I've been managing a small fleet of heavy trucks for about eight years. Not a massive operation—maybe a dozen units at peak, mix of Volvo VNLs and some older Peterbilts. Enough to stay busy, not enough to have a dedicated mechanic on staff.
And that, as I learned the hard way, is exactly the kind of setup where expensive mistakes happen.
This story starts in late September 2022. We had a 2015 Volvo VNL 670 with about 480,000 miles on it—a solid truck, honestly, but it was starting to show its age. The check engine light had been flickering on and off for a few weeks, and the drivers were complaining about a rough idle during cold starts. I figured it was a minor sensor issue. Maybe a bad O2 sensor, something we could live with for a bit.
I was wrong.
Actually, let me correct that: I was catastrophically wrong. And that mistake cost us about $8,400 in repairs and 11 days of downtime. But I'm getting ahead of myself.
The Setup: A Leased Volvo S60, a Dually Truck, and a Bad Assumption
To understand the mistake, you need to understand my headspace at the time. We had just leased a new Volvo S60 for our operations manager—one of those zero-down lease deals that had been sitting on the dealer's lot. It was a great car, comfortable, good fuel economy. And I remember thinking, 'This is the kind of reliability we need in our fleet trucks.'
The S60 lease reminded me that Volvo had a reputation for longevity. Solid engineering, well-maintained, designed to last. So when the VNL 670 started acting up, I figured it was probably something minor. How hard could it be?
I'm not a diesel mechanic. I'll say that upfront. I can do basic maintenance—oil changes, filters, brake inspections—but when you get into fuel systems and engine timing, I'm out of my depth. But we had a tight budget that quarter, and the dealer service center was quoting $1,800 just for a diagnostic on the rough idle. I figured I could save some money by narrowing down the problem myself.
Spoiler: I did not save money.
The Mistake: Timing Belt Replacement vs. Fuel Pump Diagnostics
My first assumption was that the rough idle was related to the Volvo timing belt. The truck was at 480,000 miles, and while I knew the timing belt interval was supposed to be around 150,000 miles, I had no clear record of whether it had been replaced. I figured it was overdue. A worn timing belt can cause misfiring, rough idle, even engine damage. It made sense.
So I ordered a timing belt kit from the local Volvo dealer—genuine OEM parts, not aftermarket—and spent a weekend with a buddy who claimed to know his way around a D13 engine.
We replaced the timing belt. It took us about 14 hours. My back hurt for three days. The truck still had a rough idle.
That was the moment I should have stopped and called a professional. Instead, I doubled down.
“Most buyers focus on the obvious mechanical issue and completely miss the underlying electrical or fuel system problem.”
I started reading forum posts about Volvo VNL fuel problems. Someone suggested it might be the fuel pump. Another thread mentioned injectors. The more I read, the more convinced I became that it was a fuel delivery issue.
So I decided to test the fuel pump myself. I found a guide online that explained how to check fuel pressure on a Volvo D13 engine. It involved a pressure gauge, a Schrader valve on the fuel rail, and some basic math about flow rates.
This is where the mistake went from bad to expensive.
The Disaster: How to Test a Fuel Pump (Wrong Way)
I attached the fuel pressure gauge. The reading was low—about 25 PSI when it should have been closer to 45 PSI under load. I figured that confirmed my suspicion: the fuel pump was failing. I ordered a replacement pump from the dealer—$1,400 for the part alone—and spent another two days replacing it.
After the replacement, the pressure was still low.
That was when I realized I had been reading the gauge incorrectly. I had been measuring pressure at idle, not under load. The spec called for 45 PSI at rated RPM, not at idle. At idle, 25 PSI was within normal range.
The fuel pump wasn't the problem. The timing belt wasn't the problem. The real issue? A clogged fuel filter housing that was restricting flow under cold starts. A $75 part.
I had wasted $1,400 on a fuel pump, $400 on a timing belt kit, and roughly $2,400 in labor (my time plus my buddy's time, valued at shop rates). Plus the truck was down for 11 days total between the two repairs. The lost revenue from that downtime? Roughly $4,200, based on our average daily rate.
Total cost of my DIY diagnostic: about $8,400.
The Lesson: Why a Dually Truck Deserves a Pro
This gets into territory that isn't my expertise. I'm not a certified diesel technician, so I can't speak to the engineering nuances of fuel injection systems. What I can tell you from a fleet management perspective is this: diagnostic shortcuts cost more than professional service.
The question everyone asks when their truck breaks down is: 'Can I fix this myself to save money?' The question they should ask is: 'What is this downtime costing me per day?'
For a dually truck—whether it's a Volvo VNL, a Ford F-450, or a Ram 3500—the math is brutal. If your truck hauls for a living, every day it's parked is a day you're not earning. A professional diagnostic might cost $200, but it takes two hours. My DIY diagnostic took two weeks and cost eight grand.
People think expensive service costs more. Actually, untrained diagnostics cost more. The causation runs the other way: mechanics who know what they're doing can charge more because they save you money in the long run.
Practical Takeaways: Timing Belts, Fuel Pumps, and Knowing Your Limits
So glad I learned this lesson, even if it was an expensive one. Almost decided to cut my losses and trade the truck in, which would have been an even bigger financial hit. Here's what I now tell anyone managing a fleet or owning a heavy-duty truck:
- Timing belt replacement on a Volvo D13 engine is not a beginner job. The interference nature of the diesel engine means a single mistake can destroy the entire cylinder head. If you don't have documented proof it's been done, assume it hasn't, and budget for a professional replacement.
- How to test a fuel pump properly: don't just check pressure at idle. You need a load test, flow rate verification, and—ideally—a scan tool that reads rail pressure demand vs. actual. If you don't own a professional-grade diagnostic scanner, you're guessing.
- Dually trucks (Class 8, tandem axle) have more complex fuel systems than passenger vehicles. Don't apply car logic to a truck with a 13-liter diesel. The components are heavier, the tolerances are different, and the repair procedures are entirely separate.
- If you're considering a Volvo S60 lease to save money on a personal vehicle, that's a great idea—the S60 is a reliable sedan. But that reliability doesn't automatically transfer to a VNL 670 with 500,000 miles. They're different machines with different maintenance needs.
An informed customer asks better questions and makes faster decisions. I'd rather spend 15 minutes explaining why a professional diagnostic is worth the cost than deal with the consequences of another DIY disaster.
Since that incident, I've created a simple checklist for our fleet: any engine issue that doesn't respond to basic maintenance gets sent to a Volvo-certified technician immediately. We've caught 12 potential major issues using this rule in the past 18 months alone. The most recent was a failing injector on a 2020 VNL—caught it early, cost $900 to fix instead of a potential $6,000 repair if it had failed completely.
So that's my story. An $8,400 lesson in knowing when to call a professional. Hopefully, my mistake helps you avoid making the same one.