Engine stutter at 40-50 mpg

[azakram]

I have high mileage 2012 MKT. It has developed a strange problem. When engine reaches at 41 mpg car start to stutter or knock and it does that until 50 mpg. Before and after that it run smooth with no problems.
If i press the gas hard then it crosses that 40-50 and runs just fine.
I am not sure what is causing this.

Anyone faced that kind of problem let me know what you did to fix it.

[shorod]

Which engine does your MKT have, the turbo or non-turbo?

I trust you mean "MPH" rather than "MPG." If you're getting 41 and better MPG, then you're running very lean.

When's the last time a tune up has been performed on your MKT?

-Rod

[azakram]

Yes MPH. Sorry
and it is not turbo. 3.7
Also let me correct more it is not the MPH when the RPM are between 1000 to 1500 that is when it does that.
I changed the gear to M and then drove by changing gears manually and nothing happened. I kept the rpm above 1500

Strange. I will try to clean the throttle body and change plugs. But if anyone know that will help.

[azakram]

No one? any ideas?

[shorod]

Cleaning the throttle body and changing the spark plugs (especially the plugs) are what I was thinking. I failed to see that you had responded to this thread so I apologize for the delay in responding.

On my wife's MKT EcoBoost I used Seafoam to clean the intake. I now understand that type of cleaning is not endorsed by FoMoCo, but it worked pretty well on her MKT.

How many miles are on your MKT? Have the spark plugs ever been changed?

-Rod

[azakram]

So a little update. I cleaned the throttle body and it decreases the problem. I am now filling high octane super gas instead of regular. This improved the situation.
I will be going to change the plugs next and see if this resolve the problem completely.

[azakram]

I tested it several time with regular and high octain and it is working better with high octain. 90+ still havent changed the plugs.

[azakram]

Finally changed the plugs and clean the throttle body. Now there is no knocking or any issue.
Using regular ever since and no problem.

[azakram]

It is back. And it is worst then before. Took it to mechanic and one said the problem with plugs and other said problem with the transmission. Next step change throttle body.

[azakram]

Changed throttle body put in brand new. no use

Changed plugs recommended Motorcraft made a lot of difference like 80% less missing but still some is there.

[shorod]

Have you checked for pending and current diagnostic codes? Maybe there's a code that will point to a specific cylinder, and maybe moving coil packs around will demonstrate the the issue is with a coil pack rather than spark plugs this time.


-Rod

[azakram]

Mechanic says it is random miss fire. No check engine light at all. I have bought my own scanner code with advance information. Will hook it up and drive to learn any pattern. Yesterday took out mass air flow sensor and clean it up. looked fine though.

[azakram]

I was watching some forums and someone suggested MAF sensor, so I took out the cable of MAF and all the problem vanished. Car runs perfect but checks engine light is on now. Went to dealership adn bough OEM MAF and installed. It did not work. $200
Then bought Hitachi MAf from Amazon, when I got that it was someone old Motercraft MAF wrapped in the Hitachi box.
Returned that.
No bough one from others and waiting for that to show up.
Why it is so dam difficult to get good MAF?
If still I get bad one i will be forced to take to dealership to fix.

[azakram]

Tried spark plugs, did not work
changed throttle body improvement but no fix
change MAS but no use
finally took to dealer paid diagnostics and they said it is Torque Converter. remember at this point there are no codes in the transmission or engine.
Changed the transmission fluid first with Lycos add-in work for a day.
In 2 weeks drained and fill up again the transmission fluid with slip guard from Amazon.
Improved but not completely fixed.
It is a big cost, scheduling to do that job to change the torque converter.

[azakram]

Finally changed torque converter and it fixed all problems.