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1997 4.3 s10 negative STFT around - 30 and LTFT -18 on both banks. (maybe slightly higher 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.
TPS- normal.
IAT- normal
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.
Maybe related?
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.
my 1997 4.3 chevy s10 hitting -30 STFT before entering CL. After it warms up, it will zero TRIMS on fast throttle blip but not if held steady and maintain the -30STFT and -18LTFT both banks
Hmm, there should be NO short term fuel trim while in open look! Does this also show in generic OBD2, any related fault codes present? What does the O2(s) - I "think" there is only one up-stream sensor - read and what scan tool do you use?
An expert is someone who knows each time more on each time less, until he finally knows absolutely everything about absolutely nothing.
no codes. o2s are normal. bank1 doesnt quite reach .100v but it will respond if i induce variables to make it go lean.
its a basic Maxwell scanner. i dont think theres something wrong with it.
it had dead o2s before stuck at around .200v and had psitive trims.
after i got new o2 it started showing this rich condition that the old stuck lean dead o2 were hiding
Does your map kpa say 30? Can’t tell from the pic. If you convert that to inches of vacuum it’s roughly 9 inches. How’s it run? Almost seems like the egr valve is stuck open being so rich and low vacuum at idle. Have you checked the egr to see if it’s stuck open
yes. cold start. that's why my temperature is low. i get 90 something when its at operating temperature. i didnt have the chance to drive to check if my temp. goes down because the truck is running pretty bad as you can see the negative trims at open loop.
could my STFT and LTFT be reversed? i cant se any other reason for having SHORT TERM TRIMS on open loop unless it is actually my LTFT.
im starting to suspect all my issues relate to a stuck closed IAC valve. in theory it matches all the symptoms but i dont know methods to test it completely without a scope.
i also have a overreporting MAF that i still have to chase down an electrical wiring issue because i installed my friends known good one and it made no difference
while doing some testing i saw that if i create a vacuum leak my trims will coincidently go perfectly up to zero. But also my MAF g/s reading would start reading like it should at 4g/s and not 8g/s at idle.
Why would my MAF reading be so high? Its not dirty. Could it be a restriction on EGR passage? IAC problem?
my startup high idle drops very quick. it starts up, goes to about 1 thousand and a bit up and immediately drops to below 500 and back up to 700 or 800 and than very slowly it goes to 600.
truck is running very rich. i tried restricting the air going through the MAF with a T-shirt on top of the air filter but it didnt seem to make a big diference on the my g/s reading and certainly no diference on fuel trims. is it a valid test?
im running out of hypothesis. im thinking the spider injector has a higher flow than normal and the computer is not able to reduce the pulse enough so then it starts trying to get more air by opening the iac valve causing the maf to read high. its weird because this should balance it self out or cause high rpm and its not. but its all there is left to to be wrong. disconsidering remap which is unlikely. it keeps running rich. only runs normal if i introduce an air leak or on a short period when i snap the throttle. maybe someone tried cleaning the sphere tips on the spider and worn them out. any ideas?? God bless
I believe that engine has the spider injection system they are prone to leaking and is located under the intake manifold have you smelled your oil does it smell like gas? I would look there and do a injector leak down test.
its not strong at all. just a slight gas smell i think its normal. i did check fuel pressure and it drops veeery slowly only noticable after 10 minutes. with fuel gauge and truck running fuel pressure doesnt drop on throttle. but its my first time working on a vehicle. i dont know if the tests i made are conclusive. thanks
Slight gas smell means gas is getting into the crank case the vapors by pcv system go into cylinders causing the rich condition. Along with the maf over reporting. I would trace maf sig wire back to pcm and splice in a new run of wire as well as check the pcm grounds and maf ground wire for a open / high resistance. My hunch is these two areas are your problem. The fuel smell will require removing intake and checking spider injection system while cranking it (with coils unplugged or fuse for coil unplugged along with injectors) the other possibility is ignition coils fail when under a load after being driven for a while which would cause rough idle and rich exhaust to an extent. Using a fuel injector tester and a fuel pressure gauge you could see which injector is over spraying fuel which should only vary 1-2 psi difference highest reading to lowest reading. More than that and injector/'s will need replaced/ cleaned
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