I collected parts over the last year and finally got around to putting together my pop tester. I started with info from these links:
How to build a diesel injector pop tester
DIY Pop Tester
Replace injector nozzles, set pop pressure, big improvement! - Topic
The hydraulic setup is from a kit to test a pressure relief valve in Ford Rotunda Diesel engines, sold to dealers, and same as the 3rd link above. I bought on ebay ($120 I recall), much less than the original >$500 price. The pump looks similar to an Australian model that sells for ~$450.
The fittings and pressure gage are from McMaster-Carr. From my notes:
1 5269K311 Metric fitting, 1/8" NPT-M to 6mm tube ($4.11)
2 50925K197 Steel T fitting, 1/4" NPT, MxFxF ($6.38)
1 50925K341 Steel bushing, 1/4" NPT-M to 1/8"NPT-F ($1.60)
1 50925K133 Steel fitting, 1/4" NPT-M to 3/8"NPT-M ($2.00)
1 4000K713 Gauge, Dual, 3.5" dial, 1/4"NPT-M bottom, 3000 psi range
The 6 mm fitting above is the trick, to fit the M-B fuel tube. Other people have fabricated their own, not knowing it exists if you know the PN (now you do). Use high-pressure steel fittings everywhere, not brass, and check the pressure ratings.
The pressure sensor is a GE Druck 0-3600 psig, w/ 1-5 VDC output. I got it cheap on ebay ($22 I recall) and power with a 9 VDC AC adapter. For data acquisition, I use a Dataq DL-149U ($15 ebay), which has a USB interface to a notebook PC. I used the free software, which limits to 250 Sps speed. I sprayed into a yogurt cup with a stainless strainer from 99 Cent store.
I first tested 5 injectors from my 1985 300D engine which failed 3 yrs ago at 330K miles (#1 piston beat up, others missing chunks). I bought the car at 278K miles and the prior owner had it from new and regularly took it to the dealer (at high cost), but I didn't see where the injectors had been replaced (need to review the thick file). I wondered about bad injectors, but all popped uniformly at 1900 psi on the gage. The electronic trace shows them oscillating between 1900-2000 psi, so the peaks might be too quick to catch on the gage.
I added a 0.1 mm (4 mil) shim to each, which brought each to 2000-2100 psi oscillations. Another source stated 0.1 mm = 100 psi increase, on average. A few increased ~150 psi. 4 had a factory washer of 56 to 60 mil, but one had 35 mil though still popped at the same pressure. I am guessing it was a dealer replacement. All had "Bosch" on the housings and internal nozzles. The sound and spray pattern looked just like this video (not mine):
pop-testing - YouTube
I put these 5 into the replacement 1981 engine now in my 1985 300D. They made no noticeable change in the engine. It idles smooth, but the air cleaner vibrates (like most). I then tested the 5 injectors from the 1981 engine. I bought it cheap on craigslist, so don't know the history, but it looks new on the top end and compression was 400-410 psi in all cylinders (tested with injectors out). They all popped at the same 1900-2000 psi oscillation and fine spray cone. I haven't opened to clean or shim yet. I will later test the injectors in my 1984 300D.
Finally, I read many posts with questions about buying shims, with high prices for small kits, or people buying punches to make their own. It seems I am the first to competently search McMaster-Carr because I got them for ~$4/bag of 50. I got ~4 sizes, but only used the 0.1 mm ones. They even have "Made in Germany" on the bags. Store this info:
PN 98055A105, Metric Spring Steel Shim - DIN 988, 0.1 mm th, 6 mmID, 12 mmOD, pack of 50. I also bought a bag of PN A106, A107, & A108 (0.2, 0.3, 0.5 mm).
I know this helps undercut high-priced sellers, but I enjoy helping cheap bastards like myself. I considered keeping my secrets and offering pop testing to others, but I am a well-paid aerospace engineer and don't want to fool with fussy customers, so pop your own injectors. If I lose my job, I may grovel for testing work. As for the instrumentation, that was simple for me since I once did academic experiments in liquid sprays.
One problem I ran into was that the pump started relieving ~1000 psi on the 5th injector, with nothing leaking out anywhere. Same deal when I tried an earlier injector, so assumed it was inside the pump. I took it apart and found 2 screw-in spring relief valves from the head back to the reservoir, one stamped "1000 psi" and the other "3500 psi". Before that, I had taken out the 2 ball check valves and inspected, but no change. The pump piston has two bores, so it must be 2-stage - a big piston (1000 psi) to move volume then the small piston comes into play for the high pressure (3500 psi). Anyway, I didn't find root cause, I just unscrewed the 1000 psi relief valve and inserted a 1/8 NPT plug there. I only had a brass plug, so hope it doesn't yield, though it would just go into the reservoir. The pump worked fine after that change. The diesel injectors serve as relief valves anyway, so if I only use for pop testing the pump should be safe.