10-4 Magazine / October 2026 19 The Performance Zone: By Bruce Mallinson We have another great example of how, like last month, when it comes to diagnosing a truck problem, sometimes the smallest and most unassuming part can cause some of the biggest problems. This customer had been speaking with Pittsburgh Power’s new president, Eric Moser, for the past few months about an issue that had been going on for eight or nine months. The truck is a glider powered by a 12.7L Detroit with approximately 750,000 miles on it. Over time, the customer began noticing that his fuel mileage was steadily getting worse, but there was another symptom that really stood out – his fuel pressure was reading an incredible 120 PSI at idle. For comparison, on this particular 12.7 Detroit setup, we would normally expect to see approximately 15 to 22 PSI at idle. Seeing 120 PSI was far enough outside the normal range that our first thought was to make sure the reading itself was accurate. So, before tearing deeper into the system, we advised the customer to replace the fuel pressure gauge to rule out a faulty reading. The gauge was changed, but the result wasn’t. The truck was still showing 120 PSI at idle, confirming that there was a problem somewhere in the fuel system. The question was what could be causing that much pressure. The customer had already had the truck worked on at two different shops during the months he had been dealing with the issue. He returned to one of those shops, and in an effort to eliminate another possible cause, they replaced all the fuel lines, including both the supply and return lines, to make sure there wasn’t a restriction somewhere in the system. With the new lines installed, the truck was started again, and the gauge still showed 120 PSI at idle. At this point, a considerable amount of time and effort had gone into chasing the problem, but one small and seemingly insignificant component had managed to escape everyone’s attention. On the back of the cylinder head is a relatively unassuming brass fitting. When the customer finally removed and inspected it, he discovered something that would ultimately explain the entire problem. The fitting contains a small orifice that is critical to controlling fuel flow through the system and attached to it is a secondary brass fitting that allows the fuel line to connect to the head. The two sides of DON'T OVERLOOK OR IGNORE SMALL OR SIMPLE THINGS the fitting use the same thread pitch, but there is one very important difference – one side is longer than the other. At some point in the truck’s history, that secondary fitting had been installed backwards and then subsequently tightened far too much. Because of the additional thread length, over-tightening allowed the secondary fitting to extend far enough into the assembly that it began covering the small fuel orifice. When the customer inspected it, roughly half of the opening was obstructed. That tiny restriction was enough to send the fuel pressure skyrocketing. Once the fitting was installed correctly and backed off, the orifice was allowed to flow properly again, and the change was immediate. Fuel pressure dropped from 120 PSI to approximately 20 PSI at idle, putting it right back where it should be. Correcting the fuel pressure also produced another major improvement. Once the truck was back on the road, the customer reported gaining approximately 1.5 MPG in fuel economy. After eight or nine months of declining mileage, multiple shop visits, a new pressure gauge, new supply and return fuel lines, and plenty of head scratching, the culprit turned out to be a brass fitting that had simply been tightened too far. It is another perfect reminder of something we see constantly in this industry – don’t overlook the small or simple things. A component doesn’t have to be large, complicated, or expensive to create a major problem. Sometimes a tiny restriction, an incorrectly installed fitting, or a fraction of an inch in the wrong place is all it takes to create months of headaches. There is also one final chapter to this truck’s story. After years on the road, its owner has decided to get out of the game, which means this 12.7 Detroit-powered glider is now for sale. The truck currently has approximately 750,000 miles on it. If you’re interested in learning more about the truck, give Pittsburgh Power a call. We can put you in contact with the owner for additional information. Thanks to my associate and Marketing Manager Jordan Greathouse for writing this month’s article. For more details about us (or the truck for sale), stop by our shop in Saxonburg, PA, call us at (724) 360-4080, or visit us online 24/7 at www.pittsburghpower.com. n
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