HOW-TO: Rebuild Diesel IDI Injectors
Written by Administrator   
Wednesday, 29 October 2008


This How To document was designed to assist with rebuilding Diesel IDI injectors.  Like all recipies I recommend reading the instructions from start to finish before starting the project.  I  welcome your comments, so feel free to click on the comments link below if you have a comment to add or have completed this procedure and have a tip of your own to share.




  • Use these instructions at your own risk
  • Read them through from beginning to end before starting
  • This is how I do things… it is not necessarily the right way nor the best way !
  • Using equipment,tools, and supplies incorrectly could result in serious injury to you or your property or even death


Please note:  You can click on any thumbnail below to see a larger version of the image.  After viewing the large image, you can click anywhere outside the white image border to return to your spot in the text.


Parts needed:

       4       new injector nozzles


You have many options here … current practice includes standard VW nozzles (either normally aspirated or turbo), so-called “GTD” performance turbo nozzles, or Mercedes turbo nozzles … specifically nozzles intended for the Mercedes 300D series. The ”hot” ticket seems to be the Mercedes nozzles, but others say they smoke too much.  Have a set but haven't tested them personally.

The nozzles are identified by Bosch nozzle  number:

DNO SD273 - stock turbo injector nozzle  (DNO SD293 seems to be another common number for these)

DNO SD276 - "GTD" injector nozzle  (also seen these listed as DNO SD274)

DNO SD261 - Mercedes 300D turbo diesel nozzle 


Nozzles are available from a number of sources:

- Bosch dealers

- Mercedes dealers... they seem to stock nozzles much more reliably than VW dealers

- good ole eBay 


I'm currently (2009) of the opinion that:

- the Merc nozzles flow more than we need and were not designed for our style of IDI head

- there's probably very little performance difference, if any, between the various factory nozzles, including the so-called "GTD" series

- the best nozzle for your VW might well simply be *new* stock ones!



Supplies needed:

  • thin oil  (penetrating oil works well)
  • sheet of 600 grit sandpaper
  • sheet of 1000 grit sandpaper
  • sheet of 2000 grit sandpaper
  • thin glass plate... I bought a really cheap picture frame and tossed the frame
  • anti-seize coating
  • clean solvent (paint thinner, general solvent, kerosene, etc.... something non-flammable at room temperature)
  • Can of brake drum cleaner
  • compressed air


Tools needed:

  • 22mm wrench or 27mm deep socket (Mac Tools SC141,Snap-On S6104B, Craftsmen Deep, or equv.)
  • strong vise


Step 1: Disassembly


There are two ways to get the (usually rusty) injector halves apart.  One way is to clamp the base of the injector in the vice and use a 22mm wrench to untwist the top section.







My favorite way, which seems to work well for injectors that are particularly stuck together, is to place the injector upside down in the vice (clamping the top of the injector in the vise) 







and using the same 27mm socket you used to remove the injector from the engine to remove the bottom.  A larger breaker bar may come in handy and/or heat and/or penetrating oil !   I like this way because I have tons of leverage and I’m not distorting the bottom of the injector… but be careful not to completely unscrew the halves in the vice and have pieces drop all over your garage floor.  injector_1c_disassembly_b.jpg  





You can now spread out all the parts


I like to keep the parts from each injector together throughout the process… it makes final calibration a bit more predictable and seems to result in less leaking.

I also like to take a power wire brush to the outer casings and polish ‘em up… it makes them look better, assemble easier, and most importantly keeps a speck of rust from gumming up a nozzle during re-assembly.






 Here's a schematic drawing of how all the parts fit together:











If you examine the bottom of the top half of the injector you will probably find grooves worn into the sealing surface.







Likewise for the spacer or "intermediate disk":








Step 2: Lapping


In order for the injector halves to seal properly these surfaces need to be super flat.  If the surfaces have worn at all this will require lapping, using a glass plate and successively finer grades of sandpaper. 

