Engine RPM only reaching about 2200

[Franklin]

Note: RPM gauge not working.

I've been beating my brain out on this one. Two weeks ago I noticed that I could only go 5.5 knots when my cruising speed is more like 6+ and max 7 knots under power. I didn't have my knot meter in so thought maybe I was in current. Later I thought...nope...not in a current so then I started wondering if I forgot to put my zincs on last year and my prop was degrading or maybe my prop was covered in barnicles.

Last weekend I dove and saw that the zincs where still there and prop was good and didn't have many barnicles on it but I cleaned it anyway.

This weekend I could only get 4.5 knots and had the knot meter transducer in the water to verify I wasn't in a current and it sounded like the engine wouldn't increase RPM as I pushed the throtle after about halfway. So then I thought maybe my fuel filters (both primary and secondary) were clogged so I replaced them and yes, they were nasty. Figured I solved the problem while at anchor, but the trip home proved to be it to be wrong.

So, filters aren't the problem. Prop not the problem. Fuel supply not the problem.

Possibles I can think of: linkage from throtle to engine, bad fuel (I think the filllup before Saturday was bad fuel so 1/3 of my tank now may be bad) maybe partially clogged injectors. Other ideas?

 

[Maine Sail]

If

If the motor is otherwise running fine, not rough or smoking, it is probably not fuel or injectors. My initial gut reaction would be throttle cable. It is easy to check and would take less than three minutes to fix.

The other potential cause of this slow down could be a partially clogged exhaust elbow or growth on the hull. You said the prop was clean, but how is the hull??


Check hull make sure she's clean

Check throttle cable throw

Check exhaust elbow

Check air filter

Check engine shut off cable and make sure it is closing and opening fully..

 

[druid]

Hmm.... diesels run with air and fuel in the cylinders. The more fuel you push in, the more power they put out...until there's not enough air. So if you're not getting power (ie RPM) you're either not getting enough air or enough fuel. (BTW: does it go up to full RRM in neutral? If not, it's the throttle cable!)

Now, if it's not getting enough air (which also includes blockage in the exhaust), you'd most likely get smoke due to incomplete combustion. Not getting smoke? probably not air.

Which leaves fuel, which is the most common problem. You say your filters were pretty nasty, which implies maybe some of that Nasty (often water) has gotten past them and into the high-pressure pump. Bad News - HP pumps are hard to repair and expensive. Unfortunately that sounds like the problem to me.

I'd check the obvious things, like blockage in the air intake or exhaust, how freely the prop turns by hand in neutral (checking stuffingbox and cutlass bearing for binding...), and if the engine goes to full RPM in neutral. A "hand-tach" is an excellent tool! Then look to the HP pump.

druid
www.druiddiesels.ca

 

[Franklin]

ah....didn't think about the exhaust elbow but I think Druid is right and that it is the HP pump. I had a feeling the fuel was bad when I had to replace the fuel filter right after the fillup in Dec. I mean...I didn't even get 7 miles before it clogged and it wasn't that old. Hard to swallow that an engine with only 250 hours needs a new HP Pump.

 

[sailaway78]

I would certainly check

The easier things before going after the HP pump. The cable linkage is the easiest go check and go for that first. While not nearly as easy, the exhaust elbow is certainly easier to check, and clean than the HP pump. So be sure to do those before you go after the HP pump. From the way your saying, it is getting worse, fast. Good luck, and keep us posted.

 

[CalebD]

I was going to suggest exhaust system back pressure as a possible culprit but others have already beat me to it.
Franklin,
Are you still in your Hunter 376? Last I remember hearing from you the insurance company was going to 'total' your boat and you were considering other boats. I guess I missed something.
Good luck with your engine.

 

[Rick]

Franklin, you can solve the throttle cable question at the dock. In neutral, see what RPM you can get. If she revs up to 3300 + or - 100, its not the cable. If she will only give ya 2200 rpm, and your getting plenty of fuel, you should see some smoke, probably black or unburnt fuel. Most of the time if it is the manual pump, you should get some sort of roll back. Keep in mind, mechanical pumps are widgets, they pump.... they work... they fail. Usually a leak at the diaphram is a good indicator. Its probably your linkage. The exhaust elbow, when clogged, usually gives some visual indicators (smoke).

Good luck

 

[Franklin]

Same boat. I decided to buy it back from the insurance company and fix it myself. I'm 70% done with the outside. Just need to smooth out a second section for spraying gelcoat and then do another 3' section (that's the one that worries me...it's the section the chain plate is on which will need to be removed).

