Rpm's stop increasing when accelerating

Asked by Jan 14, 2016 at 03:00 PM about the 2007 Subaru Legacy 2.5 i

Question type: Maintenance & Repair

I purchased my 2007 Subaru Legacy around a year and a half ago, and
had to have a new transmission and head gaskets replaced. The car was
ran with bad head gaskets for several thousand miles, forcing me to
replace many external engine components as well, due to damage from
the oil. The old transmission was replaced due to extremely hard shifting
from second to third gear. 15,000 after all of the repairs, my car is running
great for now, and getting serviced regularly by myself including oil
changes, new air filters, etc. The new confidence I have acquired due to
my car running so well, I am able to "carve the roads" as one would say.
Since I am no longer afraid to drive my car how i want, high RPM's are
visited frequently. The car runs amazing, and shifts well in the manual
mode of the 4EAT. although it runs and shifts smoothly, there is one minor
issue.
    When accelerating from a stop, i usually allow my car to reach 5000-
5500 RPM's before i shift to the next gear. when doing this i notice
something very strange, which has confused my college automotive
professor, and even my mechanic. First gear follows through with
accelerating perfectly, having a even increase in speed and RPM's. Second
and third on the other hand, are quite odd. When in second gear my car
reaches an area between 3900 and 4200 rpm's where the car RPM's will
stick and stop accelerating for around 1 second, and then it continues on
accelerating to the shifting range into third gear. In third gear, when my car
reaches an area around 3400-3700 the car drops about 300 rpms, before
continuing to accelerate normally. All this happens with my foot pressing
the pedal about 75% down, and doesnt happen all of the time. This occurs
about every other acceleration, especially when going up a hill. The engine
light has not appeared, so we have no way of knowing how to fix this
problem. Does anyone have any suggestions?

2 Answers

79,415

Some cars have flat spots in the power curve. Have you done a tuneup lately? Use a good fuel injector cleaner as well.

5,710

Hmmm...I haven't encountered flat spot in midband except where an intermittent MAP sensor has failed, so check its connection, and remove to carefully spray-clean, as it's easy to do quickly. I'm assuming that new plat/iridium plugs and good wires followed the HG job, so, where this occurs in the midband, without CELs, I doubt it's misfire-related either. Was the tranny you introduced new or used? Regardless, drain its ATF and replace with 3.5 qts fresh ATF. If flat-spotting recurs I'd try swapping in a used TCU. Maybe check the main harness contacts above the trans too? Ah...I have another idea: the 2006+ motor uses variable-valve actuators to readjust the cam (and timing) when a threshold about 4k rpm is crossed so as to allow faster highway passing. This system is oil pressure-regulated, using somewhat fragile pressure sensors that sit atop each actuator (left front and right rear). I wonder if a leaking sensor (in addition to dripping oil everywhere) prohibits cam actuation and therefore gets the ECU/TCU "confused" as to how to proceed? (Note that a leaking switch will NOT set off a CEL, but a failed (or unplugged) sensor will indeed do so.) So check this VVT stuff first. The switches are about $13 online when needed. OTOH, this motor really LIKES living in the bottom half of the rev curve with the VVT system, saving lots of fuel and gaining nice perceived torque bump in doing so. If you can't get rid of this hesitation, and it only occurs c. 4k then learn to drive in "D" (except on icy downhills). It'll limit fast steep hill-climbing just a bit. Lastly, there may be a faulty solenoid or valve in the "new" trannie's throttle body, but you do NOT want to go there, eh? Good luck.

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