Engine: 70 T5: LTFT (fuel trims) +18% - vacuum leak through crankcase?

[Jaaap]

My Volvo C70 T-5 from 1999 has LTFT (long term fuel trims) of +18% at low loads, various rpm's.

I have replaced:
- The MAF: new Pierburg 7.22701.05.0
- The lambda/O2 sensor: Bosch 0258007038
- Fixed some air/vacuum leaks, mostly between the MAF and the turbo

Measured fuel pressure at idle: 3.8bar / 55psi
When i put the smoke machine in the air inlet, there are no more leaks, except when i open the dipstick, then a big plume of smoke comes out of the tube.

Is that normal?
If not, what could be the cause?

[Goupil]

Could it be getting into the crankcase through the PCV ?

[SteveSarre]

Hi Jaap

How did you know that the Pierburg 7.22701.05.0 is the right part for your engine?

Steve

[SteveSarre]

Hi Jaap

That Pierburg seems to be equivalent to Bosch 0 280 218 108 so I believe it is correct.

Is the positive crankcase ventilation (PCV) system working as it should? Any leaks, and have you done the glove test (rubber glove over oil filler opening should not inflate)?

Do you have a couple of one way valves in some of the hoses, and are they working and are they the right way round?

Is the turbo boost control valve operating as it should? (Check that it is correctly opening and closing the small vacuum hoses).


Hope that helps,
Steve

[Jaaap]

Yesterday i finished replacing the PCV system. The old one didn't look too bad and there was not much crud in the oil pipes.

I also replaced 2 of the one way valves for the charcoal canister fuel vapour lines.

This did not change anything, problem still exists.

I still have to check the turbo boost control valve, not quite sure how exactly.

But i now have a suspicion what the problem may be: the brake booster.
The booster did not show any leaks when i smoked the system but it wouldn't because of the one-way valve in the booster. The brake assist is pretty weak in my car so i'm going to plug it off temporarily to see if that fixes the LTFT.

[Jaaap]

Edit: it is not the brake booster :-(
Next on the list is the turbo, not exactly sure how to check it.
Does the turbo boost control valve also have a vacuum line?

[SteveSarre]

Hi Jaap,

Have you done the glove test (either on the oil filler neck or on the dipstick)?

Also are your oil filler cap seal and dipstick o-rings in good condition? They could leak air into the engine.

Steve

[SteveSarre]

Hi Jaap

The turbo boost control valve has 3 lines which I believe are all vacuum.
https://www.skandix.de/en/spare-part...ducer/1014790/

There is also a black plastic "tree" that some of the vacuum lines go to. Check that that hasn't broken, and also that the o-ring at its base is in good condition.

https://www.ipdusa.com/products/9234...6842446-978111

Steve

[SteveSarre]

Hi Jaap

How are you monitoring fuel trims? OBD2 dongle and Torque, or VIDA DICE or something else?

If you can view while the engine is running and see a high LTFT, what is the STFT?

Can you keep the engine running steadily in that state and then spray brake cleaner in various locations and see if the STFT changes?

Steve

[SteveSarre]

Hi Jaap,

Sorry, it's me again!

These might be useful if you want to understand about fuel trims

https://www.mvwautotechniek.nl/en/fuel-trims/

https://www.youtube.com/watch?v=pIJdCZgEiys

and resetting

https://www.matthewsvolvosite.com/fo...20the%20system.

Steve