engine light flashing at 65 to 70 mph

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engine light flashing at 65 to 70 mph - 11/16/2007 3:21:14 AM    03chevyrustbucket   Posts: 22 Joined: 10/27/2007 Status: offline My 03 2500HD when going down the highway at 70mph my check engine light will start to flash. After the first time it flashes it only seems to flash when the engine is not under a load like when your going down a hill. I took it to the dealer and the code said it was cylinder #7 missfiring, they checked it and said there was nothing wrong with it and they sent me on my way less 70 bucks. Note: if I stay under 65 the light will never come on. I have changed the fuel filter and added injector cleaner to the gas with no luck. Any help would  nice. Thanks P.S. the truck is 6.0 engine with only 35k miles Post #: 1

RE: engine light flashing at 65 to 70 mph - 11/16/2007 9:06:38 AM    ZX1100F1 Posts: 2429 Joined: 8/1/2006 Status: offline If water is present in your fuel it is very common for the the #7 cylinder to misfire due to the design of the fuel rail water will pool in the #7 enjector area. The fuel rail should be drained into a mason jar and the fuel inspected for water contamination, if water is found then you should have the fuel system proffesionally cleaned and purchase your fuel from a differetn station in the future. You can also try adding several bottles of fuel system dryer to the tank, but if water has been present it may be that the #7 injector is damaged from rust.  (in reply to 03chevyrustbucket) Post #: 2

RE: engine light flashing at 65 to 70 mph - 11/16/2007 9:36:49 AM    03chevyrustbucket   Posts: 22 Joined: 10/27/2007 Status: offline Why does it only do it at the speeds I said < Message edited by 03chevyrustbucket -- 11/16/2007 9:37:33 AM > (in reply to ZX1100F1) Post #: 3

RE: engine light flashing at 65 to 70 mph - 11/16/2007 10:07:38 AM    ZX1100F1 Posts: 2429 Joined: 8/1/2006 Status: offline It is likely just slightly misfiring (always) but is probably so subtle that the ECM can't detect it until the RPM's are up while under a load when the misfire condition becomes more pronounced. (in reply to 03chevyrustbucket) Post #: 4

RE: engine light flashing at 65 to 70 mph - 11/16/2007 7:58:12 PM    santaman58   Posts: 6 Joined: 11/10/2007 Status: offline Just info that might help. I was at dealer for an idle problem and the dealer pulled a report on 6.0 liter motors for that era and there was something there about possible factory defective #3 cylinder spark assemblies/coils. If I were you I would check the price of the #7 spark assembly and if it isnt too expensive, just change it anyway. Might even find one in the junk yard for reasonable amount. It might fix the prob if the fuel cleaning things didnt work. (in reply to 03chevyrustbucket) Post #: 5

RE: engine light flashing at 65 to 70 mph - 11/17/2007 1:31:10 AM    03chevyrustbucket   Posts: 22 Joined: 10/27/2007 Status: offline Thanks (in reply to santaman58) Post #: 6

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