high fuel trim at speed but not idle my guess is an exhaust leak . Exhaust manifold bolts/gasket/crack O2 sensors not seated properly.

if it was at idle vac leaks intake leaks after MAF , injector gaskets, intake manifold leak.
my 96 the fuel trims are close to zero which does appear normal .

possible spark plugs loose ?

high fuel trim means O2 sensor sees too much air in exhaust.

 

high fuel trim at speed but not idle my guess is an exhaust leak . Exhaust manifold bolts/gasket/crack O2 sensors not seated properly.

if it was at idle vac leaks intake leaks after MAF , injector gaskets, intake manifold leak.
my 96 the fuel trims are close to zero which does appear normal .

possible spark plugs loose ?

I'll check the exhaust bolts and double check the O2 sensors.
Good call on the spark plugs! I'll check that. As far as I know they are original and so are the wires. I'm at 104,000 so I should probably think about replacing them soon.

 

Just drove home. Now bank 2 LTFT is reaching +10, sometimes more and bank 1 short term is dipping down to -8 when I let my foot off the gas. Could it be the injectors are dirty? Could it have something to do with the Lucas stuff I added? Seems like the bank 1 negative number could be due to a leaky injector. Maybe bank 2 is an erratic spray pattern? Could old plugs and wires cause this?

I checked the plugs, exhaust bolts and O2 sensors and everything feels tight. I don't hear any exhaust leaks.

I should add, I'm pretty sure the emissions hoses at the front of the car are shot and need to be replaced. And something is up with the gas tank. When it idles for a while I smell gas. Maybe air is entering the system.

 

I would look "close" for a exhaust leak on the PS (bank 2) motor. Both manifold/header to head and collector to flange.

Few tools helpful in diagnosing motor problems:

vacuum gauge
FP gauge
Temp Gun

Vacuum gauge can tell you a lot, google "how to use.." if your not familiar with using it. They are cheap to buy (Harbor Freight)

Temp gun read exhaust temps on each eyl to see if one is cold. Also front of CATS on bank 1 & 2 to see if bank 2 is significantly higher temp.

 

I would look "close" for a exhaust leak on the PS (bank 2) motor. Both manifold/header to head and collector to flange. .

I got out my stethoscope and checked for exhaust leaks and found one on the passenger side so I rechecked the exhaust bolts and discovered the rear most one was loose.

Hopefully that was the problem!

Matt, you likely found the problem regarding the fuel trims

As Jcat states replace the lose bolt with a new one. IMHO I never use lock tite on exhaust manifold bolts but do "re-torque" after a few heat cycles

your right bank 02 may have taken a hit if this went on for awhile or they are old as it saw more air so PCM commanded more fuel on that bank which can foul up the 02...and plugs, especially #8" where the exhaust leak was so check the plugs on that bank.

you may have even burned the exhaust gasket at # 8 so if the fuel trims don't level out after bolt swap you would need to replace the PS exhaust gasket itself.

 

I put my vacuum gauge on the motor and it's pulling 20" at idle and just under that in gear.

I got out my stethoscope and checked for exhaust leaks and found one on the passenger side so I rechecked the exhaust bolts and discovered the rear most one was loose.

Hopefully that was the problem!

 

when you find a loose bolt DO NOT TIGHTEN !!!!

these break off buy a new bolt at like advance auto grade 8 bolt ,3/8 in -16 thread .
I think 1.5inches long use some blue locker on them med locker..

with the engine warm tighten the bolt the end bolts have a collar. then after a couple of days re-torque when engine is warm not hot.

I had to do this on a couple of these both sides full rear.

 

OEM GM bolts comes with a MEDIUM thread locker .. helms manual also states this .

medium ,,,,,,,,,,,,,, NOT PERMANENT LOCKER. the heat cycles require the use of this or bolts will loosen ..

bolt torque 30 ft LBS . GM part #09441547
Dorman 960-215

 

sure the LTFT should be less than 10% but your readings are not all that bad to go nuts on this. replace the plugs make sure no exhaust leaks or those vacuum hoses .. PCV hoses do fail as they crack hard to see...

many do feel if its 10% or less its good over 15% is bad, 25% must fix now..

LTFT do take some time to reset . installing a new bolt and torquing a few times after use of the vehicle may seal up the exhaust leak if the manifold got distorted .. over time though the movement of the manifold will wear out the gasket so this maybe why it was loose. so try the bolt switch/torque ...

 

Good point. Thanks. Sometimes I get obsessed with stuff being perfect. You're right. 10% isn't that bad.