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Another look at my rough idle - documenting

9K views 57 replies 11 participants last post by  norohs 
#1 ·
This is kind of a bounce-it-off-wall post. My adventure using a DIY First Look Pressure Sensor to help figure out a rough idle.

My friends tell me I over think things...
I wonder what gave them that idea? :confused: :banana: :lol:

Feel free to comment...

It's been a few months now since installing the reman'ed engine and I'm still experiencing a "shimmy" rough idle.

I've heard of the expression "Thrown everything but the kitchen sink at it". Safe to say, I've actually thrown the kitchen sink at this one. Before I pull the engine, I'm down to my last ditch efforts to understand what's going on. So....

I made my own version of the First Look Sensor (too cheap to buy one). It's simply a pressure sensor that I can stuff up the tail pipe or use it on a vacuum line.

The intake waveform was unremarkable and didn't show anything unusual.

BUT... the exhaust waveform tells the story. The waveform shows 12 exhaust pulses (twice around the "block"... if you will). The first pulse is not sync'ed to any one cylinder (it could be any cylinder). The firing order is 1-6-5-4-3-2. There are no "dead" misses. All cylinders are firing, but some are weak in torque (more than one), thus more of a hum or shimmy symptom in the idle, not a dead miss type vibration..

The Torque App shows very random misfires on cylinders 3 (mostly) and somewhat on all others. Oddly noted that cylinder 6 has never shown a misfire (see below). They are only shown at idle and less when in gear. The general "hum" or "shinny" can be detected throughout the entire RPM range under load, but not under a no-load (neutral) RPM rev.

Understanding how the misfires are detected helped me a lot. It only looks for the rotational speed change in the crankshaft... it really doesn't "know" if a cylinder really fires or not, just the resulting torque for each cylinder's power-stroke.

So, here's the exhaust waveform. The way I read this, I've got 2 (maybe 3) weak-fire cylinders. I'm also looking to see if they are on the same bank. I don't care right now which bank is which right now, just if the misses are on the same bank or not. This is what I got:

Exhaust O-scope Pulses:

1 -> High -> Bank(x)
2 -> Low -> Bank(y) (misfire?)
3 -> High -> Bank(x)
4 -> High -> Bank(y)
5 -> Low -> Bank(x) (misfire?)
6 -> High -> Bank(y)
(The next cycle on o-scope repeats same)

Fire -> Fire -> Mis -> Fire -> Fire -> Mis (repeat)

So waveform pulse 2 on bank(y) is low and 5 on bank(x) is low. Knowing that cylinder 3 has been the most (by far) reported misfire, I can plug in cylinder 3 to pules 2 or 5 (the two low pulses) and count the firing order from either starting point and I come up to the same results. Cylinders 3 and 6 are weak. Now I know (best guess really :think:) the misfires (weak power strokes) are not on the same bank and are on cylinders 3 and 6.

This explains the fairly high frequency of the "rough idle" and humming vibrations when engine RPM and torque increase... and why "shimmy" might be a better way of describing the rough idle. Blurry-image-in-the-rear-view-mirror syndrome.

Now, what the heck to do next?

Done (as well as much more) spider, plugs, wires, cap, rotor, distributor, lower manifold, upper and throttle body gaskets (all twice), inspect upper manifold for crack (off-vehicle twice), vacuum 19" at idle, compression at 145-148 (not too low for new rings), PCV, idle air motor and other assorted things, like O2, clean MAF, check MAP and TPS, etc...

Maybe I should look closer at the vacuum waveform again (I didn't take picture)? I'm questioning the sweep time scale I was using on the o-scope.

Somewhere in this house I have a 250 PSI sensor (I just can't put my hands on it today). If I can find it, I'll look at the cylinder(s) with it.
 

