Misfire causing very high fuel trims
12-04-2018, 04:50 PM
#1
Misfire causing very high fuel trims
I'm a tuner with a lot of tunes under my belt but my truck has me stumped at the moment so I'm hoping for some help brainstorming. My truck is an 04 Tahoe with an LQ4 and a TSP Torquer V2 cam on 112. I expect the PCM to see misfires with this cam so I usually have them disabled. However, I recently noticed that my bank 1 fuel trims were maxed out. I could see that the O2 sensor was pegged lean.
My first though was a bad O2 sensor. So I switched them from bank to bank but the condition stayed the same.
Then I figured I had an exhaust leak near the O2 sensor that was causing it since my exhaust was hacked together due to a transmission swap I did. So I took it to the exhaust shop and got it all run nice and neat from the headers back. Then I still had the same condition.
I started testing injectors with my HPTuners scanner by disabling them one at a time. It's pretty hard to tell the difference when any one is turned off because of the choppy idle and I wasn't getting anywhere so I actually listened to each injector with a long screwdriver as a stethoscope. It seemed like injector 1 might not be firing. Also, I thought the plug felt loose so I unplugged it and plugged it back in nice and tight. Voila, It seemed I had fixed the problem.
However, on my way to work this morning, I was scanning and noticed the problem had returned. So I popped the hood and checked all of the injectors with my screwdriver and they were all functioning. Then I revved the engine real hard and my B1 O2 voltage came back up but as I watch the O2 volts graph, I can actually see misfires happening as little dips of voltage that don't look like the normal oscillations. In fact I have it in open loop so it really isn't oscillating very much which makes it easier to see the dips (misfires). Now I'm thinking about pulling my valve cover to see if something is really wrong like a broken spring or something. Does anyone have any other ideas of what I might check or how I might check it? Could a bad injector cause an intermittent problem like this?
My first though was a bad O2 sensor. So I switched them from bank to bank but the condition stayed the same.
Then I figured I had an exhaust leak near the O2 sensor that was causing it since my exhaust was hacked together due to a transmission swap I did. So I took it to the exhaust shop and got it all run nice and neat from the headers back. Then I still had the same condition.
I started testing injectors with my HPTuners scanner by disabling them one at a time. It's pretty hard to tell the difference when any one is turned off because of the choppy idle and I wasn't getting anywhere so I actually listened to each injector with a long screwdriver as a stethoscope. It seemed like injector 1 might not be firing. Also, I thought the plug felt loose so I unplugged it and plugged it back in nice and tight. Voila, It seemed I had fixed the problem.
However, on my way to work this morning, I was scanning and noticed the problem had returned. So I popped the hood and checked all of the injectors with my screwdriver and they were all functioning. Then I revved the engine real hard and my B1 O2 voltage came back up but as I watch the O2 volts graph, I can actually see misfires happening as little dips of voltage that don't look like the normal oscillations. In fact I have it in open loop so it really isn't oscillating very much which makes it easier to see the dips (misfires). Now I'm thinking about pulling my valve cover to see if something is really wrong like a broken spring or something. Does anyone have any other ideas of what I might check or how I might check it? Could a bad injector cause an intermittent problem like this?
12-04-2018, 06:12 PM
#2
I'm a tuner with a lot of tunes under my belt but my truck has me stumped at the moment so I'm hoping for some help brainstorming. My truck is an 04 Tahoe with an LQ4 and a TSP Torquer V2 cam on 112. I expect the PCM to see misfires with this cam so I usually have them disabled. However, I recently noticed that my bank 1 fuel trims were maxed out. I could see that the O2 sensor was pegged lean.
My first though was a bad O2 sensor. So I switched them from bank to bank but the condition stayed the same.
Then I figured I had an exhaust leak near the O2 sensor that was causing it since my exhaust was hacked together due to a transmission swap I did. So I took it to the exhaust shop and got it all run nice and neat from the headers back. Then I still had the same condition.
I started testing injectors with my HPTuners scanner by disabling them one at a time. It's pretty hard to tell the difference when any one is turned off because of the choppy idle and I wasn't getting anywhere so I actually listened to each injector with a long screwdriver as a stethoscope. It seemed like injector 1 might not be firing. Also, I thought the plug felt loose so I unplugged it and plugged it back in nice and tight. Voila, It seemed I had fixed the problem.
However, on my way to work this morning, I was scanning and noticed the problem had returned. So I popped the hood and checked all of the injectors with my screwdriver and they were all functioning. Then I revved the engine real hard and my B1 O2 voltage came back up but as I watch the O2 volts graph, I can actually see misfires happening as little dips of voltage that don't look like the normal oscillations. In fact I have it in open loop so it really isn't oscillating very much which makes it easier to see the dips (misfires). Now I'm thinking about pulling my valve cover to see if something is really wrong like a broken spring or something. Does anyone have any other ideas of what I might check or how I might check it? Could a bad injector cause an intermittent problem like this?
