Another MPG Thread

[CincinnatiFiero]

Why does mine suck? I am getting a best of MAYBE 25 highway about about 21 combined. In the City City I see as low as 13-14mpg for totally stop and go driving.

Within the last 500 miles:
Plug Wires
50k mile injectors

Within the last 2000 miles
Cap/Rotor
Fuel Filter

Within the past year, so 7k?
NGK UR5 plugs
NGK O2 Sensor

Past 10k
Coolant Sensor
Idle Air Control Valve
Thermostat (195)

The car does not have a cat, so that is not clogged. The muffler was blown apart by all the backfiring, so there is very little exhaust restriction.

Car has a CRX Cold Air Intake

When it warms up it idles at like 800-900, right where it should be, no vacuum leaks.

Car used to back fire and spit flames, I pressured the fuel rail off the car and the injectors leaked like crazy. I put in a used 50k mile set, I did not check them for leaks, but the car doesn't backfire and reek of fuel anymore, so I assume they aren't leaking. I've run a couple tanks of lucas injector cleaner to clean them out. But the mpg really hasn't improved.

The cold start did not leak when I pressured the line

Throttle body is nice and clean

I move the egr diaphragm around tonight to make sure it moved freely

I do not hear any ticking under load so I am pretty sure I don't have a leaky exhaust manifold.

Tonight I checked to see if the rear e-brakes were hanging up, the passenger rear was the only one I checked, it turned freely and I bled it just for good measure. I'll check the drivers tomorrow, but the car doesn't pull or feel anymore sluggish than a 2.8 should.

Someone suggested maybe the UR5s are too cold?

I have a shop scan tool, I just got it. I know I can run obd1 and 2, not sure if I can scan the obd0 fiero ecm. But the scan tool will tell me what all the sensors are reading, anything I should look for if I can scan it?

Thoughts?

[CincinnatiFiero]

First 6 are the same plug, last two are off the another plug. Only 2 plugs, both from the rear bank











They look okay to me? Especially for like 7k miles, the electrodes look really good. There is like white build up on the back, but they certainly aren't fouled.

[Cheeze]

I have the same question I guess. I bought a '87 GT 5 speed in California this summer while I was out there so I would have a car to drive. Plus I liked the whole not having a rusted terd thing. I beat on the car pretty good out there to see if anything would give out easily before the 2000+ mile trip home to Michigan. I drove it pretty hard up the coastal highway 1 (highly recommended if anyone gets out there) and got between 24-26 mpg. Drove it all over northern California and never got worse than that. Driving all the way across the country I got between 29.6 and bested 33.1. There is probably some error in there for when the pump automatically shutoff but I would say 30+ mpg easily all highway. Until I got to Michigan. I dropped to 27 mpg and have never seen over 25 recently, even driving like a granny on the highway. I honestly have no idea what happened. Colder denser air? Worse gasoline? I even tried some of those "home brew" higher ethanol blends in the corn states because I wasn't paying attention filling up in the AM hours and got better milage. Maybe my car is just saying "Fuck you, you brought me to the state with the shittiest roads in the union!" Post up if you find something that's not already said. I'd be interested.

[Aaron]

Swap a 3800sc I hear they get 45mpg. Well, only yours does.

[CincinnatiFiero]

I thought about doing a turbo 3.4, I hear they do 417whp, well, at least yours does.

[Jinxmutt]

Plugs look fine to me.

[CincinnatiFiero]

Mine doesn't even have an engine moron. But my jeep runs 12s.

Also, it's been well established that a turbo 3.4 is easily capable of 400+ whp on 10-15psi.

If you have no desire to help in this thread, kindly fuck off.

Yeah jinx the plugs looked okay to me, I've got a "bomz" (ebay) re useable filter, think I can use a K&N kit on it to clean it?

[Honest Don]

what kind of RPM are you turning at speed? Have you done a compression test lately? Maybe your right shoe is a little heavier?

[CincinnatiFiero]

I have never done a compression test on it, but since I run my oil 5,000 miles and when it comes out it is hardy black, I am assuming I don't have a lot of blowby. My oil getting really dirty would be indicative of poor sealing rings wouldn't it? When I change my oil light come through it and it has a amber color like you'd hope. I run semi-synthetic 5w30.

