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Hello all,

Ive got 1999 Devile recently started giving me trouble on start up. Turns over great but if it starts it is really rough for maybe 15 to 20 seconds. Sometimes when it does start maybe for about 5 seconds it runs really rough until sounds like small backfire through throttle body. Thinking might be throttle position sensor ????  Runs great whenever it does start with no hesitations at all. Any ideas would be appreciated !!!!!

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I had an Oldsmobile Intrigue that did very similarly when it had a bad crankshaft position sensor.

Chuck

'17 XT5, '04 Bravada........but still lusting for that '69 Z-28

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A few things come to mind here.

Pop the vacuum line off the top of the fuel pressure regulator. If gas is present in the line or leaking the regulator is bad. IIRC from my past that could cause your issue. Regulator is located at the driver side rear at the end of the fuel rail.

Are you losing coolant?

Does it misfire on every startup or only after a cold soak?

Is the check engine light on or coming on?

GM FAN FOREVER

Nice, clean, luxury= fine automobile

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2 hours ago, rockfangd said:

A few things come to mind here.

Pop the vacuum line off the top of the fuel pressure regulator. If gas is present in the line or leaking the regulator is bad. IIRC from my past that could cause your issue. Regulator is located at the driver side rear at the end of the fuel rail.

Are you losing coolant?

Does it misfire on every startup or only after a cold soak?

Is the check engine light on or coming on?

Not losing any coolant, already took heads off and replace bolts with oversized studs. Check engine light is on but havnt had time to pull codes yet. 12 hr midnights makes time limited. Just trying to get some idea where to start when I do get some time off. It only started this after sitting for few days in cold weather ????

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Ok does it occur on each startup, After cold soak, or all the time?

If it happens on cold soak but clears up I lean toward something leaking down into a cylinder(s)

 

 

GM FAN FOREVER

Nice, clean, luxury= fine automobile

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3 hours ago, rockfangd said:

Ok does it occur on each startup, After cold soak, or all the time?

If it happens on cold soak but clears up I lean toward something leaking down into a cylinder(s)

 

 

Not every time at first but last 4 times started it did. Didn’t try it tonight but wouldn’t start last time tried yesterday morning 

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On 1/3/2021 at 9:04 PM, rockfangd said:

Ok does it occur on each startup, After cold soak, or all the time?

If it happens on cold soak but clears up I lean toward something leaking down into a cylinder(s)

 

 

Ok gentlemen, replaced fuel pressure regulator and still doing same thing. Tried to pull codes but really don’t know what kind of code looking for IPC, PCM, etc ?????

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Have you checked fuel pressure?

PCM codes. The first one on the list.

Was there evidence of gas in the vacuum line to the regulator?

I would pull codes before going any further. Could lead you toward the right direction. 

Could be fuel pressure, MAP, egr,  or other related. 

GM FAN FOREVER

Nice, clean, luxury= fine automobile

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Ok thanks

was no sign of any gasoline in vacuum hose but didn’t quite get regulator seated properly first time so fuel squirting everywhere so gotta

a back out for little bit. Getting plenty fuel there. Gonna give little bit to air out then I’ll go check out codes. Thanks

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unless you check the fuel pressure with a gauge there is no way to really say what is good or not. 

GM FAN FOREVER

Nice, clean, luxury= fine automobile

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