Poor MPG. Tune-up suggestions?

[22229stingray81]

I just filled the 'vette up today with 17 gallons. I calculated the MPG to be 7.74 mi/gal, which is obviously terrible. I do believe a lot can be attributed to driving habits. I rarely drive it, and when I do, it is very short trips around the neighborhood. Perhaps 5 miles at a time, some WOT, usually no higher speeds than 50mph. Still, 7.7 is real low.... I had the spark plugs replaced 3.5 years ago and have put only ~1,000 miles on them since then, so I'd think they could be ruled out.

I also have a slightly rough, low idle that I've been ignoring for years :L 400-450rpm in gear, so it stutters a little. The car used to shake like crazy until I had the EGR valve replaced. Really smoothed out after that... perhaps it's bad again? It was replaced 9/2003, but I've only put 10,000 miles on it since then. I'm under the impression those things clog up all the time. Any thoughts?

Going through my records, it doesn't look like the cap or rotor have been replaced since I've owned it (11/1999). Perhaps start there?

Lastly, is there an idle adjustment screw on the carb, or what? Service manual says the computer does it, the internet tells me that's wrong. If there is an idle screw, where is it? Thanks....

-Tatortot

 

[Nut]

I just filled the 'vette up today with 17 gallons. I calculated the MPG to be 7.74 mi/gal, which is obviously terrible. I do believe a lot can be attributed to driving habits. I rarely drive it, and when I do, it is very short trips around the neighborhood. Perhaps 5 miles at a time, some WOT, usually no higher speeds than 50mph. Still, 7.7 is real low.... I had the spark plugs replaced 3.5 years ago and have put only ~1,000 miles on them since then, so I'd think they could be ruled out.

I also have a slightly rough, low idle that I've been ignoring for years :L 400-450rpm in gear, so it stutters a little. The car used to shake like crazy until I had the EGR valve replaced. Really smoothed out after that... perhaps it's bad again? It was replaced 9/2003, but I've only put 10,000 miles on it since then. I'm under the impression those things clog up all the time. Any thoughts?

Going through my records, it doesn't look like the cap or rotor have been replaced since I've owned it (11/1999). Perhaps start there?

Lastly, is there an idle adjustment screw on the carb, or what? Service manual says the computer does it, the internet tells me that's wrong. If there is an idle screw, where is it? Thanks....

-Tatortot

tot..... when was the last time you replaced the O2 sensor? When my O2 sensor went bad my mpg went in the toilet. Don't mess with the carb.

.......... nut

 

[22229stingray81]

According to my records, O2 sensor was replaced 9/2001, and 23k less miles than I have now. No codes are being thrown, though. Still think I should look into it? I'm sure the shop manual has a test procedure somewhere.


-Tatortot

Just looked it up in the shop manual. No test procedure; just removal and installation instructions (page 6E1-35).

 

[Peer81]

Begin with hooking it up with a dwell meter with and without the aircleaner (plug vacuumline).

Ilde mixture screws are in left and right lower front of carb (if the plugs are removed). Should be set 3 2/3 out to begin with but first the dwell meter.

Report in detail what your findings are!

Greetings Peter

 

[22229stingray81]

Hi Peter,

I do not have a dwell meter, but they seem to be relatively inexpensive. The ones I'm looking at have two alligator clips, but as I recall, there is some special green connector under the air filter to hook up a dwell meter to. Do I need one with this connector on it?


-Tatortot

 

[Peer81]

Alligator clips will do just fine.
You can also use it when driving to check the ecm A/F ratio program. The ALDL connector also has the "green connector" therminal. Connect the dwell meter to the two outside therminals and that is it (first figure out which is ground of course)

Greetings Peter

 

[22229stingray81]

Does this dwell meter look acceptable?

Just picked up a torque wrench today.... looks like I'm on a tool buying binge! :L


-Tatortot

 

[Nut]

Does this dwell meter look acceptable?

Just picked up a torque wrench today.... looks like I'm on a tool buying binge! :L


-Tatortot

Boy does that meter bring back memories. Somewhere in the bowels of the nut-garage is my ancient Strobe light too. I'm not a fan of the beam style torque wrench. Get the dial-in "click" wrench for the couple extra $$. Craftsman makes a number of nice ones and 3/8" is good for most things you'd be torqueing. Maybe a 1/2" for lug nuts.

......... nut

 

[Peer81]

Good dwell meter, take it!
And you well under way for a garage extension for all the wrenches and other stuff

Greetings Peter

 

[22229stingray81]

Thanks Peter. I'll report back when I'm able to pick it up and get around to it.

If anybody has other suggestions, please let 'em fly. Beautiful day here; took the vette out for a nice drive. Ran really well.

-Tatortot

 

[22229stingray81]

Another beautiful day so I decided to tinker with the 'vette. I figured out where the curb idle screw is, and turned it a half a turn and FINALLY it idles at 500rpm in gear, 750rpm in park. Amazing what you can do with a shop manual and a few hand tools, huh? :L

I looked around online for dwell meters last night, and I found this one. Looks like a heck of a lot more functionality over the other one. Unless anyone says differently, I think I'll get it.


