There are probably threads about this, but I didn't see one. My L98 has 156k, and looks like I'm getting about 16mpg. My drive to work is a mix of city/hwy--mostly hwy, and according to the window sticker I should be in the 17-25 range. I don't expect new car mileage, but I'd like to get closer to 20mpg. Any tips beyond the obvious (tire pressure and etc.)? Here's one tidbit. Every couple weeks I'll start it up and it will idle low and die. Once I get it fired up and going, it runs fine, and will fire right up normally for a week or two after. I wonder if my IAC isn't sticking sometimes? I have never changed the O2 sensor either in the 9 mo's I've owned this car, and I suspect it's been in there for a while. I did clean the TB a few weeks ago as well, and the air filter is a K&N that I recharged this spring.

 

If you haven't changed the O2 yet you may consider going with a heated O2 especially if you drive in cooler weather part of the year. It will shorten the warm-up times and go into closed loop earlier.

Other than that a complete tune-up with new plugs, wires, rotor and cap.

Check/re-pack wheel bearings and good alignment.

 

Might want to advance the timing slightly and see what happens. Mark where it is now so you can put it back. DO NOT get all hung up about "lights" and "marks" and "numbers"; if the engine RUNS better, it IS better, regardless of all that other.

Rear end fluid is another tweeek you can do: synthetic fluid will lower the losses there.

Make sure the engine reaches proper operating temperature. Should be around 205°. If it's less than 190° it will FOR SURE use more gas.

 

Yeah, I rarely get that hot, at least according to the dash gauge. When it gets warmed up, my gauge usaly says around 185-190. What I don't get is that the therm is a stock 195 (and its new!), so it makes me wonder if the therm is even opening, if the gauge is right, and it may be off. What would I do to heat it up? My trips are usually short, 15-20 miles, so it never gets a chance to run super hot. But the plugs/wires/cap/rotors/O2 I can do. Just had a four wheel alignment check done last month, so we're good there.

 

Here's another thing I may be working against. I have a code 32 every time I get about 5 miles down the road, that could be hurting my mileage some. I have a new ECM, new EGR valve, I have continuity through the green wire, power at the pink/black wire, all my vacuum lines are clear, and the ported line at the TB is pulling plenty of vacuum.The only two things I haven't done are replace the solenoid, and pull the intake and clean it. Pulling the intake might be a bit extreme, I think my trouble is the egr solenoid, but I want to run one more vacuum test on it first. I followed the troubleshoot flow chart the best I could, and so far most things point to a bad solenoid.

 

Most good mechanics use a scanner to monitor the coolant temp(the engine coolant temp sensor for the ecm, which is very accurate) & compare it to the gauge (mostly inaccurate).

 

Yeah, my thought too. I have an OBD2 real time scanner that interfaces with my iPad in my Mustang and 4th gen Camaro, and I noticed a significant diff between what my temp gauge says and what the ECM is telling the scanner. The actual temp was a little higher than the gauge says. I keep looking for a thirdgen ALDL equivalent that will work with my iPad, but curses, there is none! Curs-ed ALDL, foul mountebank!

 

BTW, I have no idea how to advance the timing. Never done that before, I'm pretty much a novice that's just learning a lot of this. Id have to youtube it and/or study it out just to learn how to do it! But then, it's doable. I've learned a lot in the past 9 months.

 

I would definitely address the code 32 (EGR fault) first. As soon as that SES light is on, you're losing mileage because the ECM is backing down to a much more conservative tune. I will warn you that EGR faults are the bane of 3rd Gen owners everywhere. They can be NASTY little problems to solve, often starting with a new EGR valve that does nothing to fix the problem. Here's an article that gives good backround and operational explanation before you start wrenching:

https://www.thirdgen.org/egrdiagnose

That and a complete ignition tune-up with plugs, wires, cap and rotor. Check the base timing (you do this with the timing advance wire disconnected- single through-wire weatherpack connector sticking out of the wiring harness above the HVAC box, tan with a black stripe). Stock is 6* BTDC. Usually they run better at about 10* (loosen distributor hold-down bolt and rotate distributor body CCW until the light shows 10*, retighten bolt).

 

Yeah, I've read that article numerous times, it does help with the understanding. I used the diagnostic flow chart, and it seems like it's leaning toward a solenoid issue. I'm hoping, because I've done about everything else except pull the intake off and clean it, and the solenoid is one of the easier fixes. What you said about advancing the timing is a bit over my head. It's something id have to study out for a bit first. I mean, it can be done, I just have to research it first. I also don't have a timing light, and have never used one, but can get one and learn if need be. I'm a novice here, part of why I'm on this site is to learn. The other tune up issues I can handle ok. Thx.