Around town, its probably 80% my right food, I drive a bit hard around town, but a lot of people say they can do 30-32 with a 5speed 2.8, I am 5-7mpg off the pace. At 65-70mph cruising, it can't be my right foot.

I don't know what rpm I turn at 65 in overdrive, but I want to say it is like ~2,500 or maybe that is at 70??? I remember it being close to what other people say they turn. My clutch has a lot of miles on it but it's not really slipping. I can find out tomorrow when I drive home to Cincy, but I was hoping you guys would have something I could try before I drive home.

[Aaron]

Oh, did a quick search reveal that most every 3.4 turbo Fiero is above 400whp? Because they are.

At 65-70mph cruising, it can't be my right foot.

It can easily be your right foot, even at cruising. If you're not smooth, or you are constantly in and out of the gas (Even a little bit), jamming the gas up hills, etc, just bad driving habits can cost a lot in gas mileage. Not saying this is you (Of course it isn't, everyone's a Ken Brock in their own eyes), but it's something to be cognizant of while driving.

Also, no one mentioned this, what about the brakes? Have you jacked it up and spun the wheels by hand to check for excessive drag? I had this problem on Lonzo's car when I did his swap, his e-brake was so poorly adjusted that it was "on" all of the time.

[CincinnatiFiero]

The passenger e-brake spins freely, I'll check the drivers before I head home

It's Ken Block, but yeah its a 2.8 135hp turd, it's not worth hotrodding too much, its not like I can do anything impressive lol.

[darkhorizon]

I think 21mpg was the most mpg I have ever seen anyone get with a 2.8, no matter what trans.

[Nashco]

I think 21mpg was the most mpg I have ever seen anyone get with a 2.8, no matter what trans.

Surely you can't be serious. I get that easily with mostly city driving, 2.8 and 5 speed trans. I easily get high 20s on the highway, usually 28-30 mpg. I've found that the most common source of poor fuel economy is a bad O2 sensor. Other common problems are bad coolant temp sensor, thermostat stuck open (or one that opens colder than stock), clogged/melted catalytic converter, and of course worn out spark plugs/wires/cap/rotor.

Bryce

[Atilla the Fun]

Agreed with the 02 sensor and cold coolant. My '89 S-10 4.3 gradually went from 28 mpg to 19 mpg when the 02 sensor died, and back to 28 when I remedied the problem.

[darkhorizon]

I think 21mpg was the most mpg I have ever seen anyone get with a 2.8, no matter what trans.

Surely you can't be serious. I get that easily with mostly city driving, 2.8 and 5 speed trans. I easily get high 20s on the highway, usually 28-30 mpg. I've found that the most common source of poor fuel economy is a bad O2 sensor. Other common problems are bad coolant temp sensor, thermostat stuck open (or one that opens colder than stock), clogged/melted catalytic converter, and of course worn out spark plugs/wires/cap/rotor.

Bryce

I am "serious" about what I said... its just I have not seen a whole ton of 2.8 fieros... Never said what was possible or not possible. Nobody I knew with one would drive it on the highway much either...

The few 2.8 cars in question were all 4speed/automatic cars, so I suppose if you put a 5speed in the mix somewhere, it would get marginally better.

[Atilla the Fun]

I'd say more than "marginally". The 4-speed gearing gives about 2570 rpm at 65 mph, the automatic gearing gives about 2760, while the 282 gives about 2260. Even this is still too much. Consider how the heavier F-cars, with power-eating automatics, were geared to do just 2000 rpm at 65 mpg when fitted with the 2.8L, surely a Fiero could do better if geared taller. Heck, I've driven a 2.0L non-turbo Eclipse, the second design, that could pull 1800 at 65. Heck, I had a '78 Camaro 350, with no functional centrifugal advance in the HEI, that did 25 mpg at 65 mph at 1800 rpm. With an automatic. Compared to that, a 2.8 Fiero has NO excuse for not doing better. My 28 mpg 4.3, with the aero of a brick, was doing 2000 at 65, with an automatic. My Bonneville 3.8 did 35 mpg, with an automatic, despite the weight. From all this, I'd expect a 2.8 Fiero to be capable of repeatedly giving over 35 mpg at a steady 65 mph. Otherwise, there must be something badly wrong.