5.0 Gxi-ce 270-r misfires cyl 4&7 at full throttle

[D_Nyholm]

Surveyor took my boat out for a test run and while running at full throttle for about 30 seconds, the warning chime came on and power was cut to the starboard motor ( s/n A204563). My mechanic hooked it up to the computer and it shows "cat damaging misfires" on cylinder 4 and 7. There were about 2000 on each cylinder.

After changing the cap and rotor, plugs and wires, and even a fuel pump, we are still getting them. Our next step was to be fuel injectors but after he monitored the voltages on them, they are all showing the same voltages.

The only thing we noticed in the log was that the misfires occurred at 5220 rpm. The max rpm for the motor is 5000, and I assume there is a rev limiter somewhere above that (maybe 200?). I can't imagine that the rev limiter would be to induce "cat damaging" misfires. Either it would cut fuel or retard timing to pull the motor back? We are not sure where to look at this point and don't want to keep throwing parts at a problem that could potentially be simple.

I have never experienced this issue in my 5 years of owning the boat, but I rarely ever go full throttle, and if I did, it would be for a very short time. Any suggestions on where to look next?
Thanks in advance

 

[alldodge]

Hey @muc have any ideas on this one?

 

[D_Nyholm]

Mechanic is ruling out rev limit as he spoke to his Volvo contact and they confirmed that isn't how it limits over revs. Still not sure which way to proceed at this point.

I did notice when I took it out once, no codes were thrown my first pass at full throttle. After I slowed and turned around and then went full throttle again, 15 seconds or so into that run, the misfires occurred. You can't even hear them or feel them, no popping or slowing until the computer pulls everything back.

 

[kenny nunez]

Check if the fuel rails are stainless of plain steel with a magnet. A shop owner friend told me about a Volvo that had plain steel rails that had some rust inside which was getting on the screens of the injectors which caused some problems.

 

[alldodge]

Might connect a fuel pressure gauge to rail and see if pressure drops any

 

[QBhoy]

Hi. Few things from me.
Your engine will limit at about 5150 usually. Or even a touch less. Shouldn’t show miss fire errors though when it does.
What will show them though, is the cap being bad. Even after a recent replacement. Sometimes especially so. Especially if care hasn’t been taken when installing these delicate and easily miss seated caps. Equally easy to crack them too. Never rule these caps out too quickly. I’d also ensure that the distributor has been properly aligned between the installation marks too.
A failing battery or short in the system could also cause this scenario too. Unfortunately, so could water ingress, by means of a compromised sealing interface in the top end…or far more likely, the fuel quality being thrown at her from the tank. Finally, does she definitely have the right plugs fitted ?

 

[JustJason]

Your engine should not be hitting the limiter at full speed. If the engine is rated for 5220 RPM's and it's rated for 5000 RPMS then either the power package has the wrong propellers on it or the props are slipping. I would start with a prop and prop shaft inspection first. If the props are damaged they will need to be repaired or replaced. If the props and shaft look fine, and if there isn't anything that would cause the props to slip (IE a transducer mounted to close to the drive) then change the props out for a props that have more pitch.

 

[D_Nyholm]

Just ran it again with a separate fuel tank in case the fuel was old. I didn't go full throttle to see if it would misfire below the rev limit. At about 4800 rpm the misfire kicked in.

I'm using NGK itr4a15 5599 iridium plugs. Been using them for years with no issues (these are new plugs put in this season). I can check the cap and rotor again to see how it looks on cylinder 4 and 7. Alignment of distributor looked good as well, hasn't moved at all.

Mechanic is suggesting swapping the computer from one engine to the other to see if these misfire follows. Not sure that is the best idea. Other than that we are kind of out of options besides injectors.

 

[alldodge]

Don't see plugs doing this unless the new plugs (4 and 7) look different then others. Can check gap but don't try to adjust them, Iridium coating can be damaged

I would suspect the cap before other things. You replaced it, but was it a VP, Merc or GM Ac Delco and not aftermarket?

Swapping the ECM is a good test but not in your situation. If there is something wrong with the ECM and it was caused by something on the motor (harness, sensor, other) it can damage the good one. Would be nice if you could find another boat with same motor and install yours on it

 

[nola mike]

Swapping injectors to see if the problem follows would be my next step.

 

[JustJason]

Just ran it again with a separate fuel tank in case the fuel was old. I didn't go full throttle to see if it would misfire below the rev limit. At about 4800 rpm the misfire kicked in.

I'm using NGK itr4a15 5599 iridium plugs. Been using them for years with no issues (these are new plugs put in this season). I can check the cap and rotor again to see how it looks on cylinder 4 and 7. Alignment of distributor looked good as well, hasn't moved at all.

