Engine Idles rough, misfires at idle, smells rich, but codes say it's lean on both banks

Xenogenesis

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Dec 17, 2019
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The engine codes are showing lean in both banks, but the exhaust smells rich. I've replaced the O2s and it did almost nothing for it, (I was really hoping that was it.) The other engine code is showing error in the EGR, which I have replaced both the EGR and the EGR control solenoid. I'm tempted to just block it off, and I will do it if I can't fix this. I've replaced the MAF sensor, and I don't hear or feel any vacuum leaks. My last thought is to just replace the computer. I've been at this for a long while, and the lean error used to be only on bank 1. I replaced the engine 7-8 months ago, and the rough idle has gotten worse. It used to idle fine, then if you drove it for a bit, shut it off, and fire it back up, it'd have misfires on one side. Now it's gotten to wear at the initial startup it misfires and idles rough. Sometimes it dies down to where you'd think it's gonna stall, misfiring and bouncing, but it doesn't.
 
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wmburns

SN Certified Technician
Aug 14, 2009
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Stop. Don't replace another part without testing to confirm that it's actually bad. This goes DOUBLE for the PCM. Never replace a PCM unless all other possible "causes" have been ruled out. Replacing a PCM for diagnostic reasons often creates as many problems as it solves.

Just wondering. How much $$ has been spent without a fix?

As far as what could "cause" your base symptom. What has been done to rule out an exhaust leak? What's one possible source for an exhaust leak? Hint. If you are thinking EGR give yourself a pat on the back.

Other "possible" causes that match your symptoms could be biased O2 sensors. IE the O2 sensors are not telling the truth to the PCM.

Another possible cause that comes to mind could be unstable fuel pressure.

Do you have an ODB2 scanner that is capable to monitoring operational PID's? Even better an unit that can graph? If you did it might make short work of this problem. Here's some information on an affordable ($30) Windows based unit.

ForScan ODB2 scanner w ELM327 USB
 
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Xenogenesis

New Member
Dec 17, 2019
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0
1
20
Tennessee
Stop. Don't replace another part without testing to confirm that it's actually bad. This goes DOUBLE for the PCM. Never replace a PCM unless all other possible "causes" have been ruled out. Replacing a PCM for diagnostic reasons often creates as many problems as it solves.

Just wondering. How much $$ has been spent without a fix?

As far as what could "cause" your base symptom. What has been done to rule out an exhaust leak? What's one possible source for an exhaust leak? Hint. If you are thinking EGR give yourself a pat on the back.

Do you have an ODB2 scanner that is capable to monitoring operational PID's? Even better an unit that can graph? If you did it might make short work of this problem. Here's some information on an affordable ($30) Windows based unit.

ForScan ODB2 scanner w ELM327 USB
I've never really had an ODB2 scanner personally, usually took it to AutoZone for that, but it would be nice to have one myself. I'll get it and see about that.

The money I've spent.. I think near $500 or so? It's not all in vain I hope, some new fresh sensors. I replaced the O2s ($100,) EGR and EGR control solenoid ($50 each,) and all 6 injectors (I think $350 or so.) I hate messing with injectors, but I don't think they're leaking if it thinks it's lean.

Exhaust leak, you say? I mean, we've thought about it, but I don't think it sounds like it. Then again, there's some soft noise like that, that increases when under throttle. Could it actually be that??
How do I tell for sure?
 

wmburns

SN Certified Technician
Aug 14, 2009
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How do I tell for sure?
When you get the ODB2 scanner, start first by confirming that the O2 sensors actually switch rich/lean.

Then monitor/graph the:
  • MAF
  • Long term fuel trims (LTFT).
  • RPM's
  • Fuel pressure.
Raise the RPM's to say 2,000 and watch what the LTFM do. Spray some brake clean around the intake and watch what the LTFT do. Often a problem with fuel control will give itself in HOW the various PID's react.
 
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Xenogenesis

New Member
Dec 17, 2019
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Tennessee
When you get the ODB2 scanner, start first by confirming that the O2 sensors actually switch rich/lean.