My first though was a bad O2 sensor. So I switched them from bank to bank but the condition stayed the same.
Then I figured I had an exhaust leak near the O2 sensor that was causing it since my exhaust was hacked together due to a transmission swap I did. So I took it to the exhaust shop and got it all run nice and neat from the headers back. Then I still had the same condition.
I started testing injectors with my HPTuners scanner by disabling them one at a time. It's pretty hard to tell the difference when any one is turned off because of the choppy idle and I wasn't getting anywhere so I actually listened to each injector with a long screwdriver as a stethoscope. It seemed like injector 1 might not be firing. Also, I thought the plug felt loose so I unplugged it and plugged it back in nice and tight. Voila, It seemed I had fixed the problem.
However, on my way to work this morning, I was scanning and noticed the problem had returned. So I popped the hood and checked all of the injectors with my screwdriver and they were all functioning. Then I revved the engine real hard and my B1 O2 voltage came back up but as I watch the O2 volts graph, I can actually see misfires happening as little dips of voltage that don't look like the normal oscillations. In fact I have it in open loop so it really isn't oscillating very much which makes it easier to see the dips (misfires). Now I'm thinking about pulling my valve cover to see if something is really wrong like a broken spring or something. Does anyone have any other ideas of what I might check or how I might check it? Could a bad injector cause an intermittent problem like this?
12-05-2018, 08:40 AM
#4
The thing to NEVER FORGET about an O2 sensor is, it senses OXYGEN. Not fuel, not carbon dioxide, not carbon monoxide, not "mixture"... OXYGEN. That means, if there's a misfire, then instead of exhaust gas coming out of that cyl, OXYGEN (and nitrogen) and raw fuel will come out of it. The O2 sensor will see the oxygen passing by, and respond by adding fuel.
Gotta fix the misfire FIRST. It's all but impossible to tune a motor in closed loop with that going on.
A typical thing that seems to happen is, a cyl starts misfiring for whatever reason (fouled plug for example); the ECM adds fuel because of all the extra oxygen it sees in the exhaust stream; the fouled plug gets even more fouled; it misfires worse; pretty soon it's a constant blub-blub-blub dead hole, and the motor gets the burn-your-eyes stinky idle from all the fuel.
The IR temp gun idea is good, but will only catch a more or less continuous misfire at idle. Won't catch an intermittent one, or one under load, etc. Worth a try but if it doesn't find a dead hole in progress just then, doesn't mean there's not one at other times.
I'd suggest changing out the spark plugs FIRST. Not look at, not inspect, not clean, not "they're only xxx miles old"... JUST CHANGE THEM. (yeah I know it's $50 but you know how that goes) Put in regular, normal, OEM grade and type plugs from an OEM mfr, as the book lists them for your motor... AC/Delco, Motorcraft, Champion, NGK, Denso, Bosch; NOT E3, +4, or any other kind of "gimmick" plugs. Sorry, I can't recall the part # off the top of my head, but it's not rocket surgery to come up with it. See if that has any effect on the situation.
Gotta fix the misfire FIRST. It's all but impossible to tune a motor in closed loop with that going on.
A typical thing that seems to happen is, a cyl starts misfiring for whatever reason (fouled plug for example); the ECM adds fuel because of all the extra oxygen it sees in the exhaust stream; the fouled plug gets even more fouled; it misfires worse; pretty soon it's a constant blub-blub-blub dead hole, and the motor gets the burn-your-eyes stinky idle from all the fuel.
The IR temp gun idea is good, but will only catch a more or less continuous misfire at idle. Won't catch an intermittent one, or one under load, etc. Worth a try but if it doesn't find a dead hole in progress just then, doesn't mean there's not one at other times.
I'd suggest changing out the spark plugs FIRST. Not look at, not inspect, not clean, not "they're only xxx miles old"... JUST CHANGE THEM. (yeah I know it's $50 but you know how that goes) Put in regular, normal, OEM grade and type plugs from an OEM mfr, as the book lists them for your motor... AC/Delco, Motorcraft, Champion, NGK, Denso, Bosch; NOT E3, +4, or any other kind of "gimmick" plugs. Sorry, I can't recall the part # off the top of my head, but it's not rocket surgery to come up with it. See if that has any effect on the situation.
12-05-2018, 10:48 AM
#7
I like these plugs but I did swap plugs recently to try to figure this out. I forgot to mention that in my original post. I got platinum plugs. Swapping the injectors form one bank to the other is the next step. I should be able to do it this weekend. It was trimming fine on both banks on my way to work this morning but I expect it to do it again. My injector plugs are soldered on pigtails for the EV6 injectors. I hope it's not a problem with that, but they are each soldered and heat shrinked so I kind of doubt that's the problem, but I'll have to rule it out. If I swapped injectors from bank to bank and the problem followed, I'd have my answer.
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