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Guys I would like to know the following in order to troubleshoot my turbo hissing noise
Vacuum when idle: -10.3 psi
Normal vacuum when driving: I get about -11 psi. I believe this is low. It cannot go above this value.
My top boost is about 30 to35psi (35 is rare 30 is usual top) so I have no problems with boost pressure.
Obd diagnostic for boost: I am using obdlink SX (firmware 4.0.1) with android tablet (google play: obdlink app) to get the boost/vacuum readings.
Can you please share your knowledge on this subject?? Thanks
Or can you confirm or correct the below question / statement???
Should vacuum at idle be around 24 (even if it is 20, 10.6 is too low) and when cruising around 28 psi??
This should relate to my high fuel consumption. Anyhow if I dont receive a reply, this is how one should find out if he has a vacuum pipeline problem.
I am in trouble. Note I would thing that vacuum is in mmHg (not PSi).
What I have done upto now:
1. Changed the turbo cut off valve (I had latest version but changed it anyway using Audi origina Part).
2. Have ordered the Brake booster vaccum line which includes internally the check valve.
3. Ordered a smoke detector from USA.
I will report at step 2.
Last edited by apoelistas; 06-23-2017 at 10:44 AM.
I don't know the numbers, but a hissing noise usually means a vacuum leak, which would explain low vacuum. The usual way to find that is with either a spray bottle of plain water (although Windex will work just fine) or an unlit propane torch.
You spray a heavy water mist on all the fittings and hoses that might be the problem, following the vacuum lines. Or, in an area sheltered from the wind and as much of the engine fan breeze blocked off as you can, you run the unlit propane torch around all those same lines.
If there is a vacuum leak the engine will ingest the water spray and you'll hear the rpm's stumble when that happens. If it ingests propane, it will go slightly fuel-rich and you should hear the rpm's momentarily go up. Either way, you should hear the rpm's shift as the leak is found.
The folks who make "rubber" hoses and belts (Goodyear, Firestone, Gates) all say that rubber parts should be replaced every five years because they will be aging and failing. It is very possible that you just have a rubber vacuum line someplace that's cracked from heat. That's all it would take.
My experience with vacuum gauges goes back to the 70s, so I'm dredging up stuff from way back. I recall that with a non-turbo engine, idle vacuum was generally around 20", with a more radical camshaft profile reducing that to perhaps 12-15. I never had a turbocharged engine while I was in my engine tinkering stage, but I would expect a turbo engine at idle to be in the same range, around 20". Now, your app is reading PSI, so it doesn't directly translate to inches of mercury. But at sea level standard atmospheric pressure is 14.7 PSI and rated as 29.92" of mercury. This figure of course changes based on weather conditions.
Once you get off idle, I have no experience with a turbo engine. With my non-turbos, I recall that steady state cruising on level ground would show about the same or slightly higher than idle. Acceleration would of course drop it near or to zero, while throttle-off deceleration would show 30" or higher, usually pegging the gauge.
I think we all agree that I have a vacuum leak. Thanks for confirming.
I believe that safest way is to do a smoke test (feeding the smoke through the air intake). Propane or carb cleaner are not recommended because they are flammable, especially in the hands of a non professional.
A smoke detector I believe will help me find the source.
I tried with water mist but no idle sound change. I wish I knew the location of all the vacuum pipeline.
Last edited by apoelistas; 06-23-2017 at 12:26 PM.
Uh, -10.3 psi on a boost gauge is about 20-21" H2O on a vacuum gauge.
I don't have a turbo Audi (My Q5 is a 3.0T), but I do have a high output modified 2.3L Mazdaspeed 3 turbo. I run a mechanical boost/vacuum gauge. At idle it runs about 20-21 inches of H2O vacuum.
That would appear normal. I'm not saying OP does not have a vacuum leak, but I'm saying that that -10.3 psi is equivalent to what I consider to be a normal vacuum at idle reading on my turbo Mazda which does NOT have a leak.
PERFECT thanks for correcting my results. I agree Bravo. Thanks for the conversion table link.
Idle is ok
Cruising vacuum of 22 seems to be on the low side, so this means that my leak is small (compared to 28 if this is the proper number)
Can we compare this (cruising peak vacuum) to mazda turbo or other 211/225 bhp Q5s???
I believe I need to proper Boost / Vacuum gauge in my car. Any recommendations?
Does mazda turbo or other audi Q5s hiss when on high boost/vacuum?
The hissing means there is a leak (no doubt about it) your vacuum though seems fine as far as numbers go. 10.3psi is about 20.9Hg. have to tried to see what you vacuum is while cruising (foot off the gas) and what psi are you hitting while WOT (wide open throttle). when does the hissing occur?
The hissing means there is a leak (no doubt about it) your vacuum though seems fine as far as numbers go. 10.3psi is about 20.9Hg. have to tried to see what you vacuum is while cruising (foot off the gas) and what psi are you hitting while WOT (wide open throttle). when does the hissing occur?
Idle vacuum I agree seems fine.
Vacuum while crusing is -10.6 psi only with WOT (about 22 Hg using the conversion link above)..
what are the choices of the question when does hissing occurs???? please guide me in order to do a correct test drive with the boost diagnostic. Cheers for trying to help.
when I hear the hissing sound I will get a reading I will do this 3 times. is this the correct procedure???
.................................................. .................................................. .... update from next day test.
Today I got different readings
vacuum with WOT about 11 (22"Hg)
vacuum max 11.6 psi (24 "Hg)
The hissing sound comes when on boost (not at max boost say about 20psi), usually second gear at about 3000-3500 revolutions. when on hill climb this is more noticeable.
Does this sounds like a boost leak rather than vacuum leak or even something else (exhaust leak)??? I am getting confused.
Questions:
1. Wouldn't boost leak hissing be more noticeable when say at 30psi? If yes it is not a boost leak
2. Wouldn't vacuum leak hissing be more noticeable when say at -11.6 psi??
Then it is not vacuum leak
I have no clue what to do. Can you please advice from the description?
Last edited by apoelistas; 06-24-2017 at 04:14 AM.
06-22-2017, 11:46 PM
Q5 boost/vacuum and hissing noise
