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2018 X3 M40i / 2016 X3 xDrive35i
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Discussion Starter · #1 ·
Guys, I want to save you all the headaches I've had. 5 years ago I had a 2004 XT I restored with ~110k miles and it was built, 1000 ID injectors, stage 3, etc and chased problems with hesitation in the 3k range and resolved it by adding fuel dampers on the inlet side of both aftermarket fuel rails. It took me so long to find that solution because I added an OEM fuel damper to the FEED side of the fuel, added hose length to the FEED side, etc because that's what I kept reading would solve the issue. I finally added 2 additional dampers right on the FEED sides of the fuel rails and so I ended up having 4 total dampers on that setup and it ran beautifully afterwards. I never should have needed that many. I now know why I did.

I now have a 2007 FXT with only 59k miles. Too early for fuel dampers to fail right? Well I've had a lumpy idle since I bought it and a few other issues I now know are related. Lumpy idle (felt and sounded light a slight miss but no misfires registered), some minor fluctuations in idle like 50 rpm drop and recovery, harsh transitions from off throttle to on really felt in 2nd gear when coasting and then giving it just a touch of gas...it would buck a bit. Also what could be perceived as engine growl in the cabin in low load conditions very light throttle. Not sure how else to describe it. It was pretty subtle, I had chalked that up to a pulley bearing.

Things I've replaced/done to chase this.
Plugs
Coil pack and rotate to make sure all coils were good
Fuel injectors (replaced with new old stock Denso)
MAF Cleaning
MAP Cleaning
Throttle body cleaning
Oxygen Sensor (up and downstream)
Timing belt/WP replacement (not really done to chase the idle, just did it due to age of vehicle but was good to know it wasn't a belt/timing issue)
Monitored MAF readings, Misfire counts and Fuel learning and all were accurate/good.
Fuel Damper on FEED line

So what finally truly fixed it was the RETURN damper, AFTER the fuel pressure regulator. I strongly suspect my 2004 was the same...that return damper was bad and because I never changed it, it took a ton of feed line dampers to resolve. Now, I don't really get why the return line damper is what's needed, but it seems that the fuel pump 33% duty cycle transition seems to cause some havok with fuel pressure on these cars and that return line damper is what smooths that out.

There was no discernable noise at idle on the old damper. A screw driver sound test on it produced no noise, so the ticking issue that would tell you for sure you need to replace it isn't the smoking gun.

60k miles...if you have gone 60k miles on the original dampers, replace them. iaTuning sells them for 48 bucks each and they aren't hard to replace. Do it after it hasn't been started for several hours and there will be no fuel line pressure so it's just a matter of gently prying on some stiff stuck fuel lines with small screw drivers and/or pry bars.

Here's some videos illustrating what idle conditions do to the fuel pressures taken from my 2004. I didn't measure the pressure on my 2007 but I suspect they were similar. It's now idling perfectly, the growl noise is gone, those off to on throttle transitions are extremely smooth...it feels SO much better. Even if it costs you 48 bucks wasted, do it just in case if you've not replaced it within the last 60k miles.

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