Here it is at idle with the map and maf included.

 

1st 10 frames are 30mph, 2nd 10 frames are accelerating to and driving at 40mph.

 

No not by unplugging the O2 sensors. When the truck is cold the O2 sensors are not used at start up and for a few minutes while the truck warms up and the O2 sensor heaters bring the sensors up to operating temps. While watching live data you will see the system change from open to closed loop. My question is whether the truck runs better before or after the shift to closed loop or is it the same? If you have not been monitoring for this, does it run better or worse when first started and cold vs warmed up or is it random? That helps us narrow down what’s happening. Also, you will need to check the key sensor values cold with no start up and then again at idle and at 1700 and 2500 rpm. About the only thing you need at driving speed is the speedometer function and O2 sensor graph at full throttle (with a passenger on the scanner).


George

 

Didn't see your reply till just now. I'll get more data of the ECT, and TPS. I don't think my scanner will read the CMP. When it runs good, there's no noticeable difference between open or closed loop, same with when it runs bad. It's all hit or miss on how it's going to run when I start it. I did check the TPS data the other day, but it wasn't with the engine running. The voltage went up smoothly with applied throttle.

 

So, just to make sure I get the right data. I need to:

Record the cold readings with the key on/engine off from the MAF, MAP, ECT, TPS, O2 sensors

Then record the same pids with the engine cold at idle, 1700, and 2700

Is that correct?

 

Then you have a lean or perceived lean condition that the O2 sensors are not able to compensate for even though they are trying.The O2 sensors can be the problem but are not likely the problem.

Things that can cause an actual lean condition are:

Fuel pressure
Restricted fuel filter
Poorly functioning fuel injectors
Vacuum leak
Unmetered air entering after the MAF sensor
There Are others but we start here

Replace the fuel filter.I think you already reported good fuel pressure and leak down. Make sure you don’t have any vacuum leaks or unmetered air like the intake horn after the MAF. One way to rule out a vacuum leak is fuel trims at idle and again at 2500 in park. If the positive trims go down at 2500 then good chance it’s a vacuum leak as that unmetered air becomes a smaller percentage of the total air intake. Testing the fuel injectors is a little more complicated, it involves an injector balanced test which requires a pressure gauge and pulse box. The fact that your problems switches between running good and crappy makes this low on the list.

Sensors:

ECT dead cold engine off. Is the value near ambient? Warmed up is it near 195F?

MAP engine off. Is the value near atmospheric pressure?

TPS engine off. Are the values smooth and sensible thought the full range of the pedal?

MAF is a little tricky, it outputs frequency and the computer converts it to flow. On my truck I read 6.6 g/sec at idle and 25 g/sec at 1700 in park. Do you have similar values? Here is a little more info on MAF sensors:

https://easyautodiagnostics.com/gm/4...e-maf-sensor-4

O2 sensors. These are tricky because they can fail totally and be obvious or they can get “sluggish” or marginal which is tougher to diagnose. Your first order look is to graph the pre cat sensors and see if they are switching properly and fairly rapidly between near 0 volts and near 1 volt. At full throttle driving the computer abandons the O2 sensor input for fuel delivery and enriches the mixture temporarily so the O2 sensors should peg high until you back off.

The more you can capture when it’s running like crap the better.

That will I’ll get you started. If this doesn’t do it there’s more.


George

 

Replace the fuel filter. I'll replace that today.

I think you already reported good fuel pressure and leak down. Yes. Pressure and leak down check out fine.

Make sure you don’t have any vacuum leaks or unmetered air like the intake horn after the MAF. Sprayed everything down with carb cleaner, but couldn't find an vacuum leaks. I'll try it again though to be sure.

One way to rule out a vacuum leak is fuel trims at idle and again at 2500 in park. If the positive trims go down at 2500 then good chance it’s a vacuum leak as that unmetered air becomes a smaller percentage of the total air intake. The LTFT drops, and the STFT raises when I give it gas. Not sure what rpms I was at though, I believe it was between 1500-2000. I'll check that again though.

Sensors:

ECT dead cold engine off. Is the value near ambient? Warmed up is it near 195F? Not sure on cold values, but I'll get em. Warmed up is between 195-200f

MAP engine off. Is the value near atmospheric pressure? I looked at that earlier, and with the engine off it matched the atmospheric pressure here. Don't remember if the engine was warmed up at that point, but I know it wasn't cold.

TPS engine off. Are the values smooth and sensible thought the full range of the pedal? Yes. Smooth all the way to full.

MAF is a little tricky, it outputs frequency and the computer converts it to flow. On my truck I read 6.6 g/sec at idle and 25 g/sec at 1700 in park. Do you have similar values? My MAF with the engine warm is .66 lb/min at idle, about 4.99 g/sec. Right around 25% lower than yours. I'll check at 1700 as well.

O2 sensors. These are tricky because they can fail totally and be obvious or they can get “sluggish” or marginal which is tougher to diagnose. Your first order look is to graph the pre cat sensors and see if they are switching properly and fairly rapidly between near 0 volts and near 1 volt. From what I've seen so far in testing, they are running anywhere from .3xx to .8xx. I'll double check for more accurate number's, but they're in that range.

 

Don't know if it makes any difference, but I had a mechanic friend hook it up to his scanner a couple days ago, and one of the O2 sensors kept reading back and forth between lean and rich. I think that was one of the downstream sensors.

 

The upstream sensors should switch back and forth often, but the downstream sensor should stay pretty steady.

 

There are 2 basic types of O2 sensors and up to 5 sub types. Some vehicle use one the other or both. Narrow band sensors are the older technology and simply output an oscillating voltage level between around 0.75V (rich exhaust) and 0.25v (lean exhaust) so the system is constantly reversing fuel delivery trims creating a mixture close to "stoich" based on averaging. The two front sensors in your truck are of this type and an example of what a graph looks like (but not limited to) would be:



The other basic type is a wideband sensor or sometimes called an air-fuel sensor. It outputs a drifting output signal around the target level provide a much more accurate indication of the air fuel mixture.


George