Volvo Penta AD41 low rpm at WOT

SPDauto

SPDauto

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so finally got my sunseeker in the water, it was converted from a mercruiser to a penta AD41 about 20 years ago but hasnt been in the water in 10.
Its running ok at idle and starts with half a turn of the key, but im lacking power. I have no tacho at the moment either but sounds like i am only getting approx 2500rpm at WOT. I do have some vibration under load also which could be a slight misfire, but difficult to tell.

I have checked, cleaned and replaced the fuel lines all the way back to the tank. new filter, new water catch, lift pump removed and checked and is ok.

Turbo has no play and is spinning easily and freely.

Any penta gurus on here able to shine a light on this for me? Injectors out next to be tested or is there anything i can check or clean on the pump in situ?
 
Is there a turbo boost? I had a similar problem a while back and it turned out the booster wasn't cutting in at 2500 on the port engine.
 
40/41A/B is simple basic engine, if the turbo rotor can be spun with your finger it is probably OK.
There is no electro mechanical clutch system that engages anything at certain revs, as on the later supercharged engines.
You really need a proper method of measuring revs to do anything serious.
Using your ear technology does it achieve the same revs at high idle as when you are under load ie. under way.
It can be bit scary but you need to go to WOT with no load on the engine to compare.
If you are not getting max revs when under way it may not be an engine problem but something to do wth the propeller, any smoke, is the bottom squeaky clean ?
 
As per earlier posts - you need to get a tacho on the engine - you can get cheap infra red/laser ones on amazon you can point at the crankshaft pulley etc to test the engine at full throttle out of gear. Even if you have a tacho on the boat its worth getting one as it will be accurate. Older Volvo Tachos can be a bit off. It sounds terrible and very loud but for a few moments it's OK to do this to establish the basic mechanical bits will rotate at max rpm unloaded.
Thereafter you are either short of air or short of fuel or have a fouled hull/props. Address each point individually other wise it can get very confusing. Dont underestimate the effect of fouling it takes very little to kill performance.
Vibration could be the engine could be the drive train. To check the engine and fuel injectors, start the engine let it run for a few minutes then one by one slacken the fuel injection pipe at the injector, the engines should immediately run rough. If the engine behaviour does not change you have probably found a duff injector. Tighten the injector pipe again and repeat all the way along.
 
Last edited:
oldgit said:
40/41A/B is simple basic engine, if the turbo rotor can be spun with your finger it is probably OK.
There is no electro mechanical clutch system that engages anything at certain revs, as on the later supercharged engines.
You really need a proper method of measuring revs to do anything serious.
Using your ear technology does it achieve the same revs at high idle as when you are under load ie. under way.
It can be bit scary but you need to go to WOT with no load on the engine to compare.
If you are not getting max revs when under way it may not be an engine problem but something to do wth the propeller, any smoke, is the bottom squeaky clean ?
Click to expand...

yes the turbo spins freely with no play, i know ear technology isnt ideal but she will rev much higher in neutral to the point i hate doing it to her and want to back her off again. Under load the first half of the throttle gives me progressive revs, then no change to engine note or performance if i push it to WOT. so yes revs right through in neutral.

Underside of the boat is spotless, just been antifouled and put in a few days ago, and the props are brand new, same pitch as originals also.
 
kashurst said:
As per earlier posts - you need to get a tacho on the engine - you can get cheap infra red/laser ones on amazon you can point at the crankshaft pulley etc to test the engine at full throttle out of gear. Even if you have a tacho on the boat its worth getting one as it will be accurate. Older Volvo Tachos can be a bit off. It sounds terrible and very loud but for a few moments it's OK to do this to establish the basic mechanical bits will rotate at max rpm unloaded.
Thereafter you are either short of air or short of fuel or have a fouled hull/props. Address each point individually other wise it can get very confusing. Dont underestimate the effect of fouling it takes very little to kill performance.
Vibration could be the engine could be the drive train. To check the engine and fuel injectors, start the engine let it run for a few minutes then one by one slacken the fuel injection pipe at the injector, the engines should immediately run rough. If the engine behaviour does not change you have probably found a duff injector. Tighten the injector pipe again and repeat all the way along.
Click to expand...

