When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I've been trying to track down the cause of a misfire I've been getting. Initially I got a P0300 (random misfire) code, and I was able to get it cleared and fixed by replacing spark plugs and cleaning the injectors. The issue came back with a rough idle, and sputtering with a left O2 sensor code. I replaced the O2 sensors, I have verified that I'm getting spark from my coils, spark plugs are good, I can't find any intake leaks.
I had an OBD scanner hooked up and was observing my fuel trim while putsing around my neighborhood. Bank 1 would have a normal response, but when applying throttle, it drops to zero signal. So I pulled the injectors and swapped sides, and the same issue persists with the Bank 1 fuel trim having the same issue.
I'm going to check the fuel pressure at the rails, do a compression test next, and check all fuses and relays related to fuel and air (these are in no particular order).
The car is lightly modified with longtube headers, cat delete, X pipe, axle back exhaust and recently had an RPM level VI T56 put it with C6Z06 clutch, slave and flywheel, and TICK master cylinder.
I have reached out to a few other mechanic buddies and searched the forum and it is a wide array of possibilities. Anyone have any experience with this issue? Any suggestions where to start? I do a bit more troubleshooting each day so I can update as I go.
What was the left O2 sensor code you are seeing ??…if you are running a wideband O2 sensor in your car you would be looking at an AFR (air fuel ratio) gauge in your car…if not you have the OEM narrow band sensors…if you used Denso’s they are fine…Bosch did not work well for me nor did Delphi…does your scan tool have misfire counters ??…now you said your fuel trims were normal except when accelerating and they dropped to “zero signal” ???…fuel trims will either be positive or negative above “0”…if positive the PCM is adding fuel (lean command and O2 sensors going below 450mV) or negative fuel trims where you will see a minus fuel trim number…O2’s are seeing a rich command above 450mV…so check again and see what the fuel trims on both banks are doing and also look at the 02 sensors…the upstreams should be “oscillating” between 200-800mV and downstreams will be steady around 700-800mV’s…if just one O2 sensor goes lean (below 450mV) it is not a fuel pressure issue or a MAF issue…it would be something like a vacuum leak on that bank (intake runner or injector)…if at idle your long term fuel trims are positive (more than 10%) run the engine at 3000 RPM…if the long terms decrease slowly and short terms quickly decrease you have a vacuum leak…as your throttle blade opens manifold vacuum decreases so a vacuum leak has less of an effect on those fuel trim numbers...good video below...it's 2 parts.
What was the left O2 sensor code you are seeing ??…if you are running a wideband O2 sensor in your car you would be looking at an AFR (air fuel ratio) gauge in your car…if not you have the OEM narrow band sensors…if you used Denso’s they are fine…Bosch did not work well for me nor did Delphi…does your scan tool have misfire counters ??…now you said your fuel trims were normal except when accelerating and they dropped to “zero signal” ???…fuel trims will either be positive or negative above “0”…if positive the PCM is adding fuel (lean command and O2 sensors going below 450mV) or negative fuel trims where you will see a minus fuel trim number…O2’s are seeing a rich command above 450mV…so check again and see what the fuel trims on both banks are doing and also look at the 02 sensors…the upstreams should be “oscillating” between 200-800mV and downstreams will be steady around 700-800mV’s…if just one O2 sensor goes lean (below 450mV) it is not a fuel pressure issue or a MAF issue…it would be something like a vacuum leak on that bank (intake runner or injector)…if at idle your long term fuel trims are positive (more than 10%) run the engine at 3000 RPM…if the long terms decrease slowly and short terms quickly decrease you have a vacuum leak…as your throttle blade opens manifold vacuum decreases so a vacuum leak has less of an effect on those fuel trim numbers...good video below...it's 2 parts.
Thanks for the response and video! I'll update after work today when I get crackincrat the car. I also plan on swapping sides on my O2 sensors to see if my lean fuel trim follows it, and also checking the fuelvpressure at the rail. I checked for a vacuum leak by spraying carb cleaner all over the intake bridge, MAF, vacuum hoses, injectors, etc and there was no change in RPM. I also pulled the injectors and fuel rail and hit the fuel rail with some compressed air in case there was any debris or possible blockage. I also used a stethoscope to listen to each injector, and there was a small volume difference but no rhythm difference between each. I have also had the injectors and MAF cleaned by a shop earlier this week. The car ran about 80% smoother after I got home (this original issue left me stranded 4 hours outside of home. The shop cleaned the parts to get it running enough to get me home where I can troubleshoot where I'm at now). The misfire issue came back, but it is not throwing any codes for running rough/misfiring. Let me know if you need any other info, and I'll get back to you this evening!
