The morning our diesel engine experienced a runaway started like any other. We were headed out of Monterey Harbor on our 1979 Cheoy Lee 41, Avocet, bound for Morro Bay. We fired up the steadfast if sometimes quirky Perkins 4.108 without issue, untied from the dock and made our way out to the ocean.
Just beyond the Monterey Harbor breakwater the engine revved loud and quick, a hiccup we thought was our prop getting caught on kelp—not uncommon for the area. But moments later, when we were about to raise our main, the diesel revved higher than before and kept increasing, blowing billows of white and gray smoke off our stern. I grabbed the helm and Chris dashed below, both of us simultaneously realizing the direness of the situation.
I was eternally grateful for the small seas and gentle breeze, letting Chris throw open the floorboards to reveal our angry engine, assess the issue, and quickly find a resolution. In the span of 60 seconds Chris tried to stop the engine first by manually actuating the stop solenoid on the high-pressure fuel pump, commonly called the injection pump, which would starve the engine of fuel. When that failed to work, he knew the problem wasn’t associated with the fuel system—we had a runaway. The only way he could shut down the engine was to starve it of oxygen. He grabbed a screwdriver to pry the mushroom cap off the air inlet, throwing it to the side as he placed his palm over the intake.
Silence. Cursing. The overwhelming feeling of uncertainty.
You could cut the tension with a knife. I continued to sail the boat while Chris worked. Oil was everywhere, adding a black layer atop of our varnished surfaces and covering Chris’ arms and face like war paint.
A runaway is just that—a diesel engine that won’t stop, creating a frightening cascade of problems and potentially severe damage, injury, fire, or worse. It’s fairly rare but highly dangerous and, depending on what causes it, extremely difficult to stop; we were lucky Chris was able to stop our engine as he did. Several factors can lead to a runaway, including oil seal failure in a turbocharged engine, overfilling the crankcase, incorrectly assembled throttle linkages or, as in our case, failure of the injection pump.
The injection pump supplies the injectors with fuel in the proper sequence and at the proper pressure. When it fails, diesel can flow unregulated into the crankcase. When the fluid inside the engine reaches a point where it spills into the intake manifold, the engine will burn its own oil/diesel, creating a feedback loop that can’t be easily stopped and can lead to further mechanical failure due to overspeed, oil starvation, the engine throwing a rod, or fire—none of which are ideal.
With our floorboards tossed aside and the engine fully exposed like a heart surgery patient, Chris concluded that the amount of oil inside the oil sump was much too high, resulting in the pump overfilling the crankcase. He grabbed our oil siphon and began to remove about a gallon of oil mixed with diesel from the engine, returning the level back to normal on the dipstick.
He carefully inspected the block and the sump beneath the engine, verifying that he’d caught the Perkins before any major damage had been done. Once everything was relatively cleaned up, we carefully turned the engine back on, this time prepared with a rag in hand to snuff it out if she tried to run again. After a few cranks the tough Perkins rumbled back to life, coughing and sputtering until the oil inside the intake manifold was burnt out.
We decided to continue our passage to Morro Bay under sail, since the forecast wind was favorable. It was a long night full of steady winds and an 8- to 12-foot following swell, pushing Avocet down the California coast at speeds she had seldom seen before and hitting a new speed-over-ground personal record of 12.9 knots. Around 7:00 a.m, we saw the Morro Bay breakwater off our bow. Hesitantly, we started the engine listening intently for any sudden changes, waiting to spring into action. Luckily all was well, and we made it into the anchorage safely, setting our hook, and settling into our new workspace.
The Diagnosis
After some research, we discovered that we were not the only Perkins 4.108 owners with runaway stories. Every owner who had this happen (and documented it on the Internet) mentioned that they had a leak developing from the high-pressure fuel pump, also called the injection pump, which led to diesel entering the crankcase, unregulated, causing a runaway.
Having just replaced the lift pump on our Perkins, we concluded that the only other place that diesel could make its way into the crankcase was through the injection pump. This is the heart of a diesel engine, and it’s not something that you can just fix DIY. Considering that we needed to disassemble our engine to extract the pump, we rented a slip until the repair could be made.
The Perkins 4.108 is, for the most part, a very economical engine to service and maintain. Although most components are affordable, the injection pump is pretty much the most expensive piece to replace besides the block itself. New pumps are between $2,200 and $4,000, and a rebuilt pump can set you back anywhere from $600 to $1,300.
