Tons of Data Requiring Analysis (Emissions/Misfire Related)

[TheNiceLips]

Hi everyone,

Sorry for the hiatus since my last post - I've been finishing undergrad, and now I'm married and a graduate student!

Anyways, time to get to Pontiac Bonneville Club related talk...

My beloved Bonneville now has over 82k miles on it, and with it, some issues...

Earlier this summer I replaced the UIM (and gasket), and spark plugs. The UIM replacement included a new PCV Valve & MAP sensor, UIM manifold itself, UIM gaskets, and Fuel Injector O-rings...here's a link to the kit: http://www.autozone.com/intake-system/i ... ckfit=true

Since then, the coolant level has remained steady, and the oil consumption was significantly reduced. However, after this maintenance, I realized that my fuel economy has gone noticeably down (I was averaging ~450 miles/tank of gas, now I'm lucky to get 380 miles/tank). I noticed the car was also down on power, so I replaced the Cat (the old one was rotted and crumbling). After some time had gone by, the car began to be randomly bucking and misfiring under heavy load (100% throttle). Now, it also randomly idles fairly rough.

I have an OBD II Bluetooth scanner, which lets me view live output and read trouble codes, etc. Below is the data that was logged during the most recent misfire.

Code: Select all

P0302

• Fuel System Status: Open Loop - Has not yet satisfied conditions to go to closed loop (Bank 1 or both)
  Calculated Engine Load Value: 90.2%
  Engine Coolant Temperature: 198 F
  Short Term Fuel Trim Bank: -0.8%
  Long Term Fuel Trim Bank: -6.2%
  Intake Manifold Absolute Pressure: 97.0 kPa, 18.6 inHg
  Engine RPM: 5576 RPM
  Vehicle Speed: (fast enough)
  Mass Air Flow Rate: 157.88 g/s, 20.88 lb/in
  Absolute Throttle Position: 100.0%

Now, the misfire has happened across multiple cylinders, so I don't believe it's an ignition problem (I mean, why would the coils/wires/plugs go bad all of a sudden, and could a set of brand new Iridium plugs cause this? Side note - the old plugs seemed perfectly fine, they weren't carbon fouled, oily, etc. The only difference between the old ones and the new plugs were that the old ones had a bit more of a gap, I believe due to aging (The car is 17 years old, after all...))

Screenshots: http://imgur.com/a/G70y2

The above link has 2 screenshots from the OBD II Scanner. I want to bring your attention to the emissions data. On the abbreviated screenshot, after the engine was warm, I turned the car off and on again. The fuel system was in Open Loop until I revved the engine (See the long bump in the Engine RPM data feed), at which point it converted to Closed Loop (using the Oxygen Sensors as feedback for fuel mixture). Then after I hold the engine at increased throttle, I let it idle. Notice that as soon as I step on the throttle, the mixture is unanimously lean (as in .1 V from the O2 sensors), and then the ECM bumps the Fuel trim from negative to positive (as indicated in the voltage increases in both O2 sensors.) Then at idle, the upstream sensor ("O2 Bank 1 - Sensor 1") begins fluctuating normally, but the downstream sensor ("O2 Bank 1 - Sensor 2") holds along a more "rich" mixture (it typically reads 0.7+ V). Now, either the O2 sensors are damaged/biased, or the fuel mixture is genuinely lean.

Now look at the longer screenshot, with more information. The fuel system status was "Open Loop" until the bump in Engine Load/Throttle Position made the system switch into "Closed Loop." This switch coincides with the upstream oxygen sensor's fluctuation in voltage. However, notice that the downstream sensor is reading a rich mixture, despite the upstream sensor reading a lean mixture. At this point, I wasn't sure which, if either, sensor to believe. Then I looked at the Fuel Trim values. The Short Term Fuel Trim (STFT) is only reading slightly negative (-2.3%) at this instant, however the Long Term Fuel Trim (LTFT) is a more troubling -20.3%, and I'm surprised it didn't throw a trouble code (either

Code: Select all

P0172

or

Code: Select all

P0175

). If the fuel trim were actually that low, then I tend to believe the poor performance and misfires are from the engine being starved of fuel, even though the O2 sensor(s) are reporting a rich condition.

Therefore, I deduce it's one of two things. Either:

• 1. The O2 sensors are correct, and despite the Lean LTFT, the exhaust is still genuinely rich (which would indicate either a faulty fuel regulator (too much pressure) or a leaky injector
  -or-
  2. Bad/Biased O2 sensors that are incorrectly reading a rich exhaust condition

I'm hoping someone from this forum can shed some light on this condition and give my head a rest from pounding it into the wall.

Thanks in advance,
TheNiceLips

[MattStrike]

First, I would check the plug gap, it's possible that one or more of the new plugs may have been damaged, and the iridium tip is broken off after running them for a while. But after that I recommend checking the wires and coils. Coils can fail either on the primary winding or secondary winding. A resistance check is preferred, but an open in one of the windings can be found with a diode tester.

I would solve the misfire before chasing the rich/lean issues with the PCM. Raw gas from the misfire will throw those readings off. Does the PCM set a code for which cylinder(s) it's detecting the misfire? Remember that one bad coil can set two cylinders and the random misfire codes.

