WTF is happening with my 99 4.6 AWD? HELP ME. PLS.

So I picked up another Expedition. 99 4.6 AWD.

I replaced the front diff (was blown)
Replaced Spark Plugs.
Replaced a Coil Pack.
Replaced an o2 sensor.
Replaced a the Camshaft Pos Sensor.

The passenger manifold has been replaced. The drivers side has a HUGE rotted hole in the manifold.

The mechanic was supposed to replace BOTH of them with my Pace Setter Shorty Headers but decided he didn't want to do that and just replaced one side with a stock header and didn't do the other at all.

It hard starts, like SUPER HARD. It bounces around at idle, between 500 and 1500. Sometimes it will turn right over, other times its SUPER hard to start. It has back fired both through the intake AND in the exhaust.

When driving it, thankfully only to get it towed away from scum bag mechanic, it won't get up above 55 MPH, has NO POWER driving uphill (I couldn't even drive it on to the tow truck) and drags gears out REEAAALLLLLYYYYY long. I jam it to the floor and nothing happens. Its also throwing lean codes.

When I put it in gear to move it, RPM's drop SUPER LOW and it fights to stay running or stalls. Same thing if I come to a stop.

Would the drivers side leaking manifold cause these huge problems? What else should I be looking for?

Anyone have some insight? I am pulling my hair out.

It is possible for the exhaust leak to cause a lean condition. If enough of the exhaust gases are escaping before reaching the oxygen sensors it can skew the readings. In addition, I would check for any vacuum leaks if you haven't. These can cause the same symptoms you described.

This link has some interesting information which might help you: https://www.ericthecarguy.com/kunen...ons-Answered-Here/50573-exhaust-manifold-leak

Thanks Shab!

My pleasure, let us know what you find

Check fuel pressure at the rail and if you can at the pump itself before the filter. Check it at idle and at mid throttle using the ac as load.

I had a 97f150 w the 4.6 and soundsto me like the same carp I had,so I bought a can of mass air flow sensor cleaner, fixed most of my problems right there my 01 Expo with 5.4 i use it whenever I clean my K&N

Done both exhuast manifolds and replaced vac lines, including the one that runs behind the intake. It idles fine sometimes, starts right up some times. Other times it really hard starts. Once put in to gear, still runs horribly. It will rev out and then die when it comes back down.

Idle Fuel pressure 34, Open Vac, 43.

What else should I be looking at.

I'd run a dtc check. If you don't have a scanner, their relatively cheap, most shops like Midas, and repair shops will do it for you for free. Bring a pen and paper, write down any and all codes. You could have a bad mass air flow sensor, throttle position sensor, the list is endless. You need to know the basics first. Fuel pressure is fine, so you can pretty much rule out that you have a good pump, filter, up to the rails. Keep us posted what you come up with.


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Just ran the codes and there is a code for TPS. Which wasnt there before. So of course, during the removal of ****** screws, this happened.





JUST MY LUCK!!!!!!!!!!!!!!!!!