Years ago (1974) I was helping a friend with an MB180 ? diesel sedan. It wasn't running well, and I was looking around for a diesel compression tester, when a guy in a shop said he thought he could do the job using a test routine on his Sun diagnostic machine. He connected a light wire from the tester to the starter cable, pulled the manual injection shut-off, marched the oscilloscope, and then cranked the starter. It displayed a peak/valley of voltage that corresponded to the resistance to cranking over of the individual cylinders. He said that on a gasoline engine he'd connect another wire to the #1 cylinder spark wire, and that would allow the oscilloscope to identify the individual cylinders. He demonstrated this on a V8 Cadillac in the shop, and sure enough, once the machine knew how many cylinders and what firing order, it labelled every dip.
Sure enough, the oscilloscope showed comparatively higher voltage on every forth dip, which he said corresponded to less cranking resistance/less compression. He said it didn't matter which cylinder it was, because the next step was to pull the cylinder head in any case, which we did and found a burned exhaust valve. I wish I'd then taken it back to his shop to run the test again.
Anyone else ever seen this done? I realize that it isn't as good as a comprehensive compression/leakdown test, but if it would work with a handheld or laptop based oscilloscope it would be a lot quicker when looking at a potential car to buy.
Anyway, I've always thought it was an interesting gizmo.
SEO

Yes, I've done that test many times, using Bosch equipment - mainly on petrol engines though - it's a good way to get a quick (cheap - dealer labour rates at £100 per hour!) look see.

There's also a very rough and ready method if your car has manual transmission - park it on a hill, put it in gear (on an MB diesel, you would also fix the stop lever in the stop posisition), and let the car roll down. If there's a cylinder with poor comression, because if there's a cylinder down, you get bump, bump, bump, roll........bump, bump, bump, roll.......... etc instead of a regular series of bumps as each cylinder comes up on compression.

I could see how the starter current would take a dip at the compression stroke of a weak(er) cylinder and could be viewed on an o-scope.

I would think that the same engine would suffer from a rough idle or even an audible clue when cranking.

I'm primarily interested in this as part of an inspection of boat machinery. Some of the boats I look at have 8 and 12 cylinder engines, and one cylinder with bad compressiion is pretty well hidden.

Diesel?

I wonder if it might be possible to detect an imbalance by looking at the glow plugs as a thermacouple while the engine is running?

If you are really that concerned... ask for a compression/leakdown test.

For general tshooting on any engine a valuable tool is a surface thermometer. It will immediately you which hole is not performing properly by heat from each exhaust port. That along with other symptoms is very helpful. Smoke, color of smoke, etc. It is the first tool I reach for when a miss or rough idle is present. Just aim the dot as close to the exhaust outlet on each cylinder at the same spot on all cylinders and record. Do it on a cold engine and then on a warm engine. It helps.
Bud

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1987 300D Turbo, 175k mi., 1998 BMW 323i Convertible, 1997 F250 4x4 7.3L PSD

I did this with my MB diesel soon after I bought it. I was surprised that the weak cylinder had a higher exhaust temp rather than lower as intuition would have lead me to believe.

Was his device measuring current, or voltage?

I'd love to make something like that up myself.

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1983 Mercedes-Benz 240D
Automatic, A/C, Power Sunroof, Power Right Side Mirror
231K Miles FOR SALE MAKE OFFER

That is rather a different result. That cylinder must have had much higher cylinder pressure for one reason or the other. But you must have suspected that cylinder because of the difference in heat. The tool is also handy when adjusting IP/turbos. Use it as an EGT guage though not as accurate as a probe in the exhaust flow.
Bud

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1987 300D Turbo, 175k mi., 1998 BMW 323i Convertible, 1997 F250 4x4 7.3L PSD

First off, this was 33 years ago, and my memory may be a bit foggy. But I think that was before the days of inductive current sensing in DC, and I don't remember him putting a shunt bridge in. Based on that, my guess is what he was looking at was voltage, which would logically drop when the starter is running under more load, increase under less load. I could be wrong about that. A shunt bridge wouldn't be that hard to rig up, because it could just be attached to the positive pole of the starting battery. But a 500 amp shunt isn't small or cheap.

I think that Fluke now makes a handheld oscilloscope with an inductive current sensor, that can download digital files to a computer.
This might be the best solution to my requirement, which is to be able to quickly and non-invasively provide some information on the conditiion of an engine in a boat, along with all the other information on the hull, deck machinery, electrical systems, etc.
One of the things that a marine surveyor really tries to avoid is doing wrenching work on machinery. Even if you charge enough to make it worthwhile, if the sale doesn't go off as planned the disappointed non-seller might claim that you damaged the engine in the process of doing a compression test. Not all engines have glow plugs, and smaller engines don't have cylinder pressure test ports. In that case, you're left with taking out individual injectors to test compression. On a 12-92 Detroit that's an all-day affair with good access and no surprises, by the time you've pulled all the injectors, tested compression, reinstalled injectors, hooked up jumpers, and reset the rack.
The heat test on the exhaust manifolds works, except that it might be identifying a bad injector or pump as easily as low compression. I like the infrared thermometer idea. I'm still in the old days of a candy thermometer, which does work.

Seo, you are correct that the temp test with a therm. will not rule out everything but it will at least eliminate some guesswork. It also can be used for troubleshooting radiator, AC compressors/condensors/evaporators, waterpumps/thermostats, exhaust leaks and anything that has a heat source in the home or auto. At Harbor Freight they have a basic one for around $40 I believe.
Bud

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1987 300D Turbo, 175k mi., 1998 BMW 323i Convertible, 1997 F250 4x4 7.3L PSD

A relative starter cranking current could be easily seen by looking at the voltage drop across the positive batt cable. Scope probe at the Batt +, grnd lead at the starter end of this cable.
As current rises (more compression) the voltage across this cable would peak. Might be some way to use the tach sensor as a cylinder "marker".

I may have to try this out. Just happen to have a weak #1 cylinder on the parts car for comparison.

I have spent hours talking about this with engineers on tugs, some of them very well educated operating/design engineers. They tie themselves into knots figuring out how to figure out which cylinder is compressing and mark that to one segment of the graph.
I stick with the idea that on an inline engine with a one-piece head it doesn't matter. If you show up low compressiion, the next step will either be to do an old-fashioned gauge-in-the-hole test, or you're going to pull the head.
This is different from the situation with a big engine that has individual heads for each cylinder, but these engines usually have valved ports into the combustion chamber. The last boat I was on (Last week) had a V16 ALCO. The engineer had the gizmo in his "gold locker" that would allow him to make indicator cards of each individual cylinder's firing pressure. Very cool.
SEO

Are you familiar with the Detroit 8.2?
I've been looking for a truck to use around the farm to move dirt and located one this weekend with the 8.2.
Price is right but its been parked for 8 months, needs batteries.

I hear good and bad about this engine.

Like I said, it was opposite of what is intuitive. Low compression means unburned fuel burning as it is exhaused and higher manifold temps. Backward from what I have experienced with gassers. I posted here about this, years ago.

Compression testing, injector cleaning and valve adjusting are the basics of maintaining these engines. Just do it right one time when you get the car and it will probably wear you out before you wear it out.