They Took the Sulfur Out of Diesel. Your Injection Pump Noticed.

A rental 200 kVA came back from a winter job last year with a dead injection pump. Three thousand one hundred hours on the clock. The rebuild shop’s verdict was two words: fuel starvation wear. Not dirt. Not water. Wear.

The tank was full of clean, fresh, on-spec ultra-low-sulfur diesel. The fuel wasn’t contaminated. It was just dry — and nobody at that site knew dry was a thing fuel could be.

Customers ask me three questions about this, usually in this order. Here are my honest answers.

“The sulfur’s gone from diesel. Why should I care? My generator runs fine.”

Twenty years ago they started pulling sulfur out of diesel — first down to 500 ppm, then 15 ppm, basically nothing. Good for the air. Your injection pump disagrees.

Here’s the part most people miss: sulfur itself wasn’t doing the lubricating. The brutal refining process that strips sulfur out — severe hydrotreating — also strips out the polar compounds that gave diesel its natural slipperiness. What comes out the other end is clean, legal, and dry as a bone.

There’s a test for this. The HFRR — high-frequency reciprocating rig — rubs a steel ball on a plate inside the fuel and measures the wear scar. The spec (EN 590, ASTM D975) says the scar can’t be bigger than 460 microns. Refineries dose lubricity improver at the terminal to hit that number. Usually. Mostly. The fuel in your bulk tank came from a truck, which came from a terminal, which may or may not have dosed it right that day. Nobody at your site is running an HFRR in the back room.

Your injection pump cares because it drinks what it pumps. A rotary distributor pump squeezes fuel at hundreds of bar through clearances measured in microns — the cam plate, the rollers, the distributor head. The fuel film is the bearing. Common-rail systems run 1,800 bar and up with even thinner margins. Dry fuel doesn’t cause a dramatic failure. It causes a slow one: metal polishing metal, a few microns at a time, for a few thousand hours. Then one morning the clearances are gone and the pump can’t build pressure anymore.

That rental set I mentioned? (Here’s how the two pumps in your fuel system divide the work, if you want the full picture.) The supply pump was fine. It was the injection pump — the precision one — that died of thirst in a full tank.

“So I pour in an additive. My driver uses two-stroke oil — same thing, right?”

Not the same thing. Not even close. Let me rank your options honestly, because this is where money gets wasted and pumps get killed.

Proper lubricity additives work. The ester-based ones dosed at the terminal are literally designed for this — a few hundred ppm brings a dry fuel back under the 460-micron limit, and it’s HFRR-verified, not vibes. Reputable aftermarket additives at the specified treat rate do the same. Boring, effective, done.

Biodiesel blends are the interesting one. Even B2 — two percent biodiesel — restores lubricity better than most additives. B5 is better still. The catch is everything else about biodiesel: it goes off faster in storage, it grows things in warm tanks, and it gels earlier in winter. (The storage side of that tradeoff lives here.) If your supply chain is solid and your turnover is fast, a B5 blend is the cheapest lubricity insurance there is. (And if you’re curious about the renewable diesel version of this story, that’s a separate conversation worth having.)

Now the folk remedies. Two-stroke oil: better than nothing in a 1980s rotary pump, maybe, but it’s not formulated for this, the ash content is wrong for anything with a DPF, and you’re guessing the dose. Used engine oil: absolutely not. It’s full of soot, zinc, and wear metals — you’re feeding your precision injection system a cocktail of the exact things filters and aftertreatment are trying to keep out. An old-timer I work with has done the used-oil trick for thirty years and swears by it. I asked him to show me one pump that made 8,000 hours on that cocktail. We’re still waiting on the evidence. We argue about it the way old friends argue about football.

And kerosene — the winter trick. Cut your diesel with kerosene or Jet A to stop it gelling, and you’ve just poured near-zero-lubricity fuel into a pump that was already thirsty. I watched a site do this one hard winter and lose two injection pumps in a single season. The fuel flowed beautifully at minus twenty. The pumps didn’t survive to enjoy it.

One more: more additive is not more better. Double-dosing some chemistries drops the additive out of suspension or starts attacking seals. Read the treat rate. It’s printed on the bottle for a reason.

“How do I know my pump is already wearing out?”

It won’t tell you with a light. It tells you with behavior — if you know what to watch for.

The classic is the hot restart. Cold morning, fires right up. Run it hard for an hour, shut it down, try to restart ten minutes later — and it cranks and cranks. Heat opens up the worn clearances, fuel slips past the plungers instead of building pressure, and the pump can’t make rail pressure until it cools down. When a customer describes that exact pattern to me, I don’t reach for the battery tester. I reach for the fuel system.

Then there’s the slow fade: full load used to be easy, now the set sags, hunts a little at steady load, maybe puffs black when you step-load it. (Smoke color will back up the diagnosis.) Timing drift from a worn distributor head does that. And glitter in the fuel filter element — actual metal flakes — is the pump writing you a letter. Read it. (While you’re in there, the injectors deserve the same suspicion — here’s how I check them.)

The honest math on rebuilds: a pump rebuilt at the first symptoms costs real money. A pump that seizes takes the drive, the timing, and sometimes the front cover with it — and strands the generator. I’ve never met anyone who regretted rebuilding early. I’ve met plenty who regretted waiting.

What I’d actually do on a site: buy fuel from suppliers you’d trust with your engine, keep the batch receipts, and if your supply chain runs through back-road terminals and farm tanks, dose a reputable lubricity additive at the printed rate and log it. Check the filter for glitter at every change. And the next time someone suggests cutting winter diesel with kerosene, ask them who’s buying the next injection pump.

The sulfur left diesel twenty years ago. The pumps never got the memo — they’re still built like the fuel is slippery. Give them something slippery to drink and they’ll outlive the engine around them. Give them dry fuel and they’ll die young, quietly, and expensively. Your call — and if you want a second opinion on yours, we’re around.