Rebuilding Your TDI Fuel Injectors

First, let’s get this out of the way: Fuel injectors are a wear item on all diesels, including the VE, or rotary pump TDIs sold in North America from 1996 through 2003. Although projected wear rates vary, by the time your TDI approaches a quarter million miles, odds are the nozzles are worn and the injectors aren’t delivering as much fuel as new nozzles. And delivery pressures also tend to decline with age,  sometimes not evenly across cylinders. Probably the biggest factor in injector wear rates if fuel quality. Good fuel, along with a lubricity additive like Stanadyne or Opti-Lube, will help injectors last longer.

How do I know when my injectors are worn?

Short answer is, you many not know when they’re worn. If decreased fueling causes a power loss, usually the change is so gradual that it’s hard to identify. And if one injector is fueling heavier than others, the amount of additional smoke may be minor. But low power, more smoke, and a loss in fuel economy are the most common symptoms of injector wear.

Nozzles are the biggest wear item. Impurities in the fuel change the shape of the nozzle orifice, which in turn changes the amount of fuel delivered and the spray (atomization) of that fuel. Ironically, the worn nozzle holes may deliver less fuel, not more. But not always: sometimes worn injectors will stream fuel into the cylinder.

At any rate, worn injectors can cause engine damage. Streaming injectors can melt pistons. They also wash fuel down cylinder walls, which can cause excess cylinder wear. Cars that are persistently underfueled will make more turbo boost to compensate, stressing the turbo more than normal. And in extreme cases, overfueling can bend connecting rods in some vehicles.

It’s not just the nozzles that wear out. As shown above, Injectors have multiple moving parts that operate for each engine revolution. And even though they’re cooled and lubricated by diesel fuel, they do wear out. The consequence is usually lower fuel pressure and earlier injection. Both are bad for power, fuel economy, and engine life.

Nozzle sizes

When you’ve determined that your nozzles are due for replacement and your injectors need refreshing, the next question most ask is “what size and type nozzle should I buy?” Searching forum posts will get you many different opinions. And nozzle sizes are identified in multiple measures, which makes it more confusing.  Most common size measurements are from Bosch, who were the OE maker for VE TDI injectors. Second most common is Bosio, who translate the sizes into their own numbers.

Here are some basics on stock nozzle sizes:

  • .184 Bosch nozzles were standard on early VE TDIs. Bosio uses the number 706 (preceded by DSLA 150P)
  • .170 Bosch nozzles were on later VE TDIs with manual transmissions. However, the most common replacement for these nozzles is the .184 size, same as in the earlier cars. The difference in fueling is nominal.
  • .158 Bosch nozzles were used in ‘99.5 – ’03 automatic transmission TDIs. Bosio uses DSLA 150P 442 as its number

When replacing nozzles in an unmodified TDI (not chip tuned), many owners will increase the nozzle size one step. That means .184 nozzles in an automatic transmission car, and .205 Bosch size (520 Bosio) in a manual. Others will go slightly larger, to .216 (Bosch) or 502 or 1019 (Bosio). These larger nozzles will work with low smoke and no loss in fuel economy in most cars, although they may require some minor adjustments to fueling using the VCDS diagnostic tool. They will provide a significant increase in torque and power. Larger nozzles will deliver fuel more quickly than smaller ones, which is like advancing the injection timing. This will improve starting and can, ironically, increase fuel economy even though the engine can deliver more power. Given there are few, if any, downsides to a minor increase in nozzle size, most owners opt for larger nozzles at replacement time.

In modified cars, most owners will work with their tuner to select the nozzle that will best fit their car’s hardware and tune.

What about nozzle coating?

In the past VE nozzles were not coated. However, when Ultra Low Sulfur Diesel (ULSD) was mandated in 2007, the sulfur removal process also took lubricity out of the fuel, increasing nozzle wear rates. Many new

Nozzle install and testing

As noted above, injector bodies will show wear along with nozzles. A qualified fuel shop (Bosch certified or other) can refresh and test the injector bodies to ensure they:

  • Reach full pressure before releasing fuel
  • Deliver fuel evenly across all 4 injectors
  • Spray and atomize the fuel correctly
  • Deliver correct fuel flow.

Because of this, simply swapping nozzles and not testing flow, pressure, and atomization is hit or miss. It might work, or it might not. Many suppliers offer hot swap programs where you can purchase injectors with nozzles mounted and tested and send back your used injectors as cores.

Think its time for a swap? These Upgraded Complete Injectors w/ Bosio Nozzles are a great option!




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