ok, i have an 02 trans am Firehawk, LS1, 4L60E. 74,004 miles on body and engine.

very poor fuel economy (12 in town, 16 on highway cruise set at 65)

constantly throwing 02 sensor codes, "low voltage" (however, while watching live feed, the 02 sensors ARE reading correctly... they only throw low voltage over 50% throttle, if i Pussyfoot it around town, no codes. the second i jump on it... MIL popps on).

has an idle Stumble/misfire... i assume the misfire is still there through the rpm's, but i cannot feel it... no misfire codes.

i have checked spark plugs (AC delco iridiums). AC delco plug wires (all newish... less than 25K miles) when i bought the car, it had a custom tune on it, still running like poop... i had local GM dealer flash PCM to factory tune. runs better, but the power is starting to fall off sharply, accross all RPM's. at some points, really feels like a V6. spark plugs look good, are all gapped at .040. here is the list of mods

K&N panel filter (Ram air car)
Pace setter Long tube headers
Full exhaust, no cats (3")
Bosch 13111 corvette 02 sensors in front, not sure what sensors out back... not that it matters. (i need to SIM them)
completely bone stock past this......no aftermarket LID, MAF is completely intact, still has the intake "bellows"... etc.

fresh M1 syn 5w30 motor oil, K&N Air filter has a tint to it... but not "dirty".


i cannot hear, or locate any exhaust leaks, that would cause the lean codes, which are about 98% on the Drivers side (Bank 1).

Tomorrow, i will pick up a few feet of vacuum line and start listening closely for exhaust leaks, and see if i can locate any. I will also check the Fuel pressure. if i cannot find anything here soon... i'm pulling the intake to re gasket it, and check the injectors while i have the intake up "as best i can by "looking") I would like to have the car tuned locally, to just optimize the timing curves and such. not so much for "performance" but to just make it run as best as it can, and potentially get more fuel MPG's on the highway. i don't expect it to get good mpg in town, but i know the LS1 cars are rated a lot higher than 12/16....

i know an intake gasket, could cause the lean condition, and the misfire... i know HEADERS, can sometimes change the volumetric efficiency blah blah blah, and give issues... but why does everyone else around me not have these issues... I also know even a "small" exhaust leak, could cause issues such as this. I also know an engine running very RICH, could "wet" the 02 sensors causing a lean reading...

but i am completely lost.


PLEASE help me if you think you know what may be causing this issue... its haunting me...

 

How long have you had the car, and how long have your MPG's been garbage?

 

Had the car about 3 weeks. MPG's have been crappy since the day i got it.

its a 3.23 gear'd rear end so i know that attributes some.

 

Get some MAF cleaner and clean the MAF and change the O2s. Don't assume they are good just because they are reading.

 

^ That's exactly what I was going to say....^

 

seen a few threads involving a small vacuum leak in the PCV system.

 

ok, this morning i put NEW plugs and wires in.

NGK TR5IX iridiums, with Autozone brand Duralast wires. the 02 sensors were put in by me last week. they are brand new. the MAF has been cleaned, with MAF cleaner (seems like really expensive brake cleaner) and i have searched for small vaccum leaks... cannot find any.

 

Make sure you don't have an exhaust leak at your header flange an the merge. That could cause your problem too.

 

i have sprayed Carb cleaner over every single Vacuum fitting i can find, From the PCV, the EGR stuff, the AC conector lines, up and down both sides of the fuel rail (thought injector o rings maybe) the intake bellows, both in front of and behind the MAF. NOTHING. tried a water bottle, same thing. used about 3 foot of 1/8th inch vacum line i keep around as a stethascope and listed on the top and bottom of each primary on both sides of the engine, no leaks. got under the car, listened the whole way down each primary, no leaks. there is one very very small pinhole leak, but its AFTER the rear 02 sensor (where they welded in the bung, there is a needle sized hole) but that would not effect anything.

After doing plugs and wires this weekend this is what i have noticed. the car feels great now. much more power. feels where it should be. still getting 02 sensor codes however, right now only for the rear 02 (no cats so thats acceptable).

i have noticed, the car feels sluggish... and its not due to lack of power... when i give it throttle, the TCC doesn't feel like it wants to unlock, and the car won't downshift until i give it a lot more than i feel it should in the go pedal... i am wondering.. is that common with these cars??? i know they do have a large throttle cam, but i am starting to suspect my TPS... i know its easily tested, i have to find my insertion probe for my multimeter... .65 at key on engine off and 4.5-5v at WOT

More to folllow. i will fix this car!!

