Working my way through a newly acquired 2007 XC70 and have discovered an issue (several issues? not sure) that I need help figuring out.
Quick background: PCV done, tranny done, timing belt done, thermostat new, new oil, new air filter, new fuel filter. No codes in VIDA (well, at first—more on this in a sec).
The symptom (part I): When traveling at highway speed and you give enough throttle (say, passing throttle), it stumbles. At first I thought uh-oh, tranny, or haldex, or something right on the wheels, or whatever, but the more I felt it and reproduced it, the more I thought not. It happens about 1-1.5 seconds after acceleration kicks in, and there is one stumble for a fraction of a second, and then it's back to normal.
The symptom (part II): Because I started to suspect it was something other than the wheels and things directly connected to the wheels, I fired up Torque and watched a bunch of stuff. Things that make me wrinkle my brow: (1) fuel pressure varies from about 45 to about 42-43 when accelerating... is it me, or is this low? (2) the biggie: when accelerating, the throttle seems to hunt a bit (steady pressure, but reading varying) and then also frequently jump wildly up from 20-30 to 85+ for a second or so, then fall back down to the 20-30 range. That jump to 85+ is really obvious because there is no new foot movement associated with it and that's near the top of the range, like it thinks it got stomped.
The symptom (part III): I left torque running after I got off the highway. Turns out the same thing is happening on side streets at 25mph, it's just that it doesn't seem to cause a stutter or anything like that at slow speeds. But the jump is still there. Accelerate a little and it's like 15... 17... 22... 85.5... 21... 16... 85.5... 20.
The symptom (part iv): Other things that I'm not sure are related but bear mentioning: as I was doing my test, two new things happened that I've not seen before. (1) According to torque, coolant temperature hit 221 degrees. That seems too high, should it not be about 20-30 degrees less? And yet the temperature needle in the dash remained pegged firmly in the middle — should I trust whatever sensor torque is reading, or the needle in the dash? (2) A new code popped up for a bad O2 sensor reading.
Obviously, something is amiss. Immediate candidates for suspicion:
- Throttle module (cleaned when doing PCV) — Bad module? Bad connection?
- Fuel pressure sensor/regulator? — Misbehaving / doing wild things and ECU trying to compensate and torque is reading some secondary value?
- Fuel pump? — Is the fuel pressure low and it's due to a weak pump?
- O2 sensor? — Could this cause a stumble under acceleration? But probably not a crazy throttle, right?
- New thermostat? — is the temp related at all? Is 221 normal after all and I just think it's too high?
- ECU behaving oddly? — Bad ECU seating or bad ECU fan?
Hoping someone who's seen one or several of these symptoms has some ideas. Car remains eminently drivable, I just came back from having it on the highway for 50 miles and if I stick it in cruise and just go along that way, temp is 201, throttle is rock solid and you'd never know anything was wrong.
But obviously, something somewhere is wrong...
Hoping there's an access panel now in case a new pump is needed...
