I've been slowly working through my 88 IROC replacing everything in the drivetrain over the past two years with top of the line parts. The engine is a tired 350 L98 TPI (150k miles)/700R4/2.77 posi rear.

It has a fresh tune up and it just passed California Smog with flying colors. I'm waiting for the last details to be finished on a pumped up ZZ4 replacement, so the only modifications that have been done to the original block is a mega ported plenum, 24# SVO injectors, 9th injector removed, and a custom PROM chip.

I started out with only 9mpg and have steadily worked up to my newest fuel reading of 13 miles per gallon and am just comfounded with why my mileage is almost 1/2 of what it should be able to get on the freeway. What am I possibly overlooking??? My rings are still in good shape; but my valve guides are shot and the main bearings loose- so I don't even hammer on it for fear of throwing a rod before I'm ready to do the engine install. the whole reason I picked a new ZZ4 design instead of a 383 is so I could still get great mileage for freeway trips......
Any thoughts that make a lick of sense??????
Nitro

 

Best highway mileage is usually achieved at a constant 55-60 MPH for a couple of hours. If your "highway driving" includes stopping and starting for traffic, you're not getting a true indication of highway mileage.

Passing an emission test is good, but doesn't mean the engine is in peak operating condition. As you acknowledged, the engine already has problems, and may also not be making reasonable compression. That will kill mileage faster than rich mixture or misfire. At $3/G, you can probably afford to freshen up the engine and save some money.

 

Considering the average NASCAR Cup Car gets 4 - 6 MPG (Full throttle at 190mph), and you are getting 9 MPG? I'd say you have a serious problem lol.

 

The driving has been alot of town and country style; not true highway. Fortunately I do have a hand built motor being finished shortly. But if I've got some sort of sensor issue or? I want to nail it before I transfer all the electronix over. Because I've been rebuilding it from one end to the other, a tank of fuel can last 2-3 months; but with a closed fuel system I shouldn't be getting any evaporation issues, I would think. I'm sure at a loss........ Dang! Nitro

 

I could still use some help on this please.......

 

Is there any chance that a work distributor bushing or a loose timing chain or a 160 degree thermostat can contribute to this type of issue?

Or, can the 2.77 rear gears/700R4 just be to tall for around town and country driving where the drivetrain needs to be up to freeway speeds to work out?? ( I did just drop in a 3.27 posi set up this week and next week the stage 3 700R4 gets installed).

Please, SOMEBODY give me some directions

 

If your car is operating below the normal termperature, the FI system might be going into open loop to get the temp up, so it'd run richer. I'm totally a NOOB when it comes to FI, but that comes to mind.
9mpg in town or on the highway? I get 8mpg in town, a 350 V8 driven like it should be in town isn't going to get great mileage. But 20ish MPG is normal on the highway, driving like a little old lady letting every semi-truck pass you. What do you get on the highway driving very consistantly?

No, lower numerical gears help your mileage across the board generally speaking.

 

Yes, yes, and yes !
Personally, 13mpg is my next target. Currently around 11.8, BUT....
Big block, in a 5600 pound Suburban, geared too steep.

Sounds generally like this motor is shot, and might just not do much better.