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Maintenance Q&A
Back to the basics of troubleshooting

By Paul Abelson
senior technical editor


Q: Five weeks ago, I bought a used 2005 Volvo with a Volvo VE D12 engine. It has 465 hp at 1,900 rpm. It has about 624,000 miles on it now. (Editor’s note: The reader also provided VIN and serial numbers.) Here is what happened.

On my first run loaded, the check ECU code came up but did not affect performance at all. I cleared the code. It ran fine through the Appalachians. The third week it started losing power on inclines when loaded. The same code came up. In Wyoming, I got the code again and did not have any power.

The truck is great if it is not carrying a load. Loaded and on any type of incline, it loses power and slows down to 5 mph. The mechanics at the truck stop could not determine the problem.

I went on to the Volvo dealership in Denver. There they checked injectors, changed the fuel pump and changed the EGR valve.

That still did not fix it. I had to stay with the load for three days until another driver came to take it on to California. I went home and brought it to a heavy equipment mechanic. He replaced the throttle module and boost sensor. Still no power and it is showing the code.

A: Unfortunately, your experience is more common than it should be. You can still run into technicians who replace parts until they find something that works instead of going through the basics as any good mechanic should. This is time-consuming and costly.

My brain trust, TMC Silver Spark Plug awardees Tom Tahaney and Carl Tapp, conferred with me on your problem. We concluded that you need to go through the steps the old-fashioned way.

Evidently you are not burning enough fuel to generate the power you need. Three things are required: fuel, air (oxygen) and an ignition source. If your air intake was blocked or your turbocharger wasn’t working, it would be obvious. But your engine runs fine when not under load, so we can toss out lack of air as a cause. Also, any reduction in air supply would show up as a fault code, since fuel and air devices are managed.

Compressing air in the cylinders increases its temperature above the ignition point of diesel fuel, so when fuel is injected, it ignites and burns. Again, you’re getting ignition at low loads, so compression ignition is probably occurring at all load conditions.

That leaves fuel. The amount of fuel needed varies directly with load conditions. Delivery is usually managed by the engine control unit (ECU), but that only works if the injectors are actually getting all the fuel they’re supposed to. If there’s a physical obstruction that limits fuel delivery, it might not show up in the ECU codes.

Fuel restriction can occur anywhere along the fuel system, starting at the pickup tube.

A sludge buildup from the tank or piece of dirt in the fuel can get sucked into the tube where it can obstruct flow. Even a piece of loose gasket material can partially obstruct fuel flow, flapping back and forth and limiting flow under loads.

Filters and screens are often overlooked. With 624,000 miles, when were fuel filters last changed? You didn’t mention any leaks, but with today’s ultra-low sulfur diesel (ULSD) fuel, leaks are harder to discover, especially by smell. A small leak or a loose fitting could suck air into the fuel, aerating it and reducing the amount of fuel sent to the injectors. Fuel flow can also be reduced if a line is crimped, dented or bent.

It’s easy to check the computer codes and replace what is indicated. But when there are no indications, you have to go back to basics. Just remember: air, fuel and ignition. Trace each flow in a logical manner and you should be able to fix any problem, even those that are not in the computer.

Thanks for including the engine serial number. When I have to contact manufacturers about reader problems, VIN and serial numbers are great time-savers. LL