1997 S70 T5 stock boost pressure?

Vuym · Post by **Vuym** » 18 Jun 2025, 14:11

Hi everyone,

Noticed today my turbo inlet to TCV vacuum line was torn (red see pic attached) so I replaced it and double checked all the vacuum lines were in the correct place.

Before replacing it I was consistently getting 10 psi but now I'm getting about 6-7psi max.

My question is: how much should a stock T5 be making? If my new figures are not consistent, what could be wrong to explain this loss in power? I've attached an image of my dyno result if that's any help (the vacuum line would have still been torn at this point).

There is currently no catalytic converter on the car but I think that only affects spool up time.

Post by **matthew1** » 18 Jun 2025, 14:25

11-12psi is normal. Your problem is almost certainly a vacuum line or cracked vac elbow.

Post by **scot850** » 18 Jun 2025, 20:43

The stock TCV is only good to around 12 psi. It starts to get flaky at 14 psi.

What pipe did you use? If it is on a vacuum side then it may be collapsing under boost.

Neil.

Vuym · Post by **Vuym** » 19 Jun 2025, 01:08

scot850 wrote: ↑18 Jun 2025, 20:43 The stock TCV is only good to around 12 psi. It starts to get flaky at 14 psi.

What pipe did you use? If it is on a vacuum side then it may be collapsing under boost.

Neil.

Thanks guys, was always under the impression that 10psi is what the car should be making stock. I've been using the same silicone hosing for all of my vacuum lines (https://amzn.eu/d/dyNrdBF)

Can the lambda light from decat cause the car to limit boost?

Post by **scot850** » 19 Jun 2025, 08:37

I don't know the answer to that one. Sorry. It is possible as the O2 sensor not getting a reading may think there is a fault, but I don't know for sure. Hopefully someone else will know the answer.

Neil.

Post by **matthew1** » 19 Jun 2025, 08:46

What codes are you getting? Let's start there.

Is your fuel economy suffering? Keep an eye on that.

Post by **scot850** » 19 Jun 2025, 13:22

TCV's do fail with age. So maybe it is as simple as that. I know Pick and Pull type yards are getting rare in the UK, but we still have them here and I usually keep spare from the yards just in case.

Neil.

Vuym · Post by **Vuym** » 29 Jun 2025, 13:28

matthew1 wrote: ↑19 Jun 2025, 08:46 What codes are you getting? Let's start there.

Is your fuel economy suffering? Keep an eye on that.

I'm getting P0136 & P0141 as I only have the front sensor, no rear. I get about 320 miles in a full tank 80% Urban 20% Motorway. Have been monitoring boost pressure with TCV red disconnecting and it will get close to 12psi in 2nd gear and 15psi in third. With all the lines attached to the TCV it seems to vary a lot, sometimes 5 psi, sometimes 8 etc.

I could purchase another TCV to rule it out as being the problem, I had a quick check of all the vacuum lines and the yellow-ish one that runs from the FPR regulator is quite perished / rotten so I'm going to replace that tomorrow morning.

Post by **matthew1** » 29 Jun 2025, 13:38

That's 20mpg, which is completely normal for a P80 HPT. Good even, with that split city/highway.

Disregard boost in 1st gear. 12-15psi is very healthy.

Without the back story and codes, I'd give this engine a high mark for health. I haven't looked up those codes but will.

Post by **scot850** » 29 Jun 2025, 20:54

Both those codes are for the downstream sensor on the cat. How do you not have one on your car? Did you have it removed? If it is not there it will throw a code and in these cases for readings out of spec.

Neil.