The hunt for the vacuum leak!

dikidera · Post by **dikidera** » 05 Sep 2023, 03:23

So, I unlocked my 140hp S60 to 170hp, or so I thought. The MAF readings were way below what I expect. I expected around 136g/s(as other users have logged the 170hp version) at 6000RPM, what I got instead was around 120-121 grams per second. Now there are some horsepower estimates from the tuning world which is g/s * 1.25 = estimated HP(other way is g/s / 0.82). I got ~150hp. Not terrible, but not great either.

I was wondering what it could be, clogged cat(I am getting an ECM-4201 code), bad injectors, bad tune? Well, it was not the tune for sure. As I was driving and mashing the gas pedal, revving the car to ~6000 RPM I noticed...a very prominent whistling sound only at high RPM regardless of car speed, where I expect the airflow to be the highest and yet is lower. Oddly enough the Oxygen sensor was reading 0.89 at ~6000 RPM, I would've expected a leaner mixture or even codes.

I have some suspicions where it might be coming from, but if it's not from there it's going to be difficult to find. The car has no symptoms at idle, g/s are normal, or they appear to be at 3.16-3.6 g/s, STFT is dead 0% at idle and Oxygen sensor reads exactly 0.99-1.00.

So any way I think the problem is coming from the last intake manifold bolt, just under the thermostat housing and above the PS pump. It's the only one which I failed to torque enough, because both thermostat housing and PS pump prevented me from fitting the extension and swivel properly. I only managed to get it somewhat tight, the car at the time did not show any obvious symptoms so I thought I was good. But after getting my ass handed to me by a 1.6 Lada uphill, I knew something wasn't right. And the whistling confirmed it for me, at least I hope it did, I wouldn't want to end up in a wild goose chase.

Post by **jonesg** » 05 Sep 2023, 03:27

there you go then.

Post by **abscate** » 05 Sep 2023, 04:15

You can get that one by dropping to a 1/4 inch drive 10 mm socket and extension. Not too tight on those IM bolts, only about 15 Nm

Post by **vtl** » 05 Sep 2023, 05:08

What does LTFT look like? STFT won't show signs of leak until it developed much further.

dikidera · Post by **dikidera** » 05 Sep 2023, 06:16

LtFT always at 0% or at most 0.78 until it goes back to 0. Bear in mind I always drive well below 4500 rpm, my car is optimized for fuel economy and low emissions since it's a ulev. And I have noticed the transmission trying to shift sooner into a higher gear and not stay at high rpm for very long.
What I wanted to say is that it might be a leak that occurs at higher rpm? Not sure if that is possible since vacuum in the manifold is higher when the throttle plate is closed buuut..
@abscate that is what I use but...it didn't fit properly.

Post by **vtl** » 05 Sep 2023, 06:26

Real fuel trim is LTFT. STFT normally oscillates in 98-102% (0.98-1.02) range and hunts for the best fuel trim. If it drifts too far away from 100% the LTFT is corrected, so STFT returns back to 98-102%. Small leaks are indicated by LTFT. STFT only shows severe leaks when the engine is apparently struggling to run.

Any chance you are on a significant elevation? Also high heat affects the air weight.

dikidera · Post by **dikidera** » 05 Sep 2023, 06:32

Elevation about 200m above sea level, maybe slightly higher at times depending on where on the road I am. Ambient temps were 25c at that particular day, obviously the air gets hotter underneath the hood. At those ambient temps with A/C off, I should have seen a bit more airflow.

Still doesn't explain the whistling. The belts are new with the exception of the serpentine belt tensioner so the whistling cannot be from there. The other thing could be....clogged cat and the backpressure escapes from wherever it can??

Post by **vtl** » 05 Sep 2023, 06:54

dikidera wrote: ↑05 Sep 2023, 06:32 Elevation about 200m above sea level, maybe slightly higher at times depending on where on the road I am. Ambient temps were 25c at that particular day, obviously the air gets hotter underneath the hood. At those ambient temps with A/C off, I should have seen a bit more airflow.

Still doesn't explain the whistling. The belts are new with the exception of the serpentine belt tensioner so the whistling cannot be from there. The other thing could be....clogged cat and the backpressure escapes from wherever it can??

That's possible. Can you get a backpressure tester? Or even buy one, they are not overly expensive.

dikidera · Post by **dikidera** » 05 Sep 2023, 07:21

I don't currently own one. I attempted to use a compression tester (I have the appropriate adapters), however (I only assume)it proved to have too little granularity and thus the pressure gauge did not move at all(even during high revs). But it should have showed at least some backpressure, which would be normal considering I have 2 catalytic converters.

In the image the throttle closes at that point because the gearbox likely changed gears(at which point I also eased the accelerator pedal), but the gear ratio needs about a second longer to change, from my experiments so what you are seeing is not the cause.

Post by **vtl** » 05 Sep 2023, 07:44

Your Volvo has only 1 converter. The flat thing in the catback is a resonator, not another converter.

My 02 V70 w/ B5244S wasn't particularly fast. It was perhaps a bit faster around 4500 RPMs than Seat Ibiza 1.4TDI with the stick I had right before Volvo. Don't know if Lada would leave it biting the dust, but when I test drove my XC70 w/ 2.5T my first thought was: "Holy sh*t! What would one make such a fast car for?! Is it even legal?" I certainly became acceleration addicted for a while until aged.

Also keep in mind that AW55 unlocks torque converter during kick-down, so it robs your engine at least 30-40% of ponies, depending on how thin is your ATF (less viscosity less torque transfer via fluid). Best torque transfer in my experience is with OE Volvo ATF and a few ounces of Lubegard HFM. HFM makes it roll uphill at idle revs.