About a month ago on a 100 mile trip I had an engine stumble while running about 60 for 3 or 4 seconds, then it went away.
Since that time, I have noticed that my average MPG is down about 5 MPG, no other indicators.
( Even though I know that my UltraGauge is WRONG on the avg MPG, it IS CONSISTENT, usually it shows around 32, since then I haven't seen above 27 )
Yesterday, I backed the car out of the garage, and when I started to shift from reverse, the engine died, but restarted with no issue.
( almost panicked when I got no starter at first, then it occurred to me to put it in PARK --- duh ! )

Here's what I have done today :
Changed back to the original Bosch CPS. ( sprayed connectors with contact cleaner )
Cleaned the MAF/IAT sensor.
Replaced the air filter.
Checked for any vacuum leaks.
Changed to a backup RCM.

( AGM battery is almost new, 12.5 volts static, 13.9 - 14.2 volts running )

Only time will tell if I have cured the problem, but any suggestions as to what I might have missed are welcome.

 

Any codes??? You have covered everything that I would have, and more! I was going to suggest a failing coil pack (or two) until you stated it did it shifting from reverse. I am sure you tested the spark plug boots/wires to coil? I think a few local jaunts under a variety of conditions is in order. That should build your confidence back up, or maybe duplicate it leading to more in-depth looking. Hope you got it!


PLEASE let us know if you are still having this?

 

Thanks George, no codes and had not thought of the coil packs, and I do have a spare.
I'll try that if I have to. I did check all the wires at both ends.
I'll update this post as time goes on.

 

What's the fuel trims look like on the UltraGauge ?

 

My experience with stumble/no codes is usually secondary ignition: plugs, plug wires/coils. And the XF has 12 plugs/wires.

 

Fuel injector/ leak or connection? Any hint of fuel smell?

 

mrmiata : I didn't know I could see fuel trim with the UG, I'll look into it.
padgett : thanks
dinasrt : nope, no fuel leak/smell

I'm thinking I'll chunk a can of SeaFoam in the gas tank just for grins though.

 

I don't have one myself.. but every time I mention Torque Pro.. Mark.. aka pizza.. says "same with Ultragauge".. LOL

 

Just a caution, additives in fuel sometimes creates stumbling as deposits hit the vitals...

 

My experience would say plug wire or injector. Run a can of SeaFoam through it. I had an issue with a plug wire on cylinder 6, it looked ok but it was not really snug and I suffered a stutter from time to time, usually wothout a CEL but sometimes had Cyl. 6 misfire. Good luck, I hope you find the fix.

 

How bout measuring the fuel pressure?

It's the only think you did not mention that I can think of.

 

Good catch. Will do.

 

UPDATE: Drove over 250 miles Friday and Saturday, no problems at all.
I will say it WAS the CPS that was causing stumbling.