Sucking Air
Not long ago I wrote a post where I described the vacuum gauges that I installed onto our Racor fuel filters. As a follow up, since that time I have now become an expert at bleeding air from the fuel system.
Unlike the gasoline system on our PDQ, any bit of air in the diesel system will stop it from functioning (I don’t know why there is such a difference but I’m sure someone will explain it to me). Anyway, I have always been aware of the necessity of sometimes having to bleed air out of diesel fuel systems and in fact, I have even been shown how to do it before. I’ve just never had to do it myself, especially on the Leopard. When the engine wouldn’t turn over after installing the new gauges, this was the first thing I checked. Of course, I had to consult the manual to determine the exact procedure.
Getting the air out of the system was simple enough and the engine turned over fine after that. The problem is, after stopping the engine and letting it sit for a while, I’d end up having to do it all over again. There was definitely a leak allowing air into the system and the obvious suspect was the new gauge that I installed (note that this only occurred on the port engine).
The installation instructions on the gauges did mention using some type of thread sealant (which I did not) but I don’t think they were too specific other than to say “do not use teflon tape.” I didn’t get around to addressing this problem right away as we had our guests on board and were constantly moving from place to place. As such, I just continued to bleed the air out each time before starting the engine, whether that be at anchor or under sail before coming into a harbor. Yeah, I got pretty good at the whole thing!
When I finally had some time to properly address the situation, I looked through the various sealants, etc. that I have on board and found a tube of pipe dope. I simply used a bit of it to help with the seal, both on the threads of the gauge itself and on the threaded coupler. Following this modification, I only had to bleed the air from the system one final time. It obviously did the trick as since then, all has been good (knock on wood).
Rather than being upset with this minor issue, I am quite happy for the new skill set that I’ve acquired. Let’s just hope that I don’t have to put it into use again in the very near future.
The things we learn “accidentally” usually stick with us longer than the things we learn in the more traditional ways. And THAT is yet another appealing aspect of sailing and cruising.
Agreed.
Diesel is injected into the cylinder at the top of the compression stroke – when there is a *LOT* of pressure in the cylinder.
Think of the injection pump like this: a cylinder with a solid rod for a piston. The cylinder, the pipe leading to the injector, and the injector are filled with diesel. Now, if you tap the end of the injection pump piston with a hammer, a spurt of diesel shoots out of the injector, pretty much regardless of the pressure in the combustion chamber… because liquids are essentially incompressible.
Now introduce an air bubble.
When you tap the end of the piston with your hammer, all that happens is that the air bubble is squeezed down. The impact of the hammer is absorbed in compressing the bubble and no diesel squirts into the combustion chamber.
Gasoline engines use either a carburetor (which can tolerate an unbelievable amount of air in the fuel feed because of the float tank), or a low pressure injection pump (that does not depend on the incompressibility of liquids to work).
Bob
s/v Eolian
Seattle
Thanks, Bob. Makes complete sense.
On a gas engine the fuel pump only needs to fill the bowl. On a diesel the fuel pump must raise the pressure to 5000-10000 psi to open the fuel valves; any air in the line and the pump cannot make the pressure, the valves don’t open, and you go… nowhere.
http://en.wikipedia.org/wiki/Injection_pump
In fact, the pump serves as both fuel supply and timing, like a distributor.
Those pumps are $$$. Hence the need for clean fuel.
I was confident that you’d come to my information rescue! 🙂
There’s a lot of debate about the advisability of using teflon tape in diesel fuel systems. The yellow teflon tape is made for gas, oil, propane and natural gas systems, and should work just fine in diesel fuel systems. Some people get frantic about pieces of tape entering the fuel system and clogging the fuel injection pump or an injector, but the only fittings where tape would be used should all be upstream of the filters, so that really shouldn’t ever occur. Personally, I’ve had no problem with yellow teflon tape on diesel fuel fittings. I just try to ensure that I hold the tape back two or three threads from the end of the fitting. But I’ve heard Permatex #3 is the best way to go.
I don’t know if I’ve ever seen yellow teflon tape. I definitely don’t have any.
I once owned a diesel VW. I discovered that a diesel will run off WD40. I had an air leak as well, I removed the air filter and with a friend spraying WD40 into the intake I started the engine and traced the clear fuel line back to the loose connection. Who knew!
I don’t think I’ll try that. 🙂
Mike & Rebecca,
Ceramic knives are coming down in price and are extremely sharp. I bought a complete knife set with sheaves for each knife for $25 on sale. Ceramic knives could be kept anywhere and will never rust and are guaranteed to keep a sharp edge.
I would love to have a bunch of them. I hear they are especially good for cutting the new hi-tech lines like Amsteel.