I occasionally see mention of compression tests done on diesels but I have never seen a kit to actually do it. I have an old Onan L634/Cummins A series here I would like to check. Also a small Isuzu 3 cylinder. Most of the engines I would need to check have glow plugs in the cylinders but I have no idea if they share a common thread or seat. Anybody have a test kit they can refer? Even a cheap one would be fine as long as it works. I'm probably only going to use it a couple of times a year.
Harbor freight sells an incredibly cheap kit with more compression testing adapters than you will ever need. I have a couple testers, I just buy them as needed.
You can make a quick and dirty test by checking current on the starter motor cable. I've got a cheap two channel oscilloscope. On one channel connect the fuel line pickup from my phototach kit, on the other channel goes my clampmeter on the starter lead. Crank the engine and you can see the current peaks as the starter brings each cylinder over compression. A cylinder that is low on compression will have low current draw relative to the others. The fuel line pickup lets you identify which is number one cylinder. It will not give you actual pressure readings but is perfect for identifying a bad cylinder.
X2 on the harbor freight kits.. There are afew diesels that cant be checked w/ the adaptors that come in the "master" kit.. but overall its good. I work in a fuel shop so any injector adaptor I need, I make from a core injector. I haven't found an oddball that takes glow plugs, that couldn't be tested w/ the GP adaptors in the kit..
X3 on the HF kit. I only had the occasion, to use it twice, so far. I had to buy 1 fitting for an obscure old compact tractor, off the snap on truck, and that was only like $16.00...:thumbsup http://www.harborfreight.com/20-piece-diesel-engine-compression-tester-93644.html
Awesome, I have never heard of this, I messed with oscilliscopes in school but have never really used them in "real life." What type of oscilliscope and phototach could you recommend?
I generally just observe amount of " blow by "out the tube while the engine is running to get an idea of condition and compression in the cylinders . It's quick & easy .
I've made several adapters in the past. I bought quick connects like are in the HF kit and then used an old injector or glow plug. Lots of ways to get creative here.
The scope and current clamp are from D!ck Smith which is our equivalent of Radio Shack. The fuel line pickup is from my Cat phototach kit which is a universal engine speed measurement tool. That part isn't essential, you just need a way to identify which peak on your trace is number one cylinder then using the firing order you can extrapolate the rest.
I suppose you could remove one injector/glow plug to eliminate that "bump" and determine which cylinders' bumps are remaining, without the fuel line pickup. That is, if one of four cylinders is particularly low (compression/cranking amps) then pull one injector and you should be able to determine the order of the cylinders from the one that's missing from the graph. Got to keep an eye out for a cheap oscilliscope, and probably a fuel line pickup, cause the above sounds like a lot of messing around in comparison.
Hmmm.. a 500.00 scope + accessories + the knowledge to know what your looking at. vs a 30.00 kit and a guage..?? that's only going to be used MAYBE 3x.. Really??
The scope is useful for other things, and you don't need the latest and greatest. A simple oscilliscope that would work for me was obsolete decades ago. Who knows, you might be able to get a smart phone app that does the same thing, like the audio tachometer. 'Course, then I'd need to get a smart phone...
I agree that using an oscilloscope isn't for everyone, but the beauty of it is that it is a non-invasive test. How are you going to do a compression test on a common rail diesel with no glow plugs, for example?
They have a plug-in diagnostic port. Heres another what-if> If an engine has NO compression and wont start, how is the scope going to read a spike in starter draw.? The starter is just going to spin and spin.. am I missing something? If an engine has a no start condition OR SKIPS, you HAVE TO get invasive ANYWAY.. Loosen an injector line, one at a time, to find out which cylinder it is, THEN remove the injector to have it tested or remove another one to switch it with.. IF the skip stays on the original cylinder with a different injector, whats the problem?? OH WAIT, lets ask the scope. The scope isn't answering.. whats the compression reading?? does it go up when you add oil into the cylinder?? are the valves stuck,?? are the springs broken?? are the rockers moving?? What?? I cant hear you mr. scope.. The fella wanted to know about, an INEXPENSIVE COMPRESSION TESTER.. lets just leave it at that..
