figured it out.

 

I had an issue on my 97 4.6 a couple of years ago where I would come to a stop sign or light and my engine would bog and sometimes kill. In neutral, it would idle fine. I would look at the IAC (Idle Air Control Valve) and possibly the TPS(Throttle Position Sensor). Highly unlikely that its the TPS, but changing of the IAC valve fixed my issue with the engine killing at stop signs. If its the same setup as the 4.6, the IAC is on the top driver's side of the throttle body. If I'm not mistaken, it has a wiring harness connector on the top of it and kinda looks like a small aluminum canister that points to the front of the truck. This IAC is nothing but a butterfly valve and they can get pretty gummed up and stick open/closed. I have cleaned mine before replacing, but it seemed that it didn't take long for it to start acting up again.

I tried googling a discussion on IAC valves, but this is the best I could find so far. http://www.f150forum.com/f12/idle-ai...um-leak-10203/

 

Well, the first lesson you should have learned is not to rely literally on what the code discriptions say as the store had done.
Codes tell you what system is in fault, "not what part to change".
Many store employees are not trained machanics on every make and model.
In fact in CA they are not allowed to do this at the stores.
When you need help, provide codes if possible. We will interpet them for free.
The other half of this situation is knowing what the systems are and how they work. This is where the meat of the matter hits the road.
For example; in your case the IAC "idle air controller" is not keeping control of the engine idle as it should.
Poor idle in gear is the loading from the transmission converter and anything else adding load to the motor.
Go into neutral and you remove this loading, is what you experience.
Clean this part. It's not in the codes to tell you this. You just have to know.
Good luck.

 

Check your lines too. My 4.2 was doing this and it was due to some funky rubber line that ran from under the throttle body to a junction beside the brake booster. The rubber was all eaten up and deteriorated. Also there is another hose right next to that one down there that makes a 90 degree and runs to the passenger side. Check that.

 

I would replace the IAC valve. Cleaning may do it . On a vehicle of that age , I would replace it. If you have plenty of time and not much money,
try cleaning it first and see if it improves. Then I would replace it. That
requires double the work. It is more than likely that that is your culprit
though.

 

got it. thanks

 

thanks all

 

Don't use brake cleaner.
A specific throttle body cleaner is made for the purpose.
The IAC needs to come off so you can operate the plunger to be sure it's free.
The throttle body blade and bore need to be free of any buildup. Hold the blade open so you can spray the whole area.
After reassembly do the check to see if the IAC is working by removing the plug while the motor is at idle. The idle should go low indicating the IAC was working when plugged up.
If you still have an issue, look elsewhere for a vacuun leak.

 

http://img826.imageshack.us/img826/7069/83677099.jpg

 

I am currently having the same issue with my 98 F150 4.6L. It idles fine when in P or N but as soon as I put it in gear it idels very rough. I have checked the voltage at the IAC and even replaced it, tested the TPS (Throttle Position Sensor) and MAF (Mass Air Flow sensor) and also replaced my fuel filter. None of this has helped. The probe tester I have isn't a very good one and isn't suitable for backprobing the sensor connectors on the harness side so I wasn't able to fully test the TPS and MAF but they are getting the right signal from the PCM. I am going to replace the TPS and see if that helps but first I might check my plug wires and coil packs to make sure they are dry. Is there any way to test the coil packs? Well back to troubleshooting. Just thought I would get on here and see if I could find something I haven't checked yet. Hope you all don't mind me piggybacking on your thread. I will check back periodically to see if anyone found a solution or if I find one. Happy troubleshooting!

 

well i replaced

 

Update: I purchased a new multimeter and was able to backprobe the TPS and MAF sensors and they checked out good. Also picked up new plugs so I am going to change those and see what happens. It has been quite a while since they have changed so we will see. Also, I forgot to mention earlier that my engine light has not come at all since this has started happening. Has anyone else experienced this. I haven't taken it in to the dealership yet to see if they can extract any codes. It is a pretty far drive and not sure if I want to chance the drive. And the troubleshooting continues.

 

plenty of vacuum at the PCV (Positive Crankcase Ventilation), havent checked the EGR (Exhaust Gas Recirculation) yet though

 

@ 98FordTruck,
I replied to your other post but it seems like you found the same thing here lol.

If its not the IAC, could be a vacuum leak, but if its idling rough, I'd figure it would be a pretty obvious leak. Have you checked out the fuel filter, plugs/coil packs/COP? Injectors unplugged/faulty?

 

My position still is not to throw parts at it in hopes of getting lucky.
Do this and give a report on the results.
With the engine at hot idle, is the RPM about 650 to 675? yes or no.
Then remove the IAC connector, does the Idle drop low? yes or no.
If yes to hot idle RPM and yes to RPM drop, the IAC is working ok.
Here is how the idle control works:
The PCM pulses the IAC to open it enough to detect the proper RPM from the crank sensor.
When this is attained, the Idle is controlled to within +/- 50 rpm based on this loop feedback circuit.
When there is a load change, the crank sensor detects it, sends this change to the computer and corrects it back to spec by opening or closing the IAC a small amount.
I would power down the PCM about 5 min. then reconnect and test the start/idle again to see if there is any change.
We are trying to get a handle on it for you if you will report accurately what the results are and not go into changing parts to color the issue.
Do not go to EGR system looking as a source of poor idle control. That system is not active at idle or open because it would cause you very rough idle and a code. If inoperitive, it would set a code but have no effect on idle.
If you had a vacuum leak, the idle would likey rise and set a code from OX sensor detection of it.
Until you get to the cause, changing parts is pure luck and costly.