Need help with diagnostics please . . .

This is the puzzling part. If not for this, a cracked head/leaky gasket is the obvious.

 

UPDATE:
Performed a compression test today and all cylinders >300psi
Double checked both oil and water - no contamination of either
No evidence of excess crankcase pressure - no discharge from breather etc
On sea trial observer mechanic considers white smoke actually steam. Port exhaust water notably hotter to touch than Stbd.
"Tweaked" fuel pump 1/4 turn. Increased fuel resulted in additional 200RPM but evidence of soot in exhaust. Returned pump to test bed settings.
Ran engines on no-load and both readily go off the scale (Rev counter max read 4k RPM)
Ran each engine under max load ie. underway one engine only WOT. Port max 2650 / Stbd max 3000
Bypassed fuel filters (in case hi-flow required) no change
Both props/shafts clear and clean and turn easily by hand in neutral
No noticeable difference in rev drop across the two motors when put in gear
The port motor is definitely using about double the fuel as the Stbd but no evidence of smoke or lost fuel in bilges etc. Port motor appears to be running hotter exhaust although engine temp is consistent with Stbd.
Can't think of anything more to add except I, my mechanic, the guys that did the original installation and the collective wisdom of the Gulf Harbour bar flies are now utterly flummoxed!
ALL IDEAS GRATEFULLY ACCEPTED PLEASE!

 

Well--if the engine idles and then labours as the revs pick up--I will re-state the comment made earlier. You may have something which blocks your exhaust as the gas flow increases. My advice--check thoroughly the exhaust system on the engine in question. Better than damaging the engine to the muffler and clean it out. If it has one of those plastic or rubber/plastic mufflers it has probably got hot at some stage and become distorted inside. If it is a metal one it may have a dislodged baffle. They lie down--then stand up as the gas flow increases and block the muffler. I may be wrong--but you have tried everything else--try the obvious one and see if that makes a difference. No steam means exhaust gasses are not getting hot because engine exhaust is not reaching proper temperature and combustion of fuel is incomplete. Again a property of a restricted exhaust system. The only other possible thing is a piece of something is interfering with the air intake manifold. Try putting a tube into it connected to a vacuum cleaner and having a poke about. You never know what might be in there.

 

How are you measuring the fuel flow to know you have different consumption?
You need to measure the boost as well.
egt if posible as well.

 

@mike Banks
Exhaust flow appears no different across both motors. Each system is a 4" flexible hose running to a large (circa 10"dia) barrel-type glassfibre waterlock muffler and thence a straight 4" rigid plastic pipe out through the transom.
Both engines exhaust some steam but the port (problem) motor exhibits slightly more which I had wrongly thought was white smoke - hard to tell when helming the boat.
Many thanks for your idea. Alan

 

@powerabout
Fuel consumption is pretty simple - each motor has an identical 25gal dedicated tank with sight glass. On a 12nm run the Stbd tank level dropped just under 1" whereas the port went down just over 2".
Motors are normally aspirated so no boost.
I don't have any facility to measure EGT but the galvanising on the port manifold appears to have been "burnt" (white powder deposit) whereas the Stbd motor doesn't show this. That could however be misleading as the port manifold was repaired a while back.
Port exhaust emission is much hotter to touch than Stbd.
Thanks for your inputs. Alan

 

Whats strange is you get twice the consumption at full throttle so one engine is processing twice full load fuel consumption...wow
I'd ask garret if that model pump is capable of that
Someone hasnt put both fuel returns in one tank have they?
So not losing water and not gaining oil?

 

@powerabout
Great thought but no - the boat is a cat and each hull/engine/tank is fully independent of the other. :'(
No loss of water & no change to oil level.

 

I had a problem similar but it was with temp difference between two motors. Did all the things possible senders, gauges, cleanout heatexchangers etc. could not figure it out , ended out to be tach, as one motor was doing all the work and the other was just loafing along. Have you checked the tach as they might not be accurate? and one engine is working much harder than the other and using twice as much fuel? just a thought. Steve

 

@mreoe4sure

Hi Steve. Thanks for that. My mechanic plans to check the tachos against a master next thanks but I can't get my head around that being the problem as it wouldn't account for the significant drop in performance - the boat has lost some 4kts at WOTs.

Next move is to scribe a line to show the physical position of the pump and slacken the bolts slightly to allow us to rotate the pump with the engine running at a set RPM and see if the revs come up at all. I doubt that will explain the difference in fuel consumption though.

 

I looked briefly online,and these do have some electronics which leads me to wonder if it's a crank sensor.

Perhaps switch them from engine to engine,and go for another run?

Or sign up at a Nissan Navara forum?

This is a prime example where a FLIR viewer or a laser IR ($20-$40) temp sensor would be verrrry handy.

 

OBD port?

 

A wild guess but:
When you checked the props, were they the same: Dia., Pitch, DAR, type etc. And, have they been correctly sized for same.

 

@WestVanHan
These are straight QD32 (not QD32T, Ti or ETi) so purely mechanical pump and no OBD port :-(

@JSL
Props are matched and were running fine for at least two years before this problem manifested itself

Thanks again guys

 

@WestVanHan
Just ordered a Laser IR sensor - thanks for the tip