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2008 Silverado miss and poor mpg

Old June 27th, 2019, 11:03 AM
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Default 2008 Silverado miss and poor mpg

Hello All,

I just purchased a 08 Silverado 5.3 with 170k miles. It ran fine until two miles from my house. It also used about 1/4 tank of gas to go 40 miles. When I got two miles from my house, the check engine started flashing and the truck was running real rough. I have the following codes: p0307, p0452,&p0121. I checked the compression on cyl 7 and got 150 psi. Next, I swapped the plug from cylinder 3 to cylinder 7. I started the truck after that and it ran fine. I know the problem still has to be there. Any suggestions on what to do next would be greatly appreciated!
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Old June 27th, 2019, 11:53 AM
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How did the plug from cyl 7 look when you removed it? Possibly just needs a tune up.
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Old June 27th, 2019, 11:59 AM
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It was a bit black. The plug from cyl 3 was a lot cleaner. Iím not sure why the miss went away after buttoning everything up.
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Old June 27th, 2019, 1:25 PM
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Classic symptoms of a bad throttle position sensor especially with the p0121 code. It's clipped onto the side of the throttle body. It used to only be available as a complete unit but you can now buy just the throttle position sensor from some parts suppliers. I had to change mine at 165,400 miles. Did it display any messages like reduced engine power on the console? When this started happening with my 2007 Suburban, it would flash the check engine light and display reduced engine power and would run horribly. After turning it off then starting it back up, it would go away for anywhere from 50 miles to 1000 miles. Also, at highway speed I could hear the butterfly in the throttle body slamming closed and the suburban would have a brief hesitation. Once replaced, the problems have never come back.

I can't say if that will fix your fuel efficiency issue though. You should average around 15 to 16 MPG combined. I get about 14.7 average combined. Was the fuel needle on the empty side of the gauge? To go from 1/4 to empty happens about 3 times faster than going from full to 3/4 in ours.

How's the compression in the other cylinders? Here's what I got recently while trying to determine if I had a torque converter issue or engine issue. Ended up being partially because of counterfeit spark plugs but mostly because of the torque converter clutch. The lowest number should be no more than 15% of the highest number.

Cyl Avg
1 175
2 186
3 187
4 188.5
5 188.5
6 178.5
7 196
8 182
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