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#3 ·
Norohs,
good.. a fascinating case ... (like Sherlock)

1) the pulse data from the exhaust was fascinating, never thought of looking at that before.
every third pulse is odd (actually an even harmonic), as it is not co-synchronous with a normal spark sequence, but with the crank...
2) what do you mean by a re-manufactured engine? short block, long block.. plus intake? include the distributor? ignition circuit? fuel pressure regulator in the intake?
3) I assume the wire harness is stock/original...
4) I assume you have changed your spark plug wires and cap/rotor...

My first guess is that your van has a poor wobble in the distributor timing pick-up... or your self-identified crank position sensor which leads to a poor signal every full rotation (half way through the 6-cylinder fire sequence)... 4-stroke engine 101, causes every third cylinder pulse to be off in a 6-cylinder engine.

the game is afoot,
Herminator
 
#4 ·
Looking at everything you have shown. And pondering the plug pics.
Mechanically it is fine. Compression is really close on all cylinders.

2 and 5 plugs are sooting up more than the others. Rich mixture, improper atomization, or weak spark.

Judging from the symptoms your giving. It sounds like for whatever reason it just doesn't have even fuel distribution or spark. One or the other.

Personally I would close the plug gap up 10-15 thousandths and run it see if it is running more even. Closer gap is a hotter spark. If it has weak spark that will straighten that out alot. I would check the spark color at each plug and not at the coil. See if it losing voltage in the journey.

Once you have discounted spark plug voltage as a gremlin. Next I would be scrutinizing the injection system. Looking for proper atomization.
 
#5 ·
After trimming down the spark plug gaps as Maher suggested, (bless you for extracting the plugs one more time), can you also put a timing light on the main distributor lead, to see if all 6 pulses are consistent or equal?

I think we are zooming in on a crank based sensor which is causing the poor even harmonic in your 6-cylinder.

Herminator
 
#6 ·
Herminator,

That's a great diagnosis idea. I was wondering how to check to see if the timing was different on individual cylinders. I can take a HV clamp on the main ignition wire going to the distributor cap and scope it. The pulses can be captured and a dwell timing can be read as the space between the spark pulses. About the only thing I can see that would throw this check off would be the 'normal' RPM twitches as it tries to hold a given RPM. Still, it has good merit in theory and should provide more proof for a CASE re-learn being needed.

The engine was from a re-builder, so I guess it's a rebuilt engine (long block), not a re-manufactured. It was a standard long block and everything off the old engine had to be moved over to the new engine, except for the heads, block and parts inside the block. I did find the old distributor with shaft bearings out of round, so a new distributor was installed. Nothing was changed on the wire harness. Plugs, wires, cap and rotor were changed. Delco cap and rotor replaced the new ones that came on the new distributor (it came with cheap as* ones).

Here's what I came up with that was my revelation:

Every third pulse... the reluctor ring has three stators. The Hall sensor used in the crank sensor sets the base time for the ignition and the fuel injectors. It's time is offset by the values (computer data) collected when a CASE relearn is done. The camshaft sensor is a one-pulse per cam rotation. It's only "real" job is to decide which of the two possible cylinders are to be fired. You can read up on the correlation between these two sensors and how they are used (together and separately) to perform several different functions. In normal (base) running, the camshaft sensor is only used to select the correct cylinder out of a possible two.

From what I've been reading...

The CASE (re)learn is something that happens at the factory level for every new vehicle with this design. The reason (mostly) it's needed is because of esotrensic(?sp) (out-of-round) error(s) changing the gap between the reluctor ring stators and the Hall sensor of the crankshaft sensor. Things like the crank shaft being a bit off center, a harmonic balancer causing a minor end-of-shaft bending, play in crankshaft bearing and the oversize of the reluctor ring shaft hole can all add up to a slightly out-of-round travel of the reluctor ring (slight wobble if you will). This will change the distance between the reluctor stators and the crankshaft sensor (the normal air-gap) and thus effect the exact time that the Hall sensor will toggle (indicating a timing event - like spark and injectors). Depending on the amount of change in the air-gap (between the reluctor stator and the crankshaft sensor) as the reluctor ring rotates, the amount of crankshaft rotation error between events can be significant.