 

That pinhole leak could be affecting it. Some times tiny thins like that can be the issue. JB WELD or solder that up just in case.

 

well the first thing that stood out to me was the fact that you have long tube headers with out cats.... and you had the PCM flashed back to stock. you would need to atleast have the rear O2 sensors deleated out of your tune, or get yourself some O2 simulators. When I had an O2 problem ( wire burnt by header ) , the car got extremely horrible gas mileage, and ran sluggish. It also had such a strong gas smell standing behind the car. Good Luck

 

Sounds like the throttle position sensor is going bad. It along with engine speed determin tc lock up.

 

This is kinda out in left field but has it been raining a lot where you live? I'm asking because some of those symptoms are similar to problems a friend of mine was having with his TA. We had been playing around with my Efi Live tuning it and shortly after that his started running rough. He did not have LT headers but both cats were deleted. Tried several different things and never could get it right. Wound up getting a whole new PCM for it and when I took the old one out. A whole bunch of water came out of the bottom of the PCM. Turns out the gasket on the PCM was letting water run straight through it. May jot be you problem but worth the 5 min to check I would think.

 

Also if I'm not mistaken. The down stream O2's are just basically a tattle tale for the cats they shouldn't have any really control like the upstreams do. One other thing years ago I added a basic air/fuel gauge. Didn't have a dedicated O2 for the sensor so I followed some instructions on how to with it between my two O2s. When ever I have it full throttle the damn thing would fall flat on its face like I had lost two or maybe three cylinders. What was happening was the gauge was creating a "bridge" between the O2s and they were shorting each other out at WOT. If you have an A/F gauge you might try cutting it loose. I know my two posts seem a little odd but I hope they can be of help.

 

I had the exact same problem, cleaned the maf and didn't change a thing. Put a new maf in and the problem went away. All my power is back and theres no more 02 codes.

 

Whats your fuel pressure at? Have you checked it with a manual gauge?

What are your long term and short term fuel trims? Are they maxed out left and right?


Some shops have smoke testers to help with vacuum leaks, they have saved me in the past.

Exhaust leaks sucking in air is a common issue, but to be happening on both banks is possible, but unlikely.

Base engine timing?

 

ok sorry for no replies...

here is the latest...

new plugs, and wires. Duralast plug wires, NGK TR5IX iridium plugs.

pulled the injectors... had them professionally cleaned locally, (ultrasonic cleaning with berrymans chemdip) and flow tested, all are flowing properly, within 1% of each other with very very nice spray patterns, i had two that were squirting like a super soaker however... now they are perfect.

we were driving it on the highway... getting 20-22 mpg's avg (not bad for an auto at 70-75 mph) and suddenly i lost nearly all power... couldn't accelerate from 70-75 with it floored... pulled into an exit immediatly, gas tank was 3/4 full, and you could hear a horrific wining coming from the tank... i topped off the tank on a hunch, thinking maybe the pump overheated... whining noise went away.. ran fine... normal power. well, drove it the rest of the way for hte weekend (about 350 miles) keeping hte tank topped off, and it did it one more time, a just a hair over 3/4 tank. its at the dealer right now getting "diagnosed" (fuel pump is 400 dollars through parts store, or 900+ through dealer, so letting the 100 dollar deductable warrenty cover it). i should know something in the next hour or 2.

a simple fuel pressure check should have cought this... i'm an idiot for not checking the pressure sooner... i couldn't locate a tester, and doing the plugs and wires seemed to clean everything up... the MPG's has increased tremendously however. I may try swapping the MAF sensor with a known good one from my buddies LT1 car to see if it changes anything after it gets back from the stealership.

 

Long tubes with stock tune? Anyway are the o2 sensors crossed?

 

Well my problem came back even after the new MAF. Tested my fuel pressure and that seems to be the issue.

With the car off turn the key to on, my pressure goes to 30psi and almost instantly drops to 0. Then when its running it idles constant at 30 and drops to about 27 under load.

Gonna check my FPR and the fitting to see if that the issue. and if not Im gonna buy a racetronix pump.