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post #1 of 6 Old 08-25-2010 Thread Starter
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rpms quiz

Looking for more theories on the symptoms in my 2QM20 (79' C&C 34). I occasionally have a dip in rpm's or throttle when under way. It never gets worse once the symptoms have occurred and I have noticed the last couple of years. This year, I've changed out the fuel lift pump and have fresh, clean fuel and filters. I have no fuel leaks. The head and one cylinder was rebuilt in 2006. I have minimal white/grey smoke, which get more intense under high rpms/load. I use FPPF in my fuel. run a 3-blade feathering kiwi prop.

So, might I have a small pin hole air leak somewhere (where?) that "goes away" once engine heats up? I haven't checked the fuel pick up line yet in the tank--is it possible to create a small air leak if the tank is less than 1/2 full? could a cranky blade on my prop alter rpms like this? It feels like a short fuel shortage, but the problem never gets worse.. driving me crazy trying to diagnose.
thanks for the ideas.
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post #2 of 6 Old 08-25-2010
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Evaluation of 'fuel starvation" symptoms:

Apply a vacuum gage somewhere between the lift pump and the 'racor' filter(s). Most racor filters already have a gage port to do this.

Start the engine and watch the gage during constant rpm normal operation. Trial 'should' be at 80% of maximum engine rpm, with normal LOAD (prop turning) on the engine.

If the engine then stalls and the vacuum reading *increases* there IS a fuel restriction such as collapsed filter, undersized filters, totally 'completed' /clogged filter, obstruction on the 'diptube' (blocked screen on the dip tube), kinked fuel delivery tube, blocked tank vent, etc. etc.
If the engine stalls and there is *no increase of vacuum* (gage stays steady) then look for blocked water injection nozzle, in the exhaust system, blocked exhaust system, etc.
If the engine stalls and there is a *DROP of vacuum* ... you have an air leak in the fuel delivery system: pin hole in the lift pump diaphragm, loose compression fitting, etc. (Note: compression fittings 'should' be trimmed back each and every time you open one ... that 'ferrule' in a compression fitting compresses/deforms the copper tube permanently).

BTW
To correctly monitor the RACOR filter(s) via use of a vacuum gage, go to Parker - DIESEL FUEL FILTRATION, choose the appropriate filter type .... and find the proper technical FLOW vs. ∆P CURVE for your specific filter.
You are looking for a graph that lists Gallon Per Minute/hour VERSUS 'differential pressure (vacuum)' psid.
You change the racor when the indicated vacuum, with the engine running at 80% of wide open throttle (WOT), is at 80% or rated maximum flow demand of YOUR ENGINE ... so you have to look up the specific engine data for the expected flow, etc.
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post #3 of 6 Old 08-25-2010
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When you were working on the engine, did you replace the crush washers on the fuel line banjo bolts? If not, they're likely to be a possible cause of intermittent air leaks. The crush washers are not designed to be re-used, and if re-used may not seal properly.

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post #4 of 6 Old 08-25-2010 Thread Starter
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Thx. Appreciate the new ideas and will check it out once this nor eastah winds down here in nh
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post #5 of 6 Old 09-11-2010 Thread Starter
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think i solved it..

The best things about diesels is that even the seemingly most complex issues always prove themselves out to be a basic malfunction of the operating principles. After many iterations of troubleshooting, I discovered the hose clamp on the fuel line feed from my fuel tank was partially loose---I was able to tighten it 3-4 more turns. Hard to believe this could have let enough air into the feed to create the symptoms I experienced, but since I made this adjustment, the issue has gone away (knock wood 2x). Have faith in your trusty diesel engine...
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post #6 of 6 Old 09-11-2010
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If the hose to tube connection from the tank was partly loose, the following 'may' have been acting:
The fuel delivery line is operating under vacuum, a VERY small amount of air (cc/HOUR, etc.) is drawn in, an air bubble slowly builds up in a 'high' spot in the line (top/head of fuel filter, etc.) until it partly takes up the majority of space in the diameter of the line, the 'bubble' purges downstream (called an 'air slug') which passes all the way through the system. Yanmars being 'easy to bleed', especially can handle small 'slugs' ... and you only get noticeable drop in rpm.
The design problem is that fuel delivery systems on small marine engines are 1. vacuum motive (larger marine engine are usually pressure motive), 2. small marine engines use very poor 'connectors' that can easily 'suck air'. The standard 'compression fittings' that are used are in actuality 'one time tighten only' fittings; if you open them you should remove the 'ferrule' and 'trim back' the copper so that the ferrule fits over 'uncompressed' copper. This vacuum motive configuration allows a boat builder to quickly and cheaply install the supply lines TO the engine - 'quick and dirty'.

The best in marine design is to have a fuel pump AT or IN the tank, the delivery line PRESSURIZED, the connections of the delivery lines 'double flared' or 'ring seal' connectors. The downside is that with a pressure system if there develops a leak one will fill the bilge, etc. with oil - and if the oil is pumped overboard you can encounter heavy fines, etc.
So, for a bombproof fuel delivery system on a small boat ... change out all the copper lines to stainless steel using *double flared connectors*; and, put a 12vdc pump just after the tank.
BTW - such a system will allow the filters to be more efficient in total capacity of 'crud' removed before the filters become plugged.
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