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Discussion Starter · #1 ·
I'm struggling to find the cause of low boost on my 2001 V70 T5. I bought the car with 163k on it and it had very little boost initially. After installing a boost gauge, I found that I was getting around 10 psi. I replaced some split hoses on the TCV and repaired some cracks in the charge pipe and the boost jumped to a steady 14.7 psi. In fact, it would sometimes build to 16+ initially before settling back down to 14.7. It was awesome. Now, about 10,000 miles later, the boost dropped back down to the 10-11 psi range. I found that the charge pipe had cracked again so I did a better repair on it and got the boost up to 12 - 13 psi. I then checked all the hoses again and they are fine, I did a smoke test on the intake and found no leaks, I replaced the TCV, I changed the spark plugs (which were mostly hand tight except for one that was so loose that it was wobbling when I blew out the plug well with compressed air before changing it), and I replaced the clutch position switch which was not working. I still only get 12 - 13 psi of boost.

While poking around under the car, I saw some yellow markings on the turbo so I would imagine it has been replaced at some point. The car runs awesome other than being down on power. No oil use, very smooth power delivery and smooth idle. The vacuum sits at 18 inHg at idle which will drop to 20 inHg during trailing throttle.

I'm not sure where to look next.
 

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http://volvospeed.com/vs_forum/topic/151580-ard-tune-me7/
"ME7 is a torque based system, meaning that when you depress the throttle 50% you are effectively asking ME7 to give you 50% of the torque the ECU is programmed to allow. At 100% throttle you are effectively asking the ECU to give you 100% of the torque the ECU is programmed to allow. Notice I said "asking the ECU". That's exactly what you're doing because even though you might ask for 100% torque, the ECU may not allow it. Reasons for this can be things like overheating engine, traction control has messages that there isn't sufficient traction to suppor the user torque request, CEL on for misfires, etc...

As it relates to expected boost values:
Since ME7 is only concerned with the power (torque) it is being asked to provide it could just about care less about boost. For instance, if a particular ME7 ECU were programmed for a max torque allowance of 280ft/lbs and the user throttle request was at 100% then ME7 would try to provide that full 280ft/lbs if conditions allowed (i.e. no loss of traction, no overheating, no CEL, etc..)
If by chance it was a cold day and environmental conditions were optimal it may only require 16psi to develop that torque value. However if it were a hot day and environmental factors were not in your favor it might require 18psi to deliver the 280ft/lbs of allowable torque. If the max boost limit programmed in the ECU was 16 psi then even though you were making max boost you might not be making total allowable power. This is why boost target on ME7 may seem low even though the car ECU is providing what it is programmed to provide and consequently can mean full boost isn't necessarily making full power"
So don't get so hung up on peak boost numbers.

Unless you have a tune, 10-11psi is stock boost pressure for your engine.
 

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Discussion Starter · #3 ·
I wouldn't say I'm hung up on peak boost numbers. The car used to peak at 16+ then settle in just under 15 psi and man did it pull hard. I never heard any pinging and the car ran awesome. The only problem I had was keeping the tires from spinning and worrying about burning out the TC light. I had read a post that said max boost for a T5 was 14.7 psi so I thought everything was good. Now that you're telling me it was overboosting I'm not sure what was the cause. It was very consistent and it would sit at that value rock steady the whole time it was under boost. I really miss the power. The car feels like a slug now.
 

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Where are you? Elevation plays a part in final boost pressure.
 

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Discussion Starter · #5 ·
Just above sea level.
 

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Discussion Starter · #6 ·
Update - Full boost has returned for some unknown reason. Yesterday while driving the car I noticed it seemed more responsive than it had. A few full throttle blasts showed the boost was up around 14+ psi again. I had my autocom in the car so I hooked it up and read the MAP sensor and my readings were 1990-2000 hPa spikes and 1930-1950 hPa steady. That equates to 14.6 to 14.7 psi spikes and 13.7 to 14.0 psi steady. Could it be fuel related? I run premium but if the fuel isn't high enough octane and the engine knocks is the boost limited by the ECM along with the timing?
 

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Discussion Starter · #8 ·

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I have a friend with a 99 T5 and his car is bone stock but remarkably fast, we did a boost check and it also runs a solid 15 psi throughout the rev range. Because his car is lighter he can actually keep up with my C70.

So yeah it does happen once in a while that a car just has the right combination of factors that allow it to run a little hotter than the base design. I guess it used to be really common on 850s which is why they sometimes have a reputation of being underrated by the factory.
 
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