No power under load

My 1996 3.4 has trouble accelerating during the first few minutes of driving. Once I hit around 2000 RPM in anything but 1st gear there is no power. This normally goes away within a few minutes of driving which leads me to believe its electrical and not caused by a vacuum leak. Recently I've changed the plugs, plug wires, and the MAF sensor. I had an MAF code, lean code, and engine coolant temperature code. I'll see which codes returned when I get around to messing with it again. Can the ect sensor cause this? The MAF seemed like it could have helped slightly, but it's hard to tell.

 

Any ideas?

 

the only thing I can think of is to put ascan tool on her and look at "Calculated Engine load value" and "Mass air flow rate". One tells you what the ECU thinks the load is on the motor and the other tells you how much fuel in grams is being used.
For a benchmark, at least from me, you are going to have to wait... I have not got the front bumper on mine back on yet so I cant run her down the road for you for a few days. Maybe someone else can get the data to you quicker.

 

here you go. Something to compare your numbers to.

 

That's some unfriendly data to try to be interpreting unless you're only considering idle rpms, but I guess there is some consistency to it even at higher rpms.

@mechanicjon you forgot to hammer down!

 

More then just idle rpms. And yes it was a normal drive, no hammer down that time. As far as reading it. It depends on what type of scanner and log file he can create.

 

The r^2 values for those data get pretty low on rpm range above 1k. If that's all that can be gotten from the ecu then it's better than guessing!

And the hammer down comment was a joke

 

might be time for new cats.

 

r^2 values? there's a lot more info. I just trimmed it down to what I thought would be important.

 

The statistical significance of a set of data when using it to correlate with another or have an idea of how reliable the data is in measuring a variable - basically how close all the data points are to the line that best describes the data (average of all data points in each set at a given rpm in this case). The higher the r^2 value (up to 1) the more consistent and presumably reliable the data is. You can see that MAF (air flow) has a higher r^2 value in the charts because more of the data points are close to the line.

When looking at the Calculated Engine Load and MAF (air flow) values at anything higher than 1k and especially higher than 1900k it became difficult to assess what a "good" and "normal" value for each would be. Either way it's pretty good to be able to compare that to whatever his data is and see if there's something way off - although it would be more difficult to tell at the 2k+ rpm range where he's experiencing a problem. Anyway I wasn't sure if this data was supposed to be linear or set to a curve like power and torque vs. rpm so I put a linear and polynomial line of best fit and r^2 value for each data set. It looks like it didn't make much of a difference for these data.

Here's an article explaining it better than I did. Prepare to enter the nerd zone:

https://people.duke.edu/~rnau/rsquared.htm



P.S. You still rock.

 

Ok. I fallow you now. To get a better baseline the log would need more constant rpm's for a longer time to get a more accurate number for a given load.

 

May just have to do that next time I log a drive.
Sorry OP didn't mean to highjack your thread.

 

Thanks for all that data guys. The scan tool I have access to doesn't provide feedback like that but I'll see if I can get my hands on one. I don't think it's the cats since it's intermittent, only during the 5 minutes or so of driving. I did find that there is a clear difference between the MAF sensors after switching them out each morning for a couple of days, though. I guess the next step is to replace the temp sensor. I also found some oil in the intake tube. I guess I will see if the pcv valve is stopped up and might be contributing to the problem.

 

Coolant temp sensor is reading incorrect? Hi or low?

 

I believe high, but I'll have to check on that.

 

That can definitely cause a cold running problem. ECM is running the engine as if it was operating temp. It may by going into closed loop way to early.

 

I was pretty convinced that was causing most of the issues after reading about it. I'll change the sensor and hopefully the problem stops there. If not, I'll see if I can get a good scanner and look at the air flow readings vs load and attempt to put the data in an excel graph, but i think the new (used) MAF helped. I appreciate the help, I'm not much of a mechanic.

 

Been awhile since I've driven the truck, but it still has the issue after changing the ECT and trying yet another MAF. I guess i'll do some cleaning on the TB and PCV this weekend since I had a lean code and MAF code, but I don't think that is the problem since the hesitation is intermittent and not all the time.

 

And you no longer have codes or check engine light on?

Most scanners will show the live data. Check what it thinks the coolant temperature is.

This could also be ignition related. Bad coil, cap, rotor or spark plugs. Something to consider if the other things test good.

 

No light at the moment, but there might be stored codes. I'll work on it tomorrow. I just changed the plugs, tested coils, and these are direct ignition engines so no distributor. Could this be a fuel problem? The truck runs fine after just a couple minutes of driving. I have a lean code, but if it wasn't getting enough fuel you'd think it would bog down every time you accelerate? This is why I thought it was the airflow sensor initially. I guess i'll watch what codes come back after cleaning some stuff up, replacing some vacuum lines, and i might run some fuel system treatment through it and saw off the fuel filter.