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Dutch Pete

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PostSubject: MPG question   Wed Mar 09, 2016 3:46 pm

Seen a lot of MPG topics come by, this one has to do with the replacement of the head gasket. Engine is a 93 350 TBI with 183k. Replaced the head gasket, t-stat, radiator, all other necessary gaskets, placed exhaust man gasket, milled the heads to make sure they where straight.
Now the sad thing in which I need the groups advice. Mpg went from a steady 22 to 17. I can't figure out why. Did I overlook something? Engine runs good and strong, starting both warm and cold is good.
I don't think an OBD I "learns" again if the battery was disconnected and if it does, it's sure taking its time. Did 2000mi now after the rebuild, both in and out of town. All other is equal, driving habits, cruising speeds.
Any advice?
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bigredwagon

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PostSubject: Re: MPG question   Thu Mar 10, 2016 3:44 pm

Did you check for any unconnected or loose vacuum or emission lines?
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Dutch Pete

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PostSubject: Re: MPG question   Thu Mar 10, 2016 4:35 pm

yep, did that (twice)
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Sprocket

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PostSubject: Re: MPG question   Fri Mar 11, 2016 11:29 am

a vac line could be cracked too. or an O2 sensor may have gone bad causing a rich condition.

Different beast but when I bought my 94 Buick I was getting about 12 mpg. two things were wrong.
One was a cyl not firing (turned out to be the cap had an issue causing a short on #7), the other was a bad 02 sensor telling the computer the exhaust had too much O2 so it was dumping more fuel into the motor.

I found the short first and went from 12 to about 16 and then the 02 gave me another 2 or 3 mpg....

BTW you can pull the o2 sensor and bench test it with a meter (google search it), so you don't have to buy one first.
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convert2diesel



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PostSubject: Re: MPG question   Sat Mar 12, 2016 1:30 am

Still on the vac side, check your evap system. Had similar problem on my older TBI cadillac engine. Turns out I had the vac lines mixed up with the result being the purge valve was open all the time. Not enough leakage to effect performance but enough to throw off the manifold pressure sensor. Actually discovered the solution after the gas cap one way valve failed closed and almost collapsed the tank.

Also check your PVC valve. Even if you have replaced it, at the price, there's a outside chance you got a bad one.

Anything that effects your MAP sensor will also effect your fuel mileage. Doesn't take much to throw off your mileage if the MAP is lying to the ECM.

Bill
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Dutch Pete

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PostSubject: Re: MPG question   Sat Mar 12, 2016 4:20 am

thnx for the advice
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