Poor mileage

[ssgoodman]

I just bought a 90 GMC Jimmy, 350 TBI, 700R4, NP208 and its getting horrible mileage, 9 MPG going from Minneapolis to Fargo.  after it sits for awhile i can smell the unburnt off gas, any suggestions on how to pull some better mileage with the Jimmy, 9mpg seems low.  also it does seem to idle rough and it died a few times on me, seems to be all related to a fuel issue but not sure where to start with it.

Sutton
i will post some pictures of it after i get some good ones was too busy yesterday.

[VileZambonie]

Tune it up & set the timing.

Give it a good thorough going through, make sure no vacuum leaks etc. From there you'll need someone with a scan tool to make sure all of your sensors are correct. The first thing I would check is fuel integrator and block learn. This will tell you if the ECM is adding more fuel...then check to make sure you aren't getting any false inputs...

9 mpg is pretty bad. You aren't in low range are ya? 

[ssgoodman]

man that was fast vile.  would the vacuum leak effect the gas mileage? 

i wish it was in low that could have explained the crap MPG.

[VileZambonie]

Absolutely. A vacuum leak creates a lean a/f signal and the ECM adds more fuel to compensate. A cooler thermostat or faulty ECT signal will cause it to add more fuel. High fuel pressure will cause it to use more fuel. A faulty or lazy O2 sensor signal will cause it to use more fuel. A faulty MAp Signal or leak in the MAP vacuum hose....etc...catch my drift? That's why the 1st thing I would do is check fuel integrator and Block learn after you do your prelimanries to verify if in fact it is adding more fuel.

[dumbucket1]

I only average 9 mpg in my carbed 350 blazer around town. I have never taken it on the highway though.

[Captkaos]

A TBI Blazer should get about 15 in the city.  You should be albe to get more out of a carb'd than 9mph if it is 2WD.  My built 454 got 12mpg

[dumbucket1]

not sure why I cant pull more than 9 mpg out of this thing. it runs mint, 700r4 and 3.08 gears so you would think I could get better mileage out of it.

[VileZambonie]

You'll actually get better mileage with 3.55 or 3.73 gears. The

[Stewart G Griffin]

The other thing(s) is/are, your jimmy is probably a few inches higher than a 2WD pickup or jimmy.  That alone    probably affects mpg a little.  Just the fact that it is 4x4 means probably 1-2mpg less than a 2wd equivalent (Can you guess that my truck is 2wd?)

Although 9 is low even for a 4x4 jimmy.

[Robry 87]

I have a 350 TBI 700r i get about the same but i no i've got a problem somewere also but i do not smell any gas and have no gas  leaks but i got good tip's today.

[ssgoodman]

i found a hole in my gas tank today which might explain alittle bit of the bad mileage but its not a terrible leak so probably doesnt effect it much.

[VileZambonie]

you'd be suprised. Think of it like this... 3 drops of oil from a leaky engine every 100 feet = 3 quarts of oil lost in a thousand miles. Gasoline has nowhere near the viscosity oil has... I'm sure you'll see a significant increase in mileage without fuel simply pouring out of your tank.

[ssgoodman]

i got the steel guard off from the tank and the straps are non existent, is the steel plate the only thing that holds the tank in or are there straps that either got removed or rusted off?  the leak is alittle bit less now that the plate isnt against it but i am going to have to remove the receiver hitch to get at the back bolts holding the sheet on.

[ssgoodman]

would the gas tank for a 1990 suburban fit into my 1990 jimmy?  i can only find suburban tanks at the junkyards here.

[TexasRed]

You try autozone?