98 gmc sierra-poor fuel economy

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  #1  
Old 01-17-04, 08:01 AM
reyes45's Avatar
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98 gmc sierra-poor fuel economy

I have a 98 sierra 1/2ton p/u,4.3 vortec .f/i, a/t.
For some reason , all of asudden I lost 3 to 3 1/2 mpg.I got no check engine light . I put in new ign. coil, new wires,and plugs. It also has all three o2 sonsors new(5000 mi. old) and air filter. what are your sujestions. Please help. Thank you so much.
 
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Old 01-17-04, 11:09 AM
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Did you check your air pressure,alignment and brakes for a dragging concern?If you have or can obtain a fuel pressure guage check to see if the fuel presure drops while the truck is turned off because the pressure regulator may be leaking.A quicky test is to remove it and tap the vacuum port on your hand if gas comes out it's bad.Fuel pressure regulators are getting to be common.
 
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Old 01-17-04, 04:00 PM
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Since the fuel mixture is always regulated at 14.7 -1 buy the computer, the only way to change fuel mileage is by the aggressive style of driving or the amount of work the engine is doing such as how much load, how much wind resistance, how much wheel drag due to brake drag , low air pressure in the tires or wheel alignment causing wheel drag. The engine likely isn't the issue. Does the engine feel the same way it always did? There's always the possibility of one of the TBI injectors is messed up and not spraying correctly. That's about the only engine related problem I can think of.
 
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Old 01-17-04, 11:12 PM
mike from nj
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even a torque converter not locking up could cause this, although it's not usually as severe as 3 mpg, more like 1-2 mpg. this should trip a check engine light though.

have you had the truck scanned for codes, sometimes there are some stored that won't trip the light right away, because they don't fail the monitor test everytime, but the codes could be there and would be a big help to you. also knowing your long and short term fuel adaptives values would be a big help(scan tool only)


davo----is the regulator inside the manifold? i remember seeing two of these (vortecs)in the shop over the years, one was the injector (spider) was leaking, the other was the plastic fuel line was leaking. both are inside the intake manifold, and led to a rough running truck. could either of these be just starting to leak? without tripping a light?

just curious
 
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Old 01-18-04, 04:14 AM
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Originally posted by mike from nj

davo----is the regulator inside the manifold? i remember seeing two of these (vortecs)in the shop over the years, one was the injector (spider) was leaking, the other was the plastic fuel line was leaking. both are inside the intake manifold, and led to a rough running truck. could either of these be just starting to leak? without tripping a light?

just curious

That's pretty common on those "W" motors but almost always sets a CEL when the adaptives reach their limit. Until then, it doesn't usually cause any problem. It is usually only found when looking at the data.
 
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Old 01-18-04, 06:40 AM
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Oops yes it's in the upper intake but usually what we find is the regulator leaking fuel.If it has been doing it for awhile pull the spark plugs and if the right side are black and the left side is clean the pressure regulator is leaking.The design of the intake forces all the fuel to the right side.The injectors may also be starting to clog very common problem also.This may sound funny but I don't see to many 1998 vehicles anymore because they are too old(out of warranty),but I'm already fixing broken 2004 cars and trucks.
 
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