RomRaider

What is a vacuum leak?

When the engine is at idle, or cruising speed, the throttle plate is almost completely closed, so the engine has to draw air through a tiny sliver. Since the engine is trying to draw in more air than the throttle plate will allow, the air pressure inside the intake manifold is very low - much slower than the air pressure outside. The intake manifold is said to be "in vacuum." For a naturally aspirated engine, this vacuum generally goes away at full throttle; for a turbocharged engine, the vacuum turns into positive pressure (boost) at with more throttle and higher RPM.

For decades, cars have used this vacuum source to do lots of useful things, that are needed when the throttle is closed but not at full throttle - power brakes, blow-off valves, and even (in the days before ECUs) changing ignition timing. A modern Subaru has a number of "vacuum hoses" hanging off the intake manifold, serving a variety of purposes. When these hoses come loose or crack, the result is a vacuum leak - the intake manifold sucks in extra air through the leaking hose or hose fitting.

What do vacuum leaks do?

Since the MAF sensor can't "see" the extra air that's coming in through the leak, the ECU doesn't add a corresponding amount of fuel. The immediate result of this is a lean burn at idle - AFRs above about 17:1 will cause the engine to run rough, and a big enough leak will push the AFR above 20, where the engine generally not run at all.

Vacuum leaks are most pronounced at or near idle. The amount of air being sucked in through the leak remains fairly constant when you give the engine a bit more throttle - at cruise, for example, the engine draws in more air through the larger throttle plate opening, so the leak has less impact, and the ECU is often able to add enough fuel to keep the AFR near 14.7, where it should be.

If you give the engine enough throttle to get into boost, the vacuum leak becomes a boost leak, and the engine runs rich instead of lean. The engine pulls in less air than the MAF sensor "sees" so the air-fuel ratio goes down.

Signs of a vacuum leak:

Keep in mind that none of these things are definitive proof of a vacuum leak - there are other ways to get the same symptoms. However, vacuum leaks are a typical cause of all of these things, so if you're seeing this signs, you may well have a vacuum leak. But if you've recently been messing around under the hood, where there's a good chance that a vacuum line may have been accidentally pulled off or sliced open, and you're seeing these symptoms, you should probably start looking for a vacuum leak.

The most typical indication of a vacuum leak is an "AF Learning #1 A" of 15%, or 14.99%. On most Subarus, that's the highest that an AF Learning value is allowed to get, and it indicates that the ECU is adding fuel to compensate for a leak. A few Subarus have factory tunes that allow AF Learning values to rise up to 20% or 25%, so if you see anything over 15% there's a very good chance that you've got a vacuum leak.

If you hook up a logger, you'll typically find "AF Correction #1" pegged at 25% or close to it.

If you see the engine running unusually lean right after startup, for example in the 16:1 to 18:1 range, that's probably a sign of a vacuum leak. For a small vacuum leak, the ECU can mask this, by raising AF Correction and AF Learning, so if you aren't looking at the AFR gauge right after startup you might not notice this.

If you see high AFR while cruising (over 16:1 or higher), that's another likely sign of a vacuum leak. If you feel the engine running rough, check your AFR - there are a million other things that can also cause an engine to run rough, but a vacuum leak is one of those things, and it will be accompanied by a lean AFR.

Note that it's normal for the AFR to alternate between the low 14s and mid 15s while cruising - that's the ECU playing games with the catalytic converter for the sake of better emissions. Don't panic, that's normal.

Finding and fixing a vacuum leak:

This can sometimes feel like hunting for a needle in a haystack. That's just the way it is.

The most common cause of a vacuum leak is simply a disconnected hose, so start by looking at every single vacuum line that comes off of your intake manifold. Follow them from the intake manifold to whatever is they plug into. If the far end of the hose is just hanging there, not plugged into anything, you just found your leak. Look for a nearby hose nipple and put the hose back onto it.

Also keep an eye out for cracked or sliced hoses. If you've used rubber caps to seal off unused hoses or nipples, check them for cracks.

Also keep an eye out for bare nipples on the intake manifold. If you find one, which hose was supposed to be attached to that nipple?

Also try looking at each of the things that vacuum hoses are supposed to connect to - bypass or blow-off valves, brake boosters, evap valves, PCV valves, etc. Do you see any bare nipples, and loose hoses nearby?

If all else fails, try gently pulling on every hose you see under the manifold, and make sure it's connected to something at each end. If you find a hose that's not connected to anything, there's your leak.