I would have been done by now but harbor master told me I'm not allowed to do that kind of work in the marina

 

[Jack Tar]

Diesel exaust gasses condense in the exaust system. I have had the same problem on smaller diesels. Ill wager when your all done it is an exaust blockage. Elbowsare the usual place.

 

[Hermit Scott]

I'd bet you a dollar the throttle cable has slipped somewhere in the linkage.

 

[Benny]

Franklin, barring bad fuel, it looks like a clogged exaust elbow. Is the engine mising, is it smoking?

 

[Steve Dion]

Franklin: I would also go for the exhaust elbow. This is a good thing to check since your issue is very symptomatic of this problem.

Be sure to have a new exhaust gasket on hand in case the elbow is okay, they are usually shot once you remove the manifold.

 

[sailndive345]

... I would have been done by now but harbor master told me I'm not allowed to do that kind of work in the marina

Franklin,

Which marina are you at? I used to sail in Galveston Bay - in fact I was the third paying customer at "Lafayette Landing" back in the 80s - I think now it is Kemah marina or something..) Dry sailed my J-22 out of Seabrook shipyard for a few years.

Reminds me of many memories.... Good luck with the glass work!

Sailndive

 

[sailndive345]

I imagine you have already checked for this but;

A friend of mines Catalina 32 showed very similar symptoms you described following some yard work: Turned out these folks installed the air intake so that it was almost completely blocked, starving the engine of air. In his case, it was a very simple fix - I think Druid's list is very comprehensive and it probably would be worthwhile looking over all the simpler and less costly stuff before going for the HP pump...

Best of luck!

Sailndive

 

[gmeyerga]

When I had exactly the same symptoms it turned out I had
bled the system carelessly and ended up with an air bubble
in the secondary filter. The engine ran normally up to midrange
rpm, but did not make it beyond that. The bubble apparently
restricted the fuel flow.
I expect a bubble in the primary filter can cause the same
problem.

 

[Hermit Scott]

When you guys are talking about a clogged exaust elbow, is it clogged from dried and baked salt that fills up the elbow only leaving a small hole for exaust to get through?

 

[Don12364]

Fuel Bug & Pickup Tube Strainer??

Hi Franklin - fellow 376 owner,
Have you still got the little strainer on the bottom of the fuel pickup tube inside the tank?
Mine clogged solid the first time I had fuel bug so I took it off in a fit of pique.
Next time the fuel bug arrived the whole fuel line blocked solid right up to the filter in the engine compartment and I had to take it all out of the boat and take it home and blow it out on the airline.
My fuel bug stops the engine exactly 3 days after filling up with dodgy motion lotion. Now I use 'Fuel Set' without fail at every fill up and have had no problems since.
BTW the symptoms of the bug are exactly as yours and get worse with time the revs dropping lower and lower.
Seems there is no proper answer other than fitting an in line strainer just as the pipe exits the tank with the facility to blow back any muck into the tank as a 'get you home' measure.

 

[Rick]

Re: Fuel Bug & Pickup Tube Strainer??

Hermit, yes we are. Dried salt, calcium, exhaust particles, heck whatever is in the recipies for diesel, and salt water. All that "gook" doesnt like to make a 180 turn, but beats having your boat sink.

Cheers

 

[Ctskip]

Re: Fuel Bug & Pickup Tube Strainer??

I'd go to the throttle cable. I just went thru this. I found a kink in the cable on the handle side of the cable. How it got there? I'll never know.

Keep it up,
Ctskip

 

[Franklin]

I guess I forgot to come back to this thread.

Sailndive : I'm in Kemah Boardwalk Marina now. There are some good things about this marina, mainly I have a lot of friends there. Management, even with changes, seems to be lacking, but I don't think I've ever met somebody who had been at their marina for more than a couple of years and love it. I wonder why there just doesn't seem to be a perfect marina anywhere....

I checked the Elbow and it's not clogged. The linkage wasn't the problem because I could manually disconnect it and move the arm on the pump and it didn't increase RPMs after about halfway.

I didn't think about the line having an obstruction limiting fuel. One thing I like about my setup is that the fuel can gravity drain all the way to the pump which comes in handy when I want to get some diesel out of the tank. When I changed the filters, it did seem that it was draining slower than normal but I didn't think the engine could use more fuel then what was draining but I could be wrong there. I'll try to clear the line.

Thanks all.