Mileage after lots of new and existing mods.

[4player]

So I recently did a lot of work to my rig. its 94 Blazer 5.7L. It has fairly new cap, rotor, plugs, wires, pcv (last 6,000 miles)

Existing mods....
throttle body spacer
fuel injector spacer (little plastic piece that goes between the fuel injectors and throttle body)
Ultraflow 3" flow thru stainless steel
Stock 3" exhaust

What is new is,

rebuilt Throttle body (all gaskets replaced, all springs and filters replaced)
new EGR
ultimate TBI mods
punched cat (it was plugged)

Before all this I was getting 14 on the highway and 10-12 in town

Now I am getting 12 on the highway and 9-10 in town.

Do you think I don't have enough back pressure?

 

[NotARealK2500]

I'm not going to delve into the nuances of "back pressure", but I don't think it's likely to blame. I'd rather have a smaller-diameter exhaust with higher exhaust velocity than a larger exhaust (3"+) and not be able to evacuate exhaust as efficiently. That being said, I don't think a wider exhaust or more "open" intake may necessarily be to blame.

Prior to doing similar mods (TBI mods + port/polish, TBI spacer, shorty headers to 3" pipe and 4" dump, etc. etc.) I was getting about 12.3mpg. I'm now getting about 10.7mpg over the last 5,000 or so miles, which seems pretty similar to what you're experiencing.

Didn't realize this prior to studying Automotive, but the Inland Northwest especially receives fuel tailored to the environmental climate; winter blends contain fewer heavy molecules (i.e. fuels with larger hydrocarbon chains) that contribute to increased fuel economy, and the potential RVP of winter gasoline is higher than summer gasoline (doesn't vaporize as easily). Similarly, increased resistance in lubricants during start-ups, and longer run times necessary to reach operating temperature, etc. etc.

I'd double-check timing and keep on trucking. Check again when the weather starts getting better and I'm willing to bet you'll see a difference.

 

[slowburb]

I don't buy the backpressure idea either. Off the cuff, could your old TBI have been restricting fuel flow thereby consuming less fuel? Possibly the TBI mods have increased fuel flow. Either way, I suggest you're feeding it more fuel now, hence the lesser MPG.

 

[99'Subourbon]

I don't buy the backpressure idea either. Off the cuff, could your old TBI have been restricting fuel flow thereby consuming less fuel? Possibly the TBI mods have increased fuel flow. Either way, I suggest you're feeding it more fuel now, hence the lesser MPG.

This ^. My experience has been on older vehicles you 'spruce up', they were getting better gas mileage before because they weren't running right - doing all the mods i.e. removing any restrictions (factory fresh, or otherwise), helping it burn fuel better, increasing flow in, and flow out, in turn make the system use more fuel. Especially if you aren't changing your driving habits. You'd have to drive more conservatively, like seriously granny it, to see mileage gains from doing mods.

That's my experience anyway.

 

[4player]

I am having a few other problems too. I replaced the knock sensor with a new one and the computer says it is not working. The mechanic I took it to says it si working but that the computer is bad and can't recognize it. I plan on replacing the computer here in a month when I have the funds.

 

[SkyHighColorado]

I am having a few other problems too. I replaced the knock sensor with a new one and the computer says it is not working. The mechanic I took it to says it si working but that the computer is bad and can't recognize it. I plan on replacing the computer here in a month when I have the funds.

What was going on with the old knock sensor? Just old? Code? Brand? I highly doubt the ecu is not picking it up. if that's the case though, check the connection and wiring.

If you have a engine light on: bend paperclip into a U shape and jump the last 2 upper right top row pins on the obd port under dash. Turn the key to run (last click before starter turns over) and count the flashes of the engine light. It will flash one then pause, then flash 2 time quicker then longer pause = code 12. 12 = running system check for codes. Any flashes other than that is a code. eg: 3 flashes then short pause then 2 flashes = code 32.

reply back with results.

 

[jkeaton]

I have always seen 2, sometimes 3 mpg differences when refineries switch to their "winter blend". In all my vehicles.

 

[4player]

The old knock sensor worked but it got beat up trying to get it out so I replaced it. It was tested as working. We hooked the computer up to a test rig and it shows the main computer as bad.