Need Definitive OM617 Compression Test Procedure
 

Just purchased the Harbor Freight kit and wish to test compression and will use GP holes but need a good procedure from one who has done this the right way. Valves were just adjusted -anyone? Thanks!

This is from a different compression test kit.

It needs to be done on a fully warmed up engine.

Your harbor frieght compression tester is meant to be used through the injectors holes on the 617 engines. Not sure there is an adapter in the kit that will allow you to go through the GP hole. I have the same kit.

There is some arguement wether it's better to test compression on a cold or hot engine. To me the compression on a cold engine is far more important than that of a hot engine. A hot diesel is much easier to start, even with pretty poor compression. A cold engine with poor compression is much harder to start or won't start. A hot reading will give you a more acurate reading of what the compression is when the car is warm and running, but the cold compression will tell you what you are dealing with when you go to start the car in the morning.

As for doing the test. You screw the adapter in. Plug in the gauge, and crank the engine and watch the gauge go up until it stops going up. And thats your compression. If you do go through the GP holes you'll need to remove the injector lines or wire the stop lever down, to keep the engine from starting. Also if you take all the GPs out you'll reduce the draw on the battery and starter, because it won't be fighting the compression of the other cylinders to turn over.

I think HF needs some help with their technical writing and editing.

"compression text"

Answer
 

PROCEDURES FOR: COMPRESSION AND LEAKAGE TESTING
http://cust.educ.ubc.ca/wstudents/tsed/Students06/cdemers/PROCEDURE%20FOR%20COMPRESSION%20TESTING.doc

That's what happens when you have 8yr olds in China writing the documentation to go with the part that his 9yr old brother built...

Good information, thanks but seems specific to gassers. I was hoping that someone had the actual FSM for the OM617 motor -thanks!

It's not a difficult task. Its going to be the same for any vehicle wether its gas or diesel. Disconnect the fuel supply. Remove the spark plug, glow plug, or injector(depending on you the fitting you have). Plug in your gauge, and crank the engine over several times until the gauge stops going up. Mark down the value and move to the next cylinder.

The FSM suggests doing it on a fully warmed up engine. That info is straight from the people that designed the engine. Doing a test on a cold engine will not accurately diagnose any engine problems. An engine is only cold for a short period of time. Most of the driving is done with the engine at full operating temperature. Does it make any sense to run tests on a cold engine if most issues occur when the engine is fully warmed up?
BTW, I wouldn't trust any tool from HF to accurately test anything. I'd bet their gauge would be off +/- at higher readings compared to a true professional gauge.

DJ, I'm not picking on you bro, just the concept. :D

I know thats what is suggested, it's because everything warms up expands, and seals up, which will give you higher compression, and a better picture of what's going on with a running issue. Which is important.

But I ask why then, when we all go look at a potential purchase of our beloved diesel cars, do we want to start it cold. Beacuse it tells you how good the compression is. If it fires right off on a blustery cold day, you know you've got a tight engine, if it takes some doing to get is started you know the engine is getting a little tired. And why is is getting tired, amongst other things, compression.

I guess it comes down to what you're trying to diagnose. Cold starting and cold running issues, then I'd suggest a cold compression test. Warm running issues, then a warm compression test.

Maybe, that is a U.S. General tester, made in China, of course.

Remove all the GP's or injectors to do the test, that way the is no pressure build up on the other cylinders. It also prevents a "cross reading" from a bad head gasket. Our shop teacher in HS preached that on every compression test we did.

Sorry Andrew, I didn't realize it was your post. Didn't mean to bust your cajones on the subject. You bring up a valid point.

the HF kit I got did indeed come with fittings for the glow plugs on my car... of course it was the large body glow plugs I was testing...

and the 617 procedure I used was:
1 remove all glow plugs, injector lines, and accelerator linkage.
2 wire the IP in the full STOP position.
3 connect the fitting to the glow plug hole and connect the tester.
4 Crank the engine until the pressure stops rising.
5 bleed off the pressure, and remove tester and fittings, move to the next hole.
repeat.

I found it impossible to get the fitting on with the injector lines in place.
my car kept squirting fuel out the IP, so I put the lines back on the IP, and swung them out of my way. prepare for a mess of diesel fuel. the stop lever must prevent pressure build up, but I still got a fair amount of fuel squirting.

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"

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.

Great!

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?

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

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

Answer
 

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!

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

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?

http://www.harborfreight.com/media/c...image_1275.jpg

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?

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

The threads on the glow plug are M12 x 1.25.

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

yup. all engines have leakage in the rings and the valves. a wrench would NEVER build full pressure.
also, the fixed volume of space compressed into the remaining space is the basics for a compression ratio evaluation... stricly a measurement thing.
a compression test is measuring the pressure of the compressed gas to determine wear and leakage.
550 is AMAZING pressure,
300 is minimum acceptable and all pots need to be within 10% of one another or the motor will not operate properly
steady rotational speed is needed to maintain the pressure to the gauge.

As far as the HF kit goes, I bought one and here are the pictures:

http://i83.photobucket.com/albums/j3...f/ac9b0da3.jpg

The kit:
http://i83.photobucket.com/albums/j3...f/cd92540f.jpg

The numbered image of the kit:

http://i83.photobucket.com/albums/j3...f/efc0e092.jpg

The key:
http://i83.photobucket.com/albums/j3...f/9558049a.jpg

The two closest 1.25 thread (does it matter 10 vs 12x 1.25?), next to a new beru plug:

http://i83.photobucket.com/albums/j3...f/d50a73e0.jpg

If you're doing the compression check using the injector holes I recommend disconnecting the glow plug wiring at the fuse; this will cut down on the current draw from your battery.

Kurt

I'm planning to do mine through the GP holes...

We can go on and on over the value of a compression test and if all the pressure tests are necessary and valid. Personally I agree with the validity but not necessarily the need.
What I feel is most important is what the compression test is really best able to tell you.
Simply put it is this... a comparison of one cylinder to another.
Giving this as a measure it is important to measure each one the same.
Weather that be buy time or number of times the piston rotates keep it the same and you will have a good idea of overall comparison.
If overall it is good you are fine, if one or two are low you probably have a problem.
For all but the most anlytical that should give all the info you need.

everyone here should do a compression test on their om617 using the same harbor freight tester and using the same method and post up their results. curious to know how my engine compares. i'd be willing to loan my hf compression tester out, too

Well mine is the hf tester shown above (93644), and I got the following cold:

Cyl 1 380
Cyl 2 385
Cyl 3 375
Cyl 4 355
Cyl 5 375

Number 4 I did have to do twice as I couldn't get a good seal of the parts the first time... The second time pressure held, but I'm not 100% sure it was perfect.

So now for the discussion, what does cold tell me versus doing it hot (those numbers are cold), and what am I going to miss out on if I do not do it hot (it would be a pain to remove the injector lines and all for a while...

These were done through the GP holes, btw.