i do have a tacho, and it did work after having the alternator recon'd but then stopped working again. The alternator is charging ok so just havnt got around to sorting that yet.
I think i may have a slight misfire at a certain rpm, not at idle but as i bring the revs up. iv ordered the tool to remove the injectors and have them tested. hoping the pump itself is ok as its not so easy to remove.
 
julians said:
This android app works remarkably well as a tacho as long as you configure it correctly for your engine

Engine RPM – Apps on Google Play

It listens to the engine note , for the individual firing pulses and displays rpm
Click to expand...

Never thought of that! Will give it a go! The under load WOT is def much lower than in neutral, im missing 1000rpm or more so easy to tell but i do need to sort the tacho.
 
Is the fuel in the tank of unknown provenance or is it good clean fresh fuel?

Could there be any air leaks in the fuel system, perhaps sucking air into the fuel lines under high load?

are the fuel filters the correct type /clean? ie could they be restricting the fuel flow under high load?

is the air filter on the engine clean/new? could it be restricting air flow under high load?
 
Last edited:
julians said:
Is the fuel in the tank of unknown provenance or is it good clean fresh fuel?

Could there be any air leaks in the fuel system, perhaps sucking air into the fuel lines under high load?

are the fuel filters the correct type ? ie could they be restricting the fuel flow under high load?

is the air filter on the engine clean/new? could it be restricting air flow under high load?
Click to expand...

fuel tank is clean, as i had a good look when fitting a new sender unit. theres 300l of fresh diesel in it.

I did find a kinked fuel hard line and have now repiped the supply to the engine using 10mm fuel line, thought id found the issue but no change. No air leaks as far as i can tell.

Fuel filter is new and is Recmar brand. Air filter is new and i removed it to rule it out for now. water catch is also new, and i bypassed it to rule it out with no change there either.
 
engine was fitted 20 years ago at a guess, have videos from previous owner of her flying at 25/30kts with the new engine in 2012. Props were changed by me as the originals had a few dings and bends. I changed like for like with new props
 
@springer i noticed a comment you made on another thread, about the raw water pump protruding after a rebuild. The raw water pump was rebuilt prior to the boat going in the water. Is there a specific clearance required? I did have to move the shaft so as the impeller would fully go in. There is a chance it may be too far in?
 
SPDauto said:
engine was fitted 20 years ago at a guess, have videos from previous owner of her flying at 25/30kts with the new engine in 2012. Props were changed by me as the originals had a few dings and bends. I changed like for like with new props
Click to expand...
I understood you had changed like for like, did you test with the old props before changing? The fact it fully revs out of gear but not in gear and you have vibration would lead me to suspect a drive train/prop issue.
 
they were changed like for like, minus the damage to the old props. New props are genuine volvo, exactly the same pitch as originals. Drive was visually checked over and all appears to be ok. Its not a consistant UJ vibration, more like being off a cylinder. I dont think i suspect the drivetrain at this stage.
 
You really need to check your boost pressure before anything else. A lot of these engine series suffer from a torque drop right where you estimate. That is when the turbo is supposed to start spooling up. Even minimal wear can delay the spinup and expose you to a torque drop and with a prop on the limit you just cant get those extra revs to spool up properly and keep climbing up the revs. There is a boost chart in the manual to show you what you can expect in the way of boost at your revs to compare against. I'd take a punt a few years plus plenty sitting idly has not been kind on the turbo / turbo bearings and that bthe prop was for max performance given it's a speedboat and as such you're just not developing the boost that goes on to push you through the flatspot.
 
Workshop Manuals are here

KAD Series

the files are callled KAD 42 but they cover your engine.

i agree with Bruce…I’d be looking air side. Boost leaks, intercooler needing a clean (although doubt that would stop you at 2.5k rpm unless really bad) or a combo of issues.

defo test the injectors as Kashust said too
 
oh, your turbo shouldnt have a wastegate, (belongs on the 31) but if it does (the turbos on these series are each unique but all bloody interchangeable) then these can often fail open which would dump spoolup pressure
 
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