I've never been able to get the mpg that first had ~3 years ago. 16-18 mpg; now 14-15 mpg. IIRC 16-18 mpg was before 10% ethanol added to gas. does ethanol result in lower mpg when compared to mpg from straight gas?

It will lower it a little, but not that much.

How's your tune?

five7kid, sure would like to see mpg return .. taking all suggestions -- here's what's up: 1st known problem: one vacuum leak left, the a/c heater vacuum control; when using a/c or heat, have a hissing sound and vacuum drops for about 10 seconds before coming back. that part should be in this week. 2nd possible problem: may still have cold start issues though think I'm good to go http://www.thirdgen.org/techboard/vb...ber-69994.html

Carb was redone by rebuild shop last Aug. I haven't tried to adjust the idle screws or m/c.

operating temp is consistent between 200 and 230. idles quietly @ ~500 so is running pretty nice & all tuning is by the books. Dwell is 30 when warm. reasonably recent tune-up parts. good tires, no extra weight.

outta ideas other than vacuum and possibly making a little improvement to starting

Some thoughts:

Where's your timing? A few months back I bumped mine up from 0 to 4 degrees advanced on my stock LG4. Ran two tanks of gas through it and noticed a consistent drop of about 2 mpg. Also noticed a little light pinging with throttle during low speed cruise. I suspect the knock sensor was pulling more timing out than I had added and/or the pinging was decreasing efficiency. Probably a little of both.

Converter lockup work? I drove my camaro for a few months with an inop lockup. After I got it fixed I didn't notice an improvement in mileage, but lockup at cruise DOES indicate that the system is operating in feedback mode. Your vacuum leak could be enough to bring it out of feedback, or put the mixture off too much for the carb to compensate.

Warm up can hurt. I usually see a drop of about 1 mpg during the winter months. Winter for me is 30-35 in the mornings. I start it up and drive, no sitting there running. Around 22 Winter/23 Summer. Auto/2.73 rear end.

Another note: I really don't see much difference in mileage with how I drive. Whether I baby it or scoot from stops/on-ramps (within reason, not in secondaries too much) I pretty much get the same back and forth to work everyday. 35 miles one way, same route for three years (since they built a new airport bypass).

its at 0

ok, will fix vac leak when part comes in

would take off but on cold mornings car refuses to drive nice until coolant = 110 degrees; takes a minute or so

getting on it much at all lowers my mpg

Choke working correctly? Is it all the way open when its warmed up?

good - opens up al the way when warm

how many revs you pulling at light and also at part throttle?

Im in the same boat with you and Im also averaging about 14-15 mpg, more on highway. Idle in park is 1350-1400 and in drive it is at 600. I also have the vaccum control leak, I replaced the part but it still does it, but I think the plug up might not be tight.....I had to buy a new q-jet becaue of a stripped inlet nut and I had it tuned then as well. Im not sure where my timings at(dont know how to check that or set it) Not sure about Dwell either? Once I start mine I wait a couple seconds drop the fast idle and I go no problem.

Tobias05, start at low rpms, during traffic 2300 rpm takes it to 45 to 55 (in- town driving), 55 mph on highway at ~1800 IIRC.

camaropunk, after vacuum leaks are over I am going to check my fast idle, maybe that could be lowered

Air_Adam, maybe my fast idle screw is a bit to high, will check it out like I said to camaropunk. Any thoughts on how much might fast idle screw set a little high might mess up cold start warm up?

the fast idle on higher helps on warm up thats what its for, higher also means more gas because its revving higher, now if its set to low the car wouldnt rev is high resulting in not warming up as quick.

Vacuum might make the fast idle lower id think if it even effects it

camarounk,
the a/c heater vacuum control part came in - how long did it take to install & is it basically take off plastic around radio, take out radio, & then heater control?

scooter500

That's basically it.

The O2 sensor can be weak without setting a trouble code, while richening the mixture. Pretty common cause of poor mileage.

I just discovered taking apart Berlinetta #2 this weekend that the MAP sensor line was split where it connects to the carb. The previous owner (or their shop) put a hose clamp on it, but it was split right at the end of the nipple, so I'm sure it was registering lower vacuum than reality - which will keep the computer from making proper decisions. Could explain the 20 MPG I got with it bringing it home (TH200C and 3.23 gears don't help, either).

Yeah basically, let me know if it fixes ur leak, mine is still leaking and its alot worse, I hate this thing ive wasted 4 hours on it and its still leaking

I went to the junk yard and picked up a couple used vacuum switches. I had to change it twice before I found a good one but it worked.

As far as gas mileage I've got a 91 305 with an edelbrock 600 with electric choke. In the summer I was averaging 16-17 around town back and forth to work. Now with winter here it has dropped down to 13-14mpg. My torque converter lockup is adjusted to come in around 25mph. Any ideas on how to improve the mileage? I have stock 2:73 gears.