[TheNiceLips]

Thanks for the reply, MattStrike

Okay, I'll check the plug gap as I just did a quick search and 1 Amazon review stated that the factory gaps can be way out of spec. If that's the case, what should the nominal gap be? 0.060 in?
(here's a link to the plugs: http://www.autozone.com/external-engine ... ab-reviews)

And yes, the PCM will throw misfire codes, but in apparently random cylinders. In my initial example, it was cylinder 2, but this morning was cylinder 5, and before that it was cylinder 6, and before that it was either cylinder 1 or 3... The only reason I ruled out the coil packs were because I never touched them, and these issues only appeared after replacing the UIM and plugs.

Also, what if the misfire is caused because of the rich/lean issues? There's three things that go into a proper combustion, spark, air/fuel, and compression. I think we can safely rule out compression as the closest I got to the cylinders were through the spark plugs, but they were tightened to spec. This leaves spark and air/fuel. Either the spark is bad (damaged plugs, wires, and/or coils), or the air/fuel is incorrect because of a damaged O2 sensor, faulty injector, bad fuel pressure regulator, etc.

And I believe I failed to mention this, but the misfiring is typically at its worst in the mornings after the car cooled overnight. This morning I thought it was going to stall, the engine was missing so badly. Then I go to show my boss (who has a lot of experience with the 3800's) after the car cooled for about 4 hours, and it purred like a kitten. I'm about to go home, and I'll see how it performs then.

Sorry for the rambling, I'm just trying to come up with a systematic way of troubleshooting the problems.

[MattStrike]

yeah, .06 but be really careful of you try to re-gap iridium plugs - the tip is extremely fragile and breaks easily.

At night, let it idle, and see if the wires are arcing anywhere, that would tell you if a wire was compromised during the work.

Only reason I suggest starting out with ignition parts is those are usually the cause of misfires in my experience, they are the easiest to test, and when they are bad they cause a lot of issues that the PCM can only respond to. So, example, if a bad o2 were to cause a misfire it would be difficult to tell from the data stream vs. a bad coil as the sensor would essentially tell you the same thing either way. So you have to pick one to rule out first. I feel ignition is relevant to start with as you changed the plugs; but don't rule out that it might just be a coincidence.

Usually if it's a fuel injector it either works or it doesn't. Same with fuel pressure (they make a tester gauge for that), in that it's not normally intermittent. Upper o2 sensor is the one used for fueling, sensor 2 keeps track of the catalytic converter and is only used for emissions testing.

I've had ignition problems cause both running poor cold and OK hot, and exactly the opposite. Probably depends if the coil windings are broken on the primary or secondary.

[bobc997615]

I've had coil packs where the terminal post had cracked over time that have caused a misfire.

[TheNiceLips]

Thanks for the replies - keep 'em coming!

Matt, thanks for the recommendation - I like the logic and troubleshooting path associated with it (e.g. If it's not "A", then try "B", if it's not "B", then try "C"...)

Bobc, I'll check the coil packs if the plugs and wires check out okay. Is the only way to test a coil pack by checking the resistance between the two internal windings?


UPDATE: On my way home after work, the car ran okay, it only misfired exactly twice the entire time. Other than that, it ran smooth, powerful, and responsive. This morning, after letting it cool overnight, it had a rough idle again - not as bad as yesterday morning where it almost stalled, but worse than last night's drive home. Here's a video from this morning: https://youtu.be/G1HCwa1UFqM

So now my troubleshooting logic aligns with MattStrike's as the engine performance improves as the engine gets warm. And we all know that a warm engine has a better chance for a full and complete ignition of the air/fuel mixture in the cylinder (basically helping the spark plug do its job).

I won't be able to work on the car until Friday, but I'll reply with any new information as it becomes available.

Thanks again to everyone helping me out with this, you da real mvp

[MKMike]

Are you using Bosch spark plugs?
I ask because they have been the source of misfire issues for many here.
The plugs may work for a while and then malfunction, despite being new.

If you have a look at the Pontiac Bonneville Club Maintenance list, you'll see a notation about the Bosch plugs (and sensors)
http://www.pontiacbonnevilleclub.com/fo ... 40156.html

Do a forum search just using the word Bosch and you'll find pages of complaints like these:

http://www.pontiacbonnevilleclub.com/fo ... ilit=bosch pay attention to crash93ssei and SSEBonne4evr


Then we move to http://www.pontiacbonnevilleclub.com/fo ... 39853.html

and

http://www.pontiacbonnevilleclub.com/fo ... 20516.html


http://www.pontiacbonnevilleclub.com/fo ... ch#p449704 post by 94SilverSSEi


http://www.pontiacbonnevilleclub.com/fo ... ch#p430835 Jrs3800


http://www.pontiacbonnevilleclub.com/fo ... ch#p421904 nos4blood70

Just thought you should know.

[bobc997615]

[quote="TheNiceLips"]Thanks for the replies - keep 'em coming!


Bobc, I'll check the coil packs if the plugs and wires check out okay. Is the only way to test a coil pack by checking the resistance between the two internal windings?

I could visually see a crack at the base of the terminal post or down the length of the post.

[TheNiceLips]

@MKMike, wow - how'd you know they were Bosch plugs??? I wasn't aware that Bosch plugs were of such stellar quality - I should've known something was up when they had a mail-in rebate that made the plugs only cost ~$3/each.

I guess I'll start with swapping the plugs with ACDelco's and assess from there.

[TheNiceLips]

RESOLVED: Swapping the plugs and wires cured my rough start/idle problem. Thank you all for your help.

However, I'm still getting poor gas mileage (lucky to get 19 mpg despite almost all highway miles). I can see some coolant that has leaked out from between the UIM and LIM, and is pooling in recesses if the LIM. Could this be the cause of my poor economy?