1999 mercury mountaineer runs rough when engine is cold

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ronnie310
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Joined: Tue Apr 15, 2014 12:51 pm

1999 mercury mountaineer runs rough when engine is cold

Unread post by ronnie310 » Tue Apr 15, 2014 1:12 pm

I have a 1999 mercury mountaineer 4.0 sohc v6 engine. it is hard to start and runs rough in cold weather until engine warms up, acts like old carb engine that the choke does not work on. has no codes logged. I have tried to check the iat sensor, but there is not one in the intake pipe where the book that i have says it should be. truck has alot of miles on it, but runs good when it warms up and does not use any oil. can anyone help me/ thanks

 

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carriedi
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Re: 1999 mercury mountaineer runs rough when engine is cold

Unread post by carriedi » Tue Apr 15, 2014 5:48 pm

when it is cold and running rough you might want to check for a vacuum leak into the intake manifold.
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ronnie310
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Re: 1999 mercury mountaineer runs rough when engine is cold

Unread post by ronnie310 » Tue Apr 15, 2014 6:39 pm

I will recheck for leaks, but it does not act like a vacuum leak, it idles too smooth when engine warms up. also it cranks and runs smooth when outside air temp is warm. what about the intake air sensor. thanks

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carriedi
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Re: 1999 mercury mountaineer runs rough when engine is cold

Unread post by carriedi » Wed Apr 16, 2014 2:28 am

the vacuum leak may go away as the engine warms up and parts expand with heat. Just as an exhaust leak may be present on a cold engine but goes away as the engine warms up. As the leaks worsens it may eventually become too big such that it will continue to show up even after the engine warms up.

what makes you think you have a problem with the IAT? before you worry about a bad sensor you should scan it's value. With the engine cold (like first thing in the morning before you start your engine) scan your data stream. your coolant temp and your air temp should be within a couple of degrees of each other. Once you start your engine the temperatures will change at different rates so that test will no longer be valid.

if your mass air flow sensor uses all six wire connections the IAT is built inside MAF
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ronnie310
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Re: 1999 mercury mountaineer runs rough when engine is cold

Unread post by ronnie310 » Wed Apr 16, 2014 9:09 am

thanks I will do that

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