Engine RPM variation (again)

As Gwen wrote yesterday we were finally ready to head off on our journey down the Pacfic Coast, trying to catch an elusive and narrow late season weather window. We were a bit nervous prior to departure as the forecast were for gales and small craft warnings down the Strait of Juan de Fuca. The worst of it was to be right at the Eastern end, so we made a pre dawn departure to, hopefully, avoid the worst.

When embarking on a trip like this, particularly with the possibility of encountering bad weather, you do not want to hear your trusty Lugger diesel engine begin to slow down spontaneously. We thought we had resolved the RPM variation issue by replacing the fuel flow sensors, as I described last week. Apparently not. It only took a couple of cycles of this to realize we weren’t going anywhere.

We didn’t make it very far…

OK, now you know what happened. If you don’t want to read a bunch of geeky stuff about trying to fix fuel flow issues with diesel engines, this might be a good place to stop.

Because this is an ongoing issue, I pulled out my phone to capture some video of what was happening. I compiled a short compilation of those clips in hopes that it will be helpful in trying to diagnose and finally resolve this problem. I’ll include a link at the end of this post.

What we saw and heard

  • While cruising at our normal 1800 RPM, a spontaneous drop of > 200 RPM, which was very obvious listening and watching the tachometer. The engine would slow down, stay at lower RPM for a moment, and then speed back up to the “right” RPM. Several instances of this are illustrated in the video. We also saw this when running at Wide Open Throttle (WOT).
  • As this was happening, the Maretron display showed large fluctuations in fuel flow as recorded by the inline sensors. It looked like the decrease in RPM was associated with an increase in reported fuel consumption, but it is hard to be certain because the system averages readings over a 5 second window.
  • There did not appear to be any fuel restrictions as would be measured by the fuel supply vacuum gauge, which is mounted on the fuel filter manifold. In fact, the reading we saw on the gauge was 0, surprisingly low, and meaning no fuel restriction.

Troubleshooting and attempted fixes

Diesel engines are pretty simple and very reliable. According to Wikipedia:

The diesel engine (also known as a compression-ignition or CI engine), named after Rudolf Diesel, is an internal combustion engine in which ignition of the fuel is caused by the elevated temperature of the air in the cylinder due to the mechanical compression (adiabatic compression). This contrasts with spark-ignition engines such as a petrol engine (gasoline engine) or gas engine (using a gaseous fuel as opposed to petrol), which use a spark plug to ignite an air-fuel mixture.

Wikipedia

Diesel mechanics say that 90% percent of issues are due to fuel delivery, so we decided to work our way through the fuel system from supply to return in order to identify and eliminate potential causes. Here’s what we did:

  • Inspected and replaced the Racor primary fuel filters. These are the first stage of filtration, making sure that the fuel to the engine is clean. If there is contamination in the fuel, these filters remove it. They eventually need to be replaced. The indicator is the vacuum gauge I mentioned earlier. There was no symptom of high vacuum, but the filter had not been replaced since we started our trip to Alaska, so we replaced it. By the way, the system actually has two filters, so if one gets clogged you can easily switch the the backup.
  • Replaced the Lugger secondary fuel filter. This one is mounted on the engine and is the final stage of filtration. It was replaced this summer while we were in Alaska but had not been replaced since the tanks were inspected and the sensors installed, so we did this one as well.
  • Inspected and cleaned the Algae-X filter, which is some odd contraption that a previous owner had added. It is essentially a housing with a magnet in it, through which the fuel passes on the way to the engine. I suppose it is yet another stage of filtration, but it is entirely unclear that it actually does anything. Nevertheless, we disassembled and inspected it. All clean.
  • Having done all of this, and “bleeding” the fuel system to make sure no air remained in the system after changing filters, we started up the engine. We ran it up to 1800 RPM at the dock, and still saw the RPM (and fuel flow display) variation, even when the engine was running in neutral under no load. So, clearly none of these items were the cause of the problem.
  • Next, on the advice of the knowledgeable mechanic that replaced our fuel flow sensors, we “bled” the high pressure side of the engine, which involves loosening the fuel injectors while the engine is running to ensure that no air is trapped in the system. This had no effect – same symptoms when running at 1800 RPM at the dock.
  • Finally, we bypassed the Maretron fuel flow sensors using a couple of fittings to connect the fuel supply and return lines, respectively.
The diesel supply and return lines going in to (and coming out of) the Maretron fuel flow sensors. We put fittings in place to bypass them.

At this point we were out of ideas, so time for a sea trial. Unfortunately, the variation was still present… smaller and far less frequent, but noticeable both by the sound of the engine slowing down and watching the tachometer needle drop and rise back up. This time it seemed to be less noticeable at cruise RPM, but clearly evident at WOT (as shown at the end of the video). So, I think we can conclude that the Maretron sensors are not themselves the cause of the RPM variation. Now we need to turn to the fuel system on the engine itself, perhaps the fuel injector pump or even the injectors.

Bottom line… back to the yard. We plan to limp over to Philbrooks in the next day or so, which will be our new jumping off point for the trip South.

Here is the YouTube video that illustrates the issues: https://youtu.be/UFkKP2qk-AI

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