compression gauge puzzler

I don't understand that. All you care about is the air you get into the cylinder, right?

I used to do a lot of leakdown tests, and I think I remember how they were done. All I did was pump the cylinder up to some pressure and time how long it takes to drop to some lower pressure. Do the test dry, then wet, in each cylinder. If it falls faster when dry, it's rings. If it's the same time when wet, it's valves.

Am I missing something?

Reply to
Ed Huntress
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That's interesting, and I'm sure it's faster to use than the old ones. From what he demonstrated, though, I see no way to identify ring versus valve problems. I guess you could run the test wet as well with that rig, but it's a different animal.

You could pump mine up with a bicycle pump, if you had to, but I always had a little compressor handy.

Reply to
Ed Huntress

I guess you can do it that way, but it's not terribly effective or accurate. A normal leakdown tester feeds 80psi to one side of a restriction, which feeds the air into the cyl. Guage on inlet (generally 80 psi) and on the cyl side. With no leakage at all you get

80/80. If you have a small leakdown (normal) you may have 80/75. A sognificant leak may give you 80/60 or 80/50.

Then you listen in the intake, exhaust, and crankcase to tell where the air is going.

Reply to
clare

You would use a stethoscope to locate where the leak is.

Valve leaks you would hear in the exhaust or carb. Ring leaks you would hear in the dipstick tube.

MikeB

Reply to
BQ340

OK, I can see where that's a lot faster, and it actually combines the equivalent of a regular compression test (sort of) and an old-fashioned leak-down test.

With the older equipment, you only would do a leak-down test if all cylinders looked good in the regular test but you had some reason to believe one or more cylinders still had a problem. You could do both tests dry and then wet to try to isolate the problem a bit; pressure loss with a wet cylinder meant a valve or head-gasket problem, so you had to pull the head in either case.

The only thing I see wrong with the newer device is that it only checks the rings at TDC, while a conventional compression test will show a cylinder is off from any problem along the whole stroke -- like a scored cylinder that isn't scored along the full length of the stroke (an engine that's been over-revved a lot, or which has a cracked lower ring). Also, a valve that's out of adjustment, or which has a sticky lifter, but which is still fully closed at TDC on the compression stroke, will not show you the valve problem.

Anyway, it's not something I'm likely to do again. I just thought, mistakenly, that the OP might have been looking at an old-style leak-down tester. It appears that it was just a missing Schrader valve stem in a conventional compression tester. That's what we save old bicycle inner-tubes for -- spare Schrader stems, and gasket material.

Reply to
Ed Huntress

I'm sure it's a practical solution or they wouldn't go to that complication. And in a commercial shop, time is money, and the old way of running those tests could take a whole Saturday morning. If it was a Bristol or a Jag engine, it was worth the play time. Otherwise, not.

God save me from ever having to do a hot valve-lash job on a six-cylinder Triumph again...

Reply to
Ed Huntress

Yeah, I can see that it's very quick to diagnose several problems. But watch out what you call "normal." Unless I'm mistaken, those two-gauge deals didn't exist when I was club racing, in the late '60s and early '70s. At least, it wasn't equipment that we bottom-dollar sports car types had. The ones we used were "normal" then.

I do see a limitiation of this newer device, compared to the old ones, however. I described it in one of my posts to Jon.

Reply to
Ed Huntress

It's very interesting and it looks slick. I'm sure one would be on my Christmas list if I were 45 years younger. d8-)

Reply to
Ed Huntress

Very interesting. Thanks, John.

Reply to
Ed Huntress

Actually you can use a leak-down tester through the entire stroke IF you have the proper tools to rotate the crank.

For a very fast way of checking for a dead hole in an engine you don't use a compression tester anyway. Toss a scope on there. One channel will use an inductive pick-up on the pos battery lead. The other goes to any plug wire. One channel gets set to measure the amp flow and the other acts as a trigger. Pull the power to the injectors or unhook the fuel line or pump so the engine doesn't start. Then crank it over with the starter and look at the amp readings. If all the holes are good you will see nice even amp draws across the engine. Any cylinder not pulling it's share will show as decreased amp draw.

To see it in action -

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Reply to
Steve W.

the proper

So you have to back off the rocker arms or remove the camshaft?

compression

on the pos

the fuel

and look at

Yeah, I've seen descriptions of that.

Reply to
Ed Huntress

I use a dial caliper in the #1 spark plug hole when the crankshaft pulley and belt cover TDC reference have been removed to replace the timing belt. It's close enough to align the belt in the proper crank and camshaft pulley notches.

There is a dial indicator adapter that screws into a spark plug hole. I didn't buy one when I was rebuilding motorcycles because the spark plug area was obstructed by the frame and fins, and I only had to be accurate to within one gear or sprocket tooth. jsw

Reply to
Jim Wilkins

-Won't guarantee you're on the compression stroke. The method I use

-does.

The UP marks on the camshaft pulleys guarantee that, or I can watch the #1 lobe positions. With the timing belt off I could turn the crank

360 and interchange what had been the compression and exhaust strokes. The Crankshaft Position Sensor that generates spark timing wouldn't know the difference. jsw
Reply to
Jim Wilkins

I've never had it fail to indicate a problem - it is used all the time on aircraft engines and is MUCH more accurate at indicating the health of an engine. If all cyls are 75/80 at annual this year, and one drops to 72/80 next year, you know you have a problem developing on that cyl, Usually a valve problem. If they all drop to 70/80 you suspect ring wear. The absolute numbers are not what is important - it is the difference and the trend. Can't do that with what you call the "old" leakdown tester. By the way, I wonder how "old" that is, when the 2 guage system was used already in the early twenties.

I'd say what you are refering to is more of a "shadetree" or "redneck" cyl leakage tester than an "old" leakdown tester.

Reply to
clare

the proper

No, you can test it MOST of the way down on the power stroke and Most of the way up on the compression stroke. Leakage at the bottom of the stroke doesn't amount to much as there is never any useable pressure down there

Reply to
clare

Or just pop the cambox or rocker cover off and check for the "rocking" valve pair (TDC between exhaust-intake) - then check the one opposite in the firing order. It will be TDC Compression stroke - all valves closed

Reply to
clare

Ok, "shadetree" definitely applies. I see that some poster on a Locost forum describes the same kind of tester we used, and those guys are as cheap as I am. d8-)

I'll bet I can find a description in one of my old British car books, but I don't have time to go digging into that stack.

Anyway, I'm glad to learn about the type with continuous air supply. I see that's almost the only thing described on the Web.

Reply to
Ed Huntress

the proper

Ok, I can see that. It would solve a lot for me if I actually tried one out.

Reply to
Ed Huntress

-Unnecessary work if you use the right tool.

Unnecessary expense if you know the right tricks.

I used to rock the crank and watch for valve overlap until I worked on an engine that immediately deprimed the hydraulic lifters if turned the least bit backwards - and they stayed collapsed and loud when I started it. jsw

Reply to
Jim Wilkins

spark plug

have the proper

Videos are great. I'm all for them. We just have to be choosy about which things we want to spend our time on.

Reply to
Ed Huntress

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