compression test - PeachParts Mercedes-Benz Forum

goldveg · #1 05-05-2014, 06:33 PM

Does anyone know the thread size for the injectors off of a 1983 mercedes 300td. Is it 18mm X 1.5? I am trying to buy a compression tester. Thanks in advance, goldveg

Diesel911 · #2 05-05-2014, 10:36 PM

Quote:

Originally Posted by goldveg

Does anyone know the thread size for the injectors off of a 1983 mercedes 300td. Is it 18mm X 1.5? I am trying to buy a compression tester. Thanks in advance, goldveg

18mm X 1.5 is a Spark Plug size and Compression Testers for only for Gasoline Engines have Gauges that do not have high enough pressure.

The Injectors threads for your Year and Model are M24x2 Injector/12mm x 2

There is also a Compression Tester Adapter that goes into the Glow Plug

Hole but in some areas you need a Tester that has a 90 degree elbow.

Horbor Freight sells a resonably inexpensive Compression Tester for Diesls. The have been know to have some occasional problems so if you get one besure to use it before the warranty ends.
Glow Plug thread size M12-1.25

mach4 · #3 05-05-2014, 11:18 PM

Quote:

Originally Posted by Diesel911

Horbor Freight sells a resonably inexpensive Compression Tester for Diesls. The have been know to have some occasional problems...

Been there, done that... took me three tries to get one that worked. Despite that, I'm happy with the kit and the results.

ROLLGUY · #4 05-06-2014, 12:20 AM

I have used my HF compression tester several times with no issues. They have no adapter for the newer injectors (fine thread). I had to get a spare injector and make an adapter when I did my bro's 602.

Stretch · #5 05-06-2014, 02:55 AM

Always best to buy a tester with an adapter that fits the glow plugs - then you don't have to replace the injector crush washers

strelnik · #6 05-06-2014, 09:05 AM

Quote:

Originally Posted by Stretch

Always best to buy a tester with an adapter that fits the glow plugs - then you don't have to replace the injector crush washers

GOOD ADVICE!

Look on Ebay for Motometer diesel compression testers, they have a lot of parts and adapters. I have seen several sets that fit the W123 injectors, don't know about the glow plugs.

crazy4diesel · #7 05-06-2014, 09:17 AM

I bought the Harbor Freight one, must have been lucky the first one worked just fine, and I think I had 25% off coupon, and it was on sale, ended up around $35...so not complaining.

ROLLGUY · #8 05-06-2014, 09:29 AM

Quote:

Originally Posted by Stretch

Always best to buy a tester with an adapter that fits the glow plugs - then you don't have to replace the injector crush washers

Agreed. I only did the test on a 602 through the injectors because of the pain to remove the manifold. On the 61X, it is easier to test through the GP's.

BillGrissom · #9 05-06-2014, 03:37 PM

Only problem with my HF tester is that the quick-coupling likes to pop off the injector adapter. I just held it down with my hand last pass. Those are standard air compressor parts so I should be able to replace them. It only pops off as I hit the final >400 psig. If your engine has worse compression, it probably wouldn't happen.

Diesel911 · #10 05-06-2014, 07:53 PM

Quote:

Originally Posted by Stretch

Always best to buy a tester with an adapter that fits the glow plugs - then you don't have to replace the injector crush washers

Another reason is that the Harbor Freight Set the Injector Adapter that has a large emmpty Space in side that reduces the compression pressure.

The Glow Plug Adapter has a smaller empty space.

Ritchie · #11 05-06-2014, 08:36 PM

What size is the glow plug? I am looking at a Matco
Like the original post, I also saw 18 mm x 1.5 somewhere but for the glow plug not the injector. And that Peugeot was the same as the M617. ??

OM603 · #12 05-06-2014, 11:42 PM

Quote:

Originally Posted by Stretch

Always best to buy a tester with an adapter that fits the glow plugs - then you don't have to replace the injector crush washers

Quote:

Originally Posted by strelnik

GOOD ADVICE!

Look on Ebay for Motometer diesel compression testers, they have a lot of parts and adapters. I have seen several sets that fit the W123 injectors, don't know about the glow plugs.

With crush washers widely available for less than $2 each, it hardly seems to be any bargain in terms of ease of effort to choose removing the glow plugs down in the abyss between the IP and the head with their easily dropped wire nuts, and awkward access, over removing the injectors and testing with that adapter. Just time wise alone I'd bet it takes three or four times as long to get the glow plugs out compared to getting the injectors out. And properly done a compression test should be done on an engine at operating temp so getting down to the GP's can be uncomfortable as well.

If when replacement injector return lines are installed their loops are made a couple inches longer than the minimum length required they can easily be reused by just cutting them off just above the nipple loosing only the 8 or 10 mm on each end whenever they need to be disconnected, so you can avoid replacing them in their entirety each time doing it that way as well.

OM603 · #13 05-07-2014, 01:00 AM

Quote:

Originally Posted by Diesel911

Another reason is that the Harbor Freight Set the Injector Adapter that has a large emmpty Space in side that reduces the compression pressure.

The Glow Plug Adapter has a smaller empty space.

Because the internal space of either adapter is not a great portion of the tester system's overall internal space and because the tester total system volume effect on compression pressure is reduced by an order of magnitude with each subsequent compression stroke's pressure accumulation.

I'd be very willing to bet that the tester gauge tolerance far exceeds any significantly measureable inaccuracy that could be induced by adapter choice.

Only by ignoring the insignificance of any such adapter differences and ignoring the operational functioning of the tester system could adapter choice have any effect on measurement outcome.

Test at operating temp, properly connect a functional tester system, and follow established procedure of ensuring at least 8 rotations of the engine per cylinder test, and there can be no significantly measurable adapter choice effect.

Stretch · #14 05-07-2014, 02:00 PM

Quote:

Originally Posted by OM603

With crush washers widely available for less than $2 each, it hardly seems to be any bargain in terms of ease of effort to choose removing the glow plugs down in the abyss between the IP and the head with their easily dropped wire nuts, and awkward access, over removing the injectors and testing with that adapter. Just time wise alone I'd bet it takes three or four times as long to get the glow plugs out compared to getting the injectors out. And properly done a compression test should be done on an engine at operating temp so getting down to the GP's can be uncomfortable as well.

...

As mentioned above it is a lot easier on an OM617 (rather then the OM603 I assume you are talking about)

Ritchie · #15 05-10-2014, 12:17 PM

:-) the Matco kit DCT 1553 came with three adapters that fit my 83 300d. Two glow plug (TU-15-5, TU-15-6) and one injector (TU-15-4A). This is a very nice, quality kit.

I searched the 1983 300D catalog here for "glow plug" and and got some conflicting results on the adapters.