Lapping removes metal and so should be done as sparingly as possible.  Carefully inspect the surfaces ( spacer and injector top)... if a lot of wear is evident you may need to start at 600 grit and work your way up to 2000.  If there is very little wear a polish with the 2000 grit paper may well be enough.  The official Bosch equipment is even finer (4000+) but I've had good results with good old autobody sandpaper.

The glass plate (I used an cheap picture frame for the glass). 







 The purpose of the glass plate is to give you a very very flat surface to sand on.

  Start by lapping both sides of the spacer.  Use circular looping motions, moving from one grade of sandpaper to the next (transfer the glass plate as well so that you are always lapping on a flat surface) .  Lap both sides of the spacer, keeping the metal as flat as possible.







Finish up by lapping the bottom of the top injector half







When you’re done all signs of wear will be removed and you should have extremely flat sealing surfaces







Lay out all the parts for reassembly.  I tend to keep the parts from each injector together as I work thru the process… seems to result in less leaking and also keeps the breaking pressure as close to original as possible




Step 3: Cleaning


IMPORTANT:  from now on be surgically clean… the injectors and nozzles are precision parts, and a tiny bit of dirt will ruin your whole day !  Assemble on a clean surface, using clean hands and tools.

I start by thoroughly rinsing all parts in very clean solvent/paint thinner/etc







I then spray them down with brake drum cleaner and compressed air to make sure they are very very clean.


Step 4:  Reassembly


Finally it’s time to crack open the new nozzles from their protective plastic cases… they will probably look much more presentable than the old ones







Reassembly is the reverse of assembly (always wanted to say that)… I start by installing the new nozzle







I put a very thin coat of antiseize on the threads of the top half, since I’ll be back this way again in about 75K miles.  



Be very careful not to get any on the sealing surface however.

If you look carefully you’ll see a groove filed in the round surface of the top injector half.  I do that to remind me what nozzle is installed:  no groove= stock nozzle, 1 groove = GTD nozzle, and two grooves = Mercedes 300D nozzle.

Torque them together hard… the Bentley specs are 70 Nm or 51 ft-lbs.


 Lately I've been leaving the antiseize off until pop-testing is complete... during the calbration cycle you tend to disassemble and then reassemble the injectors a couple of times as you hone in on the right pressure... and as diesel leaks out it tends to dilute the antiseize.

 All done and ready for calibration and leak testing… don’t they look pretty:








I find that the more lapping you have to do the more the stock breaking pressure gets raised... pop-testing after this proceedure is something I'd recommend.  I did a recent batch that was up at around 180 bar after reassembly... too high for proper operation in my humble opinion. 

When installing them back in the head they get torqued to 70 Nm (51 ft lbs)... new heat shields are MANDITORY as they deform as they seal.


Here's a quick video of pop-testing in action:




end of proceedure


 What about the 1.9l injectors (AAZ engine) you ask???


Well, as the saying goes, those are a horse of a different colour:








 The AAZ injectors are on the top, and the standard 1.6l injectors are on the bottom.  As you can see, the AAZ injectors are a dual-spring design and have two shims.  The dual-spring concept is designed for a two-stage injection pulse: a small inital pulse at one breaking pressure, and then the main injection pulse at a different breaking pressure.  The idea is a quieter engine.

 I haven't played with them much yet so can't comment at this point.  Lapping them should be the same job but setting the breaking pressure afterwords is a whole order of magnitude different, and of course the shims from my standard collection don't fit.



Comments?  Questions?  Tips or Tricks?  Feel free to post them. 


Comments (28)Add Comment
written by Steve Cunningham, July 04, 2016
Well I'm glad I found your site...under "diesel inj torque specs" I was getting performance figures on diesel cars! I was looking for the injector reassembly torque. I've had my 1.6 vw eco diesel injs apart a couple of times, including putting in new nozzle sets, and would crank that thing hard. However, a couple would seep fuel. I never thought one could lap them-I was afraid of scratching the surface. I converted my 78 rabbit to use fry oil, and after 150k I need to clean the nozzles about 80k. I use a water filter good to 5-10 micron, the plungers seem to be OK. I'll get some 2-4,000 paper and give it a shot. Thanks for the help! PS maybe lap the pressure shim at the same time to maintain pressure?
written by anton, May 04, 2016
Nice helpful presentation.
I've not been able to find any reference to the test holding pressure times. Often test pressure - below release pressure - is quoted for visual leak testing only but no leak down time. What tme period would you expect to see after a rebuild. Indefinately would be nice but I can never achieve that.

