The story is. I have an LT1 swapped into a 1946 chevy pickup. Just finished the project a month ago. The truck was running great but over the last week it has developed a situation were it stumbles around 2500 rpm and then feels like it's loading up until about 3500 rpm then it comes out of it. Does this only under a load, usually 2nd gear is worst. If I rev it in neutral, it does not happen. It has new plugs, plug wires, and an opti. I had the emissions removed from the computer and switched to speed density. I have recorded a data log. Nothing seems to jump out at me, but I'm not as experienced in reading this data as some of you. I'm not sure how to post it here, but I could email it to someone if they'd be kind enough to look at it. If you can offer any advice, I'd appreciate it.

Is it normal for the fuel trims to max out when under a load? When the motor is stumbling, the data coresponding to that moment shows 57% throttle with both O2's reading 0.030 and Fuel trims at +25% ltft and +50% stft on both rails.

Thanks

 

Sometimes stumbles come up on this engine. It is maddening. It could be reated to a sensor that is failing or the connecting wire is not completing the circuit to the sensor. it i hard to track down. The most likely sensor is the engine temp sensor near the water pump. The stumble may be fixeed by simply replacing this cheap sensor and if not check the 02 sensors and all conecting wires and harnesses with a multi meter or any such testor.

 

The O2 sensors shouldn't be steady at .030V under any circumstances. The PCM sees the low voltage, and boosts the BLM's to raise the voltage, but yours doesn't rise. Its either running extremely lean, or you have huge leaks in the exhaust, misfires, exhaust valves that aren't closing, O2 sensor wiring problems, etc. Since you're running speed density, its possible the MAP sensor readings are bad, not indicating the engine is under load (high MAP). Whgat does the MAP sensor read when its under load and misfiring?

What is the year of the engine - OBD-I or OBD-II? What data logging system are you using? I can review logs, but I'm so backed up I've had people waiting for weeks.

 

Thanks for the replys. It is a '96, so I'm ODB2. As far as data logging, I only have a high dollar code reader from a friend of mine. It has a record function and live data. MAP readings seem good. Under load, they are reading 80 to 96 kPa. I've noticed that when the stumble occurs, O2 readings are real low, but they do come up in the next frame. In one instance, O2's were reading 0.015 when the stumble occured and the next frame they were reading 0.925. It was still loading up in the second frame however.

After reading Injuneer's website, the following made the most sense.

"Remember, the O2 sensor can only measure OXYGEN. It can't measure fuel, or other components of the exhaust, so if you have a cylinder that misfires, there will be all the oxygen that was in that cylinder, and all the fuel that was in that cylinder, in the exhaust. The sensor will see that oxygen, and the voltage will drop, indicating "lean", but it is not really lean."

That sounds like a likely senario for me. It misfires causing the low O2 readings, then it is loading up because it is running way too rich try to correct a false lean situation.

My question now is, what could be causing the misfire? Is it normal for Ignition advance to always be negative. At idle it runs -18 degrees and under a load, is goes as far as -50 degrees. It had been running good for a while after I put new plugs, wires, and opti on trying to fix another issue. I have check all the plug wires for burns and such to no avail.

 

Yes. Normally ignition advance is shown negative to indicate degrees before top dead center. My writeup mentions that some scanners show "-" and some show nothing.