Engine: D52447: Hissing noise on power down and increased fuel consuption

[AlexKnopp]

Firstly can I just say thanks to all the posters that have answered common and uncommon questions in these forums over the years. I've regularly found myself finding the solution I've needed from this forum and that wouldn't have been possible without all of you.

I have an XC90 D5 2011 132,000. Its been a brilliant car since the day we bought it about 7 years back. I've done most of the basic servicing myself and the alternator about a year back. Excuse me if this next bit goes on a little, but I want to give you the full context of whats going on.

Part 1 of this problem.
Recently the car has started to use a little more fuel than usual and for the last year or so its had a tiny hiss from underneath the car when under load which then gets louder when the turbo dumps. Lately that hiss has got louder and from everything I can find in the Haynes and on here it would point to the seals on the resonator box. No problems with power, she still goes like a train. A little age related loss Im sure.

I bought myself a cheap £30 OBD reader off amazon and plugged that in, no codes found. Ran the on board diagnostics and no codes found. Took it to the garage and they plugged it into something that cost about 2 grand, no codes found.

So, this morning I dropped the cover from underneath and went all the way along from the inter cooler down to the resonator tightening every clip I came across. I found that the lower clip on the resonator when tightened was simply over threading and breaking the tension so I replaced that clip and tightened that up. The lower resonator joint and the plastic pipe going into it was also very VERY oily. Im guessing there was a leak there somewhere, Ive cleaned it all up so well soon see where thats coming from.

Went for a drive and low and behold, the noise was still there, if not a little louder.


Part 2 of this problem
I thought it also wise to check some of the smaller pipes involved and decided to check that the turbo actuator was working. That seems to be working. However, when I took the vacuum tester out and plugged it into the hose that goes to the back of the engine, from that first T piece out of the actuator there was no vacuum at all. The pip that goes to the front was fine, but the one to the back wasn't. I dont know if its meant to or not but, Im guessing if it is then perhaps this could be why there is a hiss coming from that direction and possibly something to do with the turbo?

I hope this description triggers all the mega minds amongst you. Any questions or clarification feel free.

[SwissXC90]

I would expect you have a boost leak, and the hiss you are hearing is charged air (boost) escaping. This leads to an incorrect air-fuel ratio, resulting in more fuel being burned and more soot being generated.

This fault will not show up on any code reader, as the car will not log a fault code until the DPF becomes full. and you want to avoid the DPF getting full, because it becomes a death spiral of the car trying to regenerate, and not being properly able to, and the DPF getting even more full, until it becomes blocked.

You should carefully inspect the charge air pipe from the turbo charger all the way to the intercooler for any splits, cracks or holes.
Check also the intercooler, and the rest of the intake system.

A smoke tester simplifies this test enormously, and makes it very easy to find the leak.

[AlexKnopp]

Originally Posted by SwissXC90

I would expect you have a boost leak, and the hiss you are hearing is charged air (boost) escaping. This leads to an incorrect air-fuel ratio, resulting in more fuel being burned and more soot being generated.

This fault will not show up on any code reader, as the car will not log a fault code until the DPF becomes full. and you want to avoid the DPF getting full, because it becomes a death spiral of the car trying to regenerate, and not being properly able to, and the DPF getting even more full, until it becomes blocked.

You should carefully inspect the charge air pipe from the turbo charger all the way to the intercooler for any splits, cracks or holes.
Check also the intercooler, and the rest of the intake system.

A smoke tester simplifies this test enormously, and makes it very easy to find the leak.

Hi
Thanks for your reply.
Good thought, so I'll get it in for a smoke test next week.
I've made some additional findings today.
I do have a vacuum tester which I used to narrow down what I think it might be. When I attached the tester to the pipe that I mentioned above in problem 2, the pipe which goes to the rear engine mount, I found there was no vacuum at all. I am wondering if the hissing noise is coming from the mount, seeing as I thought it was coming from the resonator and with both of them being in the same place this could make sense.

[SwissXC90]

Clamp of the vacuum lines to the engine mount. See what happens. if the engine mount cannot hold a vacuum, then you need a new mount

[Tannaton]

I would also say most likely a boost leak - as it's been doing it a while check for oily deposits on the inlet tract from the turbocharger, under then engine, into the intercooler and the intercooler itself, and out of the top into the EGR mixer and inlet manifold. That would be a quick check but a smoke test is the best way if you have access to one.

Intercoolers leaking is very common, and often easy to spot as the bottom and core of the intercooler gets oily.