Hi Anton... good question and I don't have an official answer as I've never seen actual specs for how *long* an injector should be able to hold a pre-break pressure.

Given that the engine has many injection cycles per second when running I'm generally of the opinion that if the injector can hold rated pressure for something 10X or 100x that... say 5-10 seconds... all is probably fine.

We are talking about bottle jacks and analogue pressure gauges here, so it is a bit of an art I guess. smilies/smiley.gif Perhaps someone else has seen an official spec and will comment as well.

written by Niko, January 31, 2015
I recently did my injector (for a ford) and I think I got the pressures within 5 bar . Is that good enough ? And is it obvious when the injector leaks ?

Hi Niko... yup for my money 5 bar is more than close enough.


written by Dan Pico, October 19, 2014
Great how-to. I need to rebuild the injectors in my 1998 E300 OM606. I was wondering if sending them out opposed to doing them my self. I don't have a tester, so i don;t see how i can properly balance them. I have only found 1 company that does this service: KermaTDI
Are they good?
Do you know of any others?

Hi Dan... you don't mention where you live, but most major centers have a Bosch Service Center listed in the yellow pages that should be able to calibrate your nozzles. I've not used Kerma myself so can't really comment.


written by jim, March 26, 2011
thanks for the info very cool smilies/smiley.gif
written by Bob, May 29, 2010
Vince, excellent site. thank you so much for providing this resource to the community!
do you know of a source for shims to adjust these guys?

thanks again! -bob

Hi Bob... thanks for your comments!

Unfortunately I don't know of a reliable source for a "kit" of shims... VW will sell you them one by one of course.

I got my stash during a group buy on a VW forum... some kind soul bought a ton of each size and then made individual kits with 5 each... but I've not seen that offer repeated anywhere else since.

Sorry I couldn't help more,

written by Brendan, December 21, 2009
Great page, I have just replaced the injectors on my 84 1.6d jetta and I can't get two of the injectors to stop blowing exhaust out through the injector holes. I have spent close to an hour on each injector hole cleaning every last spec of carbon build up out of them before I put new heat shields in but I can't seam to get them to seal properly, and I have even stepped up the torquing to 54 ft-lbs and then max 56 ft-lbs. I have bought complete reman bosch injectors including the housing so I'm not sure if this could have anything to do with it but I have never had this problem before when I have done compression tests on the engine. I am about ready to pull the last hair out of my head. If you have any suggestions that would help that would be really appreciated.
written by Duffy, September 01, 2009
Just wanted to say sweet site, I never even thought of rebuilding an injector, now I feel stupid. Thanks for the education, this is great stuff. Thanks so much for sharring
written by karl, July 24, 2009
hi i wonder if you can help me
i recently bought a set of injectors for my 2.3 tdi frontera (bosch)and noticed that a little diesel was leaking out between the top and bottom half so i removed them (2) and tightened them up (vice and 22mm spanner as they were new it wasnt too bad) will this affect the injectors in any way (the opening pressures of the jet especially as they were factory set) this is probably a stupid question but i am new to diesel and i am learning as i go if you could answer this i would be very thankfull to you
many thanks Karl
written by john, June 11, 2009
dig your write up!
in order to equalize pressures to ensure even sanding
hand lapping is best done in a figure 8 pattern!
written by Barry, May 12, 2009

Do you happen to know if 1.6 or 1.9 injectors would be better on a Frankenstein engine? I am building a 1.6 mech block that I am fitting a ported 1.9 hydro head to. I will be running the 1.6 turbo/manifolds(port matched) and injector pump. The 1.9 injectors are probably fresher and I wondered if the dual stage would be any benefit and would work ok with the 1.6 pump?
written by Scott, May 09, 2009
Do you have any resources or links to sites that are familiar with the AAZ two stage injectors. I am having difficulty in finding someone who knows how to set both stages.


written by ClintJ, October 16, 2008
Does increasing the pressure of the injector effect the spray pattern? for better or worse?
I just wondered if increasing the pressure would improve atomisation and mpg...

