Vacum Leak Smoke Test

2001 F150 V6 150k miles. ODB II codes P0171 and P0174, system too lean.
So were sucking air.

I want to perform a hack smoke test to find this leak. I do not have one of those expensive smoke testers and I don't want to buy one. Instead, I am thinking of a workaround/hack and wanted to get some opinions.

The carb cleaner spray didn't work, I don't want to use propane because I don't feel comfortable with that, the MAF checks out good, and I have no reason to believe the fuel pump or filter is the problem because the truck preforms strong/normal on the road.

I am thinking about finding/buying a can or two of compressed smoke. I hear they sell it at party city or one or those Halloween stores. Then i want to rig the air to blow into a vacuum line somewhere on the intake manifold ...seal off the main air intake. I believe the main pressure for the vacuum system is about +-8 to 10 psi so I don't want to blow strong pressure, just a slow steady flow. ...please correct me if i'm wrong.

So I am thinking (uh oh) that as long as I slowly blow smoke into the vacuum using this hack method. ...maintaining a very mild and slow flow of air/smoke going in, I should be able to spot the vacuum leak. Right?

My stupid questions are:
Is this a good or stupid idea?
Do I need to concern myself with the kind of smoke I use?
Is this hack dangerous to anything in the intake that I haven't thought of?
Should I do this on a hot, warm, or cold engine or does it matter?
If this is a good idea, what main vacuum hose should I blow into?

Thanks in advance.

You seem as if you are making it to complicated - check out these links --- http://www.aa1car.com/library/ford_lean_codes.htm and http://www.stangnet.com/images/stories/docs/sn95_TSBs/04-17-04.pdf


Good luck :loser:

@AutoService101 - Thanks for the reply. I read the information at the links you provided. I have already done everything except troubleshoot the fuel filter and pump. I dont think they are the problem because there is no performance issues. I really think the next test has to be a smoke test of some kind. Actually I don't think its very complicated to attach a can of smoke and blow it in the manifold. All i need to do is make sure I find the best possible hose to blow into.

Fuel pressure can still be low and not cause noticable driveability issues. Eventually it will get low enough to cause driveability or starting issues. It is worth a check. As far as a vac leak at intake I am assuming you have sprayed around intake with carb cleaner?? I guess I have never seen the need to use smoke to find a vac leak before. Usually they are pretty easy to find. I wish I could be of more help.....

yes, i mentioned the carb cleaner test above. Its slightly difficult because carb cleaner is not as potent as it used to be...environmental stuff i guess. Also, its hard to get a reaction unless there is a gaping hole and it seems if that test is going to work than you should be able to actually hear the air leak. So i don't think its a reliable test for smaller leaks. besides, gunning the motor to 2000 +-5 rpm and spraying carb cleaner on it just makes a whole lot of noise and creates a whole lot of heat. I know these engines are notorious for vacuum leaks as they age. I want to be sure that this is not the issue. So a smoke test seems to be inexpensive and simple. pull a hose plug it wit the can of smoke and spray. The cans of air cost about as much as carb cleaner and I can test on a cold engine. My main issue was only whether or not the kind of smoke would cause any issues, and whats the bast hose to blow the smoke into. Oh. Also, If it were fuel pressure shouldn't there be more codes than 171 and 174?

I have never done a smoke test to determine a vac leak before to be honest so I have no idea. I thought that you had to have a smoke machine (I have seen these for checking fuel system for leaks). I have never seen a "can" of smoke. I would not think that the can would beable to produce enough smoke. If so I am interested on what you find or have found.
I would have to think that if you have no driveability issues that if you had a vac leak it was fairly small and may only occur if enigne was hot or cold (probably not leaking all the time, yet). I have used carb cleaner with much success in the past and you should not have to rev up the engine at all. you should be able to check both hot and cold this way.
To be honest with you from what you say I do not believe you have a vacuum leak. I would think that if you have eleminated everything else the fuel pressure would be the next logical approach.

1. Remove air filter and wrap with glad wrap.
2. Put air filter back in.
The idea is to block all air going to the intake.
3. Remove the vacuum hose going to the brake booster.
4. Using a black and mild cigar to blow smoke into the
Brake booster vacuum hose and plug with thumb.
5. Watch for smoke leaks!
Smoke leaks is your unmetered air leaks, or vacuum leaks causing the p0171 DTC.

1. Remove air filter and wrap with glad wrap.
2. Put air filter back in.
The idea is to block all air going to the intake.
3. Remove the vacuum hose going to the brake booster.
4. Using a black and mild cigar to blow smoke into the
Brake booster vacuum hose and plug with thumb.
5. Watch for smoke leaks.o
Yes that sounds like a good way to do it, hope you like cigars tkswan (http://www.automotiveforums.com/vbulletin/member.php?u=520564)!! :smokin:

No, im not a cigar guy but that's interesting. Thanks for the ideas and I will check the fuel system later, this test seems really easy to do and it will eliminate what I suspect is really happening. I found Smoke in a can fairly easy. About $2 a can and I will get way more smoke from a can than I will from a Black and mild, and I won't even have to put my mouth on anything...:) I'll give it a shot next time the truck is here, probably tomorrow. Thanks for all the comments. That's exactly why I come here.

