Hey guys.. need some pro help, as usual... lol

Got a 99 z24 auto (ld9) thats new to me and its running pretty good.. no DTC's etc.. However.. my emissions test (high nox) and obd scanner tell me im running lean, but obviously not lean enough to trip a code.. It has a little hesitation when accelerating from a stop and feels like it jumps rather then slowly picks up speed, but sounds/runs great higher up the rpm range.. I seems like it wants to stall once and awhile at idle and has actually stalled on me once when coming to a stop..

Im looking into causes for this, and possible ways of testing components.. As i understand it.. its gotta be fuel pump > fuel filter > injectors.. my thinking is coil/plugs have nothing to do with the actual a:f ratio.. is this correct?

I plan on doing the fuel filter regardless (as i feel that is the cause) and plugs since i dont know how old they are.. But i have had a couple small issues with the gas gauge lately (check gauges light and shows empty, then fixes itself) so im wondering if its the sending unit/pump assembly (naturally the most expensive component.. lol).. i hear it kick on when the key is on.. but have no idea how to actually test it.. But if the fuel pump was going, wouldnt that be more noticeable at higher rpm's rather then less?

Just looking for a little insight, i know its impossible to diagnose properly on here but any input would help..

a check for fuel pressure would be a good place to start. possibly fuel pump or fuel pressure regulator. the injectors would be less likely, but they can get gunked up and have a poor stream. otherwise, i'd look for small vacuum leaks. you are right to assume that the ignition system is ruled out.



87 Firebird
All stock...........lol.

I have run a couple bottles of injector cleaner since i bought the car.. seemed to help..

Its at the point where i bought the car and have no idea what kind of maintenance was done on it.. this isnt a huge problem or anything but obviously something i would like to fix.. then i can just stay on top of everything, this is the one remaining issue since buying the car that i havent managed to solve....yet! lol

I run a obd2 usb interface to my laptop.. it gave me fuel pressure numbers but i dont have them offhand, but i worry thats not a reliable number to go by.. I have finally sound a good mechanic locally, but i still hate leaving decisions to them.. I prefer to tell them what to check and do..

My gas tank is seeping a bit at the seam.. so the plan is to drop and replace it in the spring, which is when i'd do the fuel pump if thats the issue.. thankfully its not affecting drivability and forcing me to fix it asap.. still broke from xmas and buying the car.. lol

no offense, but techs probably laugh at you when you tell them what to look for. if you have a good tech, they know what they are doing and don't need a whole lot of guidance as far as diagnosing a problem. that's why it's important to have a tech you can trust to actually fix the problem rather than look for your money.



87 Firebird
All stock...........lol.

ya i hear ya.. I more just meant i dont like being the person that says "my cars not running right, fix it!" and leaving them to randomly swap parts that arent broken on my dime.. not a problem with a trustworthy mechanic but this guy is still a stranger to me.. I prefer them just doing what i ask them to.. im no expert mechanic but i've come across enough guys trying to feed me BS that i know is wrong to just trust any mechanic right off the bat.. one shop wanted to replace the whole coil pack when we both agree that isnt it and that was just the other day.. lol

Your advice seems totally logical and makes sense to me, so if i were told that i'd be fine with it.. But some things just raise a red flag..

Hes a semi-retired mechanic, fabricator, weld inspector, race car builder etc etc... his wall of credentials was actually pretty impressive.. lol... does the mechanic thing at his home shop because he enjoys it, not to make a ton of money.. I think i can trust him based on first impressions, but im still a bit hesitant..

ok.. i was mistaken about being able to get fuel pressure numbers, but here are the numbers im seeing..

Short term fuel trim bank 1 - 25% (-lean)
Long term fuel trim bank 1 - 9.38% (-lean)

Im not sure what these are supposed to be..

Also noticed the "Ignition timing advance cyl #1" jumping from about 12-16 degrees advanced at 600rpm idle.. that doesnt seem right to me but what do i know, plus i've read to not put tons of faith in these obd numbers..

all help/comments appreciated..

STFT is useless for diagnosic purposes.

LTFT is useful, is shows what the computer has learned and what is his adapted to.

You can be either -10 through +10 and it is normal.

It being -9% means it is running rich and the computer is leaning it out

Timing advanced 12-16 degrees at idle, it sometimes gets to 20-30 degress advanced


High Nox, well the EGR valve is what controls Nox, it is the job of the it, to reduce Nox


- 2004 Cavalier - 124k, owned since new

Thanks for the reply..

and thats my bad in the way i typed it out... its 9%.. not -9%.. so it is running lean..

EGR valve was removed midway thru 99.. I believe my car fits into that as i never saw one... though i will look again to cover my bases..

good to know about the other numbers tho..

I Agree with randizzle, that's why it's important to always ask questions always get to know the people who are working on your car on a first name basis if you can. A lot of that stuff can add up to a significant savings down the road also keeps you from being charged for stuff you don't need done to your car. Just zero in on the problem get the problem fixed and get on down the road ya know what i mean.

I just moved a couple years ago.. so i lost any connections i had.. Took it for granted to say the least.. Its been brutal finding a good shop, which is strange because there is a huge racing / hotrod community.. lol.. guess its all who you know around here.. Niagara falls, Ont in case any guys on here are local :p

But this guy seemed really good... he just finished up some work on my parents car (guinea pigs so to speak, their decision tho).. they went in thinking motor mount went, he found a swaybar bushing was gone then found the wheel bearing was going.. mounts were fine... I took it in for them and he definitely passed on prime opportunities to make extra cash.. He charges $40 /h and per mitchell times, but passes on his discount on parts..

From meeting him and talking to him for awhile, i liked the guy.. Definitely knows what hes doing, so i'll be taking my car into him in a couple days.. I dont wanna dick around with the fuel filter in my driveway so i'll get him to do that and give er a good looking over.. see how it goes..