHg showing on gauge. Not sure how much it should under load when on plane but seems very low.

Maybe I missed it but thus far i haven't noticed 12.9inHg in any of the screenshots...Saw 17.2 under Manifold pressure at idle of 6.22 RPM?
Maybe the new TMAP sensor changed the readings and hadn't really noticed it yet?

Yes, you can run with the vacuum line disconnected for a bit to test as that ought to raise the fuel pressure up roughly 5 psi on the pressure gauge and look for fuel leakages.

Looks like the boat is new to you?

Engine serial number sometimes helps when possible on your end.