Rough idle and rpm drop 2001 Expedition EB 4.6

[genuinegemini]

Hey everyone. I have an 01 Expedition EB with the 4.6 and I'm running into an issue after some general maintenance. This is actually my sisters truck and she was having some body roll issues and an rpm drop/surge almost stall when holding the brake while in gear. I replaced all of the suspension/steering components except the springs (no air suspension) and it rides really good now. Some general research on the rpm drop/surge issue indicated several different possibilities. So I started replacing things to try and resolve the issue and eliminate potential causes. I know it's not the best path to resolution, but it is what it is. So far I have replaced the brake booster, the TPS, the EGR valve and DPFE sensor, the evap purge valve, the vent solenoid, the plugs and coils, the fuel filter, the mass airflow sensor, and the bank 1 up stream O2 sensor. Some of these were replaced based on research related possibilities and some were replaced due to codes. After replacing the aforementioned parts the truck has a rough idle, almost like a miss, and still has the rpm drop issue when holding the brake. I sprayed carb cleaner around various areas of the engine, like the intake manifold, various vacuum lines and hoses, and the brake booster while it was running and the rpm never changed indicating it wasn't sucking air that I could tell. I disconnected the battery for 20 mins and then reconnected it and drove it around town for about 10 minutes or so. The truck runs fine when accelerating and going down the road. I put my little scanner on it afterwards and got three codes indicating a lean condition on bank 1 and 2 P1071, P1074, and an excessive flow from the EGR P0420. Short of considering the new EGR valve is stuck open I'm not sure what else to look at or consider. If anyone has any knowledge or insight to share on what may be going on or something I've missed, I would be very appreciative. Thanks in advance for you time and attention.

 

[jr1under]

After replacing the aforementioned parts the truck has a rough idle, almost like a miss, and still has the rpm drop issue when holding the brake.

What about the IAC valve? Some of the truck's symptoms match what happens when that valve is failing.

 

[JamaicaJoe]

I have had a somewhat similar battle. (without lean codes) . I did find that the EVAP Purge hose from the manifold to the Purge Valve (on the firewall) was crusty and probably leaking vacuum. I would replace it just to rule it out. Also check your intake between throttle body and MAF for any leaks due to cracks, especially in the corrugated sections.. I have used FORSCAN and found that some of my troubles may be that the battery does not maintain a sufficient voltage right after cranking. Clearing the keep alive memory (KAM) by shorting the battery leads together (after removing the negative lead) restores drive ability. Right now I am focused on vacuum leaks because it seems the AC is not blowing cold, so there is some sort of blend door or vent that might not be moving.

As far as the IAC , on mine, removing the connector from the valve dropped the idle way down, so I think that means it is doing its job.

 

[JamaicaJoe]

I will add that if I run out of things to do, I may have my local shop do a smoke test because I really don't trust all the crappy hoses on the manifold vacuum of these old trucks.

 

[genuinegemini]

What about the IAC valve? Some of the truck's symptoms match what happens when that valve is failing.

Thanks for asking. I forgot to add that in, but yes, I replaced the IAC as well. I also replaced the vacuum line to the purge valve and a smaller piece of hose on the passenger side near the firewall that connected two smaller plastic lines. I also cleaned the throttle body. The noticeable drop in rpm while in gear and holding the brake seems to have resolved. Now I'm chasing down a vibration that comes and goes every few seconds. Some research suggests motor/trans mount or a torque converter failing to come out of lock.