OK so first thing what was the O2 sensor code ??...just because it is an O2 sensor code doesn't always mean it's a bad sensor...at least you have a scan tool to look at some data PIDS...first thing is to check ECT, IAT, and MAP/Baro key ON engine OFFf,,,ECT and IAT should be close to the ambient temp after the car sits overnight...MAP/BARO should read around 29.9 inches Hg if you are at sea level...after the engine is at operating temp with no loads on it the MAF should read about 6.0-6.5 grams/second...so after the car warms up what do your long and short term fuel trims look like at idle on both banks and also look at the upstream O2 sensors..I mentioned previously what a vacuum leak would look like with your fuel trims.
OK so first thing what was the O2 sensor code ??...just because it is an O2 sensor code doesn't always mean it's a bad sensor...at least you have a scan tool to look at some data PIDS...first thing is to check ECT, IAT, and MAP/Baro key ON engine OFFf,,,ECT and IAT should be close to the ambient temp after the car sits overnight...MAP/BARO should read around 29.9 inches Hg if you are at sea level...after the engine is at operating temp with no loads on it the MAF should read about 6.0-6.5 grams/second...so after the car warms up what do your long and short term fuel trims look like at idle on both banks and also look at the upstream O2 sensors..I mentioned previously what a vacuum leak would look like with your fuel trims.
Unfortunately, I don't remember the code and I cleared it when I installed the new O2 sensors, sorry. Could you tell me what PIDS, ECT, IAT, and MAP/Baro mean? I have an idea, but I want to clarify. It's snowing here in Michigan, so I will update in the evening tomorrow with the respective values. Thanks again!
Unfortunately, I don't remember the code and I cleared it when I installed the new O2 sensors, sorry. Could you tell me what PIDS, ECT, IAT, and MAP/Baro mean? I have an idea, but I want to clarify. It's snowing here in Michigan, so I will update in the evening tomorrow with the respective values. Thanks again!
On-board diagnostics (ODB II) Parameter IDs (PIDs)
ECT=Eng Coolant Temp
IAT=Intake Air Temp
MAP=Manifold Absolute Pressure
Baro=barometric pressure
"The manifold absolute pressure (MAP) is the pressure inside the engine's intake manifold (inlet manifold). When the engine is off, normal manifold absolute pressure is the same as the barometric pressure (atmospheric pressure) outside your car."
Search some, read some, it will start making sense and find a gear head friend at work to help you-
Next time you pull codes, use your smart phone and video them as they scroll so you have them documented.
If you can just screenshot those PIDS I asked with your scan tool that would help us…Key On Engine Off, Idle, and at 3000 RPM which would make it a little easier….I don’t know if you can record and graph with your scan tool but having some data PIDS under load would be great !!
If you can just screenshot those PIDS I asked with your scan tool that would help us…Key On Engine Off, Idle, and at 3000 RPM which would make it a little easier….I don’t know if you can record and graph with your scan tool but having some data PIDS under load would be great !!
I do have a video of the fuel trim of bank 1 and 2, but it was sent via message and it may be hard to make out. Bank 1 will be the red line, and bank 2 is the green line. I have it attached to this reply. Let me know if it works ok, this is the first time I am using Imgur to post a video as the forum doesn't support .MP4s
If you can just screenshot those PIDS I asked with your scan tool that would help us…Key On Engine Off, Idle, and at 3000 RPM which would make it a little easier….I don’t know if you can record and graph with your scan tool but having some data PIDS under load would be great !!
Ok, I was able to get to my car and plug a scanner in. Here are the codes from when it was cold and at idle:
And here is from when it was warmer at idle:
Here is even warmer with a better scanner at idle:
Here is it warm at around 1500rpms. I didn't want to raise it to 3000 over concerns for further damage.
I included the code as it has a pending P0301. It switches between my 3 scanners used of a P0300 and a P0301.
I hope this helps give more info and apologies if my descriptions aren't super helpful. I'm handy enough to be dangerous, lol, but I'm still learning! So any info or advice is super appreciated.
