Yeah, I bought that piece of chinese crap and barely performed 3 full tests before it broke. once cold, once warm, and again after soaking the cylinders in MMO, and driving it around for a day or two. on the third time, the fittings started going out and the hose started leaking.

I resent that "U.S. General" exports jobs to import this kind of garbage into the USA.

needless to say i took it back and explained (politely) in so many words that the device was a failure and got my money back. i have only myself to blame for my disappointment though as i recall thinking while inspecting the purchase in the car, "hmm why do they have three copies of the same attachment?" ... 2 weeks later ... "ahh, because the attachments are of inferior quality and the manufacturers know that"

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'82 240D 224K miles manual transmission
mods: wooden 4by4 bumper, EGR delete and older EX manifold without EGR port, glass pack muffler (cheapest replacement muffler), rebuilt bosch injectors with Monark nozzles

working on: aux electric fuel pump, coolant/fuel heat exchanger/filter head, afterglow, low oil pressure buzzer/LED

actually, they are different fittings, thread pitch, etc -

that is true, there are different fittings, but there were various numbers of copies of each kind of fitting. the one of which there were 3 copies - luckily they were the ones that I needed, because all three eventually failed.

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'82 240D 224K miles manual transmission
mods: wooden 4by4 bumper, EGR delete and older EX manifold without EGR port, glass pack muffler (cheapest replacement muffler), rebuilt bosch injectors with Monark nozzles

working on: aux electric fuel pump, coolant/fuel heat exchanger/filter head, afterglow, low oil pressure buzzer/LED

Great!

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1984 300td euro turbo

There are two schools of thought amongst engineers / mechanics about compression tests.

1/ They are a waste of time, either you have a problem or you don't, if you have a problem you need to get the spanners out, and the unspoken but crucial statement that any decent fitter can tell if there is a problem.

2/ They are a handy diagnostic tool, they are especially useful as a empirical tool, e.g. they will actually give reproducible numbers.

The following is what compression tests can and can not do.

1. Compression test gauges are not calibrated, this means ten different gauges will probably give ten different readings.
2. A quick and dirty way to to test a compression test gauge is to connect it to an airline with a known pressure set regulator.
3. A compression test gauge is only as good as the non-return valve.
4. Hot engine tests and cold engine tests are two entirely separate things...
5. Hot engine tests will find things like too tight tappet clearances, bad valve seats (esp with alloy heads) and suchlike.
6. Cold engine tests will find everything.

The best nearly free add-on you can give to a compression test kit is a tee connection with a PCL male (air line) tail, and a low pressure (50 psi tops) gauge to swap out for the standard 300/450 psi gauge. More on this later.

As others have said, pull ALL injectors / spark plugs / glow-plugs (whatever you are using to connect the test set) to stop cross bleed.

Atmospheric air pressure is approx 15 psi / 1 bar, an engine with a 22:1 compression ration will in THEORY therefore produce a maximum compression pressure of 22 bar, or 15 x 22 psi = 330 psi... in practice it is ALWAYS lower, there is always SOME leakage.

What nobody tells you to do, make sure the battery is in tip top condition, put a 10A charger on the battery, and leave it connected while doing the test. This is the ONLY way to keep cranking speed, and therefore compression, constant.

1/ Cold dry test, crank the engine for 5 seconds, read pressure, write down result, move to next cylinder.

2/ Cold wet test, same as the dry test, but one squirt of motor oil from an oil squirt can immediately prior to connecting the test adapter, crank for 5 seconds, read pressure, write down the result, move to next cylinder.

Even on a brand new engine, there will be a notable difference between the results of the cold wet test and cold dry test, but this should be less than 30 psi / 2 bar.

One any engine differences between cylinders, wet or cold, should be less than 15 psi / 1 bar.

Anything outside these ranges = get the spanners and your wallet out.

Now you need to pop the rocker cover... on any engine, the inlet and exhaust valves "rock" at TDC exhaust stroke.

Now swap the 300 psi gauge for the 50 psi gauge, add the tee between the compression tester and the engine.

MOST IMPORTANT

Set the compressor pressure regulator to 30 psi MAXIMUM. 10 psi recommended.

