Sorry if it gets long alot to unpack. Have a 99 1500 5.9 mag that myself and 2 shops cant seem to diagnose. Truck has 120k and had a cylinder 1 miss and high oil consumption. Went straight for a hughes plate but did a leak down and compression test as well and found it had a stuck exhaust valve so got some reman heads as well. Did the install and had lack of acceleration and bad o2 readings that were mimicking eachother as well as dropping down to .006 and speratic. Ran diagnostics on the cat (im in cali so gotta have a carb cat)and it failed heat test but didnt seem to have back preassure so i scoped it and it was shot/broken cataylist. I Installed a magnaflow c.a.r.b cat last week along with 2 new o2 sensors. Definitely got alot of power back but still seem to be lacking alittle acceleration off the line. Here is where it gets tricky .

O2 readings warmed up or first start at idle are droping down to .006 still both upstream and down. Once you start accelerating they start functioning almost perfect only thing is upstream will still drop down to .008 or .006 on the drop but pattern is fairly solid and downstream stays pretty steady around .7 with some small fluxuations maybe .08 at most. Once you get over 3k upstream flatlines at about .7 until it shifts and rpm drops then it goes back to rich lean pattern. They act as though there is a lean condition but fuel trims are pretty consistent +2 to +5 on long term and -2 to -4 short term. If i stomp it long term briefly jumps to +10.4 then drops right back to 2. Just got it back from shop number 2 and he says it acts like a pre cat exhaust leak but smoke testing and sniffer showed nothing upstream . Did say i had a few downstream leaks after o2 as i have a clamped on magnaflow cat back and joints are leaky. All he could come up with is either no backflow cuasing it to read off or making a very small leak not show up. Sugested welding downstream joints and try stock muffler or its a possibility the remans could have a crack somewhere on the exhaust ports but wasnt very confident in either and was frustrated as he was stumped . First shop said maybe ecu is acting up but was also not very confident. Anyone have any ideas? I should add when i vac test plenum im at about 17 in hg and am at 4k elevation which ive been told to ad 3/4 a point per 1k elevation but not sure on that.

Truck also has rebuilt trans new plugs cap rotor injectors timing chain water pump cold air intake . Have also changed map iac tps from my other truck while diagnosing. Thing is killing me after dumping that much into it and just cant pin it down. Any ideas are welcome! Thanks

 

What brand O2 sensors did you install?

 

Currently walker but i have 2 original mopar and 2 that om not sure what brand out of another truck all read the same

 

How is your fuel pressure??

 

Good 49psi .

 

Does it maintain that though???

 

Yes seems to not budge while from idle to 4k. Drops to 42psi when i shut the motor off and holds. Has a new fuel pump a few months ago

 

so much for that thought.....

So, it's running incredibly lean...... On both banks..... Bad map sensor??

 

Thats what's wierd and what the last shop couldnt understand is both o2 sensors are saying lean at idle at least but ecu isnt compensating as fuel trims are pretty in range. Then if you give it gas readings get pretty normal up until higher rpms on the highway where upstream flatlines right around where downstream sits at .7 or so. If just reving to high rpms it reads fairly normal but even when getting normal patterns still hits .008 on the drop. Im using a icon t7 so i dont know if it is not processing fast enough cuasong the flatline on the graph when high rpms on highway or if its really flat lining. I swapped maps with my other truck and didnt have much luck but is there a way to test it? My tablet is showing map preassure at 15 then up to 20 when reving and voltage is 1.5 to 1.7.

this is a pic of a high rpm flatline then where pattern returns is at the shift.

This one is cruising then romped on it where it flattens out

Thats at idle it will fluxuate to like .2 max but usually stays low

 

Not sure if going really lean at idle is normal, or not.... GM did that back in the OBDI days, as it helped with gas mileage.

Your graphs actually look pretty normal. The PCM will drop out of closed loop at higher throttle openings, and just run from tables programmed into the PCM.

So my question becomes, does it run good? Have adequate power? Doesn't exhibit any bad behavior like stalling at stops, etc??