Hi All. What’s been happening is the port engine randomly looses RPMs while at cruising speeds. It doesn’t shut down just drops. I throttle back both engines and wait a few seconds and then power back up. Had the fuel filters changed and no water seen. Today something strange happened. Boat was running fine and I hit another boats wake. The RPMs dropped. Any thoughts? Fuel pump? This never happens at low speeds. Thanks. 2002 41 PC with Volvo Tamd 74 engines.
Engine loosing RPM
- Thread starter jlazzfp
- Start date
If this is happening when you go over a wake then to me it seems you may have a fuel delivery issue and some crud / diesel bug temporarily jamming the intake feed of the tank. When you back off it drops whatever.
A complete aside- a colleague recently had similar. Turned out it was paper towel. Some people with twin tanks dont like crossing the salon with diesel dripping on their teak so stuff a bit of paper towel in the hose nozzle. The attendant is supposed to take it out, doesn't always happen and tissue doesnt degrade in fuel.
Another got diesel bug that formed a jelly string in his fuel lines. With enough suction it would compress and jam the flow sufficiently that he'd draw a bit of air through the secondary fuel filters. Not enough to stall the engine but enough to cause revs to peel off until it self bled the air out. The jelly string behaved erratically as a partial blockage which delayed diagnosis.
Worth checking before you start throwing money looking for non existent engine failures.
A complete aside- a colleague recently had similar. Turned out it was paper towel. Some people with twin tanks dont like crossing the salon with diesel dripping on their teak so stuff a bit of paper towel in the hose nozzle. The attendant is supposed to take it out, doesn't always happen and tissue doesnt degrade in fuel.
Another got diesel bug that formed a jelly string in his fuel lines. With enough suction it would compress and jam the flow sufficiently that he'd draw a bit of air through the secondary fuel filters. Not enough to stall the engine but enough to cause revs to peel off until it self bled the air out. The jelly string behaved erratically as a partial blockage which delayed diagnosis.
Worth checking before you start throwing money looking for non existent engine failures.
Divergirl
Member
Interesting jfazzfp…...as you know we have the same boat, same model, same year, same engines. I am having this EXACT issue on the SB engine. What the heck? Had her hauled for the high water alarm issue (as I mentioned in another post) and as I was cruising south on the ocean, down to FLL for haul out....the RPMs on SB would drop for a minute or so and then speed back up to that of the port. Did it several times and has been for a while now. I notice when I leave my dock that the port engine seems to always have more "spirit" than SB. SB was the engine for the safety recall wiring harness years ago. I think yours was port, was it not?
After 5 weeks in a marina waiting for my wiring harness to come the dealer noticed that I did not need the recall since I have split throttle and transmission levers not the single controls. I don’t know which ones you have. I agree with Bruce that this is an issue with engines getting starved of fuel. At low rpm they are fine. My mechanic is going down again to check the fuel filters. There may be crud build up due to bacteria in diesel fuel. Not sure. Hope to find the issue before I haul out this year. Let me know what your mechanic finds! Thanks.
A common fault with these engines that may cause this type of issue is the boost pressure sensor. There is a pipe that leads to it that can fill with condensation. You can drain the water via a cap, shape of a bolt, at the end of the said pipe, just below the edc unit. Try opening it and let the condensation drain out. Leave it open overnight or a couple of days if time allows but often the ECU diaphragm switch corrodes and need replacing.
Yes but as it has an impact on fuel metering and if it's sending the wrong signals your fueling will be out. It's the only sensor in the system that does not give a fault code if it fails.
Divergirl
Member
I have not started to diagnose this problem yet. As you know, I have had a bunch of crazy problems with this SB engine. VP actually replaced the computer which appears to have resolved the engine kicking into high gear while in neutral & idle. I attack one problem at a time and my last problem was the high water alarm going off in the middle of the ocean and a bunch of water pooling under the shaft of the SB engine. That has now been resolved and so I plod on. Let me know PLEASE what your guy finds.
You do seem to be having more than your fair share of issues. My condolences. These are normally reliable and well thought of engines with few well documented failings. Turbo and wastegate being one but I dont think relevant in this scenario.
So!! I was having a problem with the port engine loosing power and dropping RPMs at times. I had my mechanic doing some over the winter work and this is what he finds! 
Im no mechanic but this is supposedly a fuel line crimped beneath my valve
cover. Early this summer I had my valves adjusted at a reputable a Volvo dealer
in New Jersey. I’m sure that’s not supposed to be like that!!! We will see if the repair
fixes the issue.
Im no mechanic but this is supposedly a fuel line crimped beneath my valve
cover. Early this summer I had my valves adjusted at a reputable a Volvo dealer
in New Jersey. I’m sure that’s not supposed to be like that!!! We will see if the repair
fixes the issue.
Divergirl
Member
Wow....that is interesting for sure. Keep us posted. My regular mechanic will be changing the oil, filters, etc. next week so, I will have him check the fuel lines. There is so much that you have to watch and pay attention to. I think that mechanics, like anything we all do for a living, get tunnel vision sometimes.
Divergirl
Member
Ok, Jlazzfp...here is what Martyn from Eagle Marine sent me. There are 2 attachments I will upload. One is the part, the "black box", and one is from Volvo Penta explaining why this part was issued as part of this
service campaign. ***It says the file is too big to upload onto this site so if you want to private message me your email I will forward to you.
service campaign. ***It says the file is too big to upload onto this site so if you want to private message me your email I will forward to you.
I tried to start the boat this weekend and the port engine stalled. Saw a little diesel in the bilge under the separators. Look at the pressure gauge on the filters and watched it drop. The mechanic was there so had him take a look. It was a loose hose from the fuel tank to the separators and was sucking air! Tightened everything down now it?s solid as a rock. I wonder how long this was happening. Gonna take her out for a test this week.
So! We used the boat 4 times without issue. No loss of rpms on the port side. Today the wife takes her friends out with a captain and of course the port engine lost rpms 4 times during their trip. He is also my friend and helps me maintain my boat so he knows of the issue. He said the water was rough and was bouncing around a lot when it happened. On the way back the water was flat and no issue at all. He believes it’s electronic and not fuel related. When it’s running well it’s solid. I’m not sure where to look next. Loose connection somewhere? Also looking for a good mechanic n the New York / New Jersey area recommendations are welcome! Thanks.
