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| 85L98-84L83 | Poor milege in 85 vette 21mpg. | ||
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Guru Newb
 
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My 85 vette gets 21 mpg on the highway and my 84 vette gets about 27 on the highway. What should I check for. I have replaced plugs wires cap & rotor, fuel filter, removed air pump, removed cat, added K&N, cut airlid and even added the dreaded air foil and replaced the throttle positin sensor(because of a code).
Could it be the injectors? How would I be able to check they still on the car? Could some of the injectors be spraying extra fuel? I have no codes being thrown. When I first got the car a year ago I added the above stuff and it went from 16 mpg blowing blue smoke to 21 mpg and no smoke, execpt on the odd start up after it has sit for a week. |
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Posted on: 2008/11/11 19:02
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85 vette - 3.54 gears,pulleys,160 stat,hypertech chip,K&N,no cat,B&B cat back,cut air lid,TB bypass,air pump elim pulley,air foil,Torque thrust II wheels,maaco paint job 84 vette - K&N,renegade,2 inch TB's,adustable fuel pressure to 15 psi,air pump e... |
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| CentralCoaster | Re: Poor milege in 85 vette 21mpg. | ||
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Senior Guru
 
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Did you verify the mileage on both cars at the pump or using the dash?
4+3 or automatic? |
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Posted on: 2008/11/11 19:55
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1985 Z51, ZF6 |
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| captmike13 | Re: Poor milege in 85 vette 21mpg. | ||
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Elite Guru
 
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It also helps to know where your located. Some places have that blended crappy gas.
Auto and stick does make a difference. Here in Oregon I get about 22-25 on the highway with crappy blended fuel. Mike |
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Posted on: 2008/11/11 20:32
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Mike| 85L98-84L83 | Re: Poor milege in 85 vette 21mpg. | ||
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Guru Newb
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I am going by the digital readout, maybe it is not accurate.
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Posted on: 2008/11/11 22:20
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85 vette - 3.54 gears,pulleys,160 stat,hypertech chip,K&N,no cat,B&B cat back,cut air lid,TB bypass,air pump elim pulley,air foil,Torque thrust II wheels,maaco paint job 84 vette - K&N,renegade,2 inch TB's,adustable fuel pressure to 15 psi,air pump e... |
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| captmike13 | Re: Poor milege in 85 vette 21mpg. | ||
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I know my readout is very close to actual. Last I checked it was about 1mi/gal difference.
Mike |
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Posted on: 2008/11/11 22:52
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| Schrade | Re: Poor milege in 85 vette 21mpg. | ||
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Elite Guru
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Check your fuel pressure, and do an ohm test of the injectors. Low FP, and poor spray quality means a less-than-optimum burn... |
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Posted on: 2008/11/11 23:59
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LT5, Marc Haibeck ECM module, AUTOMATIC!!! 
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| j3studio | Re: Poor mileage in 85 vette 21mpg. | ||
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Elite Guru
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Injectors, injectors, injectors!
Swapped new Ford pink tops in this summer and got about 20% better real (not computer) MPG. |
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Posted on: 2008/11/12 2:34
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Grace, 2003 50th Anniversary Convertible |
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| CentralCoaster | Re: Poor milege in 85 vette 21mpg. | ||
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Senior Guru
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The computer mpg calc assumes a certain fuel pressure and injector flow rate. If your injectors aren't matching this, the computer won't be as accurate.
I think 27 is too high for an 84. 21 is too low for an 85. Autos will get better city mileage due to the converter unlocking. Manuals usually get better freeway mileage because they're more efficient and let you use engine braking instead of having to hit the brakes as much. But I question the 4+3's efficiency. My stock '85 4+3 got 24 mpg highway. Conversion to 6-speed bumped that to 28mpg Swap from 3.07 to 3.45 dropped it to 25mpg Cold air intake bumps it up 1-2 mpg in cold weather. I'm still running original injectors (cleaned) and stock fuel pressure. |
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Posted on: 2008/11/12 4:39
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| bogus | Re: Poor milege in 85 vette 21mpg. | ||
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Grand Imperial Pooh-Bah
 
