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gas mileage loss after tune up

  #1  
Old 01-23-2016, 01:19 PM
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Default gas mileage loss after tune up

vSo about 2 months ago my friend and i decided it was time for a tune up on our trucks. my truck is a 1994 Full-size Blazer 5.7 350. I replaced all the old spark plugs, plug wires, distributor cap and rotor, topped off fluids and while i had things off, I completely removed the throttle body to clean it. Also, I did the ti mod of grinding down the ridges on top of the throttle body.

Everything went back together fine, except that I didn't have my new gaskets so I had a roll of gasket material and just made my own for the throttle body as well as for the injector pods. Im waiting on new ones.

when i fired it up it fired right away and sounded great. the idle was better than before too. So as I drive and go thru a few tanks of gas, I notice my gas mileage dropping. I check the mileage every fill up.

It has now been about 7 fill ups and I have gone down from about 13.3 mpg, to now 8.6 mph. I don't understand what it is, and it goes down every time . I have not changed my driving style or my usual routes. A couple things I thought of, was the gaskets, if they are not perfect, will I get leaks thru there? Also, when I put the throttle body back on and connected the fuel lines, I wasn't sure which part the rubber washers/gaskets went on. Are they supposed to go between the gas lines and the coupler, or the coupler and the throttle body. If those were wrong, I'm guessing that would case a leak.

Let me know what you guys recommend. Thanks!
 
  #2  
Old 01-24-2016, 05:13 PM
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no clue?

guessing its timing. Going to try to time it later this week.
 
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Old 01-25-2016, 05:50 AM
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Originally Posted by 94psBlazer View Post
vSo about 2 months ago my friend and i decided it was time for a tune up on our trucks. my truck is a 1994 Full-size Blazer 5.7 350. I replaced all the old spark plugs, plug wires, distributor cap and rotor, topped off fluids and while i had things off, I completely removed the throttle body to clean it. Also, I did the ti mod of grinding down the ridges on top of the throttle body.

Everything went back together fine, except that I didn't have my new gaskets so I had a roll of gasket material and just made my own for the throttle body as well as for the injector pods. Im waiting on new ones.

when i fired it up it fired right away and sounded great. the idle was better than before too. So as I drive and go thru a few tanks of gas, I notice my gas mileage dropping. I check the mileage every fill up.

It has now been about 7 fill ups and I have gone down from about 13.3 mpg, to now 8.6 mph. I don't understand what it is, and it goes down every time . I have not changed my driving style or my usual routes. A couple things I thought of, was the gaskets, if they are not perfect, will I get leaks thru there? Also, when I put the throttle body back on and connected the fuel lines, I wasn't sure which part the rubber washers/gaskets went on. Are they supposed to go between the gas lines and the coupler, or the coupler and the throttle body. If those were wrong, I'm guessing that would case a leak.

Let me know what you guys recommend. Thanks!
whens the last time you`ve serviced your transmission,whens the last time you`ve changed the oil?What about air pressure on your tires?do you check the air pressure monthly.
 
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Old 01-25-2016, 10:34 PM
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Originally Posted by Toocute350 View Post
whens the last time you`ve serviced your transmission,whens the last time you`ve changed the oil?What about air pressure on your tires?do you check the air pressure monthly.
Sorry I should have added that. I have not serviced the transmission in the year and a half that I have had the truck. I was planning on getting a flush done before winter, but never got to it. I check my tire pressure about every other fill up. I am a little OCD about it. And the last oil change was 2000miles ago.
 
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Old 01-25-2016, 10:57 PM
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Piece of advice for what it's worth - DO NOT do a total trans flush drop the pan and change the filter and the fluid that you removed by removing the pan only - by doing a total flush you will be removing all of the grit in the older fluid that the slightly glazed clutches need to keep from slipping - as far as your mpg goes I wonder if your homemade gaskets are coming apart slowly creating leakage
 

Last edited by odat; 01-25-2016 at 11:03 PM.
  #6  
Old 01-28-2016, 06:43 AM
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I did some research on that throttle body modification. Engineers did a ton of work on expensive airflow equipment to design those ridges. Bean counters would not allow extra material and more complicated casting unless it serves a purpose. I do believe that mod gets you that 50 cfm increase at wide open throttle. However it also changes the airflow across the entire rpm range. It's now different than what the ecm is programmed for. OBD I systems don't cope with alterations well.
Airflow is a tricky science. Dimpled golf ***** fly farther than smooth ones. I would put a stock throttle body back on with the correct gaskets and see if the mileage returns.
 
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Old 01-28-2016, 07:33 PM
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Since the TB enters into a large(ish) plenum before the air flow goes into the runners to the cylinders, the atomization doesn't occur at the TB, and the system works on a Mass Air Flow sensor, I doubt that the ridges perform any magical 50% increase in mileage function. It does not act like a carb.


Unfortunately, I don't know what the problem is though.
 
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Old 01-28-2016, 09:54 PM
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The 94 5.7 with throttle body injection uses a Manifold Absolute Pressure speed-density system. No MAF sensor on a stock 94 K.
The MAF systems measure the mass of the air going into the throttle body, where the Speed-Density systems calculate the mass of the air going in by measuring the manifold absolute pressure, inlet air temperature (crudely calculating relative air density), and engine speed. Both measure the position of the throttle blades and engine speed in order to determine the proper amount of fuel to deliver.
 

Last edited by Rusty Nuts; 01-28-2016 at 09:56 PM. Reason: shortened explanation
  #9  
Old 01-29-2016, 09:33 AM
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Right you are Rusty, I missed the '94 part. I have another car running an aftermarket ECU running speed-density. The TPS is monitored, but it only uses the rate of change for controlling the virtual "acceleration pump". The position is not used in a steady state condition. Maybe the GM OEM ECU does, I can't say.
 
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