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Ford F250 Pickup 4x4 1995 ford f250 4x4 351 v8 5 speed.....when

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1995 ford f250 4x4 351...
1995 ford f250 4x4 351 v8 5 speed.....when its cold , right after it starts , the engine will not idle , runs rough , shakes , the exhaust smells rich but no black smoke....this continues for 2 to 8 minutes then gets smooth and idles normal....did fuel filter , ignition system perfect , coolant temperature sensor replaced , air filter changed , fuel pressure good , no injector leakdown noted , throttle body cleaned of buildup , throttle position sender replaced , no codes at all anywhere. i am a HD mechanic with gas/carburetor experience only.....there is a sender or sensor on the left hand side middle of the intake manifold threads into what i believe to be the exhaust crossover passage between heads.....the sender resembles the coolant temp sensor is appearence but has a different pin orientation....what is this sender/sensor for ???? any ideas on the rough start up ???? when it warms up 5 -8 minutes the engine is smooth and has good power and idles smooth.....horrible when it starts up cold 25 to 35 degrees F ....thanks
Submitted: 6 years ago.Category: Ford
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Answered in 4 minutes by:
3/1/2011
Ford Mechanic: Fordanswerman, Ford Motor Company Certified Tech replied 6 years ago
Fordanswerman
Fordanswerman, Ford Motor Company Certified Tech
Category: Ford
Satisfied Customers: 3,306
Experience: Ford Engine Master/Transmission Master/Chassis Master=Ford Senior Master also Diesel 7.3/6.0
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Thanks for the question.

 

The only thing I didn't see on your list above was the IAC (idle air control) solenoid. This controls all idle functions. You should have a high idle at startup and this controls the air flow to the engine.

 

I suggest that you remove it and clean it with a small brush like a toothbrush and some good carbon cleaner and then reinstall and unplug the battery for 15 minutes to reset the computer and retest.

 

Thanks

 

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Customer reply replied 6 years ago
and this IAC is located where ?
Ford Mechanic: Fordanswerman, Ford Motor Company Certified Tech replied 6 years ago

Thanks

 

It is located on the side of the throttlebody.

 

Thanks

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Customer reply replied 6 years ago
i will try this in the morning tuesday march 1 / 2011 and let you know what happens.......thanks from british columbia canada.......i will get back to you
Ford Mechanic: Fordanswerman, Ford Motor Company Certified Tech replied 6 years ago

Sure, no problem just come back to this same post.

 

Thanks

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Customer reply replied 6 years ago
i cleaned the IAC ...there was not very much carbon in the housing.......i noticed when i removed it (outside air temp 35 F and engine cold) that the valve was off its seat by a quarter of an inch....shouldn't this be closed off when engine is not running ? the cleaning out of the IAC did not help the rough startup and bad idle at all.......it will clear up and idle smooth after 45 seconds of running......you shut it off and restart and it will hard start and run rough again for 45 secs to a minute......runs great when its warms up for a few minutes.......anymore ideas what it may be ? what is the sensor i described to you in my second reply......LH middle of intake manifold and threads into what i believe to be the exhaust crossover between heads.......
Ford Mechanic: Fordanswerman, Ford Motor Company Certified Tech replied 6 years ago

Thanks

 

With the engine idling at normal temp, unplug the IAC and let me know what the engine does please?

 

Thanks

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Customer reply replied 6 years ago
i will let you know what happens............
Ford Mechanic: Fordanswerman, Ford Motor Company Certified Tech replied 6 years ago
Ok thanks
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Customer reply replied 6 years ago
thanks for waiting...i have to go into town to my sons truck to try the suggestions you have made........okay , i ran the truck for 10 miles and got it up to normal operating temp and let it idle and then unplugged the IAC harness plug...there was no change in idle speed or smoothness at all.....it was idling at the normal speed and smoothly , but there was no change.......i shut it down and let it cool off for eight hours , temp outside is 36 F and the engine was cold as the rest of the truck......i tried starting it up with the IAC unplugged and it still did the rough start and bad idling.....thought it might help your diagnosis.....any more thoughts ? when the hard start and roughness happens when cold , it will do it for 50 seconds to 2 minutes while you hold the rpm up at 1500 to 1800.......then it starts to run smooth over a 5 second period and then you know you can let it idle down and it does idle smooth and at the normal idle speed.....if you shut it off and restart again , it will fire up no problem , idles up to 1800 or so for 15 seconds and then idles down to normal , like it always does........this bad start / roughness for 1 minute only happens when cold..... i will be on the computer all night...it is 700pm b.c. time
Ford Mechanic: Fordanswerman, Ford Motor Company Certified Tech replied 6 years ago

Thanks

 

Ok, sounds like you might have a bad IAC. First make sure with the key in the run position that you have 12v at the red wire. If you do then replace the IAC and unplug the battery again to reset the computer and let me know how it turns out.

