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Old 01-29-2014
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Truck bogs down when started after sitting all night

As the title says, when I go to start my truck when the engine is cold or when it's been sitting all night, the engine turns over and fires and very briefly bogs down. It only lasts about a second and then it runs just fine for the rest of the day. If I push the gas a little while the engine is turning over, it doesn't bog down. Any idea of what it could be? The truck is a 2000 with a 4.0.

Thanks

Last edited by 00GreenRanger; 01-30-2014 at 12:26 AM.
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Old 01-30-2014
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I am going to take a stab and guess your IAC(idle air control) is sticking when cold. Happened to me on my 'oo and it fixed the problem. The problem I had lasted 5-10 seconds at first then in a few weeks it would bog and die just sitting at a stop light. The T.P.S. could also not like the cold start up
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Old 01-30-2014
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Try this first.

Turn key on count to 3 then turn key off
Repeat 3 times
Then start engine

Ford EFI systems/computer turns on the fuel pump for 2 seconds each time the key is turned from OFF to ON, this is to prime the system.

So what you are testing is the fuel pressure after sitting.
Each time you cycle the key off then on you are building up pressure, your EFI system needs 65psi of pressure.

Once engine starts computer will turn on pump again for a few seconds, so if you were leaking pressure the 2 second prime might bring it up enough to start, but it drops to low and engine stumbles until pump is turned on again.
Once engine is running you probably wouldn't notice a small pressure leak.

Could even be the check valve in the fuel pump, so no external leak to see or smell.
Fuel pressure just leaks back into the tank.


Normal startup for any EFI system should be like this
Foot off the gas
Crank engine
Engine starts and RPMs should be approx. 1,500 < computer has IAC valve open all the way
Engine RPMs should drop to approx. 1,000 after a few seconds < computer closes IAC a bit for cold idle.
As engine warms up computer will continue to close IAC until warm engine idle is approx. 750
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Old 01-31-2014
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Quote:
Originally Posted by RonD View Post
Try this first.

Turn key on count to 3 then turn key off
Repeat 3 times
Then start engine

Ford EFI systems/computer turns on the fuel pump for 2 seconds each time the key is turned from OFF to ON, this is to prime the system.

So what you are testing is the fuel pressure after sitting.
Each time you cycle the key off then on you are building up pressure, your EFI system needs 65psi of pressure.

Once engine starts computer will turn on pump again for a few seconds, so if you were leaking pressure the 2 second prime might bring it up enough to start, but it drops to low and engine stumbles until pump is turned on again.
Once engine is running you probably wouldn't notice a small pressure leak.

Could even be the check valve in the fuel pump, so no external leak to see or smell.
Fuel pressure just leaks back into the tank.


Normal startup for any EFI system should be like this
Foot off the gas
Crank engine
Engine starts and RPMs should be approx. 1,500 < computer has IAC valve open all the way
Engine RPMs should drop to approx. 1,000 after a few seconds < computer closes IAC a bit for cold idle.
As engine warms up computer will continue to close IAC until warm engine idle is approx. 750
So do you think the pump is bad? After that initial stumble on a cold start the truck runs fine. There is no external leak or smell.
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Old 05-18-2014
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Well now that I'm home from school I finally had time to narrow down the problem. I started with the inexpensive fixes. The first thing I did was check the fuel pressure and it was good. I then started to think it was the IAC like strakill said so I went and picked up some electrical parts cleaner, cleaned out the IAC and now the truck is starting up great. Thanks for the help guys.
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Old 05-19-2014
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Quote:
Originally Posted by 00GreenRanger View Post
Well now that I'm home from school I finally had time to narrow down the problem. I started with the inexpensive fixes. The first thing I did was check the fuel pressure and it was good. I then started to think it was the IAC like strakill said so I went and picked up some electrical parts cleaner, cleaned out the IAC and now the truck is starting up great. Thanks for the help guys.
Right on man. Good to hear.
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