Underboost Issue '07 XC70

JHarkness · Post by **JHarkness** » 12 Jun 2026, 07:41

Hi all,

I previously posted on an intermittent surging issue which was resolved by replacing the TCV -- though I also replaced a damaged coupler between the turbo and the OTE pipe. Immediately after replacing the TCV, the car smoothly boosted to right around 10psi. That was on a short drive into town. Next time I started the car, it would only boost to 7-8psi, and after about half an hour of driving it reduced to about 5psi. Since then, occasionally it will only boost to 2-3psi, but then return to 5psi in the same drive. Not seeing anything too odd on the live data on my code reader, other than that when it boosts especially low I'm not getting full throttle (only about half throttle from TPS); when it boosts to 5psi, I'm getting full throttle. Also seems to be running slightly rich based on fuel trims (LTFT about -5%).

Resetting the ECU by disconnecting the battery gets it back to 7-8psi but not the 10psi I initially got after replacing the TCV, so it's pretty clear that the computer is seeing some issue and reducing boost pressure and sometimes throttle.

No CEL, but I did get an ECM-640C DTC, which I understand has to do with a boost leak between the BPS and the throttle body? There are no visible damages to the pipes going into or out of the intercooler, clamps are tight, etc., but seeing how bad the coupler between the turbo and the OTE pipe was when I replaced it, I wouldn't be surprised if there is some damage here that I can't see without removing all of these hoses. Is it just the soft flexible hose between the OTE pipe and the intercooler and the hard plastic pipe from the intercooler to the throttle body, or are there other hoses that aren't visible? Does anyone have a parts diagram?

Also wondering if it's a sensor issue, and if so, which one(s) might be responsible. My main suspicion is the MAF sensor, as it has given me a CEL once when I was figuring out the surging issue, and unplugging power to it has not reliably given a CEL or any change in performance. Cleaning the MAF didn't seem to make much difference, but may have kept the car boosting at 5psi with full throttle instead of dipping to 2-3psi and partial throttle. Are there any other sensors that would be suspect here? BPS? O2? Are there any reliable ways to test them?

Post by **vtl** » 12 Jun 2026, 07:50

Do you fill it with 93 gas?

JHarkness · Post by **JHarkness** » 12 Jun 2026, 07:53

Generally 91 non-ethanol, sometimes 93 -- haven't seen any difference between 91 or 93.

Post by **vtl** » 12 Jun 2026, 07:56

7-8 PSI is the stock pressure for this engine, with 93 gas. With 87 gas I've seen it dropping to half of that, i.e 3-4 PSI. Then you have a compressed air temperature problem. If you test it in 90 F weather, you unlikely will see a full boost on warmed up engine.

Post by **jonesg** » 13 Jun 2026, 05:09

5 psi is what I usually see .

JHarkness · Post by **JHarkness** » 14 Jun 2026, 07:19

I am starting to wonder if a past owner has done something to create more boost. Last winter, the car ran for a few weeks with significantly more power than I was used to, and feeling different boost levels now with an analog gauge installed, it was clearly more than 7-8psi, probably in the range of 10-15psi. Everything appears to be stock. Up until last winter (not sure what changed), the entire time I've owned the car I would say it hasn't gone past 5psi, but an increase to 7-8psi after an ECU reset indicates that that is around the correct level and the car is reducing boost pressure in response to some issue it is detecting, correct?

JHarkness · Post by **JHarkness** » 14 Jun 2026, 07:26

I have also noticed some odd behavior with gas mileage (according to computer, haven't tested how accurate it is, but I don't think it is far off). I drove about 280 miles round trip to NY state a few days ago, same load in the car, same tank of gas (91), same route, similar speeds, etc., similar temperature and humidity. Usually my gas mileage is about the same both ways when I make this trip, but in the morning I averaged 19mpg and on return nearly 27mpg. After AM drive, LTFT was slightly on rich side, but was at 0 in PM. I've noticed that gas mileage also seems to increase along with boost pressure after an ECU reset.

Post by **vtl** » 14 Jun 2026, 07:37

Can be a tired upstream oxygen sensor.

JHarkness · Post by **JHarkness** » 18 Jun 2026, 13:51

Pretty sure I found the issue. I got VIDA set up last night and checked the O2 sensor readings (see below).

: Screenshot 2026-06-18 131621.png (87.94 KiB) Viewed 154 times

My understanding is that the front O2 sensor (black line) fluctuates in voltage from about 0.1 to 0.9 volts but reads here as a calculated value, with ~1.00 indicating correct air-fuel mixture. It reads about 1 at idle. The rear O2 sensor changes between an oscillation between 0.1 and 0.7 volts (indicating problems with catalytic converter?) and a relatively flat line around 0.6 volts. When I revved the engine to 2k RPM, the front O2 sensor quickly spiked to around the top of the range reported in VIDA indicating a lean condition. I would think this much of a spike would be enough to cause the ECM to cut boost and potentially throttle if it thinks it is running that lean, honestly surprised it's not giving me a CEL for lean condition.

I took another look under the car and found a sizeable exhaust leak from the bottom passenger side corner of the exhaust manifold where one of the nuts loosened on the stud. As I've said previously, this car has always felt slow, so I'm wondering how long this exhaust leak has been causing a problem?

Post by **jonesg** » 19 Jun 2026, 11:11

I duct taped the tailpipe snd injected compressed air, spraying soapy water I found bubbles several places,
the aft cat pipe joint, around the bracket welds on the cat itself, the top of the flex pipe weld where it meets the 90 deg bend.
welded everything up and the code went away, its been ok the past 2 years.