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I have a 2003 Ford f150 4.6 Triton. For the last several months, sometimes when I started the truck, rpms would immediately drop to around 300-400. It would then kick back up to around 800 and run fine. A couple days ago, it got where it wouldn't come back up and wouldn't idle at all.

I found that if I give it a little gas to bring it up to 1500 rpms, keep it there for a few seconds, and slowly back off, it will idle but at around 600 rpms. Then, if I give it any gas at all, when I let off, it dies.

It's not throwing any codes. I pulled the iac valve. I hit it with a little wd-40 and seems to be working ok, although I'm not 100% confident on that. I messed with the MAF sensor. If I unplug it completely, it won't idle at all, even with me starting by giving it a little gas. Its completely enclosed in a pipe, so it is difficult to mess with this too much. If I pull that pipe out completely, it really messes with the airflow and won't run.

Without it throwing any codes, I'm at a loss. Any thoughts?
 

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No codes or no check engine light?
How many miles on the vehicle?

If there are no codes typically the issue is not an electronic engine control part failure. It is more in the realm of basic engine maintenance or basic engine parts.

A little on how the engine operates. With engine running at idle and you step on the gas pedal, you are introducing more air. You are also telling the computer (TPS) send some more fuel you want to go faster. The computer checks that with engine RPM (Crank sensor), air coming in (MAF) and some other sensors. To make sure the engine is burning fuel correctly the computer checks what is coming out (O2 sensor) to make adjustments to the injectors.

You step on the gas and this helps it also raises RPM. So a thing in motion tends to stay in motion and the faster it is in motion the more it tends to stay moving.

If there is a vacuum leak down stream from the throttle body, that air is not known by the system UNTIL the O2 sensor reports the exhaust is too rich because the mixture could not be burned because it was actually too lean to support full combustion.
The other issue may be the O2 sensor is still functioning (so it is not broke or not working) but the signal to the computer is slow because the O2 sensor is nearing end of life.
A clean fuel filter and air filter are also parts that can be factors if restricted that never trigger a code. To a lesser extent older spark plugs too. Not old ones that mis-fire, just old enough to be less than fully efficient to have a total burn in the combustion chamber.

Do not use WD 40 or any oil based product on the inside of the air intake system. It can cause system issues with the engine controls. Especially with sensors. If you are going to spray any chemical to clean the air intake parts, use throttle body cleaner.

If IAC or TPS were having issues that would show up as a code. Those are moving sensors that produce a code if they do not report enough movement. Non-moving sensors like the O2 and a wire type MAF only has to produce a signal to the computer when in operation.(Any signal will do for testing by the computer) If that signal is slow, the computer will not trip a code. MAF sensors need clean air to operate long term. If you want to do anything to it use a MAF cleaner spray and follow directions. And do not touch it!!! The oil on your fingers can be enough to cause issues. Nothing you can do with an O2 sensor that is slow or worn down.

>>>>>>>>>>>Action
 

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Discussion Starter #6
No codes or no check engine light?
How many miles on the vehicle?

If there are no codes typically the issue is not an electronic engine control part failure. It is more in the realm of basic engine maintenance or basic engine parts.

A little on how the engine operates. With engine running at idle and you step on the gas pedal, you are introducing more air. You are also telling the computer (TPS) send some more fuel you want to go faster. The computer checks that with engine RPM (Crank sensor), air coming in (MAF) and some other sensors. To make sure the engine is burning fuel correctly the computer checks what is coming out (O2 sensor) to make adjustments to the injectors.

You step on the gas and this helps it also raises RPM. So a thing in motion tends to stay in motion and the faster it is in motion the more it tends to stay moving.

If there is a vacuum leak down stream from the throttle body, that air is not known by the system UNTIL the O2 sensor reports the exhaust is too rich because the mixture could not be burned because it was actually too lean to support full combustion.
The other issue may be the O2 sensor is still functioning (so it is not broke or not working) but the signal to the computer is slow because the O2 sensor is nearing end of life.
A clean fuel filter and air filter are also parts that can be factors if restricted that never trigger a code. To a lesser extent older spark plugs too. Not old ones that mis-fire, just old enough to be less than fully efficient to have a total burn in the combustion chamber.

