I've been fighting a rough idle for a while

[95 Tahoe]

I've been fighting a rough idle for a while. It was really acting up this evening. Wen I got home I decided to log the data and see if I could see anything. I Notice at one point I have a spike in engine RPM's that didn't really happen. Maybe something is up with the signals the ECU is seeing? What controls the tach signal? Is it the reluctor wheel?

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[RichLo]

1995 TBI engine I assume by your user name? 305, 350, 454? Manual or Auto trans?

How long has this been happening? Have you changed any parts since it started?

 

[95 Tahoe]

1995 TBI engine I assume by your user name? 305, 350, 454? Manual or Auto trans?

How long has this been happening? Have you changed any parts since it started?

I apologize the description slipped my mind. It's a 95 TBI 350 bored 30 over. I rebuilt it earlier this year. It has had a miss in it for over a year before the rebuild. Even after the rebuild the miss stayed. It only happens at idle and is real noticeable in gear. I do not feel a miss while driving. I replaced all the ignitions components this year. The old distributor had a cracked pick up coil. The new distributor was a rock auto special. I'm debating on if it's just a junk distributor. I also replaced the IAC, MAP sensor, EGR, ECT sensor, O2 Sensor and rebuilt the throttle body and regulator. I cleaned and replaced a bunch of grounds. I'm sure I left something off.

 

[Schurkey]

First Guess: Crappy module in replacement distributor.

TBI RPM signal is generated by the pickup coil/reluctor, then processed by the ignition module and sent to the computer. Anything in that chain is suspect.

 

[95 Tahoe]

Changed the pickup coil in the original distributor and dropped it in. It idles a lot better now. I think it's fixed.

 

[95 Tahoe]

Posting an update as it took nearly a year to figure this out. I've been running it almost a day now an it hasn't missed a beat yet. It started missing again not long after the last update on this post. I bought a whole new distributor and dropped in. I reused the cap off the old distributor since it was new and I was kind of lazy. I didn't feel like moving the wires over. It still had the miss at idle but different. Not sure how to explain it but it was different. I gave up and just drove it for a while. Chased all different possibilities. Yesterday I was cleaning up and came across the new distributor cap that I took off. I decided to slap it on just to see and the misfire at idle is gone. I'm happy it's fixed but mad at the same time. New parts should not be bad parts.

 

[Road Trip]

I've been following this thread silently since the beginning. Sincere thanks for
coming back and closing the loop on what it took to clear the misfire.

EDIT: New DOA parts are not fair. As been said elsewhere, sometimes new =
Never, Ever, Worked. :-( I sincerely hope that we as country come to terms
with how we ended up here, and more importantly, we figure out how to get
back to where we used to be. (Hint: We make it ourselves instead of being
treated as a cheap parts dumpster by offshore companies.)

By the way, would it be possible for you to post a clear photo of the inside of
the failed distributor cap? Perhaps we will see something instructional like what was
recently troubleshot to in a different thread? (Failure photo inside Vortec dizzy cap)

After doing research across all manner of forums, it's disappointing how many
interesting troubleshooting threads just stop with no resolution. So when you
are reading through someone working through a tough problem and they finally
come back and report the solution you feel like you hit the jackpot! :0)

Fingers crossed that the new cap is the missing piece of the puzzle for your
truck to stay in the fixed column.

Cheers --

 

[95 Tahoe]

I've been following this thread silently since the beginning. Sincere thanks for
coming back and closing the loop on what it took to clear the misfire.

EDIT: New DOA parts are not fair. As been said elsewhere, sometimes new =
Never, Ever, Worked. :-( I sincerely hope that we as country come to terms
with how we ended up here, and more importantly, we figure out how to get
back to where we used to be. (Hint: We make it ourselves instead of being
treated as a cheap parts dumpster by offshore companies.)

By the way, would it be possible for you to post a clear photo of the inside of
the failed distributor cap? Perhaps we will see something instructional like what was
recently troubleshot to in a different thread? (Failure photo inside Vortec dizzy cap)

After doing research across all manner of forums, it's disappointing how many
interesting troubleshooting threads just stop with no resolution. So when you
are reading through someone working through a tough problem and they finally
come back and report the solution you feel like you hit the jackpot! :0)

Fingers crossed that the new cap is the missing piece of the puzzle for your
truck to stay in the fixed column.

Cheers --

I know what you mean about the threads ending without a resolution. That's why I decided to post an update. With the miss vacuum was running around 17 with a slighlty fluctuating needle. Now it's just over 18 and is steady as a rock.

The only 2 things I can see by looking at the pictures is a black smudge at the button. Probably nothing. The is a slight groove on some of the pick ups and nothing on the others. Last pic came out slightly blurry but there is visible contact from the rotor.

I doubt I could have installed it off center. It has to fit the groove on the ICM and the screw holes need to align. Who knows for sure? I've screwed up simpler things before. On the bright side I have basically all new parts.

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

 

[Road Trip]

I know what you mean about the threads ending without a resolution. That's why I decided to post an update. With the miss vacuum was running around 17 with a slighlty fluctuating needle. Now it's just over 18 and is steady as a rock.

The only 2 things I can see by looking at the pictures is a black smudge at the button. Probably nothing. The is a slight groove on some of the pick ups and nothing on the others. Last pic came out slightly blurry but there is visible contact from the rotor.

I doubt I could have installed it off center. It has to fit the groove on the ICM and the screw holes need to align. Who knows for sure? I've screwed up simpler things before. On the bright side I have basically all new parts.

You must be registered for see images attach

It's one thing when a distributor cap fails and it shows us where it didn't hold up
under the stress. It's another when the problem occurs where we can't see it?

I took one of your photos, added some fill light and a couple of arrows:

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Don't know if what looks like a crack is just a cosmetic flaw, or there's a
subsurface crack that would cause the terminal above it to short to ground?
(Assuming new dizzy is the aluminum body version?)

And the other arrow with the question mark is pointing to what looks like
a blackened area behind the terminal? If this where the high voltage
was taking a shortcut through the insulator?

At any rate, of all the things that I've had the (dis)pleasure of troubleshooting
for sheer unpredictable weirdness, High Voltage takes the cake. By now I'm
never surprised by what happens when high voltage is involved. Especially inside
a closed cavity with a whirling rotor spinning at speeds approaching 3000 rpm.
(50 times per second.)

I appreciate your quick reply to my photo request. If possible, give it one more
look in the daylight, and include the exterior of the cap for any signs of where it
failed to contain the bottled lightning.

And if still passes a close visual inspection in the daylight then I guess that we
don't get to know where the high voltage failure occurred. But when you choose a
hobby like this you have to resign yourself to not always getting to know why
a part failed? Just like nobody gets to bat a thousand or other examples of perfection.

Again, thanks for closing the loop on this. If it stays running smooth for awhile, then
consider adding (Solved) to the end of your title. This way other folks searching for
an answer will know that this is one of the threads that has a payday at the end.

Tip of the hat in your general direction.

Cheers --