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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Old 02-22-2012
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Measuring Engine Exhaust Back Pressure

I have a fairly new rebuilt Yanmar 3GM30F and would like to know how to measure (I assume at the exhaust elbow) and would be the acceptable values of back pressure on this engine..
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Old 02-22-2012
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To measure it you'd need a tapping point and a gauge - If your elbow has a threaded fitting and a plug that'd be easy enough.. otherwise you'd have to add one.

Why are you concerned? if the RW in the lift muffler is being ejected properly and the motor attains revs does it matter?
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Old 02-22-2012
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I would expect that a marine application with a water lift muffler would have higher values than the ideal automotive application that's probably listed in the engine manual.

On the other hand, if one is experiencing low power, poor fuel economy, black exhaust, checking for a restriction in the exhaust system would help diagnose the problem. If that is a problem, it's most likely right at the exhaust mixing elbow, which can usually be removed and inspected without too much trouble.
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The concern was brought to my attention from the engine re-manufacturer as to a possible "over fueling" issue due to possible inappropriate back pressure. The system was never tested thus the question as to what are the appropriate back pressure values....
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If your aqualift has a drain plug /valve to prevent back flow to engine if something goes bad with aunti siphon (mine's on a solenoid) it would be a good place to try a pressure gauge.The numbers might mean something to your agent. Many engines work hard simply because the breathing out hose is too small.
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The concern was brought to my attention from the engine re-manufacturer as to a possible "over fueling" issue due to possible inappropriate back pressure.
how is this condition manifesting itself? How long has the problem been going on? Is the exhaust system different from the original set up? I would have some concerns that the engine re-manufacturer is diverting you attention elsewhere instead of addressing a possible problem with their motor.

FWIW, the 36 HP diesel on my boat sits pretty close to the waterline level and has to push the exhaust/water mix through approximately 15 feet of 1.5 inch ID hose. The motor is old and tired, but even this amount of lift and length in the exhaust isn't causing me to put out black smoke.
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Last edited by erps; 02-22-2012 at 08:59 PM.
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Just take the exhaust hose off of the elbow and look inside (the elbow; you may have to use a mirror), or stick your finger up inside and try to gauge how much build up there is. If the carbon build up looks like it's occluding more than 25%, or so, of the inside of the elbow then it's time to replace it.

I recently changed the elbow in my 2GM20. Not because the exhaust gases were being restricted, but because the water injection point (at the top of the elbow) had become so corroded and carboned up that the cooling water wouldn't flow through the system. (Hint: if you have the "U" type elbow, the hardest part of the whole job is getting the elbow/coupling/manifold to come apart. I let a diesel shop take care of that chore.)
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The exhaust elbow is a violent place when the motor is running.
The exhaust will be pulsing like crazy and any guage on there will be fluctuation wildly, methinks.
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I agree with "erps" in that your engine manufacturer is miss leading you from an internal engine problem and is blameing it on the exhaust. If your engine is getting too much fuel it will be pushing black smoke and it will be down on power. Your exhaust system has nothing to do with this.
As "Rockter" pointed out, measuring the exhaust pressure will be impossible as the exhaust pressure will fluctuate as water occasionally gets pushed out the tail pipe causeing the back pressure to increase momentarily.
If the engine is over fueling first check out the intake and it's filtre. Second, relace all fuel filtres. Third, check the fuel pressure to make sure it's within spec. If all of this checks out as good then I would suspect it is a bad injector or maybe three bad injectors.
Also you need to look at the condition of your fuel. Algea can grow in your fuel tank. This can cause serious issues with your fuel systems injectors and filtres.
My boat has a 3GM30F that was rebuilt 500 hrs ago. I swear by these engines and have never had a problem with them. I have used similar Yanmars in some of my equipment at work and they are a stellar engine.
Maintinance is key but I have seen over 10,000 hrs. put on these engines and they still fire up and run great all day long in some of the worst conditions.
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