Runs really rough and misfires when attempting to accelerate

Tiny
JESSICA MERTENS
  • MEMBER
  • 2004 CHEVROLET IMPALA
  • 3.4L
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 215,000 MILES
My car all of a sudden one day it started running horribly. When I start it up it runs really rough and seems like it's misfiring and when I push on the gas or I'm going down the road it will spit and sputter and misfire and it's getting worse. I went and had the codes read on it and the O2 sensor was one of them and the thermostat was another one. The thermostat cold has come up for like the past 6 months so the newest one would be the O2 sensor code and I'm wondering which one I should replace first or how I would go about knowing where the problem is coming from. And I think I also read something about there being two O2 sensors on my car how do you know which one to replace?
Do you
have the same problem?
Yes
No
Monday, October 21st, 2019 AT 9:55 PM

5 Replies

Tiny
SCGRANTURISMO
  • EXPERT
Hello,

What were the Direct Trouble Codes [DTC s that were pulled from your vehicle's Power-train Control Module [PCM]?

Thanks,
Alex
2CarPros
Was this
answer
helpful?
Yes
No
Monday, October 21st, 2019 AT 10:31 PM
Tiny
CARADIODOC
  • EXPERT
You're approaching this the wrong way. The oxygen sensors aren't even in the picture until they reach 600 degrees, which takes a few minutes at least. Even then, they don't have enough effect on engine performance to notice. It sounds like you have a simple misfire problem that is resulting in too much unburned fuel in the exhaust system for the oxygen sensors to cope with. Based on the best readings they can provide, the Engine Computer can't make enough adjustments, so it sets an appropriate fault code.

The first step is you have to know the exact fault code number. There are dozens of O2 codes that can be set, and they mean very different things. Also, no fault code ever says to replace a part or that one is bad. When a sensor or other part is referenced in a fault code, it is actually the cause of that code about half of the time. First we have to rule out wiring and connector terminal problems, and mechanical problems associated with that part.

Start by looking at the spark plugs and wires. You need to solve the rough running before you go looking for other things to replace. Once the engine is running properly, it is all the other sensors that provide information to the Engine Computer, then, based on that, the computer calculates how much fuel to inject. Once the coolant reaches a certain temperature, the computer adds the readings from the oxygen sensors to the equation, but those O2 readings only fine-tune the fuel / air mixture for best fuel mileage and lowest emissions. Even a defective O2 sensor is unlikely to cause a severe running problem.

One oxygen sensor is first in line to see the exhaust gas coming out of the engine. That is where any unburned oxygen is detected. The computer switches the fuel / air mixture between slightly too rich to slightly too lean about two times per second, then it expects to see corresponding changes in what is detected by those front O2 sensors. Over time the mixture averages out to be perfect for lowest emissions and best engine performance.

Four-cylinder engines will have one front oxygen sensor. Most V-type engines will have one on each side, or "bank". With those, the computer controls the mixture on each side independently. The rear oxygen sensors will be near the back of the catalytic converter, or a little ways beyond that point. These could be the same part number as the front sensors, but most of the time they're different because they perform a different function. While the front sensor(s) switch between rich and lean two times per second, if the catalytic converter is doing its job, the cleaned-up exhaust coming out of it changes very little between too rich and too lean. The rear sensor's "switching rate" might be once every minute or two. That is what the computer is looking at to determine if the converter is working.

When the catalytic converter loses its efficiency or the catalyst becomes contaminated, no change takes place in the composition of the exhaust gas as it goes through it. Since the gas coming out is the same as the dirty gas going in, the front and rear oxygen sensors detect the same thing, so their switching rates are the same. The worse the condition gets, the faster the rear sensor switches between rich and lean. When the rear switching rate reaches a predetermined time period, the computer interprets that as the converter has lost its efficiency, and it sets a diagnostic fault code related to that.

Too often people find that fault code, and they blame the sensor, but in fact, it takes a properly-working oxygen sensor to be able to report that defective condition. That's why there are so many fault codes related to oxygen sensors. To see why there's so many, look half way down the list on this page:

https://www.2carpros.com/trouble_codes/obd2

then, to add to the confusion, look at the list on these pages:

https://www.2carpros.com/trouble_codes/obd2/p1000
https://www.2carpros.com/trouble_codes/obd2/p1100

There's many dozens more oxygen sensor codes splattered around on other pages, many of which I've never run into, and I have a suspicion no one knows what they mean, but in this entire list you won't find a single code that says to replace a part. They only direct you to the circuit or system that needs further diagnosis, or the unacceptable operating condition.

Note that there are also P0300 codes that will list exactly which cylinders are misfiring. This capability is on all 1996 and newer cars. Logic would dictate that your computer should be setting fault codes related to misfires, meaning the rough running, but part of the strategy is to not confuse the diagnosis any more than it already is. When a fuel calculation problem already exists, showing you a fault code for which cylinders are misfiring isn't going to be of value because it isn't the individual cylinder that needs attention, (spark plug, injector, valve, etc). Once a defective condition is detected, setting that fault code stops the computer from running many of the tests it performs while you're driving. In this case, the computer expects to see rough running due to some other problem, so there is no point in setting fault codes that say the engine is running rough. If it were to set a code for, say, "cylinder number three misfire", you could be tricked into spending all your time on that cylinder and never look in the right place for the problem.

Based on the little we actually know so far, your best bet is to start with basic tune-up items, especially since the problem appeared so suddenly. At the mileage you listed, you should just be putting in your second set of replacement spark plugs and wires. Next on the list of suspects would be an ignition coil. Your engine uses three of them; each one fires two spark plugs, so a defective coil will kill one third of your engine.

GM has also had a real lot of trouble with their injectors in high-mileage engines, but not that they cause rough running, so don't suspect them yet. They are more responsible for misfire fault codes when you can't even feel those misfires.

Please keep me up to date on your progress, then we can figure out where to go next.
Was this
answer
helpful?
Yes
No
Monday, October 21st, 2019 AT 11:25 PM
Tiny
CARADIODOC
  • EXPERT
Hi Alex. Didn't mean to butt in to your conversation.
Was this
answer
helpful?
Yes
No
Monday, October 21st, 2019 AT 11:26 PM
Tiny
JESSICA MERTENS
  • MEMBER
Okay, so the codes that came up is P0300 random misfire detected, p0102 mass or volume air flow circuit low input, p0128 coolant thermostat below regulating temperature, p0137 O2 circuit low voltage bank 1 sensor 2 P0420 catalyst system low efficiency P0300 random misfire detected P0420 catalyst system low efficiency.

Like 4 days ago I changed the plugs and wires and it ran great after that for two days it didn't misfire at all and it ran perfect but now it's doing the same thing again. And every once in awhile my check engine light will flash. I checked to see if I had the housing on it lined up right and the housing that goes on the throttle body seemed to not fit that tight but I tightened up the screw but it still seems loose.
I don't know what else to do.
Was this
answer
helpful?
Yes
No
Saturday, October 26th, 2019 AT 2:51 PM
Tiny
KEN L
  • ADMIN
Hello,

It sounds like the engine has had a problem, how does it sound when you are cranking it over? Can you please shoot a quick video with your phone so we can hear the noise? that would be great. You can upload it here with your response. I can tell for sure whats going on with a video. If not lets do a compression test.

https://www.2carpros.com/articles/how-to-test-engine-compression

Please run down this guide and report back.
Was this
answer
helpful?
Yes
No
Monday, October 28th, 2019 AT 6:59 PM

Please login or register to post a reply.

Sponsored links