My experience is that for standard diesel there's not much change in the spray pattern between 130 bar and 155 bar. Up around 180 I would say the cone is more defined and less "foggy"... probably not in a good way.

Some people that run bio fuels seem to favour higher breaking pressures so ensure better atomization... I have no experience myself but it makes sense to me.

There are some theories that pump losses start to become a factor over 160 bar, and I've read a couple of places that the sweet spot for mileage seems to be 140 bar... because I run pretty high boost (20+ psi) I've stuck with the stock 150.

The main thing to keep in mind as you experiment is that the breaking pressure and static pump timing are directly related, so if you play with higher breaking pressures you'll need to ease off on the pump timing a bit to compensate.

written by cyrille, September 07, 2008
Hi,great site,by the question : can we use same injectors in a AAZ without the turbo? I rebuilding a 1,9 NA without the turbo and worry about leaving the same injectors. Can I use the old injectors from the 1,6 NA ?

Just my opinion... but I don't think the actual nozzles are all that specific. More important is that they are working properly and with a consistent breaking pressure. So, I'd go ahead and keep your same injectors with or without the turbo... just make sure that they are spraying properly.

BTW... the AAZ injector bodies are quite a bit taller than the 1.6 injectors, so if you were to switch to the 1.6 injectors you'd need to round up a set of matching 1.6 hardlines.

written by Allan Daem, August 31, 2008
Hello Well layed out and informative. Just a comment to Pete, I wonder if someone has "PLAYED" with the pump delivery screw.I have seen this a number of times. Gives more power, more smoke, and decreases fuel economy. Bought a 1992 that was played with that smoked so bad you couldn't see behind you on a hill. Turned the screw out a total of 2 1/2 turns. Runs real well and no smoke. Allan
written by Kurt, August 15, 2008
Is it normal for the pressure to "bleed down" at a visible pace on the gauge if you pause in pumping, or do I just have a leaking seal somewhere?

I finally build pressure, now it leaks down (though if I pump quickly enough, I get a very nice injection from the fresh nozzle... at 163 bar for the first shim I tried! smilies/shocked.gif We'll tap that down a notch, Mmm?

a.k.a Turbinepowered smilies/cheesy.gif
written by Kurt, August 15, 2008
Is it normal for the pressure to "bleed down" at a visible pace on the gauge if you pause in pumping, or do I just have a leaking seal somewhere?

I finally build pressure, now it leaks down (though if I pump quickly enough, I get a very nice injection from the fresh nozzle... at 163 bar for the first shim I tried! smilies/shocked.gif We'll tap that down a notch, Mmm?

a.k.a Turbinepowered smilies/cheesy.gif

Hi Kurt... the bleed-down could come from a couple of places: the injector itself or the valve in your pressure tester. If you're seeing it on every injector you test I'd suspect the tester itself... if you have an old injector body you could build a plugged body with JB Weld and see.

written by Ralph Suges, July 03, 2008
Thank you. It worked out great.

Wheatfield, IN
written by tom, June 26, 2008
Hey Vince, not the MB-Nozzles are the favoured ones, the VW-AAZ nozzles DNO SD297 are the best vor IDI and DI VWengines, also for turbo and non-turboengines
best results

Hi Tom.. thanks for your comments.

Yeah I've certainly been hearing mixed results lately about the Mercedes nozzles... would be really nice to dig up a definitive chart that gives the flow specs etc for the various Bosch nozzles but so far no one seems to have that info... maybe some day !!!

written by pete Skrabek, May 23, 2008
thanks for the info, I am new to VW diesel mechanics, i have a 94 Jetta with a 1.9 aaz. the reason for the new injectors is that when I am drivin on the highway and give it good amount of throotle i get black smoke. i installed a boost gauge and on the highway its about 8psi max, but spools very quickly. and ideas how to make it smoke free?

