North American Turbocoupe Organization



Lousy gas mileage
segfaultxr7 Offline
Posting Freak
#1
I keep reading about everyone getting such good gas mileage.

Well, I drove 130 miles on the highway and got.... 21.1 mpg. I averaged about 70 mph. There were lots and lots of hills, but I still expect better. If it's on a perfectly flat road it can get ~25-40 per the trip computer, but it will ABSOLUTELY NOT get above 20 mpg if I am on the gas any more than what is needed to maintain speed on flat land. It also seems very rich at idle, to the point that I can smell the rich exhaust a little if I get out just after shutting the car off.

It has new plugs, wires, cap, rotor, fuel pressure regulator, PIP, and O2 sensor. I had adjusted the VAM to make it run richer but reset it to stock. I haven't put in the replacement TFI yet; this one acts funny when the car is cold and at WOT, but is fine otherwise. I checked the TPS with a voltmeter, it seems fine. Is there anything left besides the coolant temp sensor? Which of the two sensors does the EEC use, BTW?
88 TC 5-speed
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Dan E Offline
Posting Freak
#2
FPR's have been known to be bad out of the box...what is your FP with and without vac ?

The is the ACT (Air Charge Temp) sensor (next to the knock sensor) but I am not exactly sure what effect it has...Jeff ?

The other thing to verify is ignition timing. If it's is too far retarded you will burn more fuel.

When I had my base timing set to 16* BTDC I was getting piss poor mileage (also that was at the highest cost of gas...bad timing on two fronts on my part...LOL !).

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Dan Eaves
88TC 5spd, Crower 5.5" Sportsman Rods, Short Wiseco Pistons bored .030" Over, Ported Head with Polished Combustion Chambers, Gutted & Polished Upper, Ported & Polished Lower, Polished T/B, Ported E6, SVO T3 .63AR Turbo, Bosch Bypass Valve, 3" DP w/No Cat, 2 1/2" DynoMax single cat back w/DynoMax Super Turbo Muffler, RR Cam, K&N in Fenderwell, Gillis Boost Valve, Centerforce I Clutch, Removed A/C, Polished Wheels, Polished Valve Cover, Polished Crank and W/P Pulleys, Walbro 255LPH HP Pump, Kirban Adj FPR, and the DAMN Heater Core is new too !

NATO MEMBER (Vice Chairman...if it's a vice...I deal with it)
Dan Eaves
88TC 5spd Vermillion Red, Polished...everything...
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Walsted Offline
Posting Freak
#3
Mine had real bad gas mileage when I got it. It's still not very good, but I have gained about 5 or 6mpg during the past year through fixing things. I don't know if any of these apply to you, but mine improved a little each time after replacing the bad computer, replacing the bad O2 sensor, replacing the rear end, replacing the plugs and wires, replacing the stuck-open thermostat with a 192 degree thermostat, cleaning out the idle air bypass, adjusting the TPS to .97 volts, replacing the EGR controller and the vacuum lines associated with it, and cleaning out the EGR valve. I did a lot of these things at the same time so some of the items may not have actually affected my mileage.

Right now, I get about 23mpg, in a previously abused '86 with no significant mods, driving mostly lightly hilly highway miles at 60mph or a bit above. The tripminder reads real close to what the tankful to tankful computation yields, and I notice the instantaneous mpg start to fall real bad above 63mph, going from the mid to high 20s to the high teens, with all else remaining constant. With my tranny, that speed roughly corresponds to 2500rpm.

Don't know if any of this will actually help you figure out what is going on, but I figured I'd post it just in case
Mike Walsted - Sold my 1986 5-speed TurboCoupe
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segfaultxr7 Offline
Posting Freak
#4
Thanks.. The speed may have a lot to do with it I suppose; I try to keep it below 80mph/3000rpm because of it hitting open loop, but my tach or speedo might not be accurate. The trip computer looks to be dead on, its elapsed miles always stay perfectly synced with the odometer, and the gallons used corresponds to how much I need to refill the tank. I haven't actually calculated in a while, but it would follow that I'm getting accurate mileage readings.

I will re-check the timing. Come to think of it, I haven't set it since I replaced the TFI last week with my old one after the new one died, and had to spin the dist around to get it off. I made a mark to line it up, but it's so touchy that it's hard to preserve the timing that way. I'm going to buy a fuel pressure gauge today or tomorrow.. It runs a lot better since I put the new FPR on though, so I didn't suspect it a whole lot.

Also, are these 30 mpg TCs I hear so much about running any mods? I try to keep it under 10 psi or so, but I'd imagine the Gillis valve may be partly to blame.
88 TC 5-speed
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Jeff K Offline
Administrator
#5
In my experience, driving on very hilly terrian will just kill gas mileage compared to a flat road. Could you also have been bucking a strong head wind? Running into a 30 mph head wind will drop my mileage several mpg out on the highway.

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Jeff Korn

88 Turbo Coupe: Intake and exhaust mods, T3 turbo at 20 psi, forced air intercooler, water injection, bypass valve, Ranger roller cam, subframes, etc., etc.. // 86 Tbird 5.0 (original owner): intake, exhaust, valvetrain mods, 100 HP nitrous, ignition, gears, suspension, etc., etc.... // 91 Escort: Bone stock winter car // 00 Windstar (wifes vehicle)
Jeff Korn

88 Turbo Coupe: Intake and exhaust mods, T3 turbo at 24 psi, forced air IC, water injection, BPV, Ranger cam, subframes, etc., etc.
86 Tbird 5.0 (original owner): intake, exhaust, valvetrain mods, 100 HP N2O, ignition, gears, suspension, etc., etc.
05 Taurus SEL Duratec daily driver
04 Taurus Duratec (wifes car)
02 Pontiac Grand Prix GT
95 Taurus GL Vulcan winter beater
67 Honda 450 Super Sport - completely customized
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