Need help with diagnostics please . . .

[Redtick]

I would double check the return lines. At high rpms it could be restricted.

I've seen return check valves fail and restrict flow.

 

[L'eau.Life]

Thanks Redtick. Have bypassed the filters/primer and associated check (non-return) valves but will try blowing back through the return line.
Cheers, Alan

 

[powerabout]

low rpm and double the fuel flow ( and non turbo engine)is the conundrum here..

 

[L'eau.Life]

Well, I'm out of ideas!
We have checked, cleaned and adjusted everything short of removing the front to check the cam timing but since that is geared, I feel it unlikely that is out and can't see how performance would have degraded if it were as if that was the case it should have always been off and more than I am seeing.
We have recovered a few revs and it now runs about 400RPM below the stbd motor. Fuel consumption is still up but not as bad (around 1.5x the other motor) but that might be just that I throttle back the good engine to match them.
The boat is booked for a haul, clean and hold on Tuesday se we can inspect the props, shafts etc in case of damage or dragging on the stern gland or strut that side. Also if the weather holds I'll be heading out for the weekend and will take my IR thermo gun to check the stern gland temps.
I'll repost if I find anything but thanks to everyone who has contributed their thoughts so far - your ideas have helped prompt my own!
Cheers ~ Alan

 

[WestVanHan]

...thanks for the reply.
I was starting to wonder about this mystery.

 

[baeckmo]

Reading through the whole story, can't see that you have adressed the exhaust back pressure issue. There are several potential traps to be checked. Start with the hose between manifold and the "damper".

I've seen flexible rubber exhaust hoses that have "delaminated", so that an inner "flap" is created. No restriction at low gas flow, but as soon as you start producing some power, the flap is falling back into the hose and blocking the flow.

This would explain both the increased fuel consumption and the increased gas temperature, since the engine is working as an "exhaust gas compressor".

 

[french44]

in the same way the exhaust valves not well adjusted (gap to small) , or a restriction on the fuel return line . Check the temp cylinder by cylinder (not easy) . The things look like overload but with engine sometimes we never know...... do you check the gear box ? no hard point at high rpm , the clutch ? it could be a crack also on the top of the engine , I got **** like that with water consumption and only over 70 deg the crack open so.....

 

[L'eau.Life]

Thanks again guys - just back from a night away and I now have dark exhaust water and much steam in the exhaust flow. Cylinders appear to be consistent temperature-wise by reading with IR thermometer. Next move is to have her lifted on Tuesday to check the props, shafts, bearings etc then to remove and inspect the flexible exhaust per suggestions above and possibly replace the muffler as I can't think of a way to check that without breaking it! It is a glass fibre cylindrical affair that I haven't seen on other boats before and indeterminate age (I've found quite a few recycled things on this boat . . . .
Cheers ~ Alan

 

[WestVanHan]

Hoping you checked the cylinder temps in the same spot on every cylinder and on both engines...like the base of the injector,right at the one bolt,etc, etc

And did this while under a good load,and not just idling?

 

[L'eau.Life]

Hi WestVanHan - All cylinders checked at the same spot under load but only one engine as access to the stbd motor is very poor without removing the entire galley which is a heap of trouble to do and to store the removed furniture. The boat is only 28' and the motors are under the floor in each hull inside the cabin. Getting to the port one is as bad but if I have to I'll gut the interior at the end of the season (which sadly seems to be approaching quite rapidly now)
Cheers ~ Alan

 

[L'eau.Life]

So; another quick update . . . .
Lifted the boat out and had her all cleaned underneath and then found two things of significance:
1. The port strut has been hit by something and it is bent such that it cants about 3deg from the vertical so the motor is having to "bow" the shaft which I reckon would sap the horses
2. The rudders are out of alignment such that with the port rudder set fore-aft the stbd one is at about 10deg to stbd. Steering is fully hydraulic so need to check for a leak in the cross hose but I think this may also be losing me a knot or two.

While she is out I plan to take out the port waterlock muffler and check the condition of that and the flexible hose to address the concept of a blocked exhaust.

Cheers ~ Alan

 

[JSL]

3 degrees would probably suck a fair amount of power. Check the strut bearing and see how it is worn. Also check the entire shaft, bearings, gland etc

 

[L'eau.Life]

So, to close this out:
Relaunched yesterday after stripping and rebuilding the port driveline. Straightened and welded a small crack on the strut and checked all the bearings, glands etc. Reassembled everything and gave it all a good treatment with PROPGOLD. Reset the rudders (being hydraulic this was really straightforward) and stripped and rebuilt the port exhaust line - no problems anywhere there.
The outcome is that I still have a 600RPM delta between port and stbd but the port motor is now pulling the correct 3600 and the stbd has shot up to what I estimate as 4200 (but the counter stops at 4!). Speed is back up to 20+kts so next time she's out I'll get the stbd prop off and have the pitch set up a bit to match the revs - I reckon another 1/2" should be about fine.
It seems the real performance issue was the extra load from a bent strut forcing a bow into the shaft exacerbated by the rudders being misaligned and adding a lot of drag but having the pump and injectors rebuilt and the engine fully checked and set up precisely probably helped too and gives me real confidence.
Thanks to everyone on the forum who chipped in with ideas and advice ~ all were really useful and greatly appreciated.
Cheers all ~ Alan out.

 

[Westfield 11]

Alan, thank you for letting us all learn as well. I love a good mystery!