Look forward to hearing what you found out :naughty:

Even if you block off the intake as mentioned, you'll never build any pressure. Because of camshaft overlap, some of the intake valves will be partially or completely open and due to the overlap, there could very well be an exhaust valve open on the same cylinder allowing the smoke, and pressure, to exit the tail pipe. Without a smoke machine, the easiest way to locate a vacuum leak is with propane.

If you know anyone even with a cheap scanner that shows fuel trim, my 159 dollar accutron does from the zone, watch the fuel trim and again do the carb cleaner. These newer engines are so touchy with the numbers that most times you can see the fuel trim changes with the carb cleaner, but may not change the engine note..

@old_master (http://www.automotiveforums.com/vbulletin/member.php?u=408290). The intake side will be sealed off by blocking air coming in from the main air intake hose off of the filter. Since the exhaust side is not an "open system" it should take much more pressure for the smoke to pass through the catalytic converter. The converter will cause the smoke to back fill and blow out of any openings first, before it can build up enough pressure to pass through. If not just plug the tailpipe. Right?

@jyount (http://www.automotiveforums.com/vbulletin/member.php?u=149924) I have an Acetron ODB II but I don't think its the scanner you are talking about. Besides, how will that test tell me EXACTLY WHERE the leak is? The Carb Cleaner test didn't work very well because the leak is obviously not big enough, and carb cleaner isn't as potent as it used to be because of the environment. Given those two factors, its very difficult to find a leak with carb cleaner unless you can actually hear the air sucking through it. All the carb test did for me was make a lot of noise, generate a lot of heat, and waste a can of carb cleaner.

...Since the exhaust side is not an "open system" it should take much more pressure for the smoke to pass through the catalytic converter. The converter will cause the smoke to back fill and blow out of any openings first, before it can build up enough pressure to pass through. If not just plug the tailpipe. Right? ...


Apparently you haven't tried looking through a catalytic converter: you can see right through it. IF the exhaust system has no leaks, and the EGR valve seals 100%, a potatoe in the tail pipe might seal it off. Then, if you start blowing smoke in the booster line today, you might build up enough pressure next year some time. Let us know how it goes ;) Sounds like a lot of "back alley" fooling around to me. Why not take it to a professional and pay him to check it? As jyount mentioned, watch fuel trim. Your onboard computer is an amazing piece of diagnostic equipment in itself, if you know how to use it.

@old_master:
Much on this post (that you surf) is "back alley" fooling around. We seek suggestions form other "back alley" experts. A few of these back alley experts :) show a wealth of expertise yet never has any answers. Not all "back alley" folks has a hi-tech diagnostic facility in the "back alley" or at their disposal so we in the "back alley" have to do the unthinkable. Improvise. Yet some of us "back alley" folks aren't not ready to bend over so quickly and take the lazy way out and hand it off to a professional just yet because some of us "back alley" folks simply like a challenge.

for example: How is reading fuel trim going to tell me EXACTLY WHERE the leak is located (i.e.manifold gasket, hose, a crack somewhere etc)? So actually, your professional "back alley" suggestion from another user doesn't even address my simple "back alley" question. Much of this forum is for learning it as you go along. So my bad if I misspoke. No I have never looked into a cat converter. I actually thought it was a little more stuffed since it has to act as a filter. Thanks for correcting that. See. I actually learned learned a little something from you this year.

If you really want to see where I get the smoke test idea look, check this out:
http://www.youtube.com/watch?v=FfiQ7qWiG-M&feature=related

Also, if my back alley post annoys you than maybe you should keep your back alley comments to yourself dude. your arrogance is unproductive... :)

Nice Day!

Forum members experience ranges from Saturday afternoon rookies to 40 year + professional veterans that make their living every day by diagnosing and repairing vehicles. The accuracy of the advice that is given here ranges widely as well. My only point of "back alley" is that with the higher technology of vehicles now, the tecniques that worked on an older vehicle, might not work as well any more, that's all. As I mentioned earlier, "Your onboard computer is an amazing piece of diagnostic equipment in itself, if you know how to use it." However, there are some things that should be left to a profesional.

Checking for engine vacuum leaks can be done very effectively and quickly by directing propane to suspect areas while watching fuel trim. With the engine at idle, fuel trim numbers will stabilize, introducing propane will affect these numbers immediately, identifying the vacuum leak. The smaller the stream of propane, the more accurately you can pinpoint the vacuum leak.

A good cigar is so much better than a $1200 smoke machine, or $80 Test!
By plugging the brake booster vacuum hose with thumb during puffs of smoke, the smoke is going take the path of least resistance.
The smoke test is so much safer, than flamable tests!

A 50cent Black & Mild shorty + a recycled grocery bag works the best!

See http://www.youtube.com/watch?v=zMok2y05jNE&list=PLE6DAAE29CD944BE2