A 3" diameter piston has a surface area of 7 square inches, 20 psi over 7 square inches is 140 pounds force pushing the piston down, this WILL crank the engine, lots of engines use just this method to start.



Now use a valve (eg dust blower) on the end of the airline, and use a short piece of rubber hose and two hose clips to secure between the dust blower nose and the PCL tail in the tee. Ideally a dust blower with a flow limiting valve such as you can find on a high pressure spraygun, if not you can use a clean HPLV sparaygun, start with the valve much nearer to closed than open.... you get this valve set "right" on the first test below, once you get this setting, DO NOT ADJUST.

Now you want to do four checks per cylinder, two dry, and two wet (with a squirt of oil)

1/ Cold, dry BDC (Bottom Dead Centre) test. No engine starter use, bar the engine over by hand to BDC on cylinder to be tested, open airline valve and see how high compression test gauge reads... if it doesn't read anything, AND if this is the FIRST cylinder/test, GENTLY open the valve on the duster until compression test gauge reads around 70% of actual air supply pressure, so if supply pressure is set to 10 psi, you want to aim for 7 psi....
Read and write down pressure.

2/ Cold, dry Middle Stroke, eg halfway between BDC and TDC, make sure this is the compression or ignition stroke, see the valves... same test as above, read and write down pressure.

3/ Repeat for all cylinders.

4/ Repeat 1, 2 and 3 (NOT the air valve adjustment, you already did this) with a wet test, again 1 squirt of oil.

read and write down pressure.

===================

If you connect the air line at 100 psi, you will be generating nearly one third of a ton downward pressure on the piston, the engine will TURN, and strong enough to cripple you for life... amputated fingers won't even make it pause...

Unlike the cranking test, which measures compression, this test measures piston blowby, both at BTC where there is minimum torque and therefore wear, and also at mid stroke, where torque and wear is highest, this is an excellent way to tell worn bores from stock/broken rings.

If you gain some experience you can put each piston at BDC and up the air pressure to 15 psi, and hold the air valve open, this will also show (assuming the piston / rings are not shot to hell) valve seat leakage, you will see / hear it coming out the ports at the top of the engine.

=============================

Top tip

People (not engineers) always approach these types of test hoping for the best, and so adopt a testing technique / regime that is almost superstitious, check for cheap to fix problems first, then, slowly, fingers crossed, work your way up to the test for an expensive diagnosis, and by then feel the same way about the test as being anally probed with a cactus.

Fact is, whatever is wrong, is wrong, the best thing you can do is find out the right answers as soon as possible, then, and only then, can you make sensible plans.

I started this talking about the two schools of thought amongst engineers about compression tests, I am of the first school of thought. Which is where I am coming from when I say the following.

Only two sorts of people buy compression testers.

1/ People hoping to avoid bad news... yay, I have 200 psi in every cylinder on my diesel... which is good enough for an old car, so there is nothing wrong with my WVO conversion.

2/ People hoping to avoid bad news... yay, I have 200 psi in every cylinder on my diesel, that means I don't need to refurb the entire injection system, which frightens me even more than the prospect of a blowing valve.

What none of this helps with is a total and utter lack of experience, and rebuilding your 603 lump every weekend for 5 years until you can do it upside down by touch now a well doesn't necessarily give you experience...

The 603 had hydraulic tappets.

My car had an intermittent noisy tappet when I bought it, add 1 litre of ATF to the lube oil and it cleaned out.

Shade tree mechanics will tell you to go to the OEM supplier and buy the complete set of 12, it's a 2 to 3 hour job for the home mechanic.

EXPERIENCE will tell you than when more than two hydraulic tappets on a 603 are noisy, chances are it is the oil gallery supplying them made by the head gasket that is leaking, e.g. it needs a new head gasket...

The tests I have written down above don't give you experience, but they DO give you empirical numbers, which you can then show to someone with experience.

Cylinder #4 is down 2 bar on the rest tell the experienced mechanic a lot.

Cylinder #4 shows 10 psi at BDC in the airline test and 3 psi at mid stroke tells the experienced mechanic a lot more.