 

Thanks

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Customer reply replied 6 years ago
the key in the run position......is this with the engine not running ? ...... key on -engine stopped ?....... when you say unplug the battery to reset the computer , do you mean disconnect it and reconnect it. ?? .......... if the IAC is faulty , why does it clear up and run smooth after it warms up for a minute or so ? would it not run bad all the time or is this intermittent......
Ford Mechanic: Fordanswerman, Ford Motor Company Certified Tech replied 6 years ago

Great questions!

 

Yes, key on engine off just for safety sake.

 

Yes, disconnect the battery for 15 minutes this will reset the computer to factory default.

 

The reason it clears up after it warms up, is kind of misleading what is going on. When the engine is warm it doesn't need a high idle any longer. With a cold engine it needs a high idle or it will do what yours is doing. By you unplugging the IAC and it making no difference, the IAC is doing nothing, it is dead.

 

Thanks

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Customer reply replied 6 years ago
what is causing the richness of the exhaust ?....... did you determine what the sensor on the intake manifold i described , is ? ......it was all black and sooty when i removed it.......does the IAC control the increase in idle and return to low idle after startup ? ....the reason i ask , is because when its warmed up and is running smooth , if you shut it off and restart it will still do the high idle/return to low idle thing ,like normal , which would mean the IAC is functioning. ......when you got me to disconnect the plug from the IAC after a long run , it was hot and idling smooth and it did not change rpm or smoothness..........does the IAC remain inactive after the engine is warm/hot ? .....is the IAC only functioning and moving the valve , after cold starts ? .....
Ford Mechanic: Fordanswerman, Ford Motor Company Certified Tech replied 6 years ago

Thanks

 

The sensor I think you are describing is the IAT (inlet air temp) and yes this can cause issues and might not set a code, so go ahead and replace that if it looks bad.

 

The IAC when unplugged during idle should almost cause the engine to stall. So either the IAC is not working properly or someone has messed with the throttlebody idle screw and jacked up the throttle plates. So you can try to turn the set screw for the throttle plates back so the throttle plates close more so the IAC can do the job. If after that you unplug the IAC and the engine doesn't change and almost stall the IAC is bad and will need to be replaced. The IAC is part electrical and part mechanical. I think the mechanical part has failed.

 

Thanks

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Customer reply replied 6 years ago
tomorrow i will check for 12 volts at the IAC plug and reset the computer......then i will close the throttle plates more and unplug the IAC and see if it stalls or changes rpm......if i return the throttle plates adjustment screw to where it was originally , is this close enough or is there a procedure to set it exactly......i have read in manuals not to touch this factory setting..... is it that critical ??....can it simply return it to where it was ?
Ford Mechanic: Fordanswerman, Ford Motor Company Certified Tech replied 6 years ago

Thanks

 

Just back the screw down 1/2 turn at a time. This screw was never meant to be messed with it was suppose to be set from the factory.

 

Thanks

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Customer reply replied 6 years ago
i will let you know what happens friday night if you are online.................
Ford Mechanic: Fordanswerman, Ford Motor Company Certified Tech replied 6 years ago

I will most likely be online then.

 

Thanks

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Customer reply replied 6 years ago
i replaced the IAC and the engine still runs rough and rich on startup for one to two minutes......you have to hold the rpm up at 1800 to 2000 to keep it from shaking and stalling.......at some point the engine runs smooth again and you know that you can release the throttle pedal and it will idle smoothly at 700 rpm....any other suggestions??
Ford Mechanic: Fordanswerman, Ford Motor Company Certified Tech replied 6 years ago

Thanks

 

Earlier in the post you said the fuel pressure was good, what is the actual fuel pressure?

 

And there is still no codes?

 

 

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