Help with diagnosis....loss of RPMs port engine

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Apologize for the long note but could use some help. And yes a VP service technician is on the case with no luck.

Okay, I have a 2007 Formula 400 SS with Volvo Penta D6-350s and DPH-A drives. Engines have 420 hours on them. About a month ago, my family and I were out with some friends for a cruise in the boat. With the exception of some shifter cable issues, the boat has run flawlessly all summer which account for 65 hours of use.

While we were leaving the beach, I idled through the slow no wake for 10 minutes and went on plane for a mile or two. Brought the boat back to idle and then up to 2,000 RPMs or so to create a bit of a wake for some jet skis. Continued to do this for about 2 miles and maybe 10 minutes. Brought the boat back to idle for a slow no wake. About 2-3 minutes into that slow no wake I got a high temperature warning. Never had one before.

Not sure at what point the EVC alarms you for hot temperature, but if I recall it read about 203 degrees on the port engine. I immediately shut it off. A couple of minutes later, I turned the ignition key back on (not starting it) to see what the temp of the engine was. It was back to 185 degrees. Weird?! I instantly thought an impeller.

So, I started the port engine and everything seems fine. Continued to idle for another 5 minutes with no issue. Then I began to bring the boat on plane when I noticed a loss of RPMs on the port engine versus the starboard. It was rather difficult to get the boat on plane as a result. So, I turned the boat around and idled back to our slip.

I should note that I lost about 1/2 to 3/4 gallons of coolant from that heat spike. It must have burped out during the heat spike. The service tech thinks I may have sucked in something temporarily or even a plastic bag that covered the water intake.

Since that temporary heat spike, I´ve not had another issue with temperature on the port engine -- it stays right at 185 degrees. The loss of RPMs to the port engine has been very frustrating and one that we cannot seem to figure out. I believe the loss of RPMs and that temp spike are related somehow, of course there is always a chance they are not, but it seems like cause and effect.

Exact symptoms are that the port and starboard engines stay right together up to about 1,650-1,750 RPMs. As I give it more throttle to get the boat back on plane, the port engines stays a good 300-500 RPMs under the starboard engine. No matter what I do the port engine just cannot match the starboard. Feels like a loss of power with having less RPMs.

On a couple of occasions (after doing some testing), the port engine has stayed almost 800 RPMs under the starboard while coming on plane before catching up to starboard withing 400-500 RPMs at WOT. Again, it stays consistent with the starboard engine up to about 1,650-1,800 RPMs.

At WOT the starboard hits 3,650 RPMs and the port is lucky if it can get to 3,200. WOT speed is down about 8-10 knots. Also, sitting in the slip and revving in neutral, it feels like the throttle response of the port engine versus starboard is just not as crisp.

So, below is what we´ve done to diagnose the problem with no luck. I would appreciate any and all help with this.

01. Checked impeller - GOOD
02. Checked sea strainer - GOOD
03. Felt out-drive and propellers under water -- GOOD
04. Swapped intake manifold temperature sensors from starboard to port - NO DIFFERENCE
05. Installed new belts and torqued to spec -- NO DIFFERENCE
06. Volvo Penta service tech performed a Vodia compression test -- GOOD
07. Volvo Penta service tech measured boost pressure for turbo -- GOOD
08. Volvo Penta service tech measured injector pressure -- GOOD
09. Volvo Penta service tech changed clutches in superchargers -- NO DIFFERENCE
10. Volvo Penta service tech said no fault codes were stored

Unfortunately, the boat is being pulled out of the water on Monday for the winter. I will have to wait until spring to do more diagnostics. The VP service tech said we should do a more elaborate compression test and a fuel pressure test in the spring. What do you guys think this could be?

Service tech thinks it´s a mechanical issue at this point versus electrical but who the heck knows at this point.
 
This is a complete stab in the dark,but when other (and myself) have had similar issues it was due to the prop bushing having failed on the sidebof the engine that will reach max revs, IE your starboard engine.

IE the engine with the failed bush is doing much less work because it's not actually spinning a prop, so will rev to max easily,leaving the other engine to do much more of the work, which it cannot do ,so it can't reach max revs.
 
julians said:
This is a complete stab in the dark,but when other (and myself) have had similar issues it was due to the prop bushing having failed on the sidebof the engine that will reach max revs, IE your starboard engine.