What I’m seeing is your left upstream O2 sensor is reading 1.105 volts…the upstreams oscillate between 200-800 Mv so when you take a picture I don’t know if that voltage is stuck at 1.105 or is the voltage changing…you normally graph O2’s (see below pic) but if your scan tool doesn’t have that feature you have to look at the voltage while the car is running…is the voltage constant at 1.105 or if it is changing what are high and low voltages are you seeing ??…what we are seeing is a rich command (that is why your short term fuel trims are -31.3…eventually your long terms will adjust to bring that short term back to 0…if the upstream is really rich the B1S2 downstream should be reading rich too…it is not !!…it is .445 so that tells me your upstream sensor is “lying” !!…you said the upstreams were changed so you have either a bad sensor or a wiring issue…what brand of sensor did you use and where did you buy it ??…to check for a wiring issue remove the O2 heater fuse (fuse 15 -OXY SEN )…hook up your scan tool and turn the key ON engine OFF…what does your B1S1 voltage read ??…if it reads 450mv’s the circuit is good…if it reads 1.105 you have a wiring issue…the bottom pic is what you see when you “graph” the 02 sensor…I can actually see what it’s doing when graphed.
What I’m seeing is your left upstream O2 sensor is reading 1.105 volts…the upstreams oscillate between 200-800 Mv so when you take a picture I don’t know if that voltage is stuck at 1.105 or is the voltage changing…you normally graph O2’s (see below pic) but if your scan tool doesn’t have that feature you have to look at the voltage while the car is running…is the voltage constant at 1.105 or if it is changing what are high and low voltages are you seeing ??…what we are seeing is a rich command (that is why your short term fuel trims are -31.3…eventually your long terms will adjust to bring that short term back to 0…if the upstream is really rich the B1S2 downstream should be reading rich too…it is not !!…it is .445 so that tells me your upstream sensor is “lying” !!…you said the upstreams were changed so you have either a bad sensor or a wiring issue…what brand of sensor did you use and where did you buy it ??…to check for a wiring issue remove the O2 heater fuse (fuse 15 -OXY SEN )…hook up your scan tool and turn the key ON engine OFF…what does your B1S1 voltage read ??…if it reads 450mv’s the circuit is good…if it reads 1.105 you have a wiring issue…the bottom pic is what you see when you “graph” the 02 sensor…I can actually see what it’s doing when graphed.
You absolutely nailed it on the wiring issue. I was just poking around underneath and saw that the plastic sheathing of my O2 extension wire was peeled back around where it looped above my header. I peeled it back and saw a ton of dust, dirt and debris. I disconnected it from the main harness and found the wire in this state...this definitely has to be the cause....
When I disconnected it and removed it, the bare wires were fused together in a big clump. Looks like the hanger it was on to support it above the header was bent. Maybe the RPM guys might have bent it when reinstalling my exhaust, or it slipped off over time 🤷♂️
Anyways, orderd new pigtail extensions and they should be coming in this Sunday. I'll update when they're installed. Thanks so much for the help so far, it's definitely been a learning experience!
Your O2 signal wire was shorted to the 12 volt wire of the O2 heater circuit so that’s why you saw that high voltage…1.105 volts is the max voltage the PCM will read…with all that fuel being “pulled” you basically had a lean misfire…sometimes you will see all 4 cylinders on that bank misfiring…I’d wrap that wiring in high temp Tesa Tape and maybe covered with a high temp loom for added protection.
Your O2 signal wire was shorted to the 12 volt wire of the O2 heater circuit so that’s why you saw that high voltage…1.105 volts is the max voltage the PCM will read…with all that fuel being “pulled” you basically had a lean misfire…sometimes you will see all 4 cylinders on that bank misfiring…I’d wrap that wiring in high temp Tesa Tape and maybe covered with a high temp loom for added protection.
Just wrapped the O2 wires ans extension wires with reflectix tape. The car runs perfect now! While I was down there, I think I found the root cause:
When I installed my LT headers, there is a small gap slightly above where the headers and frame rail run on the driver's side. There are 2 lines in close proximity: O2 extension, and the clutch line. When I installed my Tick master cylinder line, I was sure to tuck the O2 wire between it and the frame rail to keep it from flopping about and resting on the close by header. I wrapped my headers in titanium wrap, but it still gets roasty with my spirited driving. I believe that when RPM installed my new transmission, they must not have left the wires in the same orientation, because when I was looking, the wire was laying between the clutch line and the headers. This adds up. I didn't have issues before going to RPM, but the 3 hour drive home, the 6 hour drive to work, and the 2 hours back was plenty of time for the wires to melt together into the mass that found. I'm not necessarily blaming RPM, I could see it being something that's easily missed in rush. In the end, it was a $50 fix and a bit of my father-in-laws and my time which we had fun scratching our heads so I think everything turned out alright.
Thanks everyone for the help! Definitely learned a lot and was definitely humbling in the matter of "are you SURE you checked all of the wiring?" Haha!
2001 Base model Misfire help!
