rough idle
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Joined: Jan 2013
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rough idle
my '97 f-250ld with 4.6l really idles rough at start-up but runs good after a minute or so, had issues in the past with coolant in one of the spark plugs blown head gasket i was told put in some blue devil head gasket repair and fixed it.all this after changing the thermostat. and now sometimes it acts like the fuel pump is going out because after driving for an hour or so and shutting it off and leaving it sit a while it will turn over but not start right away. left me stranded at the lake last week. had it towed then after unloading it from the roll-back ,it stared right up! any suggestions guys? love you guys!
Lead Driver
Joined: Sep 2006
Posts: 7,907
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I hope i'm not waisting my time typing but you have a couple issues.
Cold start is complex. One fault and you can have issues.
Here is how it works.
At key-on time, the computer looks at the values of the coolant sensor and the intake air temp sensor as the only way it has to know it's to be a cold start.
This sets the amount of fuel, opens the IAC and advances the ignition timing.
The fuel pump is separate. At key-on the pump runs about 1 second then shuts off. At 'cranking' the pump turns on again.
After the start, the idle can go higher according the ambient temp because the IAC was opened and timing advanced.
After about 1 minute +/-, the OX sensors get hot enough to take over fuel control.
You can see from this that you have to investigate the whole system to uncover and solve the issues. No guessing but go with how it works.
Good luck.
Cold start is complex. One fault and you can have issues.
Here is how it works.
At key-on time, the computer looks at the values of the coolant sensor and the intake air temp sensor as the only way it has to know it's to be a cold start.
This sets the amount of fuel, opens the IAC and advances the ignition timing.
The fuel pump is separate. At key-on the pump runs about 1 second then shuts off. At 'cranking' the pump turns on again.
After the start, the idle can go higher according the ambient temp because the IAC was opened and timing advanced.
After about 1 minute +/-, the OX sensors get hot enough to take over fuel control.
You can see from this that you have to investigate the whole system to uncover and solve the issues. No guessing but go with how it works.
Good luck.
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