IE the engine with the failed bush is doing much less work because it's not actually spinning a prop, so will rev to max easily,leaving the other engine to do much more of the work, which it cannot do ,so it can't reach max revs.
Click to expand...

That's an easy check with a swapover.
Blockage in fuel and/or air filters ?
 
I recently had an issue where my boat lost power on one engine ( Volvo D6-330) with a matching loss of performance. It was a slipping propeller on the engine that rev’d freely. The propellers have hubs which are help in place with a chemical bond (glue) this hub sits on the splines of the drive shafts. Once the bonding agent starts to break down the hubs slip and the the propellers do not turn at the same speed as the shafts. This will occur at higher torque leadings and so you won’t see it at low speeds.

You can either get the propellers rehubbed or buy new ones.

You can test if the propellers are slipping by having your mechanic check the engine loads using the VP Vodia.
 
From experience running on one engine at a time you will still get one reving to nearly full rpm and the other (the good one) reving to around half of the full rpm. It’s ok to check by running one at a time but you have to know what is normal behavior.
 
The fuel injection system relies upon a lot of data and it has a limp mode which cuts power or shuts the power off completely by stopping the engine.

Based upon the information and the running temperatures I would suspect the temperature sender first, a simple variable resistor which sends signals to both the engines ECU and temperature gauge and excess temperatures would drop the engine into limp mode to protect them, so this would be my first port of call. Swapping the senders between engines is readily checked by swapping them and if the engine still has the fault then dismisses it, if the opposite engine gest the same fault, it confirms it. In addition check the wiring as often it breaks next to the sender or breaks inside the wiring plug if fitted and often cannot be seen.
 
My first thought is the one which is low on revs is doing all the work while the other one has a slipping bush, this happens a lot with the torque of the D6.

If you lift the boat see if the gap between the duoprops on both sides has the same gap or if they have been touching.

Other wise possibly turbocharger?
 
julians said:
This is a complete stab in the dark,but when other (and myself) have had similar issues it was due to the prop bushing having failed on the sidebof the engine that will reach max revs, IE your starboard engine.

IE the engine with the failed bush is doing much less work because it's not actually spinning a prop, so will rev to max easily,leaving the other engine to do much more of the work, which it cannot do ,so it can't reach max revs.
Click to expand...

Winner, winner, chicken dinner!

I know several others have pointed to the same problem. Turns out you guys are darn good! I posted on several sites and this one is the ONLY one that pointed to this as a possible problem. Awesome!

And, that's exactly what the problem was. Turns out that one of the starboard-side propeller bushings on the duo prop was shot. I effectively only had one of the two propellers spinning on that side. That poor port engine was fine the entire time. It could never get to WOT RPMs as it was working to compensate and carry the load from the starboard side.

Thanks again fellas! Best boating site hands down!
 
Advantage said:
Winner, winner, chicken dinner!

I know several others have pointed to the same problem. Turns out you guys are darn good! I posted on several sites and this one is the ONLY one that pointed to this as a possible problem. Awesome!

And, that's exactly what the problem was. Turns out that one of the starboard-side propeller bushings on the duo prop was shot. I effectively only had one of the two propellers spinning on that side. That poor port engine was fine the entire time. It could never get to WOT RPMs as it was working to compensate and carry the load from the starboard side.

Thanks again fellas! Best boating site hands down!
Click to expand...

Good news, glad to have been of assistance.
 
Advantage said:
Winner, winner, chicken dinner!

I know several others have pointed to the same problem. Turns out you guys are darn good! I posted on several sites and this one is the ONLY one that pointed to this as a possible problem. Awesome!
Click to expand...

Other forums? are there any? :)
OK I know there are others but for the best advice this is the tops.
 
DavidJ said:
I'd be really interested to see what a shot bush on a prop looks like.
Click to expand...

I think you need to find another website for that?! :D Seriously, I didn't take any pictures but there was obvious slop in the propeller and rubber bits strewn about. The prop will need to be re-hubbed.
 
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