Hello,

My 70 El Camino has a 1999 GMC Sierra 5.3 liter donor engine and PCM and until this weekend, had persistent Bank 1 and Bank 2 Long Term Fuel Trims maxed out at +25. The values for Short Term Fuel Trim were generally low at less than +10. I thought it was a vacuum leak but found none and two different MAF sensors made no difference until I cleaned the tiny heated wires with CRC MAF Sensor Cleaner. And then the cleaning only partially solved the LTFT problem. Turns out that my K&N air filter was somewhat over-oiled and the large plastic elbow and PVC pipe that my air filter attached to caused too much air turbulence for the MAF to handle. Once the K&N got mounted straight onto the MAF, the STFT and LTFT dropped to normal levels of zero to +5.

But here's the question. Why didn't any of this set a Check Engine Light / Malfunction Indicator Light? The original 1999 tune and the changed tune operating inside the PCM today both have the boxes checked for P0171 and P0174 set to "MIL On Second Error" but I never got a CEL or MIL and no DTC's or Freeze Frame data for anything ever got stored. For as long as I had the trouble with a maxed out LTFT, I certainly believe that a second error should have occurred and something should have been indicated.

The vehicle was easy to start with a maxed out LTFT, idled well, accelerated well . . . no problems. However, the spark plugs were a little too white for my tastes on the internal porcelain.

So . . . why no CEL / MIL with this problem? Please see attached screen shot of my DTC check boxes situation.

Many thanks,

Rick

 

My first guess would be that your 1970 El Camino does not have a check engine light.

 

Good answer but no. My EC has a set of Dakota Digital HDX gauges and there is a check engine light in the face of the speedometer. And it self tests successfully during engine start.

And even if it didn't have a check engine light / malfunction indicator light, that still would not explain why DTC's for lean operation don't show up on the scanner. I don't get those DTC's for either bank, no freeze frame data, no MIL, no CEL. Nothing. Yet, I have gotten DTC's and MIL / CEL for other things unrelated to lean operation.

Why not? This is puzzling to me.

Rick