HTH etc

Some things I forgot, having re-read that.

Do a cold dry test, write down the results, do a cold wet test, write down the results, do the "MB official" hot dry test, write down the results.... compare and discuss... if you haven't ever done this, don't try and tell me about "MB official" being the gold standard.

Compression can leak past rings, past valves and through the gasket... there is a correlation between the RPM of the engine and compression because there is a correlation between RPM and event duration and the amount of leakage that can occur in that time....

A good battery will crank faster, therefore generate higher compression, which equals higher compressed gas temperature, which equals easier diesel ignition which is easier starting.... a lot of ****e old diesels with 12 VDC can be started by disconnecting the alternator and putting 24 VDC across the starter... much faster cranking... this should tell you the importance of a GOOD battery, ample cold cranking current capacity, solid electrical connections and a good starter.

Cold air is denser, and denser air compresses harder than rarer air, see all the above, but humid air is also dense, but the thermal capacity of the moisture in the air saps loads of heat from the compression process, so a freezing dry day can be an easier start than a foggy day 15 degrees warmer.

Fuel follows the same rules, any given fuel will atomise better if it is hot than of it is cold, that (injector nozzle) heat can come from a glow plug or compression.

Atomised fuel will distribute itself better in a hot compressed gas than a colder gas at the same temp, or a damp gas at the same temp, or the same temp gas at lower pressure.... gas is a fluid according to all the equations.

Flame fronts (combustion) happens by free electrons triggering the unburnt fuel to burn, releasing more free electrons.

Aluminium absorbs more free electrons than iron (hence alloy heads allowed more powerful engines, eg anti knock) and hotter metal absorbs free electrons more slowly than colder metal, so again the heat of compression has to warm the surface layers of the piston and combustion chamber.

Gas or Diesel, 99% of the engine is in the cylinder head, take that away and you have a workshop compressor, or a vacuum pump, exact same thing and principle minus combustion and fuel.

Hence the old adage, any diesel that starts cold with no fuss has bugger all wrong with it.

12 VDC into a load of 0.1 Ohms will drive 120 amperes, Volts x amperes = watts so 12 x 120 = 1,440 watts, or about 2 bhp.

12 VDC into 0.2 ohms will drive 60 amperes, 12 x 60 - 720 watts of just under 1 bhp.

0.1 OHMS of resistance in your starting circuit will rob your starter of 1 bhp, and therefore your cold start compression of 1 bhp.

HTH etc

Who sells that compression tester that connects to a GP fitting?

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

Harbor Freight does. Here's mine, highly modified. Fastest compression test yet I dare say.

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

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

http://www.harborfreight.com/12-piece-diesel-engine-compression-tester-93644.html

http://www.harborfreight.com/14-piece-diesel-compression-tester-set-46800.html

I looked at the 14 piece kit and it does not include a glow plug adapter! $149 bucks? Ouch!

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

uh. sure it does. full length probe insertion as well... two of them. one for the #22 head as well...

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

I assume you have the $149 kit? I looked at one (Dropnosky's 14 piece set) and mine (12 piece $19 set) side by side and did not spot a glow plug adapter that has the same thread as mine (M12 x 1.5?). Maybe I missed it. Can you point out which is the glow plug adapter in the pic?

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I understood most of that diagnostic procedure posted above with the air fittings and such, but I'm confused on one point. What does the cranking speed have to do with compression?

Compression is the result of squeezing a fixed volume of air (what the cylinder can hold) into a fixed space (The remaining space when the piston has reached the very top of its stroke), true? The air volume is going to be the same, the cylinder capacity is going to be the same, and the top point of the piston's travel is going to be the same no matter how quickly or slowly it reaches that point. So why is having steady cranking speed so important for a compression test? Theoretically it would make the same compression even if you turned the crank with a wrench instead of the starter -- UNLESS you've got wear letting some of that compression pressure leak down as it's produced.

What am I missing?

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~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL

Because the rings and valves do not provide a perfect seal.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

The threads on the glow plug are M12 x 1.25.

If it's a pencil style for a 616 or 617.

Attached Thumbnails

 

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83 SD

84 CD