Problem Accelerating

[2007Limited]

Wondering if anyone else has encountered this issue.


I have a 2007 Limited. 5.4 with 3.31 rear end. I first noticed problems accelerating when pulling out into traffic. My Expedition would downshift, pull around 4000 RPM's and struggle to gain momentum. I changed my sparkplugs (keeping original coil packs), air filter and fuel filter. This fixed my low speed acceleration issues.



Now I’ve discovered that if I’m on the highway driving around 70mph and accelerate hard to pass, I have the same issue with the downshifting to the 4000 RPM range and struggling.


What’s weird is at speed it accelerates better at ¾ throttle than wide open. If I floor it from a dead stop, it will pull good past 70mph without any issues. This issue only shows up at highway passing speeds.



Thanks in advance for the help,


Chris

 

[grumpyoleman]

No mechanic here, just anecdotal experience....had an '06 Navigator with the 5.4 and it was doing something that sounds almost identical, even progressed to the point that at moderate to hard acceleration it threw a CEL. Ended up it was a bad Cat Coverter. Once replaced, ran perfect.

 

[2007Limited]

I will have to look into that. Is there some kind of test performed to check the Cats?

 

[grumpyoleman]

If memory serves...the CEL was what did the trick. The code pointed to the CAT.

 

[Plati]

https://axleaddict.com/auto-repair/How-to-Test-a-Bad-Catalytic-Converter

 

[LRNAD90]

Failed cats can cause considerable back pressure, depending on how the interior failed/deteriorated, which can essentially strangle the engine.. I remember my parents bought an '81 Chevy new, back when Cats were brand new to cars, and they had three cats fail in the first year of ownership. When they did the thing wouldn't go more than 10 mph, no matter how long you held your foot to the floor.. LOL!