Jonota

Hey all... I tried fishing in PCM diagnostics, but that didn't net me anything. As it may have nothing to do with tuning, I thought I'd give it a try here.

I bought the car end of December. My typical commute is ~30 miles each way, 1 stop only near the end, 65-75mph speed limits. I'm currently averaging about 18-19mpg NOT driving crazy at all. This is my first LS1 f-body - my LT1 convertible w/ the same setup got me 26-29 average all the time, my LS1 Vette would break 30 no problem.

However, I'm not getting any relevant codes.... and looking at O2s I have cyclic waveforms (with strange sawtooth spiking on one), but they seem to respond up and down to changes in fueling. So what gives? Or should I just expect this kind of fuel mileage from only a Borla catback and a lid?

Jon

gMAG

iTrader: (1)

When I had experienced vacuum leaks, my LTFT would spike....an indication that in response to extra air being added after the maf (through the leaking hose). In response to the extra air, the O2s would go lean, and then add more fuel, to compensate, which keeps the A/F ratio aligned with that programmed in your ECM. After correcting the leaks (there was one in both sides of my exhaust, as well), the LTFTs now read 0 (zero)-bank 1, and +1.3-bank 2. Mileage went back up, and no more extra rich exhaust. Oh, & there were no codes.
Normally, when watching your O2s, you should see them cycling rich, then lean, then rich, then lean, etc.

TransAmBlue

Best place to start imo is go over everything, does it need wires and plugs? Fuel filter? Dirty Injectors? Carboned up? Excessive carbon will kill fuel mileage.

Certainly change the O2's If it has over 60k on it. Tighten the intake bolts with a proper inch lb wrench. There isn't 1 ls1 car that i've looked at with over 25k on it that didn't have loose intake bolts. Another thing to look at is the fuel trims, this can tell you alot about whats going on in the fuel system and vacuum leaks.

Another good thing you can do is a compression test, that will tell you if its a engine condition issue or just a tune maintenance issue. If compression is down all the tuning in the world isn't going to get mileage back.

I'll tell ya what helped my mileage in 20 degree weather was blocking off the ram air in the hood. It made a huge differeance just taping over the inside square opening in the hood. All that super cold air tells the pcm to dump more fuel.

Jonota

I hadn't thought to look for vacuum leaks, I'll add that to my investigation list. I have done a thorough look for exhaust leaks and don't believe I have one. I have scanned and the O2 waveforms were sinusoidal, and went from lean to rich and back consistently. One was very "smooth", and the other had some spikes to it.

I did pick up some plugs, pulled one and it looked absolutely perfect. It was then that I remembered dealing with an LT1 car running poorly with bad gas mileage that the passenger rear 2 spark plugs had never been changed out - it didn't register a misfire but they were simply carboned up nubs. I mean to get in there and check those out soon. I do plan on doing a fuel filter just because (and then I'll know when to do one again), running some cleaner through the fuel system and the engine. I had NOT heard about the intake bolts, and especially if I have loose ones predominantly on the passenger side, that could definitely be a problem!

I'm thinking compression check will come out good - it feels like it has tons of power. I can tell it is running super rich though, as my injector pulsewidth was nearing max and the car is almost bone stock. I hadn't thought of the cold air coming in either... that is an easy fix.

gMAG

iTrader: (1)

One other item you could look at is the coolant temp sensor.
See if it 'warms up' consistant with the engine heating up.
When off, even a bit, can put the A/F out of whack.

ds98formula

iTrader: (3)

Clean maf too. Check tire pressure. Make sure no breaks are hanging up

Jonota

Slight update: removed AIR system connections to the manifolds, used block plates. Felt "crisper". Re-scanned and that must have been one of the problems, now I have clear driver's LTFT at 20+. Put the car up on the lift running, and the manifold to downpipe connection that looked terrible (but JUST got replaced by a "professional" shop) is blowing exhaust gases out at the flange. Terrible work. Ordered some long-tubes and ORY, we'll see where that gets me.

Jonota

I'm hoping this is the genius statement of the year. I admittedly overlooked this thinking "no way". After doing LTs and ORY (had a blown out manifold to catalytic converter union), tune, and still only getting 23 mpg I decided to give it a try. Only 1 was even close to torque, the rest took over 2 turns to get to 89 in-lbs. That was today, so will take a bit to see if that does the trick or not.