Rough Idle and stumble under low load - TCCoA Forums
 
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post #1 of 9 (permalink) Old 05-04-2016, 01:37 PM Thread Starter
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Rough Idle and stumble under low load

Hey all, I have a 97 with a 4.6 PI motor. It is idling pretty rough and sometimes when under a light load it will stumble and seem to want to stall out. I was recently in a drive through and the car died when I tried to turn the wheel at rest. I just changed the plugs and wires, coil packs are about 3 years old, I cant find any vacuum leaks, maf is clean, IAC is new , all O2s are new, and fuel pump is relatively new. I have an xcal2 and datalogged my fuel trims and there aren't any instances where the adjustment is over 10%. Also just tested compression and the highest was about 148, lowest about 141 and there were no oil fouled or fuel fouled plugs. I'm pulling my hair out here trying to figure it out. If there is something else I should be datalogging let me know and I'll do it, that area just isn't my forte.
Thanks for reading my book, please help.

I love my V8, but I'd love it more if all 8 cylinders worked. (they do now)

My T-bird is like George Jetson's wife, you try to give her a little money, she takes the whole wallet.
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post #2 of 9 (permalink) Old 05-04-2016, 01:59 PM
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Fuel trims ideally should only be 2-3 percent off across the board (Both LTFT and STFT) when in closed loop - if you get on the gas the STFTs will show commanded lambda (the desired A:F) - some number usually between .8 and 1.

In any case - I get this with the T-bird mainly because of the cams. In short, there's too much EGR in the cylinders and I have a hard time giving it enough spark to keep it from misfiring. It's always possible you're getting more EGR than you should and there isn't enough spark to compensate.

Datalog rpm, airflow #/min, spark source, spark advance, IAC duty cycle, and ISC integrator. Let us know what you find.

-Brandon
97 Laser Red Thunderbird LX 161k, Stage 2 4.6L 2v N/A | 300 BHP (255 RWHP, 290 RWTQ) | 13.95 @ 97.58 | Build details | Pics at the Lorain Assembly plant
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post #3 of 9 (permalink) Old 05-05-2016, 10:44 AM Thread Starter
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Okay so I datalogged what you suggested, I just don't know what I should be looking for. I attached 2 datalogs The first is start and idle, the second you can see I bumped the throttle, then went from park to reverse, to neutral, to drive, to park.
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File Type: png tccoa2.png (152.4 KB, 7 views)

I love my V8, but I'd love it more if all 8 cylinders worked. (they do now)

My T-bird is like George Jetson's wife, you try to give her a little money, she takes the whole wallet.
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post #4 of 9 (permalink) Old 05-05-2016, 02:18 PM
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Can you upload and send me the .csv files? Did it stumble at all for you during either of those runs?

-Brandon
97 Laser Red Thunderbird LX 161k, Stage 2 4.6L 2v N/A | 300 BHP (255 RWHP, 290 RWTQ) | 13.95 @ 97.58 | Build details | Pics at the Lorain Assembly plant
98 Black Mark VIII 155k, stock daily driver
07 Redfire Fusion V6 SEL 178k, for the wife
Gone but not forgotten: 96 Mark VIII, 94 Cougar XR7, 93 Mark VIII

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Once you have tasted flight, you will walk the world with your eyes turned skyward, for there you have been and there you long to return. -Leonardo da Vinci
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post #5 of 9 (permalink) Old 05-05-2016, 04:26 PM Thread Starter
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Originally Posted by theterminator93 View Post
Can you upload and send me the .csv files? Did it stumble at all for you during either of those runs?
It was a rough idle throughout. PM me your email address and I'll send you the files.

I love my V8, but I'd love it more if all 8 cylinders worked. (they do now)

My T-bird is like George Jetson's wife, you try to give her a little money, she takes the whole wallet.
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post #6 of 9 (permalink) Old 05-05-2016, 06:13 PM
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Well, the idle airflow values in the tune are a little off but not so much that the PCM isn't compensating like it should be. IAC duty cycle looks good as well.

Spark looks like it's where it should be; fuel trims indicate that the PCM is having to add about 3% more fuel than programmed at that RPM range, but again - within tolerable limits. There are other things that could be logged but I don't know how much more insightful it would be at this point.

In short - I didn't see any smoking guns in the info I saw to indicate what would be causing it to idle rough. It's probably an A:F imbalance but I can't be sure.

Try a couple more simple things. Unplug the battery for several minutes to reset the memory in the PCM - observe the idle behavior with the MAF unplugged, unplug all the vacuum lines to the plenum (except fuel pressure regulator) and tape them off and observe again (with MAF attached if it doesn't get better with it unplugged).

-Brandon
97 Laser Red Thunderbird LX 161k, Stage 2 4.6L 2v N/A | 300 BHP (255 RWHP, 290 RWTQ) | 13.95 @ 97.58 | Build details | Pics at the Lorain Assembly plant
98 Black Mark VIII 155k, stock daily driver
07 Redfire Fusion V6 SEL 178k, for the wife
Gone but not forgotten: 96 Mark VIII, 94 Cougar XR7, 93 Mark VIII

TCCoA's resident pilot since 2014
Once you have tasted flight, you will walk the world with your eyes turned skyward, for there you have been and there you long to return. -Leonardo da Vinci

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post #7 of 9 (permalink) Old 05-05-2016, 06:29 PM Thread Starter
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Thanks for checking it out! I'll definitely try everything you mentioned, and hopefully find the issue soon. This idle is driving me crazy.

I love my V8, but I'd love it more if all 8 cylinders worked. (they do now)

My T-bird is like George Jetson's wife, you try to give her a little money, she takes the whole wallet.
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post #8 of 9 (permalink) Old 05-05-2016, 07:28 PM Thread Starter
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I taped off the vacuum to the EGR and holy hell it idles much smoother. I still feel like I'm getting a bit of a miss. Every once in a while it will seem to skip a beat so to speak. I have no codes btw but it seems to be an excessive egr issue for now even though its all pretty new. I'll swap some stuff around and go from there. Thanks for the help.

I love my V8, but I'd love it more if all 8 cylinders worked. (they do now)

My T-bird is like George Jetson's wife, you try to give her a little money, she takes the whole wallet.
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post #9 of 9 (permalink) Old 05-05-2016, 07:31 PM
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Could be one of a few things there - a leaky vacuum line along the EGR path, a leaky solenoid (letting vacuum through to the diaphragm), or a diaphragm that won't close all the way letting a certain amount of EGR in at all times.

Good to see you found the path to chase.

-Brandon
97 Laser Red Thunderbird LX 161k, Stage 2 4.6L 2v N/A | 300 BHP (255 RWHP, 290 RWTQ) | 13.95 @ 97.58 | Build details | Pics at the Lorain Assembly plant
98 Black Mark VIII 155k, stock daily driver
07 Redfire Fusion V6 SEL 178k, for the wife
Gone but not forgotten: 96 Mark VIII, 94 Cougar XR7, 93 Mark VIII

TCCoA's resident pilot since 2014
Once you have tasted flight, you will walk the world with your eyes turned skyward, for there you have been and there you long to return. -Leonardo da Vinci
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