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I recently towed a trailer with a car on it about 900 miles. As soon as I hit the mountains in PA, the SES light would flash slowly when coasting down long hills with the throttle closed. One I got back on the gas, it would stop flashing and the light would stay on steady. I have a ScangaugeII which showed P0300 for random misfire. I reset it a few times but it would always come back on most downhill runs. Once I got out of the mountains and reset it, it never happened again. On the way home with no trailer, it never came on once even in the hills. I did get some light pinging going up some long hills but otherwise, the truck ran great and never missed a beat whether the light was on or not. I did notice a very slight bumpy idle when I got back from the trip that wasn't there before, though......

My theory is an intake gasket leak somewhere. With the load of the trailer pushing me down the hills with the throttle closed, the suction being produced is causing a weak part of the gasket to leak and perhaps make it lean out a bit and miss a little as it coasts. Sorta like an old motorcycle when they are coasting and it "pops" out the exhaust.

Is my logic sound or does it fail? :lol:
 

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‘01 Silverado 2500HD 8.1/Allison 5sp xcab long bed
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3,857 Posts
Hey, Matt. Welcome to the forum. This is a ten year old post and the last post from TJ was 8 years ago.

You‘d do well to start a new thread describing your vehicle, the conditions, and what, if any, modifications you’ve done to your truck.
 
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