Question about 1999 Chevrolet Silverado 1500

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Silverado 1500HD 6.0 idles rough ONLY when temperature outside is cold,around 40 and below. warmer outside temp no problems at all. replaced mass air flow after scanning computer. no codes are flashing. replaced whole fuel pump assembly 3 weeks ago including new pigtail,pump went bad. this idle problem has been going on long before pump went bad.

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Clean throttle body and air by-pass valve

Posted on Feb 04, 2010

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2003 1500hd chevy silverado 6.0 liter. I had a



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1 Answer

99 silverado wont start when below 35 outside when it hit 40 outside it starts and runs fine ????


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Jan 21, 2010 | 1999 Chevrolet Silverado 1500

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After fighting with mine (2003 GMC 5.3 liter), I believe I have it solved. Mine was throwing P0171 and P0174. Also, there was an occasional p0101. I had a rough idle sometimes right when I started it, but usually after 5 minutes when the fuel system went in to closed loop mode (about 200 RPM swings at idle that slowly went away). Days below 10 degrees were much worse.

I cleaned the MAF sensor, checked fuel pressue, changed spark plugs, and cleaned the throttle body. The code basically means that more fuel is being dumped in by opening the fuel injectors longer. The code is set by the long term trim on the injectors be over 23% (opened 23% longer than they should be based on sensor readings). The extra fuel allows the engine to run with the right air to fuel ratio (not necessarily lean as the code suggests). Causes could be low fuel pressure (from pump, fuel regulator, clogged injectors, and such). Running a scanner on it for several days, everything looked normal.

That is all background. The other cause is that air is being introduced into the intake after the MAF sensor. Extra fuel has to be added for the air that the MAF does not see. I did some digging and found out that the gaskets between the plastic intake manifold and cylinder heads are prone to leaking. I tried to spray throttle body injector around where it might leak to monitor the engine speed, but the air leak seems to be less on warmer days and leaks can occur on the inside edge of the gasket where it is hard to reach with the spray (there are foam ends that open the inside edges to atmosphere). I replaced the gaskets and my long term trim dropped to about 6% from 10% on warmer days to pegged out at 25% on cold days with a cold engine. The work isn't too bad as long as you are patient enough to get the plugs off of the ignitors. I have yet to test it out in cold weather, but I am pretty confident.

Jan 06, 2009 | 1999 Chevrolet Silverado 1500

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