Question about 1999 GMC Sierra

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Hard start when air temp is cold, backfires when warm

The engine is a 5.3 liter. Truck wont start or has a hard time starting when the air temp is below 32. Sometimes the truck starts and stumbles at idle and will die doesnt matter what the air temp is.
When truck is under load sometimes it will stumble or backfire when you go to take off. this problem doesnt happen all the time off and on.

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  • levin2008 Feb 01, 2009

    i have replaced the maf and didnt seem to help the throttle plates are clean. This problem doesnt do this every time 2out of 10 starts.

  • levin2008 Feb 04, 2009

    i have put it on a scanner and no codes all sensors where normall. The engine has 120k. I have had on fuel pressure gauge with 55 to 60 psi then did a fuel clean out. Has new plugs, also has individual coils i did a cylinder drop test and all where working. The only thing i can think of is a air leak under the intake and i have sprayed the top of the intake with starting fluid no increase. please help

  • levin2008 Feb 04, 2009

    If it doesnt happen all the time how could it be the fuel pump. Why wouldnt it start when it is cold and backfire when it is warm. is there a way to find out if the fuel pump is bad.

  • levin2008 Feb 05, 2009

    thankyou i think that it is a crankpostion sensor possibly. what sensor require for start up? map, temp,throttle position, crank, does my motor have a cam sensor. i have had a boat issue where it would sometimes start and the crankpositon sensor was faulty but no codes. this problem is driving me nuts. please help

  • Anonymous Mar 20, 2014

    engine turns over but it tries to start and also vilolent backfire

  • Anonymous Mar 26, 2014

    It's sporadic, sometimes loud, sometimes soft. Continues once key is out of ignition. Potentially air diverter or blend door? Seems to stop once temp is below 70, but not always. Help!

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Try this, remove (MAF) Mas Air Flow sensor,(be sure key is OFF) so you don't set a trouble code when disconecting wire conector, with some throttle body/air intake cleaner from the outlet side of MAF, spray the sensor clean, let dry, reassemble.Remove snorkle from throttle body, and with a old tooth brush, clean throttle body& plate with cleaner, may take a couple of cans if throttle is badly 'coked' up. Hope this helps.

Posted on Feb 01, 2009

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  • gasitjr Feb 02, 2009

    Sorry that that didn't work.Should get a diagnostics scan performed to help pinpiont this issue.Other possabillitys; coolant temp sensoor, fuel presure, old tune up, weak coil, crank position sensor. Good luck! I will see if any TSB'S exsist for this. How many miles on engine?

  • gasitjr Feb 04, 2009

    Even 1 or 2 psi to low on fuel pressure can cause this issue.I believe 62 is minimum.GM's are notorious for fuel pump modules. Most have updated modules and wiring harnesses.

  • gasitjr Feb 05, 2009

    A fuel pressure gage is how I usually check for pump issues.Should come up to full rated pressure with key turn to run from off position for about 5 seconds then drop only a couple psi and hold with no drop off for 30 minutes.With vehicle running pressure should be within normal range of pressure regulator. If pressure holds, pump check valve is ok.There is a known issue with the knock sensors under the intake, but this shouldn't cause a not start issue. When they fault there is a timming control issue that would set a code.There is also a TSB on dirty fuel injectors and a cleaning process but you said thats been done.Wish i could access the diagnostic program from here at home, boss tends to frown on the 'side work' at work if you follow my drift! This a good one with no codes and normal data! I will keep looking.

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