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Salty Possum · 27-08-2019, 16:04

Detected the problem the first trip out into the bay after an offshore sail. Scraped prop. Changed Racor and the engine fuel filters which looked clean. About 1 tsp of water with a few particulates in the Racor. Sampled fuel off the bottom of the tank which looks clean. Washed air filter which didn’t look so clean but not too bad. No resolution of the problem.
Thought I had a bad fuel pump because I pumped the lever forever without seeing a flow and ended up using the bulb installed between the Racor and the engine to bleed. Opened up the fuel pump and there was a loose broken spring in it from one of the valves. Decided to replace the fuel pump with a new one. Rebled engine. No change. This time when bleeding the engine I never got any fuel flow on the high pressure side at the cylinders so I just went ahead and cranked it rather than forcing it with the manual bulb. It cranked fine. It always cranks fine. Cracked fuel input to each cylinder individually and it dropped rpm equally on each cylinder telling me that it is firing on all cylinders.

What it is doing: Where before it would make 3400 rpm full throttle under a load, now it will not go past 2000-2200 and it starts clattering at that speed. In neutral it will make about 2500 rpm and then begin clattering. Up to 2000 rpm it sounds and operates normally as best as I can tell, I get the same speed at 1800-2000 rpm now that I alway did.

1980 hours on a rebuilt engine. 180 of them under my ownership over the last year. Very few of them in the last 8 months. Was in the Caribbean for two year and the previous owner indicated he ran it to charge a lot. I had an oil analysis done when I bought the boat which didn’t reveal anything in particular.

I checked for blowback and don’t feel air flow.
Water output looks same as always. No unusual amount of smoke. Any ideas? Huge waiting list for a mechanic.

admiralslater · 27-08-2019, 16:07

I had a problem with a valve on the fuel return line once . It really effected the way it ran

DeepFrz · 27-08-2019, 17:08

Check to see if there is a screen on the end of the pickup tube. If there is remove it, stomp on it, give it to someone you hate and then see if it runs better. Just a SWAG.

geoleo · 27-08-2019, 17:11

Take the valve cover off and inspect the valves and clearances--duh

Salty Possum · 27-08-2019, 17:21

Thanks for the ideas!

TheOffice · 28-08-2019, 05:06

Blocked exhaust elbow.

**Wotname** · 28-08-2019, 06:45

Another vote for checking the mixing point and the exhaust elbow.

Scubaseas · 28-08-2019, 09:11

X3 on mixing elbow/exhaust restriction.

Howler · 28-08-2019, 09:30

Agree on elbow/exhaust, but don't discard DeepFrz's comment on the pickup tube. Easy to inspect and replace and I've seen this as root cause of low RPMs too.

Scubaseas · 28-08-2019, 09:54

You can measure back pressure on the exhaust at the mixing elbow. Shut of raw water, remove hose to mixing valve, install pressure gauge (0 to 5 PSI) on mixing elbow, start engine and rev to 3K or as high as possible for about 5-10 seconds. Do not run longer as you have no water cooling the exhaust. If you get anything more than about 1 to 1.5 psi at wide open throttle your exhaust is clogged. If the previous owner did a lot of idle or off idle charging you probably have exhaust stacking (aka slobbering) which is carbon build up in the exhaust. Like having clogged arteries. This should have come on gradually. If your power loss was sudden you might have an injector issue, injector pump issue or fuel supply issue.

DNK · 28-08-2019, 16:01

This is exactly what happened to me. My Yanmar wouldn’t get over 2100 rpm. After a lot of $$ and frustration I finally found a mechanic which had the smarts to put a vacuum gage on the Racor to see what the fuel pump was doing. It registered a very high vacuum showing a blockage in the piping to the Racor. Cleared the mess and that was the end of the problem. I would suggest checking the vacuum on the fuel system between the filter and the tank. You can look up what it should be and see if yours is OK. This is a really good way to check different parts of the fuel system. Good luck!

kmcshane · 28-08-2019, 18:33

Mixing elbow x 4 (I carry a spare)
I added a Racor vacuum gauge (Ebay $44) takes the place of the T-lock.
Detects blockages between tank and through the filter.

Salty Possum · 01-09-2019, 07:58

Took the fuel hose off the connection to the tank and put it down into a bottle of diesel—no change. So it isn’t a clogged strainer in the tank. Took the mixing elbow off. Some restriction with black concretions and rust. I don’t know how much is too much so I just ordered a new one. We will see what happens when it is installed. My Racor is mounted where there is not adequate head room for the T-handle vacuum gauge but I think I may install an in-line gauge between the Racor and the engine. I have no prior experience with diesels but am used to having more information about my engines. An actual oil temperature guage would be nice as well.