My example drawing:
 

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#10 ·
Update:

I think I'm on the right track. :banana:

A CASE relearn failed on a Snap-On tool. I'm going to try a friend with HP Tuners. :confused:

I put a timing mark at 120 degrees and 240 degrees on the harmonic balancer. I used some clear mylar wrapped around the balancer and cut to exact length for one full wrap (49mm) then placed two white tape stripes at 16.33mm from each end. The joint (ends) of the mylar are exactly in the center of the normal timing mark. I double checked the markings on the old balancer to insure accuracy.

I then used my strobe light and checked the timing of all cylinders. The timing light has an advance adjustment. The commanded timing advance (after 10 minutes at run temperature) is a steady 17 degrees (at idle) per my Torque App OBD2 reader. So, I adjusted the timing light so that #1 cylinder timing mark was aligned to the "V" mark on the timing chain cover. Sure enough, the strobe light adjustment agreed... 17 degrees advanced.

So, time to check all the cylinders. This is what I got:

(EDIT - Oops... data was transcribed incorrectly, but it's still the same in the end. It's correct now.)

Fire order: 1-6-5-4-3-2

Cylinder #1:- +17 deg
Cylinder #6: +21 deg
Cylinder #5: +24 deg
Cylinder #4: +17 deg
Cylinder #3: +21 deg
Cylinder #2: +24 deg

That's a span of 7 degrees :eek: . That's close to my earlier best-guess of 6 degrees. It's safe to say that the timing is the cause of the rough idle. It also affects the fuel injector timing too.

So, another way of looking at this is:

Cylinder #1 to #6 +4 degree difference
Cylinder #6 to #5 +3 degree difference
Cylinder #5 to #4 -7 degree difference
Cylinder #4 to #3 +4 degree difference
Cylinder #3 to #2 +3 degree difference
Cylinder #2 to #1 -7 degree difference

What a mess. I sure hope the CASE Relearn works at the next shop. :)
 
#12 ·
Norohs,

Well done. I think every crank wheel should have several timing marks on it, with a small number next to each mark indicating which cylinder it is valid for.
My timing light also has the "delay" timing feature or dial... so it is easy to "back check" what the Deg BTDC are for the target spark plug. Very slick job.

But for the uneducated, can you define the CASE accronym? and what needs to be "re-learned?"
Is there an issue in the memory of default values for each cyliner's offset timing in the ECU?

It could just be your timing pick-up device which has some issues.

Herminator
 
#13 ·
CASE stands for (I think) Crankshaft Angle Sensor Error, or something like that.

It's to compensate for normal but differing mechanical conditions of the engine. Every engine is unique. It's done at the factory and won't change unless the engine, crankshaft, camshaft, ECU (re-flashed or new), crankshaft Hall sensor, timing chain cover or harmonic balancer are replaced. This CASE data is stored in non-volatile memory in the ECU (VCU on an Astro).

Short story:

Original engine replaced at 89K. It was "glove smooth". This new (second) engine is when the vibration was first noticed. I did a lot of this and that trying to find/fix it and finally gave up, condemning the engine block. I lived with it for another 107K. Third engine was put in late last November. It still had the same issue. Thus this hunt and destroy mission was started.

It's amazing how many professional mechanics don't know or have never done this CASE relearn. I just came from a pro shop and the mechanic knows what a CASE relearn is, but has never done one. He was working on a Z28 Camero with a Stroker 383, double shot of NOS, custom everything (and I mean everything), making 1000HP... for a customer who's spent close to $200K on this car... still never done a CASE relearn.

That's ok. I'll bring instructions. ;) :thumbup:

BTW, it's Gary. Norohs stands for No RoHS (No Reduction of Hazardous Substances) - RoHS laws hurt my industry ;)).
 
