July 12, 2022 at 3:06 pm #287724BRUNOPECEGOParticipant
1997 4.3 s10 negative STFT around – 30 and LTFT -18 on both banks. (maybe slightly worse on bank 1)
I recently purchased with it not reaching over 3.500rpm and figured it could be exhaust restriction or low fuel pressure. Idle was great. Had good power quick throttle response and at the low end brakes were good, clutch, trans, etc etc and low mileage so i bought it.
Only a couple things i didn’t like. At some point it had a CNG(natural gas) kit installed and i could see some of the repairs were poorly made by someone who didn’t know what’s he was doing at all. So the truck is in a partially rough shape.
Before buying the seller told me the truck died on him so he replaced Fuel pump, FPR, fuel filter.
The day i went to pick it up it revd over 3.500rpm normally. After driving it for a week I started having a surging/ looping at idle. An INTERMITTENT jerking or bouncing with light throttle while driving that was very bad and occasionally it would die on an uphill. WOT, no problem.
At that point FTrims were very positive at even 50 sometimes I think. O2s had bias voltage but were dead at 0.200. So I got 2 new upstream O2 sensors and fixed that false lean condition. No more looping idle.
But now I have extremely high negative trims even -30 STFT at startup OL and CL truck starts idling rough and reaches -18 LTFT.
It has a slight delay on throttle blipping if rpm is low but FTrims immediately go to 0 and then very negative again quickly. If held steady at 2000-2500rpm FT zero at first and then steadily start going negative until -30 and -18 and stays. It is still dies occasionally. There is a small amount of white smoke and fuel small on exhaust but almost no fuel smell on TBI so i dont think its a false rich.
Bad pcm? I checked :
Fuel pressure- is on the low side at 4.2Bar running but no pressure drops so no fuel leaks.
MAF- over reporting 8g/s but makes no diference if I unplug it will still run rich.
O2- responding to induced variables but when normal operation B1 tends to run a little higher volts compared to B2 even though FT is the same for both.
EGR- mechanically closed by me with blocked gasket and still rich.
EVAP- new valve and not commanded to stay open.
PCV- looks good. Despite leaking some oil there’s no oil on exhaust and no blowby on oil fill cap.
Compression- cranking sounds good and even and has good power.
IAC valve code- vehicle starts funny, but every time. Rpm drops very quick.
Vehicle speed sensor code- intermittently idle high when moving and back normal during complete stop
MAF- reads 2g/sengine off key on
Gas contaminated oil? (Not very strong smell though)
Power steering? ( when turning wheel O2B1 oscillates normal full range and FT goes down to -28 )
Dead downstream O2 Stuck at bias?
PCM is not memorizing anything
Fuel gauge reads full always.
Bad spider injectors not spraying correct pattern?
I’m thinking its something electrical but im stuck now. Im studying a bit every day to figure this out.
Thanks from Brazil, cheers!
- You must be logged in to reply to this topic.