jonnydclar

Questions:
Does replacing the MAF require any re mapping by a technician / computer or can you just R & R it?
Same question for fuel pump?

So my 99 GS was running fine till it ended up "running out of gas" at 1/4 tank on the freeway 2 times on my trip to Florida. After the 2nd tow I found this thread https://www.clublexus.com/forums/gs-...to-normal.html

and decided to change my fuel pump. I replaced the filter and screen as well.

It ran fine for about 3 - 5 days and then started the current symptoms...

Mostly does NOT throw a code but when it does it is a P0171 (Lean fuel on bank 1)

Cranks for several seconds before firing when cold. Starts quickly when hot. When cold and I just turn the key to run (not start) I hear the fuel pump for 4-6 seconds (can't say I really listened for that with the old pump and you can't really hear it with the door closed and seat installed).

Huge power loss between 2,500 rpm and 3,200 rpm. Happens in almost every gear ALMOST every time you pass through that rpm range but occasionally it pulls through strong all the way up. It has tons of power (like normal) about 3,500 rpm and the break between running lean and running normal is very clear. Feels like hitting a power band on a 2 stroke dirt bike.

It does not misfire in that rpm range, rather it feels like 3 cylinders turned off and it takes 3 times as long to get through that range but the rpms do climb.

I suppose it is possible my new pump is bad but I can't understand why it would run good at 4,000-6,000 rpm if the pump was weak.

I have cleaned my MAF (well with MAF cleaner) & even tried a new MAF in it. The car ran identical with the new MAF. Then I tried disconnecting the battery for 10 minutes and running with the new MAF. The car ran horrible and the rpm range where it ran lean was much larger. I then put my old MAF back in and it ran pretty bad for a few minutes but seemed to get better as I drove. After a few starts and 10 miles around town it now runs exactly as it did before (as described above).

Unfortunately the trash man already took my original fuel pump because it was running fine for a bit and I chucked it. If I still had it I would swap it back out to test.

jonnydclar

So I left it with a mechanic today and he confirmed fuel pressure is good. He said the codes showed Bank 1 and Bank 2 lean and out of parameter and wants to change both upstream sensors for $500.


I said no thank you. I really believe that this is a symptom, not a cause but he says he can't continue to troubleshoot without replacing them. I may replace them myself after a little more investigation.

Have you ever heard of O2 sensors CAUSING it to run lean and lose power?

I can imagine they got damaged by it but even that seems unlikely unless it was running rich.