Cold Start-Up Misfires – 2003 Chevrolet Tahoe 5.3
This case study will involve a couple different and effective ways that I use to diagnose fuel trim issues. I’m going to use a 2003 Chevrolet Tahoe with a 5.3 liter with just over 60,000 miles on it I did for a used car lot. This Tahoe was setting a P0171, P0174, and a P0300. We all know the P0300 is misfire code which is probably being caused by a lean condition that set the other two codes. The P0171 and P0174 code set criteria is that the long term fuel trim cell must be above 23% for 6 seconds.
Three things came to mind when I saw these codes. A mass airflow sensor problem, plenum to intake gasket leak, or a fuel supply issue. The first thing I did was hook up the Tech 2 to look at the data stream, especially MAF data and fuel trims. At idle the MAF data looked to be in the ballpark at around 4.5 g/s at around 1000 ft. above sea level on a GM 5.3 liter. Now be forewarned that you can have a faulty MAF and still be reading very close to what a good one would be. I saw that it was close to what I would expect so I moved on to the fuel trims. The long term fuel trims at idle were at +10 to +24 showing me that the PCM was adding fuel. This engine usually has -7 ltft at idle. I then revved the engine to 2500 rpm and the fuel trim went down to around +8 – +10. Since the ltft didn’t go down that far, I was thinking it might be a MAF or fuel supply issue. A small vacuum leak usually doesn’t affect ltft at 2500 rpm to much, if at all from my experience.
I forgot my fuel pressure gauge and was really hoping I didn’t have to pull this out of the shop and bring it on a test drive. Since I couldn’t check fuel pressure I pulled the MAF out and checked to see if it was dirty. It wasn’t dirty at all but I sprayed it down anyway. That didn’t affect my fuel trims at all so I decided to smoke the intake just to see since they are known to leak and I could still keep the truck inside. As I filled the intake with smoke I waited to see if any would rise from below the plenum and it eventually did.
Two ports, one on each bank, were smoking a little bit but I was expecting a bigger leak for how high the fuel trims were. Dang it! I wasn’t convinced that the small intake leaks were going to be the whole problem, so I decided to check my fuel supply and MAF sensor another way but I had to go on a test drive. On the test drive I watched my ltft at a steady cruise and they were around 0, this showed me that the intake leak may after all be the only problem. I then did a couple of full throttle runs and my MAF showed over 100 g/s which shows me the MAF is probably good along with the engine load pid hitting 100% which also tells me the MAF is reading correct at full throttle and the engine is breathing well. For more GM engine load pid information visit www.flatrater.com. I then graphed my two front O2 sensor voltage pids. They both pegged over .8v at full throttle which pretty much put the fuel supply issue to rest. I now could confidently say that the plenum gasket leak was the only issue with this Tahoe even though my ltft at 2500 rpm didn’t go down as much as I expected for the two vacuum leaks being as small as they were. Also check this case study for more info.
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