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Rough idle misfire when started warm


it-broke-again

it-broke-again

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I have a 95', 4x4, 5-speed, 4.0l with 308,xxx miles

- I start the truck cold it starts up and runs smooth.
- if I run it for a while then shut off and shortly after, restart it it still runs fine
- if I run it for a while, shut it off, and restart it 15-40 minutes later it kind of pops and lopes but clears up after no more then 30 seconds of running

* I replaced the fuel pump and filter
* I checked compression and it's even across the board
* no codes have been thrown
*only happens on start up

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RonD

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What did the spark plugs look like?

Start up is usually a Richer mix which can foul a weaker spark plug when warm, it does come back and work as you describe.


IAC(idle air control) Valve is what computer uses to set the idle, idle screws can't work on fuel injection.

When you turn on the key the computer will open the IAC valve all the way, maximum air for starting.
So if you DO NOT touch the gas pedal, then when engine starts RPMs should go up to 1,500+ then drop down fairly fast as computer closes IAC Valve to set "target" idle for engine temp.

Manual trans warm engine target idle is 625-650rpms(AC off), if higher you may have a vacuum leak.
Cold idle is based on temp, usually about 1,100, colder higher, warmer lower

When you start the engine warm does the idle go high then drop down?
If not then clean the IAC Valve.
If IAC valve isn't opening all the way then fuel mix is even Richer, less air, so more likely to get warm start misfires.


If warm engine idle is 700+ RPM then with engine warm and idling unplug the wires on IAC Valve.
Idle should drop to 500 or engine may stall, either is good, it means no vacuum leak, if idle doesn't drop then you have a leak.
 
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