My '07 XC70 (244K miles) has started experiencing a couple of interesting issues that I can't quite seem to put my finger on and rather than starting to replace parts, I figured I would ask here in case anyone has had some or all of these issues, since I think they may be related. Car has been reliable with very few performance issues or CELs in the six years I have had it, except for that it seems slower on hills and accelerating at highway speeds compared with another P2 XC70 -- the difference felt like that between a turbocharged and non-turbocharged car of the same model.
An intermittent power surging started about a year ago but was very minor and infrequent at first. It has since become more frequent and more severe, being noticeable almost every time I drive the car. It is most noticeable when under load going up a steep hill, but as it has gotten worse, it is also sometimes present during acceleration on the highway. Seems to only be at low RPM, such as just before a downshift, which would at first clear out the surging. Twice there was a noticeable gain of power at which point the surging would stop, one time this was accompanied by a loud pop from the engine area (my mechanic suspected it was the gasket in the oil filler cap failing, which was soon replaced). The car would run smooth with no surging and increased power for at least a few hundred miles afterwards, then one day it would have less power and the surging would return soon after.
Through all of this there has also been an issue with sudden oil consumption. The oil level will stay almost constant for at least 1-2k miles at a time, then suddenly drop by 1-4 quarts (not exaggerating) -- add oil and it's fine for a while again. This has only happened during normal highway driving, not when pulling a trailer, during hot weather, high RPMs, etc. when I would expect there to be some oil consumption. Once I could smell oil while driving and noticed the level was low afterwards.
Last winter the surging got to the point of it being a violent bucking motion, accompanied by some power loss between surges, and seemingly a misfire (no CEL, and didn't check misfire counts at time). At the time the car also started and idled rough.
I replaced spark plugs (probably overdue at time), ignition coils, and found wiring harness for coils was missing a significant amount of insulation. Surging stopped, car ran smooth for a while but with noticeably less power, then surging started again and power increased, though surging was not as violent anymore and the car started and idled smoother.
Suspected a turbo issue, so I started by inspecting TCV -- cleaning it had no effect on performance but did flush out a drop or two of oil from valve (is this normal?).
Finally got a CEL that seems to be related to the surging issue, P0101 Mass or Volume Air Flow Range/Performance and ECM 121B Mass Air Flow Sensor. Disconnected power to MAF sensor, no change at idle but gave a MAF low input code within a few minutes -- drove a few miles with it and surging continued same as before.
Also finally was able to replicate surging with scanner plugged in -- noticed fluctuation in MAF (see image), also in short-term fuel trim (didn't get image of this, but it was a similar pattern around a baseline of ~+5%), while throttle was maxed out at 86.3% (which I understand is what some scanners report at full throttle).
LTFT stays right around 0% at idle and when driving. STFT at idle seems to quickly climb to 12-15% then quickly drop back to around 0%. Mostly stays between 8% and -8% while driving, but occasionally dips as low as -26%, seemingly when I let off the gas pedal after a hard acceleration.
Fuel pressure is a constant 43-45psi, which I at first dismissed because it was constant, but it seems that this is too low when read with an OBD-II scanner and instead should be around 55psi? I have also read that the pressure at the fuel rail should be 43-44psi, so is the sensor in my car at the rail as opposed to somewhere else?
O2 sensors seem to be behaving more or less normally -- though the upstream sensor oscillates between 0 and about 0.7 volts instead of 0 to close to 1 volt.
Replaced air filter, removed and inspected intake and intercooler pipes -- found that intake pipe was loose where it was attached to turbo, also blue colored line between TCV and intake pipe was loose at intake pipe. Some oil seeping from end of intake pipe, as well as around PTC valve grommet, so still another likely air leak.
Noticed film of oil at downstream end of intake pipe and throughout intercooler pipe. I understand some oil in pipes is normal, but this seemed a little on the excessive side. This work has not changed any sensor readings noticeably.
Yesterday, I was not able to replicate surging, but car had noticeably less power again, lower gas mileage, and TPS will not read higher than 40% when floored in 4th or 5th -- not until it downshifts into 3rd does it suddenly reach 86.3% when the other day it would reach at least close to 86.3% before downshifting and could be forced to reach full throttle by putting the car in 4th or 5th manually and flooring it. Is the car sensing an air leak and restricting how much the throttle can open? Why only sometimes?
I have suspected the PCV in the past (it's never been serviced, as far as I know), but it was tested around 230K and passed -- it has also passed a couple of glove tests since then.
I have gotten two P0420 Low Catalyst Efficiency codes in about 20K miles, both times they have quickly cleared themselves. Could the catalytic converter or another exhaust component be clogged? I found a forum post elsewhere about a similar surging and low power that was caused by clogged cat, but that person seemed to experience a constant and more severe loss of power.
I'm really not sure what to try next, but I don't want to just start replacing parts and hoping the issue will be resolved. Currently, my list of suspects for the surging are: bad MAF sensor, air leak at PTC valve, also potentially causing ECM to reduce throttle, bad TPS or throttle body issue, FPS, fuel pump (especially if this fuel pressure reading is actually too low), TCV, failing turbo. And for the oil consumption, I think it would have to either be a blown turbo seal or a PCV issue? Potentially stuck rings? Is there anything that sticks out to you as most likely culprits? Or anything else I should look into?
Have any of you had any similar issues and what were their causes? Do you think the surging and oil consumption may be related? Or am I looking at multiple unrelated issues due to age of vehicle? I'll try to post some more photos of scanner output, but I'm awaiting a new phone at the moment, so more photos will have to wait another day or two.