I might agree with you pump guy but at the same time there's a whole bunch of guys, me included, who never thought of using a scopemeter in this way. Now we've all learned something. No tool is an end to itself, they all require a human to operate it and hopefully that human has the knowledge to understand it. This is what bothers me about so many of the college educated mechanics coming on scene. If the computer don't tell them the problem they just start switching parts. Seems there are no troubleshooting skills taught anymore. JM2CW
The scope test works well as a very quick relative compression test extremely fast way to find out if further compression testing is warranted does it replace compression tester no but is a great thing to know about
Feel better now?
Well I got a $70 kit from ebay, looks identical to the HF but cheaper. Now I am wondering what will happen if I use the hose from my leakdown tester which has spark plug adapters to gas engines. The hose is about 1 foot long and has a 1/4 or 5/16 ID, seems like it might add a lot of volume and lead to lower compression figures on gas engines. What do you guys think?
The volume of compression gas is going to fill the hose and stop when it reaches max.. reguardless of how long the hose is.. if u let the engine spin enough. Some people let it spin till the guage stops moving.. and other give it a 10 count.. just be consistant.. BUT the material the hose is made out of, may NOT have the strength to hold the HI diesel compression.. Just DONT USE a guage made for a gas engine ON A DIESEL.. A GAS engine compression is ALOT LOWER than a DIESEL engines compression.. The diesel guage should go to about 500psi?? and the gas only goes to 200psi?? You will or should exceed the 200 on a diesel compression test and the guage OR hose may blow..
The adapter I was referencing is a spark plug adapter so it won't go into any diesel. The adapter has no check valve until 1' back in the hose so it will add to the combustion chamber volume. I estimate around 8-10cc which I think would probably lower the compression ratio too much. I have another one I made by welding an air chuck directly into a spark plug shell. Much less volume but a tight fit sometimes. We will see.
I may be off a little here but don't see a major problem with the "slight" increase in chamber volume posed by this hose. I would think the most important thing about a compression would be to show the difference between cylinders than an exact reading.
THAT &.. are we talking diesel or gas here?? The thread started, asking about diesel compr. testers and now we're talking about gas spark plugs and adaptors?? I'm lost.. & who gives a DAM about a gas engine anyway?? LOL
Hey NOW! I'm working on a 7 horse Tecumseh in my spare time! Would be tricky to do a compression test on that as there is an automatic compression release device built into the cam gear! But yes this thread is getting a bit confusin!
I brought it up because I already have a gas engine cylinder leakdown setup that uses what looks like the same connector. The kind that feeds shop air into the cylinder. Put the two kits together and you can measure dynamic compression and also feed shop air to the cylinder on anything gas or diesel. Obviously the volume of the hose is no concern when you are feeding air to a static engine. I will have to do some cross comparisons when I get something to compression test in the future. Hope this makes it more clear.
Not everyone on this forum works just on diesels. A number of years ago I started working on lift equipment and have had to learn and re-learn gasoline engines since most everything has changed since I last worked on them in the mid '80s. And I've forgotten a lot.
Well, what did the oscilliscope tell you, Ken? You did have the dial set to Tecumseh right???
No oscilloscope, but did use a dial indicator and Digital Multi-meter to set timing!
I have always been able to tell by the sound of the starter if there was a compression problem. In the past Cat never recommended compression checking its engines and did not publish compression figures. Instead they recommended a leak down test for diesel engine diagnosis. I think there was a bumping against compression test, maybe with the fan.
I know what you mean about the sound of the starter although I have seen many a skippy sounding engine continue running to the customer's satisfaction also. Leakdown testing is useful but it doesn't tell you if there are cam timing issues and such that might also lead to low compression. I look at one as static, the other as dynamic. They are all tools in the tool box.