Poor MPG. Tune-up suggestions?

[GerryLP]

Okay, I think I understand better now Gerry, thanks. We know that the EGR was bad, so at least I didn't waste time/money on something that wasn't broken. I will still say, just to be clear, it is not the RPM change that bothers me, but rather the engine vibration when the idle drops. Regardless, a consumption test is indeed next.

And as for if it passed the dwell test... I dunno! :L This is what I do know: After going for a drive (so it is sufficiently warmed up), there is a constant 2.9 degrees dwell. Then, every minute or two, the rpm drops slightly and the engine vibrates, and THAT'S when I get the dwell "flux" between ~27-37 degrees. So, the dwell appears to be changing with the engine vibration I'm chasing my tail over. This happens every time I've checked it. Engine smooth = 2.9 degrees. Engine rough = normal "flux."

Consumption test is next. I promise

-Tatortot

Tattor,

I am willing to bet that if you followed fig. 6E1-17-Chart 1 (page 6E1-12), you'll find that the left side of the chart will reveal the problem. A steady dwell of 10 deg or less can be isolated with this chart. By the way, when it says to,"choke engine", what it means is that you cup your hands over the carburetor Venturi to physically reduce the air flow.

I am also thinking the possibility of hose routing causing a problem, but it seems less likely.

 

[22229stingray81]

As usual, you da man, Gerry. I don't have the shop manual with me but I'll look at it as soon as I get home.

I'll go over the hose routing too, but I did give it a good once-over when I was replacing the rotted lines. Thanks for keeping the suggestions coming!


-Tatortot

 

[22229stingray81]

Houston, we have an odd, and new, problem.

I went out for a drive today, but I didn't get a mile from home before sensing a problem. When I let off the gas pedal, I didn't feel much speed decrease. I get to a stop light, and the engine is reving (in neutral) at about 1300 rpm. I turn around and bring it home, take off the familiar air cleaner and housing, but I can see no binding or kinking of the throttle linkage. My only thought is perhaps the choke has stuck on? I haven't been messing with the choke at all.... But that's what it feels like; like the choke has stuck on. The choke light on the dash is off (as it should be). Should I warm it up tomorrow and check to see if I'm getting 12v at the choke wire?

Thoughts anyone? Maybe I should start a new thread :L


-Tatortot

 

[GerryLP]

No need to warm up engine. Just turn the ignition key to on, and then with a multi-meter, check voltage from green wire to ground. It should read 12 VDC +/- 0.5 volts. If you do get 12 VDC, then start engine and watch the chock plate. Kick the gas pedal, and see if chock carb plate opens. If it does not, then the bi-metalic coiled spring could be broken or even not installed correctly.

 

[Antz81]

Did you get your other problems sorted? (dwell around 2.9degrees, possible vacuum leak?)

 

[22229stingray81]

After taking a break and writing the above post I went back out to see what was what. I'm getting over 12 volts at the choke coil pigtail; closer to 14v if I unplug it. It's cracked so it was taped in place. Perhaps it lost contact for a bit.

Anyways, after checking the voltage and moving around various vacuum lines and checking all throttle connections, I started the engine and RPM is back to normal. Who the hell knows what the problem was. I'll start it up tomorrow and see if I can replicate it. I suspect a kink somewhere on the passenger side. I actually have a spare choke coil laying around, so if I can get it to do it again, maybe I'll switch it out.

Antz81,

Not quite sure if I fixed the original problem yet. Idle still is a bit rough at times, so I suspect more work to be done. My EGR was bad, and I had a vacuum leak from a torn vac line. I need to do a consumption test to see if it's better. The weird thing is I couldn't get the dwell below normal today, even with the air cleaner off. It was up around 38 degrees, and I couldn't get it to do the solid 2.9 degrees like it did before. More testing is needed and I'll report back.

Thanks for everyone's help.

-Tatortot

 

[22229stingray81]

Well, the fast idle problem is gone. Took it out for a quick trip today and all was well. I think the problem yesterday happened after a WOT burst. Perhaps something stuck open? Anyways, the choke heater wire is re-seated and vacuum lines are moved out of the way if it was a binding problem... now back to my original issue It may be fixing itself. I need to do a consumption test (which is what I was on my way to do when the choke thing happened), and see if I can get the dwell to go to a fixed 2.9 degrees again. It ran well on my quick trip and didn't appear to do the idle speed change. Cross your fingers, gents!