Since the Perkins 4.108 is no longer in production, not many suppliers carry spare parts for the engine, instead special ordering them from the United Kingdom where Perkins is located. Luckily, there are a handful of diesel shops in the U.S. that could service the pump. Since the Perkins was originally built as a tractor engine, we found a lot of resources in California’s agricultural Central Valley. The shop we chose to work with was on the way to Chris’ hometown where we were visiting for the holidays, so we dropped off the pump and hoped for the best.
After three weeks we finally got our pump and all four injectors back from the shop, but the experience was not hassle free. When Chris picked up the pump, he found that the throttle assembly had been flipped 180 degrees from original. And, the input shaft still had diesel leaking from it, disappointing since that was essentially the main reason why we needed the pump rebuilt. Once the shop corrected these issues, the pump showed no signs of leaks and everything was as it should be, with Chris signing off on the repair after watching a bench test that pressurized the inside of the pump with diesel to simulate it in operation.
Back aboard Avocet we were eager to get the engine back up and running so we could continue cruising south. Chris dove head-first into the job, carefully attaching each piece that was removed, rightfully starting with the starter motor that we decided to replace entirely “while we were in there”—a phrase we find ourselves saying with every project. One of the benefits of having the injection pump rebuilt rather than replaced is that we didn’t need to retime the engine with the pump. Using the markings that Chris made on the block and the pump housing before removing it, he was able to reassemble the pieces, retaining the factory timing of the engine.
The pump went back into place accordingly and we continued with adjusting the valve clearances on the head and reinstalling all four injectors as well as the heat exchanger, mixing elbow, and raw water pump. The last thing to do was fill the engine with fluids and hope everything worked as it should.
I sat at the helm by the ignition waiting patiently for Chris’ OK to crank over the key and listen for the rumble below. Positioned above his patient, Chris was ready with tools to make necessary adjustments. After 30 long seconds of cranking and bleeding the injectors, the engine came back to life, her roar echoing across the marina in addition to our cheers.
Final Thoughts
There are a couple things we want to mention regarding the aftermath of a runaway diesel event. After a runaway, be wary of running an engine that contains diesel mixed with oil—not only does this cause runaways but it also breaks down the oil’s viscosity, which can be damaging to not only the bearings and pistons but also to the seals inside the engine.
Also, understand that diesel can easily damage plastics or rubber materials in or near the engine. We learned this the hard way; when the runaway happened, it resulted in oil and diesel fuel high enough in the sump to reach the rear main oil seal and damage it, resulting in a substantial leak that ultimately we had to repair later during an engine rebuild.
Diesel engines are not meant to be run at the rpm associated with a runaway. Because of this excessive exertion, the rings on the pistons can wear down the cylinder walls quickly without sufficient oil. If you are hopefully able to stop the engine before anything catastrophic happens, it’s a good idea to do a compression test before making a decision whether to repower or rebuild. Since we had good compression (between 430 and 500 psi) we decided that the engine would be worth rebuilding.
Finally, when oil is being burned instead of diesel, oil goes into the entire engine—intake manifold, combustion chamber, exhaust manifold, mufflers, exhaust hose, etc. Since oil is very dirty when burned as fuel, there’s a good chance that your injectors will foul, and a thick layer of oil and sludge will line your entire exhaust system. Not only will this not burn off by itself, but it lines your system with oil, creating an eco nightmare. We painstakingly dismantled each piece of the exhaust system, cleaned it, and reassembled it—disposing of the cleaning materials and biohazards accordingly—before running the engine again.
Not all engines are the same; they all have their own quirks just like the boats they serve. Our Perkins 4.108 is strong but not immune to her own oddities. Through our research we have concluded that two of the most frequently reported causes of a Perkins runaway are either the lift pump or the injection pump, so if you are a Perkins owner, check your systems accordingly.
Probably the best to way to prevent engine problems—including a potential runaway—is to check your engine oil level before you start your engine, every time. If the level is above the normal mark, find out why. Either water or diesel may be entering the crankcase, driving the fluid level up. In either case, something’s wrong that needs to be addressed.
Although runaway diesels are a statistically rare occurrence, it’s important to understand that they can happen. Fortunately, we understood what we were seeing and didn’t waste a second doing what needed to be done to save our engine, and our boat.
After the runaway incident, Marissa and her husband Chris continued south to Mexico where they ended up hauling their engine and rebuilding it at anchor in Banderas Bay—not necessarily by choice, but that’s a story for another time. In June 2023 they hauled out in Puerto Penasco, where they started their season in January 2024. You can follow their journey in real time on svavocet.com or on YouTube Sailing Avocet.
March 2024