dummy injector for compression testing - PeachParts Mercedes-Benz Forum

scottmcphee · #1 08-01-2011, 06:33 PM

The dummy injector that came with my el-cheapo compression tester had malformed threads and no way was I going to screw it into my head and booger the hole... Tossed it out.

Has anyone taken a stock Bosch type injector and "hollowed it out" for making compression readings? I have some spares.

The el-cheapo tester has a bunch of other kinds of adapters and things for testing all kinds of engines via glow plug or injector holes. Glow plugs are to hard to get at with my engine, I want to do it injector style. All of the adapters have a schrader valve in them for holding pressure readings at the dial gauge. Each adapter has a quick release type coupling (like air compressor tools) to mount the gauge on any adapter.

I can find no combination of adapter parts in the kit that mate with the top of a Bosch injector where the nut screws down onto it.

Maybe somebody has done something?

Jetmugg · #2 08-01-2011, 07:37 PM

Not personally, but a buddy of mine took apart a Bosch injector, drilled and tapped some threads for 1/4 NPT, then put a regular "air line" quick disconnect fitting on the top side. It worked great.

SteveM.

Diesel911 · #3 08-01-2011, 07:48 PM

Yes, it just depends on how much work and expense you are willing to go through.
If you already have Drills, Taps, and Dies and such it might not be much of an issue.

scottmcphee · #4 08-04-2011, 12:13 AM

Well, I can't believe how lucky I was to find the perfect brass fitting to make this happen!

In the pressure washer parts section of my favorite hardware store here in Canada "Princess Auto" (like a Harbor Freight in USA), I found a brass adapter fitting with the needed metric thread pitch (22mm x 1.5mm) for screwing into the injector bottom half, and has a 1/4" NPT female on its other end. Then a very standard brass fitting dropping the 1/4" to 1/8" NPT female and ta-da!! The quick change compression tester fitting screws right into that. The overall height of this is about the same as a standard injector. Total cost about $7 for the brass parts, no tap&die, or welding.

A little more detail: I separated and tossed the top half of the injector, removed and tossed the pin from the nozzle, and enlarged the nozzle tip hole to about 1/8" using a diamond coated cone shaped Dremel tip. (No drill bit would bite into the nozzle metal, dang it's hard!) I put the nozzle body and its crowning cap in the injector in their normal orientations, but tossed the spring seat pin, the spring, and the spring shims. Then I dropped a couple standard metal washers in next so the brass fitting would screw down onto this stack of parts to keep the nozzle body located firmly in its natural position. Thread sealant and wrenched all the brass fittings down firmly, done!

Now, I can finally test compression, something I've never done with my car.

Diesel911 · #5 08-04-2011, 12:23 PM

I had forgotten about this when I made my previous post but there is dead space inside of the Injector where the Spring goes.
The dead space will decrease your compression reading a little. This is not a big issues unless you end up with a border line minimum compression.

scottmcphee · #6 08-04-2011, 02:42 PM

Yes, keeping air space inside the false injector to a minimum to get truer readings... I thought of that! I filled space inside with metal bushing parts laying around, just to fill the void with something uncompressible.

I had another idea about putting a thin soda straw through the brass fittings and fill the void with epoxy / JB weld, then when it dries trim the straw.. but I didn't go there.

funola · #7 08-04-2011, 02:51 PM

You did it the hard way. I wonder how low a reading you will get with the extra volume in the injector?

Glow plug hole is much easier.

http://www.youtube.com/watch?v=a5AB-Es6MZk

http://www.youtube.com/watch?v=G1MB1ygVqXM