Mechanic is suggesting swapping the computer from one engine to the other to see if these misfire follows. Not sure that is the best idea. Other than that we are kind of out of options besides injectors.

Never swap ECM's.

Make sure your mechanic understands live data on the scan tool and understands how fuel trims work. There could be sensor issues that are not throwing codes.

You can definitely still have bad plugs. You can take a brand new spark plug and drop it on the ground just 1 time, and that plug will give you issues. Bad plug wires and a cap/rotor issue will cause similar symptoms

If your mechanic has a good lab scope, like Pico or similar, they will be able to tell if it's an ignition related misfire or a fuel related misfire, but that is getting into the advanced stuff.

 

[D_Nyholm]

Still playing around with this issue. Swapping parts between the two engines and it doesn't seem to be ignition related though we honestly don't know. Didn't realize the evc (evc-d) shows faults on the screen! We are showing MIL 126 FMI 14 on both SID 4 and 7. Can't seem to find anything on the Google that tells what they really mean. Only thing is maybe injector has a bad ground? Any ideas on that one?

 

[D_Nyholm]

Took it out early this morning. Engine would only go up to about 4500 at full throttle. Fuel pressure at 52 at idle and 54 at full throttle. Shows no misfires at that speed but you can tell something is wrong. Tried a few more times, rpms max at 3k and starts back firing. Use computer to check injectors and spark and cylinder 5 does have much effect. Fuel trim on bank one is maxed at around 30% while bank 2 is at 5% or so. Looks like somehow cylinder 5 injector is not working or clogged. Mind you I just had the injectors rebuilt last week!

By the time we gave up, rpms wouldn't go much above 1100 or so, bank went into open loop since fuel trim couldn't compensate.

Very confused where all these gremlins seem to be popping up from!

 

[alldodge]

From what I find about the GXI is it does Batch firing of the injectors and has a single fuel rail. Don't see how a scanner can do a misfire or tell which bank has more or less fuel. Motors like the 8.1 can because individual coils and injector control

With the motor getting worst the more it runs, it has to be fuel or heat related. There is no sign of heat so it's back to fuel

 

[D_Nyholm]

Well it is the CE (I assume means cats and electronic something). The computer shows the 4 O2 sensors and bank one and bank two fuel trim percents.

Pulled the injectors and am going to send them back to be checked yet again. Going to have to wait until Monday for that though.

 

[alldodge]

Being CATs there will be exhaust bank O2's as you mentioned.

The injectors need to be tested by someone that knows how to test. If they go to a auto place they may not test them at high rpm over time and high flow.

The best guy I know is an OSO member that goes by ArticFriends (Smitty). Or go to BAM which is mercruiser parts dot com

Boats unlimited in Texas but he has been turning folks away for being so busy

 

[nola mike]

Well it is the CE (I assume means cats and electronic something). The computer shows the 4 O2 sensors and bank one and bank two fuel trim percents.

Pulled the injectors and am going to send them back to be checked yet again. Going to have to wait until Monday for that though.

So your misfires/problems changed cylinders? I'd still try swapping out that #5 injector with something on the other bank. How do you know it's #5? Also not just lack of fuel causes high FT--vacuum/air leak is pretty common as well. But that would affect the whole bank, and not just 1 cylinder.

 

[dubs283]

Kv meter on the plug wires will show if the spark plug(s) are getting proper voltage, especially at speed/load

Otherwise, I'd be suspect of fuel supply as others have mentioned

 

[D_Nyholm]

This motor is possessed. Injectors were numbered and sent out. Guy said they were 30% clogged. Checked fuel filter and it was clear. Checked regulator on fuel rail and there is no filter screen in it and it was clear. Put 1357 injectors in 2468 spots and vice versa. Went out to run and still get 4 and 7 misfires.

On a whim, I swapped plug 4 and 6 and plug 5 and 7. Went out and got a misfire on cylinder 6. Thinking that has to be it, replaced the spark plug in #6. Went back out and go about 60 seconds at idle and start getting misfires on 6 even with the new plug. Swap the plug and wire for that cylinder and I'm still getting misfires at idle (in gear) for cylinder 6. Not sure what to do! The only correlation is that the injector for cylinder 5 that gave us issues before is now in cylinder 6. But this also seemed to be an unrelated issue to the misfires on 4 and 7. It was a new issue that popped up on cylinder 5.

None of this is making any sense at all. Basically all fuel and spark side has been replaced or refurbished on the motor. Fuel looks clean from tank to motor, even running on a separate tank to the motor we have these issues.

 

[alldodge]

Who did your injectors?
Did you get a printout of before and after?
Are you sure RPM's are correct, checked with hand held?