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  #31  
Old 11-09-2021, 10:52 AM
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Join Date: Aug 2010
Location: Sacramento, CA
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Quote:
Originally Posted by Alex E View Post
Guys, could it be the MW injection pump itself? Some kind of cavitation?
Speaking as a mechanical engineer, cavitation is greatly misunderstood. It is liquid vaporizing temporarily due to a low pressure region, such as behind a spinning pump impellor. It quickly condenses back to liquid as the vapor bubbles reach a higher-pressure region (releasing energy in a small point to make noise and even erode metal). Cavitation does not form air bubbles.

There is a related issue where dissolved air can be drawn out of solution by applying a vacuum (or lower pressure region) to the liquid. That can draw air bubbles out of solution. But, there would be no excess dissolved air in the fuel drawn from the fuel tank. If the fuel tank was pressurized to say 1000 psig, it could have appreciable dissolved air, but unless testing liquid rocket engines or working in a chemical plant (me), you wouldn't run into that.

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  #32  
Old 11-09-2021, 11:02 AM
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Join Date: Aug 2010
Location: Sacramento, CA
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Originally Posted by Theseus View Post
1. Bosch injector pump service manuals have a section on plugging the fuel bypass and all ports. To check for air bubbles, it must be pressurized, then placed in an oil bath. This should show you if there is an opening outside. It is possible, however, that an o-ring could be faulty somewhere on the delivery valve body holder.
Just thinking out loud. Since the fuel supply to the injection pump runs ~12 psig, there shouldn't be any place in the fuel passages within the I.P. that is below atmospheric pressure to suck in air. Indeed, if there were bad seals, you should see fuel dribble out, though it could leak into the IP crankcase (flows to engine oil pan). But, if there was a blockage plus a bad seal, it might suck air on say a piston refill cycle. But the bad seal would also have to act like a check valve, allowing air to get sucked in but not fuel to dribble out. I would think that if the IP was susceptible to letting air into the system, this would be a known problem after 6 decades of Bosch IP use.

People here have long fretted over air in the injection system, but I haven't heard of anyone proving that causes a rough idle. If too much air, the pressure in the injection line will not reach 1950 psig to pop open the injector. Try loosening the nut at the top of each injector, one at a time, and verify a little fuel dribbles out with no puff of gas, and each one should slow and roughen the idle by about the same amount. If that proves OK, I would move on from worrying about the gas in the injection return hose and try other things like verifying the injectors with a pop tester, especially for good spray pattern, and verify injection timing, as both of those can greatly affect idle. If you ever drive out to CA, try the "renewable diesel" you can buy at Propel and Union76 stations. My engines idle much smoother on them, due to the high cetane rating (~75), so don't neglect your fuel quality.
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  #33  
Old 11-09-2021, 01:43 PM
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1984 300SD
 
Join Date: Jun 2021
Location: Murfreesboro, TN
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Its been on pause but will start back up. Had to get sunroof working and rear window seal replaced. I have the parts to replace seals on oil filter. Leak down test first. If that is fine, I will install injection pump next.

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Instragram: doss_project
1984 300SD - 930 Blue, OM 617
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Theseus owned a ship in Greek mythology, and when a part of the ship needed replacing, it was replaced. Eventually, everything was replaced. Is it the same ship? (I own the car of Theseus. )
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