Hello All,
I have a 99 F250 S.D. with a 6.8 in it. In general it runs great but I have a couple strange issues that neither I or my friend who is a Ford Technician can figure out. First on the list is when driving and once it is warmed up I go out on the highway and it runs great. Once I get off and driving in surface street traffic it accelerates and drives nicely but when I come to a stop the engine will not idle down to an normal idle. It sits at around 1400 - 1600 rpm. If I give the gas pedal a quick hit it settles down to about 800 but it will do the exact same thing as I get to the next traffic light. I have checked everything I can think of all of the sensors, the throttle body and also everywhere and anywhere for vacuum leaks. I have also had my friend who is a technician over at the local Ford dealer run a complete diagnostic on it and everything came back normal. We both agreed it seemed very strange that when you get to a stop light it idled so high but if you held the brake and gave the gas pedal a quick short hit it would idle down to a normal range. So I am looking for any other suggestions or possible ideas of what may be causing this issue as it is driving me crazy. So the other issue is with the DPFE as I have replaced it and the connector hoses, twice now, but I keep getting the code P1406 which says the hoses are either plugged or off !!! I have replace the DPFE itself twice and new hoses and also cleaned out the small tubes that run down to the EGR tube but it does not seem to take care of the issue so I am not sure what direction to go with this either. So again any suggestions or ideas for this would also be greatly appreciated. So thanks for reading an I look forward to hearing any possible ideas you all have !!

 

The Idle Air Control (IAC) valve may be sticking, try cleaning it. There are no diagnostic codes for it. It controls airflow into the engine at idle.
Also, inspect the Throttle Position Sensor (TPS) which might be worn giving intermittent incorrect readings. I don't think there are diagnostic codes for a slightly incorrect reading from the TPS.
It could be a small split in a vacuum hose. The bend in the PCV hose by the throttle body is a common fail point because the hose is not reinforced and connected to a hot fitting.

 

Hey thanks for the input !! I actually have already replaced the IAC and also the TPS. I tried the replacement with parts from Rock Auto and my buddy told me if they didn't work to get Ford OEM so that is what is on it now. The TPS can be tested for voltage as we got a Ford factory bulletin on it and it was within the parameters of the specifications !!

 

I wonder what happens if, as you are slowing through 20MPH, you put it in Neutral and continue to come to a stop as described above.

(Boilerplate: Have a professional driver do this for you on a closed course, don't try this at home, results may vary, past results not indicative of future outcomes, , yada yada)

 

If you are slowing down and put it into neutral at say 30 - 40 mph until you come to a complete stop it will sit at around 1400 rpm and slowly and I mean really slowly go down to around 800. We had the same thought and tried it out a number of times !!!

 

Have you cleaned the throttle body? Maybe the throttle plate is not completely closing.

 

Yep the throttle body has been off completely disassembled and cleaned, actually looks better than new lol, and all the clearances on it have been checked. I thought it may be a vacuum leak around the shaft but the clearances were tight there !!

 

So sounds like the same behavior. I wonder if the throttle pedal and/or cable are sticking. Plug a computer into it and see what throttle percent is showing while it is having the issue and while it is not.

 

Have you tried resetting the PCM by taking off the negative battery cable , insulate negative cable with a towel and short out between positive battery cable and negative to drain capacitors in the system....go through several complete drive cycles?

 

ECM has been reset a number of times !! When that didn't work I replaced the ECM !!!

 

Have you tried to see if the cable is sticking at all? Any obstructions on the pedal itself?

If you are back probing the TPS to check for smooth needle sweep on an analog meter, do it with the pedal and not by hand under the hood.

 

I know you said you checked and replaced the IAC, but do you by chance have a tuner device, like a sct x4, if you do you could check and see what the IAC valve is at in terms of the amount it is open, other than that I’m not sure