Diesel Compression tester....which one is good?
 

I'm looking at a few places for a good and tough compression tester.

Who makes the most complete kit for a Mercedes?

I've had good luck with the Harbor Freight unit but some guys haven't. You might want to look online for a good used unit, like from MAC tools or similar.

Dan

I have the Harbor Freight unit and it works fine for my needs. Unless you have a fleet of diesels, it's a tool you're only going to use a couple of times. No point in shelling out a fortune for some shiny tool that's going to sit in a drawer for decades. Most of the problems people have with the HF set are due to lousy schraeder cores in the fittings. There are several fittings with the cores and can be quickly and easily swapped if that's the issue.

It's also worth asking WHICH diesel you're working on? If the 61x it's easiest to use the glow plug holes. 60x are WAY easier to use the injector holes. CDI's are a different animal entirely.

Not a big fan of the HF/Chinese stuff but for the occasional use it's probably fine. I'd trust it for variances between cylinders but not for actual P.S.I. readings.

I'm a Snap-On guy myself but that is because I did it professionally for 40 years. Way overkill for the average guy.

OTC, Matco, Mac, etc. tools can be found in pawn shops for a fraction of their usual cost.

You can buy the individual adapters. The Mercedes type is usually included in the sets.

X another for the HF unit. I have had mine for years, and it has worked fine every time. The best tester for your $.

Especially when the kit is something like $30. For the shade-tree mechanic, it's a bargain, even if it only works once. It doesn't have to be precision, you're more concerned with the balance across cylinders rather than the exact PSI rating. The El-Cheap HF kit is accurate enough for the average need.

Mine didn't work out of the box. It didn't hold pressure but I may go try another.

One of the members here has a youtube vid where it looks like he uses air brake line or similar instead of the stock hose. This lets him do the test without pulling the injector lines. As is, the hose is large and removing lines makes it a lot easier.

Faulty schraeder core in the adapter. Same ones tires use.

Harbor freight tester has no schrader valve in the test fittings and is therefore useless unless you tap and install one yourself. Tire valves will give you a low reading, I've tried them. The schrader valve for a compression tester has a special 1ounce spring where a tire valve has a much higher tension spring.

Correct. The Schraeder valve in compression gages is a special type w/ very weak spring pressure, so it works as a low-resistance check-valve. I bought a bag of them at an auto specialty shop long ago. They have white plastic collars. A regular bike or car one will give an incorrectly low reading because it might take 20 psid to push it open. We speak of the Schraeder valve in the tip of the adapter. The one used as a "pressure release" can be a bike one because you press that in w/ your thumb. I have no idea how the HF version could work w/o Schraeder valve. I will look at mine again to verify.

The main thing in getting accurate readings is that the adapters displace exactly the same volume in the cylinder head as the parts you removed. That is why the HF kit has a collar you must use around the "injector adapter". Without that, your readings will be falsely low. Of course, the glow plug adapter makes less difference, but I found it too hard to use. The HF kit works fine for me, and I recall ~$35 on sale + coupon.

Every one of the adapters in mine has one. This was why I suggested just robbing one out of a different fitting if the one supplied is no good. Maybe the quality control is so bad they let some out without valves?

Well that explains how others were able to get a reading with the HF unit. Mine not only does not have any valves, the fittings were never even threaded to take them. That led me to believe they were just junk.

I have a MAC tools tester now anyway. It only comes with the pencil and loop glow plug fittings for the Mercedes. No injector fitting but it works well.

You're better off with the glow plug adapter. You don't have to fuss with the return lines using it. It can be a pain with carbon-ed up plugs though.

It's always something.

Have a look at my 'pop testing' thread, would highly recommend getting one fitted with a gauge suitable for your injectors.


Mine has a 150bar max which makes the 115bar pop area very viable, I think the original was something like 2000bar so you can imagine the graduation is tiny.

Indian Machine tools will swap out to your spec, it does take a while for delivery though - mine has been used quite a few times now without issue, it dies however have a slight leak where the reservoir is attached, I remember seeing another post about this also.

I've been using a harbor freight one at my job working on mercedes for the past few months. I've used it 4 times and it works perfect. I paid 30 for it.

My Harbor Freight compression tester is probably close to 30 years old, last time I used it it worked fine but that was probably a dozen years ago! If I ever need it again and it is dried out I will just buy another one which should last me until my nursing home days. ;)

Guess I'll head to HF and get one!