98 4X4 4.3L

I hooked up a diagnostic tool to the truck since MPG is around 14. Found the engine temp only reaches 160F. A new thermostat will take care of that problem and hopefully the MPG issue.

Here are some other readings that I do not know the correct parameters for:

(what are these first 2 sensors?)
ST FTRM2 % = -6.3
(other ST1&2 and LT1&2's read from 2.3 to 3.1)
ST FTRM21% = -5.5


O2S11 (ox sensor) .1 to .9
O2S13 .2 to .6
O2S21 .1 to .9 (readings were not steady on all 3 O2 sensors)


The truck was idling at 626 RPM and had an engine temp of 149 at time of readings. MAF was reading .73 and the Calc load(?) was 3.1

Thanks for the help! I know this may be a bit technical for some people, but this type of info should be really helpfull.

 

Ok the sensors that have deal with the fueling of the engine are O2S11 & O2S21 in your scan tools nomenclature. Normally, O2S11 = B1S1 (Bank 1 Sensor 1), O2S21 = B2S1, O2S13 = B1S3 (post cat O2 sensor - emissions testing only), and on some B1S2 (pre cat sensor - emissions testing only). To bouce between 0.1mv & 0.9mv is about normal, but there should be A LOT of activity. The sensors should read differently (ie. B1S1 might be higher than B2S1), but they shoud cycle at about the same frequency. One sensor cycling slower than another is a good indication that it is becoming lazy.

Now, you ST FT is the short term fuel trim. The short term fuel trim is a PCM erasable memory register. The neutral value for the short term fuel trim is 0%. Any deviation from 0% indicates the short term fuel trim is changing the injector pulse width. The amount of pulse width change depends on how far the short term fuel trim value is from 0%. The short term fuel trim is rich when the scan tool indicates -99%. The short term fuel trim is lean when the scan tool indicates 99%. The short term fuel trim changes the pulse width by varying the Closed Loop factor of the base pulse width equation. As the PCM monitors the oxygen sensors input, it is constantly varying the short term fuel trim value. The value is updated very quickly, therefore, the short term fuel trim only corrects for short term mixture trends. THe correction of long term mixture trends is the function of long term fuel trim.

The LT FT is your long term fuel trim. The long term fuel trim is a matrix of cells arranged by RPM and MAP. Each cell of the long term fuel trim is a register like the short term fuel trim. As the engine operating conditions change, the PCM will switch from cell to cell to determine what long term fuel trim factor to use in the base pulse width equation.

While in any given cell, the PCM also monitors the short term fuel trim. If the short term fuel trim is far enough from 0%, the PCM will change the long term fuel trim value. Once the LTFT value is changed, it should force the STFT back toward 0%. If the mixture is still not correct (as judged by the HO2S), the STFT will continue to have a large deviation from the ideal 0%. In this case, the LTFT will continue to change until the STFT becomes balanced. Both the STFT and LTFT have limits which vary by calibration. If the mixture is off enough so that LTFT reaches the limit of its control and still cannot correct the condition, the STFT would also go to its limit of control in the same direction. If the mixture is still not corrected by both STFT and LTFT at their extreme values, a Fuel Trim Diagnostic Trouble Code (DTC) will likely result.

In order to get a better picture of what's going on, you really need to log the data over a run (aka while driving down the road).

 

WOW! I had no idea what all of that stuff indicated. !

Thanks for explaining all of this without going over my head. I'ts good to know that the numbers are in an acceptable range.

I will hook up the reader again and watch for trending on the 02 sensors. I was suprised not to find any codes set on the computer since it has 129,000 miles.

 

No problem.

The lack of codes doesn't surprise me. So long as routine and preventative maintenance have been kept up on a vehicle, it *should* run a VERY long time before having problems. No vehicle will have an infinite life, but through routine/preventative maintenance, you can significantly extend its life while minimizing the amount of money you actually spend on it.

By routine maintenance I mean maintaining tire pressures, fluid levels, greasing the chassis, checking your brake pads, etc. Preventative maintenance would be pulling the plugs to inspect them and check the gaps (possibly replace them if they don't look good), checking the wheel bearings, universal joints, tierod ends, pitman/idler arm, as well as checking/replacing belts and hoses. One HUGE preventative maintenance would be a schedule of replacing your coolant and t-stat, which should be done every 2 years for the green stuff and every ~4 years for the dexcool (red/orange stuff). For the price of a t-stat, just replace it whenever your coolant is drained. Other preventative maintenance would be cleaning your MAF (mass air flow) sensor and EGR valve (new gasket required most of the time for the EGR).

All of this will help to minimize your expenses in costly 'break-down' repairs as you will tend to notice things before they get out of hand and ruin sensors needlessly.

Good Luck