Return to Service

Rather than spend the weekend playing in the snow with Boo, we both spent our respective weekends in the valley.  Today's post will be more technical than usual, since I spent all weekend on the bus.  Consider yourself warned :)

Glow Plugging Away
The glow plug is a mystery to non-diesel folks.  I know it was to me, when I first got into diesels.  I figured it was just a diesel version of a spark plug with some fancy lingo around it to confuse people.  Actually, it is core to the difference between a diesel and gas engine.  A gas engine depends on a spark to create a small explosion in the combustion chamber (by setting the vapor on fire), creating force to drive the piston down, and create movement.  A diesel engine depends on compression for this function.  This sounds a little strange to the newly inducted, I know.  Now, gas engines need compression too, but not nearly as much.  In fact, when you start losing compression in a gas engine, you suffer "blow-by", but the engine will still run, while you slowly start polluting your oil with un-burnt gasoline.  An early indicator is a smell of burnt oil (rings going bad) or your dipstick smelling like gas. Once a diesel engine's compression falls too low, or should I say once the pressure created within the combustion chamber falls below a certain level, the engine won't fire at all.  So, enter some chemistry....

Pressure is a function of temperature and the vapor contents.  The presence of water in air, for example, reduces the vapor's ability to compress.  A low temperature of the vapor increases its ability to compress, or should I say decreases the amount of pressure created within a cylinder.  If the temperature remains sufficiently high and constant, pressure is equally high and consistent for a static vapor make-up (atomized diesel fuel, cooking oil, bio-diesel, eg).

glow plug image borrowed from
autoengines4842.blogspot.com

This is where the glow plugs fit into this picture.  They are little combustion chamber warmers, but hardy enough to withstand the pressure and heat of an active engine. Once the ambient temperature falls below about 40*F, the pressure created within the combustion chamber is insufficient to create an ample burn for the engine to run consistently.  As the temperature drops from there, the engine becomes, eventually, inoperable.  The glow plugs create the heat necessary to warm the chamber so the engine can start and run until the combustion warms the chambers directly.  Then, they shut off.

With this "science" in mind, we can better understand why it was so important for me to get the borrowed harness (without a glow plug circuit) replaced with a fully functioning harness.  I completed that in my last post.  On Saturday, I discovered that 3 out of 4 plugs in my engine were bad.  I had lots of old plugs, but only one good one, so I popped it in and hoped for the best.  Since it was over 40* in my garage, I wasn't too worried about the plugs.  Turns out I was right, though starting on the mountain may be interesting if I don't have 4 operable plugs.

Okay, So Why Won't It Start?
Aside from pressure, a diesel engine needs something pressure-combustible within the vapor.  As I indicated earlier, this ranges from diesel fuel, home heating oil, bio-diesel and vegetable cooking oil.  I've heard of other alternatives, but I'd stay with those 4 (and home heating oil only if you aren't afraid of getting ticketed in the US - its a road tax aversion thing).  So, I have temperature, I have compression, so I must be missing the fuel.  Since I had the entire system open, I probably failed to get all the air out of the fuel system.  I attack this in 3 steps: priming the lines to/from the filter, priming the pump, and priming the injectors.

Priming the Lines
With a Mity-Vac, simply apply vacuum to the fuel line that runs from the big fuel filter to the Injection Pump (IP).  Block the return line from the pump to the filter.  The filter is large and holds a considerable amount of fuel, so you can find yourself second-guessing whether you have it right.  I have a small clear filter between the tank and the main filter so I can see fuel enter and move through.  This little $1 filter extends the life of the spendy stock filter too, so I encourage others doing this too.  Once you have fuel collecting at the Mity-Vac bottle, connect the IP feed line back to the IP and start priming the pump.

Priming the Pump

IP image borrowed from
Dieselgeek.com

At the pump, remove the little return line which comes from the injectors.  Apply vacuum with your Mity-Vac to that little nipple until fuel collects in the bottle.  Do this carefully, with low-pressure vacuum.  You don't want to damage any pump innards (read: rubber seals) by applying too much vacuum at once.  I found keeping it under 5 pounds was sufficient.  Once you have fuel in the bottle, re-connect all the lines and move on to priming the injectors.

Prime the Injectors
Now, you have fuel at the pump and the pump is full of fuel.  All that's left is making sure there's no air in the hard and return lines between the injectors and the IP.  With a wrench, loosen the nut at the end of the hardlines where they connect to the injector.  Do this for all 4.  I completely loosened mine, but just "cracking" it should suffice.  Now, start cranking the engine.  Keep cranking until you see diesel fuel start to pop out of the loosened injector connections.  Once all 4 are popping, you're air-free.  Tighten it all back down and the engine will start.  Mine did.

I took a test drive over to Boo's house to prove out all of my handiwork.  This is a few miles, including a highway on-ramp/off-ramp and some city-ish traffic driving.  The new tires felt fine, and I couldn't even hear the noise of the studs over the racket of the engine and rattling windows.  The bus drove well, reached 185* and effectively leveled off there.  The heat works (though the defroster air was a little funky), and I think we're back in business.  I need to top-off the coolant, and double-check the oil, but I think the bus is now "back in service".  Now, to plan the first trip to the mountain, and the next "real" road trip: Dark Star Orchestra...

As always, thanks for following along, and I'll post more as there's more to post about...