Help, POOR gas mileage

[84GNwith87eng]

Well its almost a year and a half, I've been trying to get my 84 GN running well, it has a 86-87 motor + trans, G80 posi, etc...its pretty much identical to an 86-87...

what is has: 30# lucas injectors, new sparks plugs/wires/02 sensor (ac delco), pcv, iac, high-flow cat, factor dp/turbo, maf pipe w/ stock maf, k&n 9" filter, rjc powerplate, pretty much close to stock <except it has an alky chip installed for tuning purposes)

the boost is turned down to 10psi while I'm getting it tuned and little things fixed here and there and doing spring cleaning

I get about 100 miles to a tank of gas. Just had fuel lines replaced under driver door, new walbro 340 pump + hotwire installed, and did most everything on gnttype's spring cleaning section, including cleaning out the intercooler.

why is my gas milage so poor?

 

[VadersV6]

Dude, welcome to the club. Ive been at this for almost a year now. When I started, I was getting 8mpg on the freeway. After replacing, tweaking, fixing a million things, Im up to 18 1/2mpg if I keep the speed down. This is without lockup working.
What are your rpm's at freeway speed. By that you can see if you're getting lockup.
You could have exhaust leaks that you dont hear. I did. I had a cracked drivers side header that I didnt hear a peep coming from, and a blown out gasket where the header mates to the turbo. A leak before or right at the O2 sensor (where mine was), will hurt mileage.
Replace your ignition module and make sure all misfires are gone. they kill mileage, and at cruise speed, you could have misfires and not know it.
I made my own PCV oil vapor catch can. I have a 90 degree angled PCV valve that I got off the shelf at a Kragen store. No idea what its for but it fits in the intake. I run a hose from there to the catchcan. From there it goes to a factory PCV valve, and then to the vacuum block. When the catchcan is full, my mileage goes down about 3mpg. Dont know why, but its consistent. It definetely helped with mileage.
I gained mileage by cutting off the rubber inlet scoop for the intercooler. I gained 3mpg, even though everyone claims BS. There is another guy on here who gained the same.
Do you keep your foot off of it?
I run lean cruise with my extender and it helps alot.
Does your fuel pressure stay up for awhile after shutoff? Mine takes 3-4 hours to bleed down to zero. Thats a sign of good injectors that dont leak. Ive actually had mine hold pressure for a whole weekend. This has happened maybe 5 times (that I noticed).
You didnt mention replacing the fuel filter.
Just look through all these things and your mileage should improve.

 

[buickpower]

bad o2 sensor can cause poor mileage, also it seems obvious but double check all fuel lines for leaks, can't hurt.

 

[84GNwith87eng]

well i'm sure the alky chip is hurting it a lot, but new 02 sensor should have helped it...i also had that driver header crack, had it welded and its fine now. can't see any vacuum leaks right now, and my intercooler shroud is off at the moment, and it'll prolly stay off. My idle A/F is 6 and fuel pressure, i'm not sure, should prolly get a gauge

I did have a lot of work done on the fuel lines, and i'm sure leaks ARE possible, but the tank was pulled and inspected when the new pump went in.

Would not having a fuel filter temporarily seriously hurt my MPG? Its going back in tomorrow... Sorry, forgot to mention that :frown:

 

[87gninpa]

first of all, get a fuel filter on there, thats just crazy to run without one. second, make 10% sure you dont have any exhaust leaks before the turbo, that will kill mileage. then if you have a scantool, make sure everything is in spec, especially 02 numbers and BLM. if you cant figure it out, im nearby in bensalem, you can bring it by if youd like.

 

[Hacksaw]

An idle A/F of 6? As in 6:1? That would be too rich. Has the differential ratio been changed? If you go numerically higher it will kill mileage. Vacuum leaks on the sensors are bad, especially to the MAP sensor, & leaks can confuse the ECM.

What codes do you get when you run the test on the ECM?

You're not running leaded gas, or gas additive with MMT, are you? They will kill the O2 sensor.

Does it shift into 4th, as well as the mentioned TCC lockup?

-Mike H.

 

[87gninpa]

wow, i didnt even see that....6:1 A/F??? it should be around 12:1 or 13:1.....if you get a chance give me a call and i can take a look at it for you.....i went through this last summer and now my mpg in the city is up to 18, highway 27. 215 206 3630