HP OfficeJet 510N + HP 901 Refilled Black Cartridge Fading and Skipping Lines: Vent Holes, Tape, and Air Bubble Fixes

Question

I have an HP 510N printer using an HP 901 cartridge. I'm using a refilled black cartridge, but the print is fading. I checked the ink level with a toothpick and it looked like there was about 1/2 inch of ink left. I refilled the cartridge and covered the holes with electrical tape. I primed the cartridge and put it back in the printer.

Should all the holes be covered with tape, or do I need a vent hole open? Now the printer is skipping a narrow line about every 1.5 inches. Could this be bubbles inside the cartridge?

Answer

1) Should All Holes Be Covered With Tape? (No-You Usually Need a Vent)

For most HP-style cartridges like the 901, the cartridge needs proper venting to print correctly.

• If you seal every hole tightly, the cartridge can develop a vacuum as ink leaves.

• That vacuum prevents ink from flowing at the right rate.

• The result is often fading, intermittent printing, or random gaps-especially after a few inches of printing.

So in general: you should not completely seal the venting path.
Many refill procedures use tape temporarily during refilling/priming, but then a vent must be reopened for normal printing.

Why venting matters

Ink must leave the cartridge smoothly, and air must replace the volume of ink leaving. If air can't enter, ink flow becomes inconsistent.

Common symptom of no vent:

• Prints start okay, then quickly fade.

• It may recover briefly, then fade again.

• Cleaning helps only temporarily.

2) Electrical Tape Can Create the Wrong Kind of Seal

Electrical tape is flexible and can seal too well. It may also:

• shift with heat,

• deform,

• or block airflow unpredictably.

If the tape covers the vent, you can get ink starvation, even if the cartridge contains plenty of ink.

3) The "Narrow Missing Line Every ~1.5 Inches" - What That Pattern Usually Means

A repeating missing line at a regular interval can happen from a few different causes, but the top suspects here are:

A) Ink flow interruption (venting problem or air ingestion)

If ink delivery stutters, the printhead nozzles misfire in a repeating pattern, especially during continuous printing. When the cartridge briefly loses pressure or pulls air, you'll see:

• thin missing vertical lines,

• repeating gaps,

• and often a pattern that becomes more pronounced during longer prints.

B) Air bubbles in the cartridge (yes, possible)

Bubbles can form if:

• air was introduced during refill,

• priming pulled foam/air into the outlet,

• or the cartridge is not properly vented and starts "burping" air.

With bubbles, you often see:

• fading,

• intermittent gaps,

• and inconsistent density.

C) Dirty encoder strip or carriage timing issue (less likely, but possible)

On some HP printers, a dirty encoder strip (clear plastic strip with tiny markings) can cause banding or repeat artifacts. However, your specific sequence-refill → tape → priming → new repeat gap-makes ink flow/venting the more likely cause.

4) Why "Toothpick Ink Level" Can Be Misleading

Many cartridges have internal sponge/foam structures. A toothpick dip can show "ink present" but still not indicate whether:

• the sponge is saturated evenly,

• the outlet area is properly primed,

• or air has been trapped near the printhead feed path.

So it's possible to have "1/2 inch of ink" and still have ink starvation at the nozzle.

5) Practical Takeaway: Refill Tape Is Often Temporary-Venting Must Be Correct

While different refill methods vary, the consistent rule is:

• Refill holes can be sealed after filling (to prevent leaks).

• Vent path must remain functional for printing.

If everything is sealed, the cartridge can't breathe and ink won't flow reliably.

6) Why Priming Helps Only Temporarily When Venting Is Wrong

Priming forces ink through the outlet, which can temporarily restore flow. But if the vent is blocked, as soon as you begin printing:

• negative pressure builds again,

• flow slows,

• and you get fading and line dropouts.

That's why you may see: "I primed it, it looked good, then the print started skipping again."

Addressing printer issues can be a complicated affair due to the hands-on nature of the problems. So, we're not able to provide remote troubleshooting, suggestions, or support for printer repairs. We offer an in-person evaluation and repair service via our local diagnostic facility: BCH Technologies Printer Repair Service (https://bchtechnologies.com/printer-repair-service). Given the high demand, we operate on a first-come, first-served basis. So, it might take a few weeks before we can get your printer to drop it off. Our services are structured to repair either a whole printer or specific parts, with clear instructions on how to proceed. However, we acknowledge that our rates aren't the most economical. Thus, we highly recommend that you resort to self-help via online research. You can start by checking out YouTube or visiting our YouTube channel's homepage: BCH Technologies on YouTube (https://youtube.com/@bchtechnologies). Look for specific videos using the search icon next to "About" on the right-hand side of the menu bar. I receive dozens of queries every day asking about videos for specific topics. Having created videos over the past nine years, it's challenging to remember every single one. Therefore, using YouTube's search function would be most efficient. Plus, YouTube might suggest relevant videos from other channels that could assist you.

Thanks again for reaching out and for supporting BCH Technologies. I hope this clarifies why sealing every hole can cause fading and repeating gaps, and points you toward the most likely cause: venting and air flow in the refilled cartridge.