Injector rebuild question
 

I've rebuilt injectors in the past and never had a problem but this time out of 8 injectors I have 4 injector housings I can't get right. They are for my 240d. The ones I'm having problems with pop and spray perfect when I pump the handle of the tester steady but if I pump real slow they squirt before popping. When I say squirt it's a steady stream of fuel. I've rebuilt them with new Bosch (India) and new Bosio nozzles and the problem follows these 4 injector housings. Any suggestions? The other 4 even when I pump the handle real slow pop perfect at the correct PSI I have them set to. Should I even worry about this, it may not even be a problem. I lapped them a couple times and cleaned the housing multiple times including boiling them in soapy water and soaking them in acetone.

When you lap an injector body you alter the pop pressure because you decrease the distance between the spring seats. It's just like adding tiny shims. Lapping is to make the body seal tight and I lap to make minuet adjustments to pop pressure.

Stop pumping slowly. Bring the pressure up quickly and to the popping point. Practice.

When you are working the handle on the pop tester really slow and hoping that you will get the Injector Nozzle to suddenly open and spray like it is supposed; if the pop/opening pressure is correct you are checking how well the seating area for the Injector Pintel/Needle inside of the Injector Nozzle is seating well.

When you work the handle you only want to make a short slow stroke trying to approximate the amount of fuel that would actually go into the injector. If you take a really long stroke it is likely to pee out.

If the Pintels/Needles are not kept with the outer Nozzle as an assembly (they were mated together) they can pee out like that.
The seating area can get damaged if dirt got in and the Pintel/Needle needle went up and came down on the dirt.

The Nozzles can simply been poorly made and pee out like that.

There is another test. Once you know the pop/opening pressure of the Injector you are testing you slowly apply pressure to the handle till you are about 200 psi below the pop/opening. What you want to see is no dripping at all (another test of the seating area). However, I believe the manual allows like one drip but per x amount of time.

What you have is some nozzles are better then the others. I believe the manual has that if they pass that drip test they are usable. If you don't want any nozzles that pee out then you need to return them to the seller and hope that the new ones you get are better.

If you apply pressure steadily then the injector should pop at the desired pressure. If they pee before that then it has an issue. You can try to swap the nozzles and redo the tests to see whether it follows the nozzle. We always talk in bar, not psi.

I seriously doubt lapping would change anything. That is a 1000+ grade sandpaper.

Lapping with sandpaper DOES change the pop pressure minutely. I've lapped existing shims AND the injector body to tweak pop pressure ~25PSI. If you're patient, you can get all of your injectors popping +/-25PSI.

Regarding the slow stroke causing the "pee", get real. The injector should positively open and close when it reaches and drops below the pop pressure. A slow stroke should have the injector "chatter". If it's peeing or squirt-gunning, it's hanging up or failing to seat fully.

I've been able to get the pressure adjusted with shims but the peeing seems to follow the injector housing not the nozzles. If I apply pressure steady and firmly they spray perfect if I apply pressure slowly they pee before they pop. I thought maybe I had contaminated fuel but the other injectors test perfect. What doesn't make sense to me is the problem seems to be with the injector housing not the nozzles.

I've had the same issue with bad injector holders. The issue is that they are not perfectly "square" anymore and allow the pintle to lift unevenly causing that "pee". You can attempt repair by polishing out any rust or carbon internally to the lower holder half. If that doesn't do it, don't use the injector holder.

Are you using a centering cone when installing the nozzles?

Well shoot.
 

I submit that all five of my injectors produce a pop pressure of exactly 1975PSI. I lapped them with 1200 grit to get the difference between 1975 and the closest to 1975 the shims could produce. Some were lapped to produce as much as 9 psi. Yes it takes patients and hard work but I'm retired and had nothing else to do at the time.

I'm sorry you don't believe me.

Good for you and I believe you. Your pressure gauge must be a very accurate DIGITAL unit. I don't need that kind of accuracy for a 30 years old Car. Give or take a few bar is good enough for me.

My pressure gauge is a cheap chinese product. I should have clarified that numbers were the same across the board and let it go at that.

I may have embellished the quality of my work a bit but I assure you, sir, if you lap a body many times you'll change the pop pressure. The OP seemed to struggle with thinking that lapping the bodies would remedy the peeing syndrome he's experienced.

Try new springs, these wear along with other parts, not sure if there is tolerance test but cheap to replace anyway.

Injector helical compression spring
 

Old springs should not be reused since they were subjected to metal fatigue and permanent deformation. New helical springs from Mercedes Benz cost about $5.00 (PN 000-017-04-17).
References:
https://www.physicsforums.com/threads/how-to-tell-if-a-spring-is-fatigue-creep-or-degrade.891665/

“extremely rapid cycling over an extended period can result in internal heating of the spring wire that can result in a loss of spring rate and free length”

https://www.benzworld.org/forums/w123-e-ce-d-cd-td/1316064-injector-rebuild.html

The original springs are good for hundreds of thousands of miles and multiple injector rebuilds. We're talking about a movement in the sub-millimeter range. If anyone here has ever had an injector spring break from fatigue speak up now...