I would change the MAF too. It’s relatively inexpensive.

i would also try another scan tool. I’m not sure those numbers are valid.
If those LT and ST trims are for real I think it should be throwing a system too lean code.

 

I agree with MaxSteel, those trims should throw both P0171 and P0174. Buy a bluetooth OBDII reader from Amazon and the torque app for your android smartphone to read the data. Total cost less than $20, and confirm the numbers.

 

Ok..waiting for the maf to arrive now...cheap scan tool used..functions are limited and inconsistent..

 

Bear with me, I have similar questions after reading up on this all morning.

The wife and I just doubled our Lexus fleet by buying a 17-yo grocery getter with 200k on it. The seller replaced the MAF sensor at time of purchase. 2000 RX300.

Two problems: (1) after a cold start, the car often dies when put in gear unless it is warmed up for awhile OR i give it some gas; (2) the engine is throwing code "P0172 fuel too rich Bank 1."

Questions:
- Could problem 1 be related to problem 2?
- Regarding the P0172 code, is there any way knowing which Bank 1 sensor to check first (or rule out)? I'm leaning away from the MAF since the seller just replaced it and seemed like he knew what he was doing.

My initial ideas are:
- remove and inspect MAF to assure it really does look new
- test Bank 1 sensor 2 for resistance. But if its within range, does that mean I can for sure rule it out?


Thanks in advance

 

This doesn't sound like a MAF problem. The P0172 could indeed be an B1S1 problem. If you have a scan tool, check your short and long term trim levels for bank 1 and compare to bank 2 levels.
The car dying while cold sounds like an idle air control valve problem. At this mileage it would not be unusual for the valve to be sticking and causing the idle to be too low when cold. Try turning the steering wheel all the way to one side and see if the idle rpm rises when you put pressure against the stop, It should rise to compensate for the load of the power steering pump. Similarly, the rpm should rise when you turn on the A/C.

 

With a code of P0172, can I rule out Bank 1, S2? It is downstream of the Cat, so I am hoping it means it cannot govern the air/fuel mixture being too rich. If I can rule it out, this means I can focus just on Bank 1, S1.

 

Yes, that is correct. The B1S2 sensor does not affect fuel trim, it is used for monitoring catalyst efficiency.

 

Thanks for the help. I ran my fuel trim diagnostics like you said and then did some forum searches, but am afraid I don't fully understand what the standard is supposed to be and found some conflicting statements in the search). One thread said each bank should total to +/- 35% (in which case my ratios are fine), another said the LT should be single digits (less good for my ratios).

Here is what I have at idle (708 RPM) after driving stop and start for ~ 10 miles. The ST/LT ratios changed a little over time at idle, but held pretty constant relative to one another.

Bank1
ST = 0.8
LT = -18.0
V (B1S1) = 3.298
V (B1S2) = .745
EQ_RAT (B1S1) = 0.995

Bank2
ST = -0.8
LT = -14.1
V (B2S1) = 3.303
EQ_RAT (B2S1) = 1.997

Other data
TP% = 9.8
MAF (g/s) = 4.89
IAT (c) = 21 degrees
SPARKADV = 2.5 degrees
FuelSys1 & 2 = CL
LOAD_PCT = 19.6
ECT = 82 degrees

Also:

• When warm, car idles ~ 730 RPM. Every 90 secs it shudders and drops to ~630 momentarily, then shudders again after 20 secs and rises to ~850 momentarily. Then reverts to normal idle of 730 for another 90 secs and repeats the shudder cycle.
• Last night (before the fuel trim readings), I removed the MAF to inspect and temporarily pulled the 20a EFI fuse. Upon replacement (the MAF was new and clean), the CEL light went off and has stayed off for 30 miles.

What are your thoughts on fuel trim data (and any other info above)?

 

Post the same data at 2000 RPM in park.

 

Those are pretty high negative trims. Both are high but bank 1 is a little higher so it makes sense it was the one to throw a code first. The higher RPM numbers will help to understand what is at fault. But if that was all the data I had, I would guess the MAF is bad.

 

Long term trims are pretty high but the number that jumps out for me is the MAF (g/s) = 4.89. This is way to high for idle. Should be about 3.1 to 3.3 g/s at 700 rpm.

 

Just checked my MAF and at 650 rpm it is reading 3.5 g/s

 

If the MAF is reading high, the ECU will think the engine is running at a higher rpm then it actually is. Consequently it will send more fuel than is required causing the engine to run rich. The A/F sensors will then detect this rich mixture and tell the ECU to compensate by leaning out the mixture, thus generate the negative LTFT's.

 

DATA AT 2,000 RPM
*this is after changing a dirty air filter and driving ~12 miles

Bank1
ST = 0.0
LT = -10.2
V (B1S1) =
V (B1S2) =
EQ_RAT (B1S1) =

Bank2
ST = 0.8
LT = -6.3
V (B2S1) =
EQ_RAT (B2S1) =

Other data
TP% = 13.7
MAF (g/s) = 9.45
IAT (c) = 21 degrees
SPARKADV = 36 degrees
FuelSys1 & 2 =
LOAD_PCT =
ECT =

Note: These are the latest numbers. I've actually taken them 5 times now at 2,000 RPM since changing the filter and they continue to improve.

However, I now have a pending code of P0420 (I incorrectly said 0404 earlier).

Note that the seller just changed out the MAF thinking it was causing some kind of problem. Very likely it is thus something that LOOKS like the MAF, but isn't. The MAF is a brand new Duralast (higher shelf price than Denso).

 

No change when I crank against the stop. RPM actually rises when turn back the other way. RPM also rises when I start (or stop) the AC.