Pzindy

I have a 1999 RX300 AWD with approximately 195000 miles.

It had an exhaust leak that was worsening over the last few weeks. About 2 weeks ago it began throwing a P0171 code. About a week ago, I had the exhaust leak repaired by a local independent shop. I have been doing much DIY on this car (including knock sensors), but crawling around on a creeper under jack stands dealing with rusty exhaust components does not appeal to me. In that case I paid someone who had a lift. :-)

Anyway the shop welded up two leaks, and reset the CEL. As I had read that an exhaust leak was one possible cause for P0171, I waited to see what would happen. The P0171 came back the next day.

About a year ago, I purchased an Actron CP 9680 scanner. I had only been using it so far to read codes, but I knew it had more capabilities. I had an opportunity to go on a 300 mile round trip to visit my parents, and my lovely wife volunteered to drive so I could play with the Actron.

I had seen the P0171 occasional before and had cleaned the MAF before, and it helped for a while. But I had also purchased a new MAF sensor which I had ready to install. But I wanted to experiment. The trip down, the CEL was on, with the P0171 code.

I had been reading about p0171. I had just watched a couple of Youtube videos about fuel trims (as I started knowing almost nothing about them).
is part 1. I found it very helpful.

On the trip to visit my parents,. On the trip down, I had the CEL on, and had the original MAF installed. At my parents, I installed the new MAF and reset the CEL. With my newfound basic understanding of fuel trims, I captured some of the data on the Actron.


Before changing MAF

CALC LOAD(%) 33.3
ENG SPEED(RPM) 2616
FUEL SYS 1 CLSD
FUEL SYS 2 CLSD
LT FTRM1(%) 28.9
LT FTRM2(%) 32.8
MAF(LB/M) 3.55
ST FTRM1(%) -1.6
ST FTRM2(%) -3.1
VEH SPEED(MPH) 63

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After changing MAF


CALC LOAD(%) 37.3
ENG SPEED(RPM) 2476
FUEL SYS 1 CLSD
FUEL SYS 2 CLSD
LT FTRM1(%) 1.6
LT FTRM2(%) 3.1
MAF(LB/M) 3.86
ST FTRM1(%) 2.3
ST FTRM2(%) 3.1
VEH SPEED(MPH) 60

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Note the long term fuel trims being near 30% before MAF change, and well under 5% after the MAF change. Those numbers seem to vindicate that the MAF has resolved the P0171.

Only bad news, is about 10 miles from home, the CEL came on. This time with P0441 and P0446. That will be a new thread.

maxSteel

This video is awesome. It explains in detail how to use the trim data to identify the cause of the issue.
Replacing the MAF resolved my issue.