Injector testing questions, etc - PeachParts Mercedes-Benz Forum

Tubularfab · #1 09-20-2012, 08:53 PM

Ok, I tore apart the recently acquired 1987 300d to do some diagnostic work. Started with a hot compression test yielding 320, 340, 340, 300, 320, 320. Not really bad while not great either. I also decided to pop test the injectors while they were out since I have a friend with a tester. Engine has 250,000 miles on it, with some svo expiramentation from PO in very recent miles. Has a good bit of blowby, so that plus the wvo and lowish compression led me to an MMO soak for the last 2 weeks.

Anyway, I was wondering if any experts could give me input on the injector pop test I just did. 5 injectors gave reasonable spray patterns and were close to spec. Here is an example:
Injector testing - good - YouTube

One was rather stubborn at first and yielded this:
Injector testing - worst one - YouTube

After pumping a bunch of fuel through it it cleaned up, but still streamed a bit. Cleaning did not help. Then I checked some of the others again and noticed with a slow pump I could get a stream at about 75 psi under the actual pop pressure. Is this a bad sign, or just a bad way to test? The bottle jack tester can flow a bunch of fuel, so would be nice to know how to simulate exactly what the IP puts out.

Diesel911 · #2 09-20-2012, 10:10 PM

I can only speak in general.
The Injector is not supposed to leak any Fuel at 75 psi.

One of the tests is that you bring the pressue to 200 psi below the opening pressure (the actual measured opening pressure of the individual Injector); if your Pop Tester has no internal leakeage you should be able to hold that pressure unless the Nozzles is leaking. If the Nozzle is leaking you work the Handle slowly to keep the pressure 200 psi below the opening pressure.

At that point you are allowed x amout of drips. If it is a really good Nozzle it will not drip at all. If it squirts out the Nozzle is shot; same if it drips too much.

Also on a good Nozzle when you work the Handle slowly up to the opening pressure the Fuel will spray out suddenly with no dripping and cut off suddenly. If it is exceptional it will make a chattering or squeeling sound when the Nozzle sprays.