Then monitor/graph the:
  • MAF
  • Long term fuel trims (LTFT).
  • RPM's
  • Fuel pressure.
Snap the throttle and watch what the LTFM do. Spray some brake clean around the intake and watch what the LTFT do. Often a problem with fuel control will give itself in HOW the various PID's react.
It's gonna be a few days before I get the ODB2, so in the meantime, I'll see if it's an exhaust leak around the EGR. Thank you.
 

Xenogenesis

New Member
Dec 17, 2019
12
0
1
20
Tennessee
When you get the ODB2 scanner, start first by confirming that the O2 sensors actually switch rich/lean.

Then monitor/graph the:
  • MAF
  • Long term fuel trims (LTFT).
  • RPM's
  • Fuel pressure.
Snap the throttle and watch what the LTFM do. Spray some brake clean around the intake and watch what the LTFT do. Often a problem with fuel control will give itself in HOW the various PID's react.
Okay, so I feel really stupid if we found the issue. I just don't understand the codes on the check engine, perhaps something could still be getting to it, but we found out that the idle was practically off, so it was running off the IAC alone, which I imagine could of been a major contributor to how bad it was running. We upped the idle to about 750-900, and it idles pretty decent. I think the idle is supposed to be 750ish? Correct?

Otherwise, there's still sometimes it drops, like something is dragging on the motor, and it misses quite a bit when it drops like that. It usually clears up after a bit. She runs and drives good usually, though. I think my issue could've been I never really knew where the idle was supposed to be set. Just would like to know that.

Hoping the low set idle was the issue, but why would that cause a lean error code? Must be something else, just the low idle made it worse, or maybe caused it.

I'll be trying the OBD2 to see about all that.
Thanks for the help.
 

wmburns

SN Certified Technician
Aug 14, 2009
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519
204
Houston Texas
Hoping the low set idle was the issue, but why would that cause a lean error code? Must be something else, just the low idle made it worse, or maybe caused it.
A low idle isn't going to CAUSE a lean DTC. It's far more likely that the lean DTC is causing the low idle.

IMO you haven't really fixed the problem. Likely masking around the problem by upping the idle.

Just wondering. HOW did you "adjust" the idle? By turning up the idle set screw? If so, likely what will happen is that the current idle setting has been baked in with the throttle body stop screen and the IAC isn't really controlling the idle speed. What is the effect of this? Expect the idle to be unstable when the outside weather changes.

Remember in a correctly working idle control system does not change the amount of total air entering the motor. The IAC works by redirecting (bypass) air that was originally metered via the MAF. Sooooooo if changing the IAC also changes the amount of total air entering the motor, THEN there had to be a leak of un-metered air somewhere in the IAC bypass path.

Here's some more information that might help you to understand HOW the IAC works to control the idle.

Troubleshoot IAC idle problems 1996-2004
 
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Xenogenesis

New Member
Dec 17, 2019
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Tennessee
A low idle isn't going to CAUSE a lean DTC. It's far more likely that the lean DTC is causing the low idle.

IMO you haven't really fixed the problem. Likely masking around the problem by upping the idle.

Just wondering. HOW did you "adjust" the idle? By turning up the idle set screw? If so, likely what will happen is that the current idle setting has been baked in with the throttle body stop screen and the IAC isn't really controlling the idle speed. What is the effect of this? Expect the idle to be unstable when the outside weather changes.

Remember in a correctly working idle control system does not change the amount of total air entering the motor. The IAC works by redirecting (bypass) air that was originally metered via the MAF. Sooooooo if changing the IAC also changes the amount of total air entering the motor, THEN there had to be a leak of un-metered air somewhere in the IAC bypass path.

Here's some more information that might help you to understand HOW the IAC works to control the idle.

Troubleshoot IAC idle problems 1996-2004
Yes, it was the idle screw we turned up. Snapchat-1179030606.jpg This is what happened after I snapped the gas. I'm unsure what any of these number are supposed to be, just wondering if you do?