-Tatortot

 

[22229stingray81]

Update!!

Well, I FINALLY got around to doing the consumption test. Drove 30 miles after a fill-up and re-filled and I got 12.3 mpg. That's a helluva lot better, for sure. I might even say normal, considering my heavy foot and lack of highway driving. Dwell is right around 32 degrees and fluxes as it should. I cannot get it to drop to a solid 2.9 anymore so I guess it was all a vacuum leak? Wooo, I think I done fixed it! Thanks to everyone that helped me through this. I'm still probably going to dive into the vac system and test each individual line, but I think we found the culprit. I'll see you in hell, EGR :L


-Tatortot

 

[GerryLP]

Well, I FINALLY got around to doing the consumption test. Drove 30 miles after a fill-up and re-filled and I got 12.3 mpg. That's a helluva lot better, for sure. I might even say normal, considering my heavy foot and lack of highway driving. Dwell is right around 32 degrees and fluxes as it should. I cannot get it to drop to a solid 2.9 anymore so I guess it was all a vacuum leak? Wooo, I think I done fixed it! Thanks to everyone that helped me through this. I'm still probably going to dive into the vac system and test each individual line, but I think we found the culprit. I'll see you in hell, EGR :L


-Tatortot

Tator,

I hope you are not ready to throw the towel. It perhaps improved mileage, but just as illustration, I bumped Gail's compression ratio to 9.5:1, and around town I get 14.5 MPG....albeit with a 4th automatic gear. Nevertheless, I was never getting 12's MPG when stock.

You should be able to improve on this. Although your headlights may be popping out normal to you, any bit of hesitation from the headlights to begin moving is being cause by a leak. If the engine runs rougher while the headlights are deployed, then that is another sign of vacuum leak. So just keep at it. The Thermac could also be leaking. After warm up, disconnect one of the accordion snorkel ducts (preferably the one on the driver's side), and see if the flap is open. If it is not, the Thermac's temp sensor may be by passing vacuum signal.

If while pressing the brakes you hear a slight hiss inside the cabin, then that's your power brake booster leaking (and yes, they are a huge PITA for someone my size :L ).

So keep improving it. Before you know it, you'll be having your ride purring like a kitten...or leopard. :L

 

[22229stingray81]

Nope, not throwing in the towel, but I sure am pleased with a 5 mpg increase! I think the best I've ever recorded is something like 17 mpg, so we're getting closer. Still has a little bit of a rough idle, though.

I'll look at the headlights next. The passenger side is slightly slower coming up than the driver's side.

Do you remember my snorkel situation, Gerry?

That flap never closes! I'll check the other one, though. It appears there was some sealant foam on the flaps at some point? Completely degraded now. Oh, and no hissing whilst applying brakes.

-Tatortot

 

[GerryLP]

Nope, not throwing in the towel, but I sure am pleased with a 5 mpg increase! I think the best I've ever recorded is something like 17 mpg, so we're getting closer. Still has a little bit of a rough idle, though.

I'll look at the headlights next. The passenger side is slightly slower coming up than the driver's side.

Do you remember my snorkel situation, Gerry?

That flap never closes! I'll check the other one, though. It appears there was some sealant foam on the flaps at some point? Completely degraded now. Oh, and no hissing whilst applying brakes.

-Tatortot

Ok, so if the Thermac is not working properly, then disconnect and plug the hose that goes to the stuck-open vacuum motor, and that way you don't loose vacuum signal. You could perhaps just leave them open, but the problem is that they are spring-pressure closed, so the passenger's side may still be operational, and if it is closed all the time, it will cause a rich condition.

Ebay have a few replacements for sale:
Corvette N O s Snorkel Vacuum Damper Flap Acc 6487372 | eBay

With Thermac functioning properly, it does improve driveability.

Here is the sensor:

Corvette Air Cleaner Temp Sensor 1980 81 8997493 New in The GM Box | eBay

p.s. Temporarily, you could use a standard air cleaner, but in extreme temperature ranges, specially in cold weather, you won't have a way to help the carb to stay within the designed ranges. In over 100 deg air temperature, the open style cleaner may be more helpful.