Look at all of the couplings in your intake tract - are any of them loose? This will usually cause a relatively small vacuum leak, and not a boost leak. The leak will get worse as the engine gets into boost, causing you to run lean at high boost (this is generally a very bad thing).

If it's not a vacuum leak:

If you have the symptoms above, but you absolutely cannot find a vacuum leak, there next things to consider are in the tune:

* Injector latency too low - this will also cause a high AF Learning #1 A, with little or no impact on the cruising AF Learning (usually the "C" range).

* MAF scaling too low - bad MAF scaling can cause AF Learning values to range all over the place.

* Injector scaling too large - this can cause large AF Learning values, but typically (if the MAF scaling is good) it will cause all AF Learning values to increase together.

Thank you for this post. I just had to trace down a vacuum leak last week. My MAP sensor line had blown off the intake manifold after I hit 20+ psi boost. It took quite a while to find it because it is on the front underside of the IM. I had the BPV line blow off multiple times and had zip-tied it, and thought I had zip-tied all other lines, but missed the MAP sensor line.

I also had a ton of issues that I thought were vacuum leaks when I changed to my DW 1000cc top feed injectors. It turns out the latency values they provide are Cobb spec and need to be multiplied by .64 when they are entered into RomRaider.

Very nice nsfw, well done.
dm

Spec B Dream: You're welcome!
dwmoss: Thanks!

What if the fuel trim in A jumps to rich then lean on occasion? Does that mean a problem with the vacuum system ?

If there's a vacuum leak it will typically go to +15% and stay there. If it's not maxed out, it's probably not a leak.

They all wander around a bit on my car, and the A trim does it the most. I haven't investigated it but it might be related to IAT variation and incorrect IAT compensation.

I've been also searching for a vacuum leak in my car, but didn't find any...
My A/F column A goes to something between +6.5 and +9 %... B is between -0.5 and +0.5, C goes between +1 and +8...D is always 0.

Tried another MAF - same results.

But anyway, thanks for a useful writeup

A good way I have gone about finding vacuum leaks in the past is this:
Start the car and have your laptop hooked up logging AF correction (short term fuel trim)
Watch what the AF correction is doing for a little while, it should be fairly consistent (+-10%)
After you've got a good idea of where it is normally at idle:
-Take a can of carberator cleaner, starting fluid, etc. (anything flamable) and spray around one area of the intake manifold or vacuum lines (be careful not to spray around the turbo or exhaust (I've never had it combust, but be SAFE))
-Watch what the AF correction does while you spray the fluid, if it starts pulling fuel while you're spraying it then you know that the fluid is being sucked into the engine and burned- and you're close to whatever is causing the vacuum leak.
-After you have found the general area of where it is coming from you can spray individual hoses or around gaskets to pinpoint the leak

If it is a bad vacuum leak you can usually hear a "hissing" from whatever is leaking, but for small leaks you may not hear it (especially if you have a loud exhaust)

You can also do a boost leak test to find the leaks- typically they will be more pronounced under boost than in vacuum, but if there is a leak in the brake booster line it will not show on a boost leak test because of the check valve that is in the hose.

Hope this helps!

Great post NSFW and nice follow-up 02rexwi. Thanks guys.

Using fuel trims to spot a vacuum leak is very useful and always works. For example, a 2.0L subaru engine takes in about 2g/s of air at idle. If you have a vacuum leak letting in, say 2g/s of air(just a guess), that's a 100% increase in air entering the engine at idle that is NOT being seen by the MAF sensor. As NSFW said, subaru PCM's are limited to, at the highest, a 20% correction in either direction(positive or negative). So the PCM will give max, or near max correction in the positive direction(to compensate for the extra air once it's been "read" by the o2 sensor). As the RPM's increase, the leak because less significant or less of an impact because there's more air entering the engine. A 2g/s leak at idle is more noticeabe than say when there's 50g/s entering the engine. So, if fuel trims(AF Learning #1) at idle are high or maxed out, but decrease as RPM increases, you have a vacuum leak.

hi RmblWagon, my A/F learning #1 % stored A (0 - <5.60) reads 23.50 which I understand would be a vacuum leak but the wide band O2 is reading 4.87 to 5.22 during the idle time. It seems that they should be well above 14.7 with a leak? any ideas why? thanks

You are drawing in more air after the MAF. The high correction means the ECU is adding that much fuel. That makes it rich and the WBO2 is confirming that for you.

thanks dschultz, I`ll start the looking for the leak

^Mikey, u can try logging AF correction and spraying carb spray/starting fluid near where you think the leak may be.

thanks 02rexwi, I will do that