-Tatortot

 

[Antz81]

Another beautiful day so I decided to tinker with the 'vette. I figured out where the curb idle screw is, and turned it a half a turn and FINALLY it idles at 500rpm in gear, 750rpm in park. Amazing what you can do with a shop manual and a few hand tools, huh? :L

I looked around online for dwell meters last night, and I found this one. Looks like a heck of a lot more functionality over the other one. Unless anyone says differently, I think I'll get it.


-Tatortot

Looks like a better choice of meter to me.

Once you confirmed the dwell on the m/c solenoid, and everything else is ok, you may find yourself adjusting that idle speed again. (Assuming you change something) but at least you know where it is now

 

[22229stingray81]

Roger that, and understood! Thanks for the input.

-Tatortot

 

[Wajulia]

That mileage is exactly what I expect if the ECM goes open loop (17mpg Closed loop, 7mpg open loop). This may be obvious, but is the car throwing any codes. If there are no codes, is the ECM working -- can you get code 12 when you jumper the test leads?

 

[22229stingray81]

That brings up a great question.... a year or so ago I was throwing nonsensical codes, and couldn't get a code 12. I unplugged the ECM, cleaned the leads, and plugged it all back in and it worked fine. I wonder if it went completely kaput. It is a replacement ECM; I know that for sure. I'll see if I can pull codes again. Thanks for the suggestion.


-Tatortot

 

[22229stingray81]

Okay, I just jumpered the test leads and pulled codes. Code 12 flashes repeatedly like it's supposed to. No other codes after it. So I guess the ECM is okay.


-Tatortot

 

[22229stingray81]

Okay, I got the shop manual in hand, and I got my new spiffy dwell meter, and I set it on the 6-cylinder scale. Now... a couple of DUMB questions:

1. How do I hook this thing up? For simplicity, can I just put the black lead to any ground, say, the alternator bracket, and the red lead can I connect to the green dwell meter connector (assuming I can jam the lead in the connector)? I coulda sworn I read how to do this in the shop manual but I can't find the section I "thought" I read now :L

2. Should the engine be sufficiently warmed up when I check the dwell? Engine cold but choke off? Choke on?

3. Peter: You said to test it with the air cleaner on and without. Just to be clear, when I take the air cleaner off, I need to plug the two unconnected vacuum hoses, correct?

4. We're looking for something like 30 degrees, right?

Thanks again fellas.


-Tatortot

 

[Antz81]

Okay, I got the shop manual in hand, and I got my new spiffy dwell meter, and I set it on the 6-cylinder scale. Now... a couple of DUMB questions:

1. How do I hook this thing up? For simplicity, can I just put the black lead to any ground, say, the alternator bracket, and the red lead can I connect to the green dwell meter connector (assuming I can jam the lead in the connector)? I coulda sworn I read how to do this in the shop manual but I can't find the section I "thought" I read now :L

2. Should the engine be sufficiently warmed up when I check the dwell? Engine cold but choke off? Choke on?

3. Peter: You said to test it with the air cleaner on and without. Just to be clear, when I take the air cleaner off, I need to plug the two unconnected vacuum hoses, correct?

4. We're looking for something like 30 degrees, right?

Thanks again fellas.


-Tatortot

1. You got it. remember to set your meter to the 6 cyl scale.
2. Engine should be up to normal running temp
3. Yes you will need to plug those vacuum lines, you'll have other problems if you don't.
4. You got it again. 30 degrees is what you're after.

 

[22229stingray81]

Thanks! I should get to it this afternoon if the weather cooperates.

-Tatortot

 

[22229stingray81]

Okay, here we go.... I took the 'vette out for a drive around the neighborhood. Got operating temps up to where they should be. Side note: just as I was rounding the final turn to my house, THUNK, something hit my floorpan right under my feet. I thought I saw something shiny in the road out of the corner of my eye, but I circled around and couldn't find it. I put the 'vette up on ramps when I got back and can't find anything that would have fallen off... all suspension components are there.... yikes. Totally my kind of luck. But I digress....

Hooked up the dwell meter with and without air cleaner. And we do in fact have a problem. I got 17 degrees both times. Here's where it gets weird: If I remove the black lead, it still reads 17 degrees.... Any thoughts why that could be? I shot a picture with the black lead removed so you could see.... I moved the black lead to all sorts of grounds... dip sticks (yes I know that's not a good ground), valve cover, etc. It would fluctuate around 40 degrees as soon as I removed the lead, but would immediately settle on 17 degrees... EDIT: Please read response below. I accidentally had it on the TACH setting.

I should note that it does run a bit rough. The idle blips a little between 4-500 rpm in gear and the car slightly shakes, even when the idle isn't erratic. Anyways... where do we go from here? Thanks.

-Tatortot