I returned to the boat today determined to find the problem with my engine. As an aside, I had the first good sleep since before Xmas last night, such is the healing power of sailing. I saw a couple of sailboats mooching upwind on Haro Strait this morning and felt a little jealous, but I have a job to do.

First thing is to check compression. Bad compression will make a diesel hard to light, but once it starts, heat will often expand parts and increase cylinder compression, allowing the engine to run well. Unlike a gas engine, on a diesel you have to measure compression through a glow plug port or the injector port. I bought a cheap diesel compression gauge off ebay because I knew that the odds of ever using it again were slim, and good ones can cost over $200.00. Besides, total compression is less important than variability between cylinders, so even if it's 10 or 15% inaccurate, it's good enough.


First thing to do is pull the 4 glow plugs.





Once the glow plugs are out, you have to screw in the correct sized fitting into the port. This cheap gauge had a whole set of adapters.

The gauge snaps onto the adapter. You have to make sure it fits tightly as the pressures are enormous.

You then turn over the engine until the gauge maxes out. Note - remember to pull out the engine stop lever or the engine will blow diesel fumes out the open glow plug ports! Don't ask how I know this...

I did this for each cylinder. Below was a typical reading. I found 450 - 400 - 430 - 450 PSI. It occurs to me looking at this, I probably should have worn safety goggles; with those kinds of pressures inside the thing and it's dubious Chinese provenance, there might have been a risk of it blowing up in my hand!
The reading certainly shows why you can't use a gas engine compression tester!

At any rate, this is very good news. The rule of thumb is that no one cylinder can be off by more than 25%, and these pressure are very healthy. So the engine is in good shape. If you wanted to be a perfectionist, the next step would be a leak-down test, but I don't think it's required.

My research tells me that if it's not compression then air in the fuel system must be culprit. This can enter through the lift pump, bad seals in the fuel filters, or loose connections somewhere. One way to check for this is to put a piece of clear tubing in the fuel line where it enters the primary filter and watch for air bubbles to pass through.

Checking for air bubbles.

Well, the results are inconclusive, but I think I'm getting very close to something. There were a few large bubbles that drifted through, but I'm not certain if they got into the system when I cracked the line to insert the clear tube. Certainly once the engine was running there were no bubbles.
But it occurs to me - once I installed the clear tubing, I bled the system of air by loosening the central mounting nut from the primary filter and manually pumping fuel through it. What's unusual is that the engine lit instantly, so there's a chance that by doing the bleeding I removed air from the system that normally makes starting so difficult. Maybe those few bubbles that appeared in the beginning is what makes starting so hard. If that's the case, when the engine sits overnight air is getting into the lines. That's a different problem then something like a leaking lift pump as that would show up all the time, even when running. This problem seems to occur when it sits rather than when it runs.
This is a relief. I've been banging my head against this problem for 6 months. But when it started today, it did so better than it ever has, and ran better as well. I'm so close!
My next step is to come back to the boat tomorrow, bleed the filter again, and see if it lights up just as easy. If so, I know the problem and just have to track down the source of the air intrusion.