How would one quantitate how rough an idle is?

IF it's bad enough to hear or feel it that's usually enough to make the judgment.

I was looking for how to tell exactly how rough it is. If you change somet hing and you think MAYBE it helped a little bit, you can't be sure if you d id anything without some kind of numbers. How do you keep track? "Sounds a little bit rough. Now after this change it sounds a little iddy bit roug h," etc.

There isn't much to keep track of. If an engine is running bad you work through the various systems and determine the cause, then repair it.

It doesn't take long to narrow down if you know how. Take your scope for instance. Ever done a cranking balance test for relative compression? Also possible to set up a scope to check injector pulse vs ignition firing amplitude to see if a cylinder has a problem.

I'll tell you what. If you REALLY want to learn this stuff hit you-tube.

Paul is an instructor and he has a LOT of videos teaching modern diagnostics. Much easier than trying to type out the methods.

Thanks for the youtube link. I'll definitly check that out.

Your response on not much to keep track of now makes me think that my quest ion on the roughness measurement seems to be more a question on the capabil ities of scopes, than on practical diagnostic techniques. On that (maybe n ot very useful to a mechanic) question, take this hypothetical scenario - 3 different 1969 Pontiac 400 cubic inch V8's idle at different "roughnesses" , brand new, because of their cams. At one end you have a Ram Air IV GTO w ith a lumpy idle; you can smell incompletely burned fuel out the exhaust be cause of the overlap, and at the other end you have a 2 barrel carb-equippe d smooth idle in a grocery getter, say a Catalina or Tempest. What could y ou measure on a scope to tell the difference in idle quality? Only some ki nd of averaged vacuum reading, that you wouldn't even need a scope for, jus t use a vacuum gauge and read it? How would you know (from a scope) when th e Catalina's idle, because of stuff failing, had deteriorated to the point where the GTO had started?

You just identified the items you would look at to determine idle quality based on internally different parts.

However the comparison would need to be between identical engines if you want to determine if something internally is causing a problem due to failure.

Starting with a new engine, broken in so the rings are seated, and the bearings are freed up. To give you a baseline you would measure idle vacuum, rpm, actual flow rate through the carb at idle, cylinder compression and leak percentage, oil pressure, engine temperature, exhaust flow and exhaust gas composition. You would also note the fuel and oil used during this test as well as the type and gap of the plugs. Actual atmospheric pressure and the listed compression ratio, cam numbers, actual displacement and timing get written down as well as type of transmission. (you would also record the listed HP and torque numbers BUT remember that they are not the true numbers)

No real simple way, other than experience with hearing and working on many engines to shortcut this down on an older engine.

On an OBD II vehicle with all the ECM hardware you can actually pull a lot of this real-time from the sensors on the engine. Which is how the ECM actually manages the powertrain, it knows what the baseline should be for each item and tries to keep the engine at those numbers.

"stuff failing" as you eloquently put it, causes an actual misfire, not a lumpy idle like the low vacuum high profile camshaft RA IV. Experience knows the difference. Rough and broken are not the same thing. I often get work orders that say "check runs rough" when an engine is obviously misfiring.

As for quantifying roughness on two identical engines where one has a brand new crap-china aftermarket solid engine mount and the other has the proper hydraulic factory style mount.....I would suggest the Jenga Method. See how many blocks you can stack-up on the hood.

GW

Real hands on experience trumps pretty much anything else when it comes to mechanics.

Sounds like your writer is like a couple I worked with, don't ask the customer any real questions, just let the tech figure out what's going on...

My favorite was "Customer Complaint - car acts strange" WHAT????????????

Many years ago, writers were mechanics who graduated to writing. Then they slowly evolved into "advisers", in other words, salesmen. After that, almost all writers I worked with had between very little and zero technical knowledge, but they could sell. I even had one service manager who knew *nothing* about cars, and another writer with a similar level of knowledge who replied to questions about what the tech found with, "you don't want to know". It worked on most customers.

My strangest writeup was, "car curves while driving". When I asked the writer what that was supposed to mean, she replied, "you know, it, uh...", and then started moving her hands around in a curving motion. Oh well.

I love it! I could lean my head against the side of the open hood and feel the vibration at idle. I bet there's a way to measure that, but the point by Steve and GW is well taken - no substitute for experience. Your storie s about the writers are great. Thanks.

"check noise" is my favorite.

I have yet to see a legit customer complaint, he just writes what HE thinks we should do......groan. I guess he doesn't mind me waiting around to ask him, "WHICH NOISE? The vrrroooom? That's normal"

GW

what's flat-out ffffing sad, is that Step One, in the diagnostic process is "verify the customer complaint". That's not my opinion, that's on the ASE tests, and taught in all classes. So without a customer complaint, the work order is, well, INVALID.

Every article I have ever read on service writing talks about getting a GOOD descriptive, customer complaint....I'd settle for ANY complaint

Customers seem to want to tell you what they think is broken rather than what's happening. I've been meaning to make a big sign:

Please just tell us:

1. My car DOES ______________ and it shouldn't.

or

1. My car DOES NOT _____________ and it should.

and

p.s. I don't want to know about your feelings or your plans for the day.

GW

Usually used for high speed digital signal analysis. But this is an analogous situation at much slower speeds.