4 hours...! ok, i've been warned & will probably take off plastic then radio during week to keep Saturday work short


five7kid, how much do those 2055 headers help with mpg?



scooter500

I have no idea. I put them on the same time I put on ported Worlds and a Crane 2050 cam, along with 3" exhaust to the muffler. My mileage dropped with all that, but even with the 350, it sounds like I'm getting better mileage than you are.

That stuff really didnt get in the way, its easy to change, i spent all that time trying to get it to quit leaking and taking it apart and lubricating it and everything I could think of

Depending on how much of a mess your stereo wiring is, it shouldn't take more than a few minutes to pull the unit. I recommend getting the entire HVAC control at the boneyard. I tried to fix mine and it was never right, but a used one from a buddy solved it easily.

Until you can work on it, look at the vacuum diagram under the hood. It should have a line listed as 'HVAC'. Pull and block that line. It will cause your heater to get stuck on defrost, but it will solve the vacuum leak temporarily.

Well I actually did get another control unit and same problem, Im gonna seperate the plastic harness todayand connect each line directly and zip tie them tight and see if that fixes my problem, ive tried two differnt switches ( new ones and old one) took apart and cleaned and lubed both, maybe the plastic harness is bad causing one of them to leak, if it doesnt worked i hacked a good harness and just made it harder to reconnect a different switch.


Scooter, what kind of oxygen sensor are you running? I have a bosch one which I hear is crap for out cars, Im plannng on changing it once I get a stock delco one, Ive heard they are much better. Im hoping that should help some and im gonna also do plugs and wires and cap and rotor.

Have you ever used sea foam, I ran some through the carb and my car seemed to acelerate better. what octane are you running? what does your temperature usually sit at?

I need to learn how to set my timming so I can just do it myself and figure out what its at which is no telling.

Bosch sensor works fine for me.

Yes, you do. That's one of those basics that until you know what that is, you can't do anything else (except perhaps fix vacuum leaks).

Yeah it works for me too, but from what ive read they kill the gas mileage on third gens and cause cars to run richer, research it on here a little
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I can do just about any tune up on my car besides brakes and setting the timming, its one thing ive been afraid to mess with, I dont want to do it and cause it to run like crap and then haft to take it to the shop and tell them I dont know how to set the timing.

You're on the right track. I did the same thing, replaced the climate control switch thingy several times, greased it, bent the spring to put more pressure on it, etc. It turned out the problem was the clear plastic "harness" and its seal to the plastic lines. I pulled each line out slightly, one at a time and my leak has entirely stopped. I think most people who are changing the plastic rotary housing are changing the wrong part -- it's probably the plastic lines leaking to the harness just like mine.

You know you are the number 3 person to say that now, Ive tried adjusting the spring and even using two springs and still no luck, im just gonna chop it and see if it fixes it. I think i changed the wrong part now too.

If you want to set your timing and the car currently runs, get ahold of a timing light. Unplug the vacuum advance. Set the timing to 6 degrees btdc. Drive the car and see if it spark knocks at light load like going up a hill. Start backing it off a little at a time until the spark knock goes away. That should give you optimal timing. Or just advance it from where it is now until you can tell you've gone too far i. e. pings or poor acceleration, hard starting when hot.

This won't work with a car with a knock sensor and ECU, because the ECU will start pulling timing way before you "hear" knock. This was the old way to set timing with no ECU to worry about.

Hmm... guess I got lucky because the control unit seemed to fix my problem. When I had it apart though it occured to me that a little RTV should be able to permanently fix any problems with the small plastic lines.

Ok, i clipped the harness and zip tied each one on, It no longer constantly hisses, now it hisses once its changed then I hear a vent in the dash close and it stops and switches the setting all in under a couple seconds, longa go I know it didnt hiss at all, but guess this is as good as its gonna get.

though about rtv but it would be so easy to plug up a line

The hissing while changing means you probably have the same type of hard line-to-rubber connector leaks at each vacuum door on the HVAC system. Good to know I am not crazy...

It will hiss momentarily as the flaps in the dash move. The trick is that it stops hissing.

Hey guys will you go to my other thread to talk about this, I kinda feel like I jacked scooters thread and I feel bad. sorry scooter

http://www.thirdgen.org/techboard/co...ontrol-va.html (AC/heater control va. leak)

Aaron, does yours do this hissing??

which vaccum doors you talkin 327? yours still have a little hiss until they open I assume?

camaropunk, no problem, similar a/c heater vac control problems and 327_maro_man may have posted our fix ... I'm using a heated bosch O2 & recently changed it, haven't used sea foam but have used berryman b12, oil is staying fairly clean

Hey, I was wondering if you ever got any better mileage out of yours?