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

boneheaddoctor · #8 08-04-2011, 04:13 PM

Instead of going through the Injector hole that needs a new heat shield every time it comes out.....why not the Glow plug hole that doesn't.

Diesel911 · #9 08-04-2011, 04:31 PM

Quote:

Originally Posted by boneheaddoctor

Instead of going through the Injector hole that needs a new heat shield every time it comes out.....why not the Glow plug hole that doesn't.

"Glow plugs are to hard to get at with my engine." 87 300D.

scottmcphee · #10 08-04-2011, 06:21 PM

..thanks Diesel911, beat me to the punch.

I *wish* I could just pop a tester into a glow plug hole, but this is a OM603.

Update: I re-did the innards of my false injector.. I had a hunch this would work out, and it did: two nozzles fit back-to-back in there. For some miraculous reason the machining of the brass fitting was such that the second nozzle slid up into it... perfectly. So here's how it's built now: Lower injector half, drop a gutted nozzle in as you would normally position it, dropped a thin copper crush washer (one size that comes with each oil filter I buy fits), drop a second gutted nozzle down the hole backwards (butt to butt), screw on the brass fittings.. etc. The second nozzle tip hole is Dremelled larger, like the first.

..and regarding that heat shield question... I was wondering... There's a nylon washer that came with the compression test kit, that is the same size as a heat shield. Is it meant to drop down the hole (like a shield) for doing the compression readings? Or would it be "OK" to re-use the old shield for making a compression test reading.

(Of course, re-install of the injector always gets a fresh shield, I know that much.)

boneheaddoctor · #11 08-04-2011, 06:39 PM

Quote:

Originally Posted by Diesel911

"Glow plugs are to hard to get at with my engine." 87 300D.

I'll agree.....I was thinking OM617 from someone elses thread.

funola · #12 08-04-2011, 08:00 PM

Quote:

Originally Posted by scottmcphee

..thanks Diesel911, beat me to the punch.

I *wish* I could just pop a tester into a glow plug hole, but this is a OM603.

..and regarding that heat shield question... I was wondering... There's a nylon washer that came with the compression test kit, that is the same size as a heat shield. Is it meant to drop down the hole (like a shield) for doing the compression readings? Or would it be "OK" to re-use the old shield for making a compression test reading.

Wanna trade?

Which HF kit do you have? Mine came with a copper washer, not nylon. Problem with the HF kit is there are too many connection points. Each one is a potental leak. I have heavily modified mine and the only parts used are the gauge and the tips. Mine don't leak no more.

scottmcphee · #13 08-05-2011, 08:43 AM

The compression tester kit was an ebay acquisition, very China brand. I think it was $50 all in.

So, is it fair to say the nylon washer that came with my kit is meant to drop down the hole - like a heat shield - when taking injector hole readings? Are you recommending copper is better?

funola · #14 08-05-2011, 09:12 AM

I don't have a clear picture of what your adapter looks like, but if you have an actual nozzle to pass the compression to your gauge, then you need a good seal between the nozzle and the head, i.e. a new heat shield or equiv, otherwise it would leak out the injector body to cyl head threads if it was not well sealed.

Oh and BTW, if your gauge slowly drops after you stop cranking, you have a leak

Diesel911 · #15 08-05-2011, 10:24 AM

Quote:

Originally Posted by scottmcphee

The compression tester kit was an ebay acquisition, very China brand. I think it was $50 all in.

So, is it fair to say the nylon washer that came with my kit is meant to drop down the hole - like a heat shield - when taking injector hole readings? Are you recommending copper is better?

Most Indirect Injection Diesel Engines use a Copper Washer (or an Aluminum Waser) to seal the Injector at the Precombustion or Squish Chamber.

You can re-use the Copper Washers as long as they are not too distorted by heating them red hot and letting them cool.

The Nylon washer may also work for checking compression. Some Hardware stores sell Nylon Whashers (incase it splits).