Snapchat-1361060285.jpg And this is when I leave it idling, and eventually it'll drop like something's dragging on it, this is when it drops. I snapped the throttle, and the LTFT's did nothing.
Also, the graphs on the right are misfire detections for each cylinder, and we've found out that if you disconnect number 1 from the coil pack, number 4 misfires goes up, and if you disconnect number 3, number 6 misfires go up. I'm sure the timing is hooked up correctly, or else I think this thing would be running way worse, or even not at all. Maybe whatever is reading the misses is on the wrong side, or?
 

wmburns

SN Certified Technician
Aug 14, 2009
5,890
519
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Houston Texas
This thread does not state the model year of your car. Why is this important? Because if this is a 1996-1998 V8, the ignition system uses a "waste" spark system. Why is this important? Because certain cylinders are "married" to each other. But the cylinder combinations are NOT the normal cylinder pairing. This offers up the possibility of incorrect spark plug wire routing and wires connected to the incorrect coil pack. The fact that a different cylinder begins to misfire when the spark plug wire is pulled would almost "prove" incorrect spark plug wire routing (or you do not understand HOW Ford numbers it's cylinders).

Note, for a coil pack system when referring to the PRIMARY side of the ignition, the numbers do not always refer to cylinder numbers. But instead to coil pack circuit numbers. A dead giveaway is when letters (A, B, C, D) are used instead of numbers.

V8 coil pack waste spark cylinder pairing.
1,6
5,3
4,7
8,2

V8 coil pack diagram

Note, A CLASSIC sign of a vacuum leak is large positive LTFT's at idle. That decrease towards zero as the RPM's go up (with lower intake manifold vacuum).

So that others can learn from your experience, also tell us about your experience with ForScan.
 
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Xenogenesis

New Member
Dec 17, 2019
12
0
1
20
Tennessee
This thread does not state the model year of your car. Why is this important? Because if this is a 1996-1998 V8, the ignition system uses a "waste" spark system. Why is this important? Because certain cylinders are "married" to each other. But the cylinder combinations are NOT the normal cylinder pairing. This offers up the possibility of incorrect spark plug wire routing and wires connected to the incorrect coil pack. The fact that a different cylinder begins to misfire when the spark plug wire is pulled would almost "prove" incorrect spark plug wire routing (or you do not understand HOW Ford numbers it's cylinders).

Note, for a coil pack system when referring to the PRIMARY side of the ignition, the numbers do not always refer to cylinder numbers. But instead to coil pack circuit numbers. A dead giveaway is when letters (A, B, C, D) are used instead of numbers.

V8 coil pack waste spark cylinder pairing.
1,6
5,3
4,7
8,2

V8 coil pack diagram

Note, A CLASSIC sign of a vacuum leak is large positive LTFT's at idle. That decrease towards zero as the RPM's go up (with lower intake manifold vacuum).

So that others can learn from your experience, also tell us about your experience with ForScan.
Oh, I'm sorry, it's a 1995 3.8L V6. I put the engine in myself, and it seemed to run great at first, until sometimes after you'd drive it for awhile, then turn it off for like a minute, and start it up, it'd have a pretty consistent miss on one cylinder, then clear up after a bit. It slowly progressed until the startup idle started missing inconsistently on one side of the engine, (I have true dual exhaust) then I believe it hopped so the other side was missing more, and now it's both are just missing inconsistently. I don't know if these problems came together, or progressed into this, I'm unsure.

Snapchat-1139354981.jpg Here's what's under the hood.
Snapchat-1536974245.jpg My coil pack and the sticker that was on the original coil pack.
Snapchat-1054944400.jpg And this is a paper telling me the firing order I got from AutoZone.

Also, I love the OBD2 so far, and anxious to learn more about it. I'm really excited about it.

My LTFT's are 00 until I snap the throttle, then they seem to be lower, and eventually increase to 25% until I press the throttle, then it goes down, until I lift up and it goes back to 25%. So that is bad?

Also, I only meant on ForScan, it thinks cylinder 4 is misfiring when it is cylinder 1 is disconnected. Of course it's the correct cylinder misfiring I believe, just I wonder why ForScan is reading it's flipped. What sensor does the OBD2 use to determine which cylinder is misfiring? Is that hooked up correctly?
 
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