Do not use WD 40 or any oil based product on the inside of the air intake system. It can cause system issues with the engine controls. Especially with sensors. If you are going to spray any chemical to clean the air intake parts, use throttle body cleaner.

If IAC or TPS were having issues that would show up as a code. Those are moving sensors that produce a code if they do not report enough movement. Non-moving sensors like the O2 and a wire type MAF only has to produce a signal to the computer when in operation.(Any signal will do for testing by the computer) If that signal is slow, the computer will not trip a code. MAF sensors need clean air to operate long term. If you want to do anything to it use a MAF cleaner spray and follow directions. And do not touch it!!! The oil on your fingers can be enough to cause issues. Nothing you can do with an O2 sensor that is slow or worn down.

>>>>>>>>>>>Action
Great information! Thanks. I track some of these down and see what happens.
 

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That's a great video...now I just need to find someone who has a computer like that
Did you pull the IAC off of it and look at the pindle valve to see if it is seating properly
Some have a rubber tip on them and if the seat is uneven
It needs replaced
 

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Discussion Starter #10
Did you pull the IAC off of it and look at the pindle valve to see if it is seating properly
Some have a rubber tip on them and if the seat is uneven
It needs replaced
I looked at it. I cleaned the pindle valve a little because it appeared to stick a little. I didn't think about the rubber tip not seating well. I'll have to look into that.
 

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How many miles on the engine?

>>>Action
 

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If the oxygen sensors have not been replaced, replace all of them. Here is the reason;
you will not get a code for O2 sensors as long as they return a test signal. However O2 sensors are tasked with sending a signal (continously) to the PCM about the condition of the exhaust. They wear out over time (just like a spark plug) and the signal they generate slows down as they wear. The PCM uses the O2 signal to make air/fuel adjustments. However it the signal is slow the adjustments being made are not in sync with the actual situation. O2 sensors are accurate for about 100,000 miles.

Make sure the air and fuel filters are clean. The air filter is EZ. The fuel filter not so much. But of the fuel filter has not been changed in a couple years change it out.

if you have a gauge to test the fuel pressure do so at idle. Key on engine off I would want to see 40 to 45psi or more. After engine is started I would want to see 35 psi or more. Still might run at 30.

if you have dual tanks is there a difference switching tanks?

>>>Action
 

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Discussion Starter #16
If the oxygen sensors have not been replaced, replace all of them. Here is the reason;
you will not get a code for O2 sensors as long as they return a test signal. However O2 sensors are tasked with sending a signal (continously) to the PCM about the condition of the exhaust. They wear out over time (just like a spark plug) and the signal they generate slows down as they wear. The PCM uses the O2 signal to make air/fuel adjustments. However it the signal is slow the adjustments being made are not in sync with the actual situation. O2 sensors are accurate for about 100,000 miles.

Make sure the air and fuel filters are clean. The air filter is EZ. The fuel filter not so much. But of the fuel filter has not been changed in a couple years change it out.

if you have a gauge to test the fuel pressure do so at idle. Key on engine off I would want to see 40 to 45psi or more. After engine is started I would want to see 35 psi or more. Still might run at 30.

if you have dual tanks is there a difference switching tanks?

>>>Action
The O2 sensor was on my radar to deal with too but haven't made it there yet. I will definitely deal with the air filter. I don't have a gauge to test fuel pressure, so I will assume (and hope) it is good to go. I just have the single tank, so the dual tank is N/A.
 

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If the fuel filter has not been replaced now would be an excellent time.

The filters I usually always get Motorcraft brand filters.
The O2 sensors not so much. I usually buy those at Ebay for under $20 a piece. just make sure you buy the correct ones. O2s are non-moving electrical parts. Generics work just fine in my experience.

The parts above are non-electronic engine control parts except the o2 sensors. Because of that generally they don't trip a code is they are questionable. The O2 sensor doesn't trip a code either for the reasons stated above.

>>>>>Action
 

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Does the check engine light come on during he engine start routine?

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