I have a 1998 Ford F-150 super cab step side lariat 5.4L automatic 4r100 transmission 4x4 with off road package. I purchased the truck a few weeks ago. The body has 203,000 miles on it. The engine has 36,000 miles on it and was replaced before I purchased it. The truck runs great when driving it. The gas mileage could be a little better. The issues I’m having is at idle the entire truck shakes and feels like it has a misfire. The exhaust smells like gas. When I turn the steering wheel a little in either direction the rpms dip way down around 300 to 500 rpms. The rpms also dip down when I change the climate setting from panel to floor or defrost or any other setting. The idle seams to be normal when starting cold and it appears to only have the issues when the truck is hot and has been ran for a bit up to temp. Additionally when I shift from park to drive, or drive to reverse the truck wants to stall and will stall unless I step on the gas pedal and throttle it up. The truck will start right back up with no issues after it stalls. I have no check or service engine or mil lights on. I pulled the codes with my scan tool and the only code I get is p1000. I have replace the idle air control sensor and the throttle position sensor on the throttle body. I swapped my mass air flow sensor with another one I have that works without issues. The transmission fluid looks to be at the correct level and looks good. I’ve ran some fuel treatment in the gas. I checked the alternator with my scan tool and all is good there. I replaced the battery about 2 weeks ago when I purchased the truck. I did have a few broken vacuum lines that I repaired. I reset the battery after the repair. I’m still having the same issues. What am I missing? Any help or suggestions would be greatly appreciated. I’m at a loss.

 

The HVAC system issues almost certainly stem from too low intake manifold vacuum values---check that and report back. Also check the elbow at the back of the throttle body base---those as old as your truck are highly susceptible to cracking and causing issues such as you decribe.

 

I will check into the vacuum leak theory in the hvac, and will report back on that. I’ll use a vacuum pump and check all the lines. I really don’t think it’s a vacuum leak in the hvac. I say that because all the actuators doors work. Example defrost, panel, floor. And it doesn’t go from panel to defrost under heavy accretion. Maybe there is a check valve and that’s why my controls work and I have a rough idle, so I’ll check for the hvac leak. I have checked the elbow on the very back of the throttle body. It looked new, but I replaced it anyway. That didn’t help the issues. I decided to pull the throttle body off and clean it. I thought maybe the egr passages were clogged. The passages looked good, a little dirty but not clogged. While I had the throttle body off I also replaced the egr valve. I also replaced all 8 spark plugs with oem motor craft ones, and replaced all 8 coil packs. So far Nothing has fixed the stalling issue or the rough idle.

 

Ok so I checked into the hvac vacuum lines. Everything there is working as it should. I did discover a small hole In the big black vacuum line that leads to the throttle body. It has another smaller red vacuum line on it at the throttle body that goes to the fuel pump regulator. I fixed that vacuum leak and it did help some. I check all other vacuum lines with my vacuum pump and all is good. The truck doesn't appear to stall anymore when changing gears. What I do still notice is that while parked and idling if you change gears while holding the brake and have the wheel turned at the same time it still drops the rpms day to about 300-400 rpms. It’s feels like it’s about to die but it doesn’t. all is good. Also the truck still shakes quite a bit when idling in park. Any other suggestions?

 

Does the power steering pump start whining when under these conditions? This is an unusual situation to be sure.

 

I was thinking maybe the power steering pump was going bad also, but the power steering is not whining. Plus the rpms drop when changing gears even without the wheels being turned.
I had the same issues on a 1993 Ford thunderbird 5.0, and the power steering pump was whining. I replaced the powered steering pump on the thunderbird and that solved it. I don’t recall the thunderbird dropping rpms when changing gears. It only had that problem when turning.
I’m going to check the long term and short term fuel trim with my scanner and see what’s going on there. I’m starting to think it’s a fuel issue maybe pump, injectors, or plugged fuel filter. Maybe that will help give a clue to the rough idle. Any idea what’s considered normal a normal short term and long term fuel trim for the 1998 Ford F-150 5.4l vin L?
Also anyone ever hear of a transmission filter falling off the 4r100? I think the AODE trans had a tsb about that issue causing similar issues I’m having. I’m just not shure if that covers the 4r100 trans.