Yanmar 4JH-TE RPM Issue

[msulc]

Hi, I was hoping someone might be able to point me to some things I could try to resolve an issue that has started with our engine. A lot of things have changed (new fuel tanks and associated fittings, new turbo charger, heat exchanger, tachometer). Our max rpm while underway has gone down from about 3300 to 3000 and wondering if anybody has any suggestions on what I should check. Here’s some detailed background…
The boat is an ’88 Tayana 42. Then engine has about 4300 hours on it and is an ‘88. We bought the boat about 3 years ago. At the time, the max rpms were about 2800, but the max prop was re-pitched and the max RPMs became about 3300 (Yanmar specs).
We recently noticed the RPM decline but are unsure if it was a gradual or sudden decrease, but suspect it started the last few months. We left Annapolis, MD in November and headed south and currently we are close to Georgetown in the Bahamas.
We recently replaced our fuel tanks in October due to a fuel leak from rust. The tanks on each side go to a valve which selects where the fuel comes from. From here, it goes to an Algae-X unit, then the Racor 500 Filter, and then an older Racor fuel pressure gauge (which seems partially broken). From here it goes to the secondary fuel filter and pump.
The fuel pressure gauge seems to be partially broken. It stays at 0, and only really seems to move if I cut off the fuel supply, and then it moves to red. So I don’t think checking this really seems to tell me much about if the fuel has any blockages to that point.
I had different fuel in both sides (one tank filled at Nassau and the other at Riviera Beach) and neither seemed to improve the performance. I also changed the fuel filters without any difference. The prop and shaft are clean and the bottom is fairly clean. In neutral, I can turn the shaft either way without a lot of effort.
We changed the heat exchanger after the RPM issue was discovered because the engine seemed to be overheating at 2800 rpms (which was our typical cruising rpms) and had always seemed to have an overheating issue anyway (2900 would start to overheat the engine). The overheating issue seems to now be resolved up to our max RPMS at 3000. The temperature stays at just over 80C.
The RPMs in neutral are 3600.
The oil was changed about a month ago. Our oil pressure has always been pretty high, and I don’t know if this is an issue, but it didn’t seem to be an issue for RPMs before.
Along with the new tanks, new hoses were added from the tanks to the fuel supply. I tried to spray some lightly bubbly water on the fittings and nothing really seemed to be bubbling, so I don’t think there is air getting into the fuel system. I had someone pretty knowledgeable over listening to the engine in neutral at all rpms and they did not hear any obvious noises that seemed to relate to an issue. We did not take the boat out however.
The boat seems to run fine up to the 2900, but as we add more throttle the boat does not increase in RPMs. However, the smoke seems to be coming out a little black at this point (not a ton of black smoke) and it seems like soot was coming out into the water.
Along with the new tanks, we had a new coupler added because the people installing the new tanks lost the old coupler. The prop seems to turn fine. There does not seem to be a large amount of vibrations or anything.
During the tank installation, we also put on a new turbo charger. The exhaust was leaking because we couldn’t tighten the studs (the old ones were turning in place and we used a helicoil for the holes to fix this) and so we took the opportunity to put in a new one. I believe the turbo charger was working fine based on what the mechanic said but not really sure how to check that.
I also replaced the tachometer in November because the old one stopped working. The speed of our boat at different RPMs seems to be about the right speed for the RPMs, so I think that the gauge is showing correct RPMS (2900 is a little over 6 knots, which is what we had before).
That’s about all I can think of… Does anybody have any ideas that I might check?
I know there is a boat diesel forum but not sure if I should pay for that or not as I can’t really see the quality of the responses…

[Charlie]

Two things you didn't mention. Is there more growth on the bottom of the boat now? Also is there growth on the prop? Either of these could cause a loss of RPM's.

Not sure about this but if you had a leak in your turbo could that have caused a decrease in your power to the point where replacing the turbo has made it so your prop is no longer pitched correctly?

[bstreep]

We have pretty much the same engine. 75hp turbo charged.

The symptoms you have point to a problem we had 6 months ago: no turbo boost. I pulled our turbo charger, cleaned it (it was completely packed in carbon, not turning), and reinstalled. Now, it works better than ever.

You should be able to pull the air screen/cleaner off of your intake. Remove the whole thing, not just the cover. CAREFULLY see if the turbo spins freely (don't damage the fins - if they are stuck, remove the turbo for cleaning or repair). Then, let us know.

[msulc]

Thanks for the responses. The prop is clean and the bottom is pretty clean. Some very, very minor growth, I just cleaned it a couple of weeks ago and not much is there.
I was going to pull off what I could to get to the fins in the turbo charger. That will have to wait until our visitors leave so it will be a week or two unless I come up with anything else before then. The turbo charger is brand new, but I guess you never know...

[djmarchand]

Read this paper written by a SoCal marine diesel dealer: Seaboard Marine - Tony's Tips | Understanding Low Power Troubleshooting

In addition to these tips, check the exhaust injection elbow which is prone to fouling and plugging up on old Yanmars. It might also be the root cause of your overheating problems.

And if you still can't figure out your problem, post on boatdiesel.com. There are some real pros that hang out there.

David

[bstreep]

Pulling the air cleaner should take just a couple of minutes: 3 hold down clamps (typical of air cleaners), then a hose clamp to remove the bottom from the intake. That should give you direct access to the exhaust side of the turbo.

[msulc]

Thanks again for the replies. I took off the air filter today and spun the turbo and it spins freely.
I also pulled the exhaust injection elbow and that is pretty clean from what I can tell. I don't think the issue is there.

I checked the web link for Seaboard marine. My prop pitch has not changed as far as I can tell, I think the turbo is ok and I think the air going into it is fine.

Any other ideas?

As a side note, I now have an issue with leaking exhaust where the mixing elbow connects to the turbo charger. I took the mixing elbow off of the turbo chargers, cleaned both the mixing elbow and turbo charger with a paper towel, changed the gasket with a fresh one, and tightened the bolts. I couldn't get them tight enough to stop the leaking. I also put the old one back on and tried it too. Is there any tricks to putting the gasket on or getting the bolts tight enough? I tried to get the mixing elbow stud holes centered in the stud before tightening.

AIR issues: Not getting enough air can be caused by a dirty/collapsed air cleaner, a leaking turbo hose, a filthy aftercooler or air restriction inside the aftercooler, a worn out or partially stuck turbo, or a poorly ventilated engine room. Any combo of these can restrict air to the engine causing dark smoke and further, low power.