They're all related. I noticed my car has an EPA highway fuel economy rating of 26mpg on the recommended premium fuel. The first thing I want to ask is, does anybody believe this to be realistic? Seems to me it depends who you talk to. Some guys can't break 22 or 23 yet others are (inexplicably) obtaining more than the EPA rating. The best I've seen out of my car is around 23 and within the last few months (maybe "winter" fuel), I haven't even broken 20. My car also has a bit of the hesitation problem and general lack of instant response (and when the response comes, it seems reduced) in the idle-2300rpm range. When the car is stone cold, it drives great - it will break the tires loose literally from idle, in gear (M6). When hot, if I stab the gas, the rpms need to come out of the "hesitation/stumble zone" before they eventually break loose.

Back in July I had the car emissions sniffed in RI and the results were that it barely passed:

All figures are in GPM (grams per mile)
HC: 2.17/2.25
CO: 12.29/23.74
NOx: 2.96/3.25
CO2: 699.22

These results struck me as particularly dirty in an engine that has a reputation (at least in the b-body community) of being just as clean as brand new engines. Despite the fact that my cat probably wasn't fully warmed up yet, these results are indicative of a problem.

I have thown numerous parts at the car in an attempt to increase my mileage as well as get rid of the stumble/hesitation down low, all over the past few months. Things that I can remember:
Fuel filter
EGR valve and cleaned ports with carb cleaner
Cleaned TB, IAC
Seafoamed the intake
Fuel Power (by Lube Control) fuel treatment (like Motorvac)
Accel coil
new GM ICM
Replaced a broken ground strap from a coil stud to the body
Innovative Wiring upgraded battery cable kit (2ga and 0/1ga)
Replaced air filter
Removed MAF and cleaned crud off wires
and most recently, took out what look like good Bosch Platinum +4s and replaced with NGK TR55s

The opti, wires and O2s have about 25k on them. The car is stock. None of these things made an appreciable difference in power or mileage, which is disappointing. It seems to run a little rough, which is a combination of maybe ignition and a rotting harmonic balancer. Do I really need Delteq to get the rated mileage in this car? Am I overlooking something, like maybe the charcoal canister? I'm running out of options on this bone stock Z28...

I disabled EGR in software and my mileage didn't budge at all, though it hesitated less. Still had a lack of torque in the low end when hot. My next plan may be to enable speed density in software and see what kind of mileage I get. I'll also check drivability/torque. Thoughts?

 

i dont know, but i have that same problem as you, 93 a4, did pretty much everything you did and i got a delteq, just put in a new fuel pump + regulator also. the pump doesnt seem to be running right though. i have an exhaust leak i gotta look into but nothing ive done so far has helped at all. search for it, many people have problems like this, no one knows why.

my car also failed emissions worse than yours, which pisses me off cause i had an 80 camaro with a mild 350 in it that had no cats and that did better than this car did.

 

Have you looked under the hood in a very very dark place?
I too have a lower rpm stumble but slightly different symptoms (happens only on the highway at a certain speed), a lot of people who have had the stumble from idle to 2x00 rpm seem to have ignition related problems. Look for arcing on the coil and at each plug really carefully in a very very dark place, it doesnt hurt to just look.

I've done a motovac and it seemed to help for a few days, but it came back. For me i'm now leaning towards injectors.... It's either injectors or my maf now since i've replaced almost everything else.

When i removed my EGR it DID help a bit, but like yours it was still there. If you do figure out be SURE to post it back in this thread how you fixed it I'm going to want to order some new injectors soon if i can find a good deal on them.

Check your fuel pressure, check your wires and electrical for arcing (again). If those are all ok, it could still be your injectors

 

slp sells stock replacement injectors for $100, i might buy those, im also going to look into getting a chip for my car, see if that does anything. -dave

 

I just went thru a similar problem with my car, it turned out to be a weak plug wire, believe it or not. It was weird, would only stumble under a load at low RPM (maybe once it got some RPM up the load wasnt as big?)

And MPG wise, my formula gets 28.5 running freeway only, cruise set in the mid 80's running accross Nevada. Running down I5 thru CA I will get 26 to 26.5. i thought this was weird too, could it be low elevation makes more power, and uses more fuel?

 

I have tried spraying water and looking for arching but I don't know what I'll see at idle. I'd need it to run on a dyno in the dark for best results. Could my GM wires with 25k on them be causing this?

The fuel treatment hasn't seemed to do anything. Although it may be responsible for these soft carbon deposits:
http://kmoore.phzero.net:8081/digita...d/number_1.jpg

I am going to reflash for forced speed density mode and see what kind of mileage I get. Does anybody know what I could do to troubleshoot the charcoal canister? I hope it's not my injectors, but then it wouldn't be the end of the world. It's also important not to consider the possibility of concurrent issues. For example, maybe my 25k GM O2s are causing the bad mileage and the hesitation is injectors.

 

I just went out and screwed around. I flashed the car with speed density mode and drove around. Noted something interesting. I was getting some pretty nasty spark retard, even with knock counts remaining static. At higher loads, the knock counts increased and so did the knock retard. I could DEFINITELY feel this, SOTP. Re-flashed with MAF mode and did the same tests. Not as much spark retard or knock counts. I did some low RPM loading by riding the brakes in 3rd gear at like 1300rpm. Every once in a while I heard a mis-fire-like pop from under the hood or the exhaust.

I pulled over to the darkest area I could find (no overhead lights), opened the hood and revved up the engine a few times. There was some slight glowing from the coil wire and a little faint "fireworks" from the passenger side. I don't think I had a good view of the driver's side. So maybe I'll throw a set of Bosch Premium wires at the car this weekend. Unless someone knows of any other off the shelf wires for a similar cost that outperform these (I doubt it).

I can see this causing bad drivability (I have some surging in the low rpm range) and bad mileage, but spark retard/knock counts? How do I get rid of spark retard that wasn't caused by knock counts? Is it the low octane learning feature? Or burst knock?

 

there is your main problem, is one of the wires close to something? just moving it away from the ground source might pretty much fix it.

 

ive always had bad luck with boschs platinum +4 plugs in anything i put em in, tried em on an older kawasaki 750 and my 94 Z28 neither one liked em at all, but good luck gettin the problem fixed cause my car doesnt have this hesitation you seem to have but i do have a stumble sometimes.. its extremely rare though never does it enough or under certain conditions to find out what causes it =/

 

The wires are in their stock locations, I think.

I haven't heard rosey things about them either. That's why I decided to change them when I pulled #1 in my driveway and saw that it was a +4. But they all turned out to be fine. It's sounding like I need new wires, anyway. But the knock count problem is perplexing. I need an 8051 expert.