Hi Pete...

Good for you for trying to figure this out... many people think that "diesels just smoke" and it's too bad because it's one thing that gives them a bad rap... a diesel in proper tune should be pretty much smoke free *except* for a bit of white-blue smoke first thing cold winter mornings and perhaps a bit of black haze under full power.

Black smoke means your engine has is getting more fuel than it has time to burn. It also means your EGT temperatures are higher than then need to be.

My first recommendation is always to start with the basics: timing. Make sure both your cam and injection timing are bang on... somewhere in the 0.95 mm range for an AAZ seems to be the sweet spot but you can move to 1.00mm if the engine doesn't seem too noisy.

Second is injectors: how many miles have they got on them ?? If more than say 100K it's time to have the nozzles replaced and the breaking pressure calibrated. A local diesel shop should be able to pop-test them and recalibrate if you're not sure how old they are.

Third is airflow: adding more air will help the fuel burn better and reduce EGTs. Is your air filter in good shape ?? 8 psi is probably on the low side of normal for an AAZ turbo... you could try dialing up the boost a bit (screw with a locknut on the wastegate)... a couple of psi might reduce the smoke and add a bit more performance.

Best of luck... let us know what you find out !!


written by pete Skrabek, May 22, 2008
Hey Vince what a great page, just wonderin, what is the stock breakin pressure on 1.9 aaz injectors. I got a new set of injectors with merc 300d nozzles that are set to 155 bar. Just wonderin what my stock settings are. And if these injectors are the 1.6 design (single spring) maybe I would be better off putting the merc nozzles on my AAZ injectors(2 stage). Thoughts?

Hi Pete...

Stock breaking pressure for AAZ dual-stage injectors is 150 bar. I've got a set of Merc nozzles but have not had time to put them in my AAZ yet so can't really comment. I've actually gravitated more towards the "stock nozzles are fine for all but the most extreme performance needs... it's all in the pump" point of view and am getting very good performance out of my stock AAZ nozzles. Someday I will play, but when I do I will likely switch my AAZ over to single spring injectors... much easier to calibrate, if "playing" is my objective.

written by Andy, March 13, 2008
Vince, this page is a great resource...well done! I have a question though. When adjusting the pop pressure on these injectors, I assume you change out the washer above the spring. Where can you get thicker washers from? Thanks.
written by Neil, February 12, 2008
Cool pop tester. It appears to be based on a bottle jack. Did you buy or make this?

Hi Neil... yup it's home-built.. a 20 dollar bottle jack and a 10 dollar gauge plus a little welding. OK, so I soon upgraded the gauge to a 30 dollar one but still a bargain considering that they can be 10X that on eBay, and it's a pretty important step in the process. I'm amazed how often I actually use it... lots of diesels in the neighborhood I guess !

written by Bert Carrier, January 15, 2008
Great page Vincent, I threw in a good word for you at the forum site:
Do you know how to make an injector tester?
written by vince w, November 23, 2007
I think it depends on what you are after. In general, if the injectors were accurately calibrated in the past, and if you keep all the parts from each injector together in a group as you are rebuilding them they should stay fairly close to each other overall.

However, the lapping process tends to remove a bit of metal and so I've noticed that my nozzles are usually 10-20 bar over their normal pressure after lapping... so I'd say pop testing is probably important.

I personally want to make sure that they are all as close as possible... another reason calibration is an important step. And, I hate discovering leaks on the engine... much nicer to discover them on the pop tester !!
written by rob, November 23, 2007
After rebuild, how necessary is testing/calibration ??
written by Duane Keaton (blkboostedtruck), November 15, 2007
I like what you've done here! I'm trying out a small vibrating tumbler system from Harbor freight to clean the rust off of the injectors bodies! it seems to be working next i will get thier ultrasonic cleaner from them to do the final cleaning! trying to find something to lap them would be priceless! smilies/grin.gif
well thanks Vince
written by Simon Baxter (Mr Brick-Yard), October 22, 2007
Nice one, tidy page. smilies/smiley.gif

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Last Updated ( Saturday, 13 February 2010 )