How would one quantitate how rough an idle is?

I imagine you'd use a scope and some kind of measure about how much the idle rpm varies during the course of a few seconds, no?

wrote:
> I imagine you'd use a scope and some kind of measure about how much
> the idle rpm varies during the course of a few seconds, no?

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

--
Steve W.

I was looking for how to tell exactly how rough it is. If you change something and you think MAYBE it helped a little bit, you can't be sure if you did 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 rough," etc.

wrote:
> I was looking for how to tell exactly how rough it is. If you change
> something and you think MAYBE it helped a little bit, you can't be
> sure if you did 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 rough," 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.


https://www.youtube.com/watch?v=ZnFSUNjwDPQ
Paul is an instructor and he has a LOT of videos teaching modern
diagnostics. Much easier than trying to type out the methods.


--
Steve W.

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 question on the roughness measurement seems to be more a question on the capabilities of scopes, than on practical diagnostic techniques. On that (maybe not 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 with a lumpy idle; you can smell incompletely burned fuel out the exhaust because of the overlap, and at the other end you have a 2 barrel carb-equipped smooth idle in a grocery getter, say a Catalina or Tempest. What could you measure on a scope to tell the difference in idle quality? Only some kind of averaged vacuum reading, that you wouldn't even need a scope for, just use a vacuum gauge and read it? How would you know (from a scope) when the Catalina's idle, because of stuff failing, had deteriorated to the point where the GTO had started?

wrote:
> 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
> question on the roughness measurement seems to be more a question on
> the capabilities of scopes, than on practical diagnostic techniques.
> On that (maybe not 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 with a lumpy idle; you can smell
> incompletely burned fuel out the exhaust because of the overlap, and
> at the other end you have a 2 barrel carb-equipped smooth idle in a
> grocery getter, say a Catalina or Tempest. What could you measure on
> a scope to tell the difference in idle quality? Only some kind of
> averaged vacuum reading, that you wouldn't even need a scope for,
> just use a vacuum gauge and read it? How would you know (from a
> scope) when the 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.


--
Steve W.

wrote:
> ...
>
> (a) response on not much to keep track of now makes me think that my
> question on the roughness measurement seems to be more a question on
> the capabilities of scopes, than on practical diagnostic techniques.
> On that (maybe not 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 with a lumpy idle; you can smell
> incompletely burned fuel out the exhaust because of the overlap, and
> at the other end you have a 2 barrel carb-equipped smooth idle in a
> grocery getter, say a Catalina or Tempest. What could you measure on
> a scope to tell the difference in idle quality? Only some kind of
> averaged vacuum reading, that you wouldn't even need a scope for,
> just use a vacuum gauge and read it? How would you know (from a
> scope) when the Catalina's idle, because of stuff failing, had
> deteriorated to the point where the GTO had started?
>

"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

Geoff Welsh wrote:
> wrote:
>> ...
>>
>> (a) response on not much to keep track of now makes me think that my
>> question on the roughness measurement seems to be more a question on
>> the capabilities of scopes, than on practical diagnostic techniques.
>> On that (maybe not 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 with a lumpy idle; you can smell
>> incompletely burned fuel out the exhaust because of the overlap, and
>> at the other end you have a 2 barrel carb-equipped smooth idle in a
>> grocery getter, say a Catalina or Tempest. What could you measure on
>> a scope to tell the difference in idle quality? Only some kind of
>> averaged vacuum reading, that you wouldn't even need a scope for,
>> just use a vacuum gauge and read it? How would you know (from a
>> scope) when the Catalina's idle, because of stuff failing, had
>> deteriorated to the point where the GTO had started?
>>
>
> "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????????????


--
Steve W.

On Sun, 16 Mar 2014 17:16:03 -0400, "Steve W." >
wrote:

>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.

On Sunday, March 16, 2014 3:12:09 PM UTC-4, Geoff Welsh wrote:

> I would suggest the Jenga
> Method. See how many blocks you can stack-up on the hood.
>
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 stories about the writers are great. Thanks.