#14 ·
Herman Wiegman said:
Norohs,

But for the uneducated, can you define the CASE accronym? and what needs to be "re-learned?"

Herminator
The computer needs to learn the cylinders firing are evenly spaced just not willy nilly as it is doing.

Oh I think i will fire one now. No reason. Maybe the mechanical parts changed since it was made.

One thing for sure it explained why number 3 cylinder keeps misfiring for everybody.
 
#15 ·
chevymaher said:
One thing for sure it explained why number 3 cylinder keeps misfiring for everybody.
That was my thinking too. I've see the posts about this, but wanted to see how mine plays out before suggesting people get the CASE relearned.

BTW, I found this manual process for CASE relearn on 1998 and later trucks. It's going to be a challenge to do it in this 'big city' without getting a DWI stop. :cheers: :driving:

CASE Relearn - Manual

Turn off all accessories

With outside and water temp within 5 degrees C of each other, start the engine and idle for 2 minutes in Park. Do not touch brakes.

Accelerate to 55 MPH at part throttle. Cruse at 55 MPH for 8-10 minutes until engine reaches operating temperature.

Cruse at 55 MPH for another 5-6 minutes,

Decelerate to 45 MPH without using brakes. Maintain 45 MPH for 1 minute.

Perform 4 deceleration cycles without using brakes and foot off the gas, of 25 seconds each where no specific speed is necessary. Return to 45 MPH for 15 seconds in between cycles:

First cycle

Second cycle

Third cycle

Forth cycle

Accelerate to 55 MPH and cruse for 2 minutes.

Stop vehicle and idle (still in gear) for 2 minutes with brakes applied.

Completed.
 
#18 ·
"... could you have gotten some bad gasoline?"

No, this has been a problem forever (so it seems).

My '94 Taurus, though, did get a tank of major alcohol over the Christmas week. It was pinging and detonating so bad I swore it was cooked. Last time I heard that much detonation, I cook a 1978 van for running it out of water. Thankfully, I only put 1/2 a tank of bad gas in it on an empty tank. I finished filling it up with super gas and it still knocked and pinged like crazy. It took over a week of slow and easy driving to finally get enough bad gas out of it to get a full tank of good gas. Driving it everyday now and lucky to not have any damage to it.

I'm not saying where I got the bad gas, but lets just say it will be a long time before I pull into a certain Chevron station in my neighborhood. ;) :hand: :naughty: :snooty:
 
#20 ·
Gary,
Where did you find that "manual" relearn schedule?
It sounds a bit tiring... and repetitive.
Should the ECU be "triggered" some how, or is the spark advance always trying to improve the cylinder angle?

A GM dealership has to be somewhat aware of this procedure or adjustment.

the web doesn't seem to have too much info.. most say the PCM does the adjusting..
only a few links take a look a the distributor timing wheel and talk about CMP Retard Offset data.
http://www.gmtruckcentral.com/articles/distributor.html

The PCM may have some default or corrected offset values that can written over...

Herman
 
#21 ·
Not sure where that procedure is from, but the case relearn I have done uses a scan tool. You enter that mode once motor is warmed up. start the learn, tool tells you to nail gas, and once it revs up it stops it at rev limiter, or before if its happy.. than its done.

CMP retard also uses a scan tool, and with revs at 1500 when checking, you rotate distributor for 0 degrees on scanner. +/- 2 degrees.

That procedure that was posted actually sounds like an idle relearn/IAC relearn.

Scott
 
#22 ·
Well I re-read the rest of this thread.. You've already tried a proper CASE relearn with a tool but it failed.. Did it let you start it and then didn't take? or not enter the mode at all? I know your P/N switch has to work properly, and engine temp about 180 or so. Might be a third condition too, I cant remember.. From what else you have posted, this relearn might just fix you up.

