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5.7 vortec mileage?

I love my Chevy Chevette!
Posts: 4
Joined: 12/12
Posted: 12/09/12 03:19 PM

hi first time here. i have a 97 1500 pickup 5.7 got 18 mpg also have a 98 2500 work body 5.7 got 10 mpg no matter what kind of driving. i put the 2500 engine in the 1500 and expected better mpg in the lighter truck nope 10 mpg empty or hauling a trailer. i put that same engine in a 97 tahoe 10 mpg. what cud cause this? maybe the cam setup for the 2500?  

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I mow my lawn and find Chevys
Posts: 2253
Joined: 07/08
Posted: 12/09/12 05:43 PM

Sounds like there is a problem with the second 5.7.
I had a 93 5.7 in a dually, almost 3 tons, and it got better mileage than that.
I think there may be a fuel problem, do the plugs get black?
Or the cam installed is not right for that engine.  

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I love my Chevy Chevette!
Posts: 4
Joined: 12/12
Posted: 12/09/12 06:08 PM

motor came from 98 2500 with service body guy i got it from bought it brand new like that and i have swapped the intake from the 1500 thinking must be something with the fuel injection but runs the same. it has 306000 miles and is still strong but wont do better 10 or 12 mpg  

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I have an SS396 tatoo
Posts: 448
Joined: 03/12
Posted: 12/09/12 06:55 PM

well.... i am going to give you several things ..

do you have an OBD2 scan tool that displays live data and graphs the data stream...

plug into the better running truck...   observe the live data stream from the passenger seat..  while somebody else drives..

observe the short and the long term fuel trims... they should be during cruise.. + or -  5 to 8%.  if they are farther out than that .. you have issues..

please also check for trouble codes..

you will also want to look at the oxygen sensor wave forms at a fairly steady 2500 RPMs.. just get it close and hold the pedal steady... so you are not moving up or down..  the oxygen sensor wave forms should make a really nice repeating waveform.../\/\/\/\/\/\/\/\/\/\/\/\/\/\/\  instead of a /------------  or a \_________________________  

next... who set the distributer????    it takes an advanced scan tool ... generic versions usually won't do it...  there is a test to check the cam sensor offset from the crank...    i don't have the tests handy.. i will look them up...

on Vortec motors.. the crank sensor that controls the ignition timing is in the BOTTOM of the TIMING COVER...  picking up pulses off the tone wheel on the crank...  the computer sets the timing...

continuing..   the sensor in the distributer... is the CAM SHAFT position sensor..  it signals the ECM when the cam is in the proper position for the fuel injection to start its sequential spray order..  the CAM sensor to crank sensor offset has to be PERFECT...  and they can only be checked with an advanced scan tool.. a TIMING LIGHT DOES NOTHING .. as you are not adjusting the ignition timing.. you are adjusting the relationship of the trigger signals between the cam and the crankshaft position sensor..

with the distributer not properly set you will get usually one of these codes..

P1324 - Crank RPM Too Low
P1335 - CKP Circuit
P1336 - Crankshaft Position (CKP) System Variation Not Learned
P1345 - Crankshaft Position (CKP)-Camshaft Position (CMP) Correlation

you could have a worn timing chain...  allowing the cam to be retarded or the distributer to be retarded..

you could have a worn distributer drive gear. people have found them worn as thin as paper.. throwing p1345 's   and then with dead engines..  where the distributer drive gear stripped completely..  grab the rotor and twist it.. if it moves more than a fraction.. you may want to set the crank to TDC with the rotor pointing in the proper direction.. there is a \8/ on the inside of the top flange of the distributer housing..  there is also a \6/ there for V6 models.  the rotor should point to that and be right at it.. when the crank is at TDC..  this allows you to remove the distributer and either replace it with a new one or reinstall it.. and know where its going to line up to start it..

you can check the slop in the timing chain with a 6 point 5/8 socket on a breaker bar by rocking the crank back and forth from under the front.. if you get more than about 3/8 or 7/16 of an inch of movement on the outside of the damper  i usually put a plastic bag over the end of the breaker bar and put the extension on it or deep socket.. and another piece in the end of the socket to make it fit really tight on the crank bolt..  then you can really feel how loose the timing chain is...

lastly.. did you change the wiring harness with the engine??? and the computer with the engine...

did the ground wires get reattached to the back of the cylinder heads properly.. each of the individual ground wires supplies ground/electrons to different circuits..

if you have a scan tool.. post that fact.. i will dig out the Vortec scan tool values..  of what a normally running motor looks like...

you could have a worn distributer housing bushing..

this image covers a lot of the scan tool values.. some of the terms have changed..


but it will get you close.. the block learn and the integator have been replaced by short term fuel trim and long term fuel trims..  

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