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Check the injectors. The easy test is to remove the rail (ok, not that easy) and put little paper cups under each injector. Have someone crank it and watch the injector spray patterns. Also look for ones dripping.
I would replace the cat, tho. Modern high flow cats don't rob much power. A better idea would be to install a pair of smaller cats and do a true dual system (with x-pipe). |
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Posted on: 2008/11/12 5:56
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The single biggest problem with communication is the illusion that it has taken place. - George Bernard Shaw Education is the best tool to overcome irrational fear. - me |
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| j3studio | Re: Poor milege in 85 vette 21mpg. | ||
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Elite Guru
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Quote:
I agree with these observations. I'm getting about 24 mpg in 80% highway driving with my 1985/A4/3.07 and I think that's about all she's got for me. The 1985 computer output is actually fairly close to reality, but I'm really close to stock... |
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Posted on: 2008/11/14 14:53
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Grace, 2003 50th Anniversary Convertible |
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| bogus | Re: Poor milege in 85 vette 21mpg. | ||
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Grand Imperial Pooh-Bah
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Dude... you are so close to stock, the ghost of Zora has blessed your car!
 I get about 12~14 in my normal daily commute. Most of that is because the car barely reaches op temp before I park it again. Now, with that said, when I get it out on the open road, it does do better. Upwards of 22~23. There is no real "open road" here in LA... that's also part of the problem. Several years ago, when I had access to a real open road, I would drive from Dover, DE to Sebring, FL for the 12 hours. The one time I measured this, I averaged, for the entire 1070 mile trip, a touch over 25 MPG. That includes driving through Washington, DC, Jacksonville, and Orlando. Now, I did a second test to that and when I tanked up in NOVA, (after the worst of the DC metro area), I tracked it until I got to Sanford, FL, and that, pure interstate, 75~80 MPH, I was getting over 27 MPG. This is a 1992 LT1/MN6. |
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Posted on: 2008/11/14 15:32
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The single biggest problem with communication is the illusion that it has taken place. - George Bernard Shaw Education is the best tool to overcome irrational fear. - me |
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| Mekanic | Re: Poor milege in 85 vette 21mpg. | ||
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Registered Vendor
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As many have stated. I would call it injectors. Those old multecs are probably on their last legs.
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Posted on: 2008/11/14 23:26
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~Rich Polo LT4 Yea tho I walk through the valley in the shadow of rice, I shall fear no turbo, for torque art with me. |
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| CentralCoaster | Re: Poor milege in 85 vette 21mpg. | ||
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Senior Guru
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85s use 24# Bosch injectors. I sent my orignals in to get cleaned and am still running them.
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Posted on: 2008/11/14 23:29
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1985 Z51, ZF6 |
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| Mekanic | Re: Poor milege in 85 vette 21mpg. | ||
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Quote:
As long as the coils check out good that is a great option. |
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Posted on: 2008/11/14 23:34
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~Rich Polo LT4 Yea tho I walk through the valley in the shadow of rice, I shall fear no turbo, for torque art with me. |
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| crash | Re: Poor milege in 85 vette 21mpg. | ||
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Guru
 
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find somebody with with a bar 90 exhaust analyzer have hc and co readings checked and see if its running rich
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Posted on: 2008/11/15 1:49
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IM NOT COMPLETELY USELESS I CAN BE USED AS A BAD EXAMPLE |
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| BillH | Re: Poor milege in 85 vette 21mpg. | ||
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The Stig Moderator
 
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Quote:
I've checked the computer on my 1992/M6 quite a few times because I didn't think the milage could be as good as it is. I'm getting 19-21 around town, fully warmed, keeping the revs reasonable. Reno to Sonoma, over Donner Summit, all interstate, with the cruise on 70-75, I get 31.5, checked with a calculator. Stock motor with chambered exhaust. The computer's plus/minus one mpg. |
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Posted on: 2008/11/15 14:35
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Every man dies but not every man lives. |
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| Schrade | Re: Poor milege in 85 vette 21mpg. | ||
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Elite Guru
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Pressure wouldn't matter would it? In the calculation? Seems to me it's just flow rate - fuel in, fuel back, compared to vehicle speed... |
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Posted on: 2008/11/15 15:20
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LT5, Marc Haibeck ECM module, AUTOMATIC!!!
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| Mekanic | Re: Poor milege in 85 vette 21mpg. | ||
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Registered Vendor
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Th computer counts injector pulses to calculate mileage.
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Posted on: 2008/11/15 17:27
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~Rich Polo LT4 Yea tho I walk through the valley in the shadow of rice, I shall fear no turbo, for torque art with me. |
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| CentralCoaster | Re: Poor milege in 85 vette 21mpg. | ||
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Senior Guru
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Yup. So if fuel pressure is greater than the computer is calibrated for, then the car is using more fuel than it thinks it is. Same applies if injectors aren't flowing what they're supposed to. The computer can't directly measure the fuel. |
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Posted on: 2008/11/16 3:26
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1985 Z51, ZF6 |
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