I use EFILive, and when I have done a relearn sometimes I have to try it a couple times to get the PCM to get ready. Not as good as a Tech2 but still works.
 
#23 ·
Herman Wiegman said:
Gary,
Where did you find that "manual" relearn schedule?
It sounds a bit tiring... and repetitive.
Should the ECU be "triggered" some how, or is the spark advance always trying to improve the cylinder angle?

A GM dealership has to be somewhat aware of this procedure or adjustment.

the web doesn't seem to have too much info.. most say the PCM does the adjusting..
only a few links take a look a the distributor timing wheel and talk about CMP Retard Offset data.
http://www.gmtruckcentral.com/articles/distributor.html

The PCM may have some default or corrected offset values that can written over...

Herman
Yes most computer setting procedures are tiring and repetitive.
They don't work. You can't get the information you need. Dealer purposely deletes the hard to get information after it is out of warentee.
You can't get new parts only used which are in just as bad of shape as what you got except they work once they are refurbished. Wiring is shot and needs replaced and they don't make it. Or the resistance values in the wires upsets the computer. Cost a fortune to have it fixed by a trained specialist at the dealer. Here is the catch if they even know how.
List goes on and on.

Now some computer supporter run it by me one more time how these things are more reliable,accurate, better and easier.

Seems like once they are out of warentee, if they last that long. They are the problem. Some cars have computer issues and are considered totals because they are so expensive to fix.

I am having a rant. because I was given such a hard time for not liking computers. Insulted because if you understood them they are easier. Somebody jump up with the answer. I know if it was a HEI this issue would have been fixed in a half hour.
Instead we have a torture marathon going on that is threatening a otherwise good vehicles existence.

:whack: :angry-soapbox:
 
#24 ·
I'm not sure how the CMP would fall into this problem. I've read about it before, but I thought it was for those "hitting the wall" (out of internal compensation room on the ECU), especially on performance tunes. I need to look at this again. Thanks.

A local shop tried the CASE Relearn using the latest/greatest Snap-On tool. I can't give any details because I was in the waiting room. But, I did hear them hitting the rev limiter for 5-10 seconds at a time on 3 attempts to get it to take. Seems there are subtle variations in the CASE Relearn process. Some require WOT punch and release, slow to a two-teared fuel cutoff and other assorted variations. Who the 'flip' knows what this thing wants.

I know I should just take it to the stealer, but the local stealer has a bad reputation and has personally reamed my posterior without so much as a thank you kiss.

I'm going to do one more thing. I'm going to scope the Hall crank sensor and make sure it's sending out well timed pulses. If it is, it's 100% sure an EUC (VCU... what-ever) problem. I may have to go out-of-town on a day trip to get to a dealer that's highly recommended for the CASE relearn.

My current line of thought is that I'm caught in a catch-22 loop. The current CASE data is so far out of whack that the ECU fails the CASE relearn. I may look at getting a stock re-flash on the ECU to get things back into acceptable range so that it can do a CASE relearn. According to what I've seen, I wouldn't be the first to have to do this.
 
#25 ·
So, I had the guys at the performance engine shop try a CASE relearn today. Good news, bad news.

The good news is that it took. The bad news is that it's made no difference that I can tell. I might recheck the timing later to see if it moved at all.

On this 2002, the CASE relearn is done with punching the throttle WOT and releasing as soon as the rev limiter. Holding it open banging against the rev limiter is NOT the way to do it. I thought so much when the first guy tried it, but it looked like he was having a bad day. Far be it for me to point out his error - I KNOW what it's like to have a bad day too.

So, I'm going to look at the cylinder timing again (to see if it changed) and check it against the reluctor wheel pulses from the crankshaft sensor. If it's all stable, I'll have no other options but to condemn the ECU. If the reluctor wheel pulses follow the timing errors, I'll have to consider the crankshaft sensor, reluctor wheel or worse... something in the engine, like a cracked crankshaft (though unlikely).

The saga continues...
 
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