 

[22229stingray81]

I was being a bit cheeky, Gerry The door doesn't close because the snorkel is dented. The Thermac is probably still working, but I will check. Doors closed at warm-up, doors open after temps are up, yes?

Yikes, these prices.... I just looked at used air cleaner bases on eBay. $150 if I'm lucky!


-Tatortot

Ok, so if the Thermac is not working properly, then disconnect and plug the hose that goes to the stuck-open vacuum motor, and that way you don't loose vacuum signal. You could perhaps just leave them open, but the problem is that they are spring-pressure closed, so the passenger's side may still be operational, and if it is closed all the time, it will cause a rich condition.

Ebay have a few replacements for sale:
Corvette N O s Snorkel Vacuum Damper Flap Acc 6487372 | eBay

With Thermac functioning properly, it does improve driveability.

Here is the sensor:

Corvette Air Cleaner Temp Sensor 1980 81 8997493 New in The GM Box | eBay

p.s. Temporarily, you could use a standard air cleaner, but in extreme temperature ranges, specially in cold weather, you won't have a way to help the carb to stay within the designed ranges. In over 100 deg air temperature, the open style cleaner may be more helpful.

 

[GerryLP]

I was being a bit cheeky, Gerry The door doesn't close because the snorkel is dented. The Thermac is probably still working, but I will check. Doors closed at warm-up, doors open after temps are up, yes?

Yikes, these prices.... I just looked at used air cleaner bases on eBay. $150 if I'm lucky!


-Tatortot

The sensor was the culprit in my Vette. It would be operating correctly at idle, but at power, it would close the flap doors and cause a rich condition ([sorry, closing the flaps would NOT be a lean condition] Dyno proven). The mixture ratio would fatten.

 

[22229stingray81]

Cripes. That little thing has that much influence, huh?

The sensor was the culprit in my Vette to be operating correctly at idle, but at power, it would close the flap doors and cause a lean condition (Dyno proven).

 

[Antz81]

I was being a bit cheeky, Gerry The door doesn't close because the snorkel is dented. The Thermac is probably still working, but I will check. Doors closed at warm-up, doors open after temps are up, yes?

The door doesn't switch from closed to open all at once. But this basically what you're looking for. it should be starting to open at around when the thermac is around 100degf (+/- 20degf).

 

[22229stingray81]

Danke!

The door doesn't switch from closed to open all at once. But this basically what you're looking for. it should be starting to open at around when the thermac is around 100degf (+/- 20degf).

 

[22229stingray81]

Another update:

I meticulously went through the headlights and tested EVERY vacuum line, both actuators, and both relays. Prognosis: The passenger side actuator has a slight leak. Thanks for pointing me in that direction, Gerry! Now I need to decide if I'm going to buy the whole actuator, or just the seal kit. I need to remove the old one first to see if I can free the link clevis, or if it's rusted on there.

Side rant: $9 shipping for a couple of rubber seals? Come on, parts suppliers. ad


-Tatortot

 

[Antz81]

Side rant: $9 shipping for a couple of rubber seals? Come on, parts suppliers. ad

I went to order some parts from an american supplier (around $200 worth) and was quoted about $250 for shipping.
Ended up getting them locally for about $210 and shipping was $8
Try to explain that difference.

 

[GerryLP]

Tator,

The actuators in my vette are the originals, and the seals did the trick. So it is worth trying those first before incurring the cost on the actuators themselves.

Antz, vendors over here automatically add a premium as if it takes any more efforts from them. It is almost as if they take all the shipping mark-ups from each company (UPS, FedEx, USPS), and add their own mark up. I get frustrated about it as well. ad

good news from you both. You were able to solve the problem.

 

[22229stingray81]

I got the actuator out today. As I expected, the link clevis is rusted to the link rod. I'm soaking it in WD-40 now. I have vice grips on the link rod TIGHT, but whenever I try to turn the clevis, it slowly turns with it. So I may have to pick up a new actuator and clevis.... ugh. Maybe I'll pick up a torch at the hardware store and try that. Or shall I put the whole thing in the freezer? :L Any tips are much appreciated.

Oh, also, how do you open the damn thing up? Am I missing something here?

-Tatortot