How do you find a vacuum leak? - Mercedes Benz SLK Forum

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#1 Old 09-05-2013
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How do you find a vacuum leak?

CEL came on and threw a 0170. I am in the process of cleaning the MAF. The recommendation after that is a vacuum leak. Since I just changed the breather hoses, how do I find if leaking/location of vacuum leak?
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#2 Old 09-05-2013
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There are a few ways I've approached this over the years:

One is to use a length of hose small enough to fit closely to your ear. Use the other end as a probe and move around the suspected areas, listening for a hissing sound.

Another way is to use spary carb cleaner that is compatible with catalytic converters and spray it in the suspect areas. If any of it is sucked in the intake tract, the engine speed will change SLIGHTLY.

Hope this helps.
Doc
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#3 Old 09-05-2013
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You can buy a Party Smoke Machine from Walmart for about 50 bucks that works pretty well and have fun at Halloween later. Be careful if you go the carb cleaner route or use any other flammable spray. There is also the cigar method which is kind of crude. Most good Indy shops have a professional smoke machine.
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#4 Old 09-05-2013
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Just to clarify; none of the breather hoses (which deal with crank-case ventilation) would cause a vacuum leak. But working around the old hoses on the engine may have caused a breakage, and this is what I understand the question to relate to.
And (apart from the brake servo breakage mentioned in a different thread) the vacuum system is typically not used for the engine running. (It may be modulated to drive the turbo control in a VVT turbo, as in my Laguna)
So checking for hoses that affect tickover is one thing (one sort of vacuum).
And checking for the loss of vacuum reservoir is a different thing (and its leakage will be apparent for a few seconds after the engine has stopped making its row)
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#5 Old 09-05-2013
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Tolak- Would an improper seal on the breather cover(crank-case) itself cause the issue? So if not the breather hoses, do I just start tracking all of the hoses starting at the engine-test at the engine and then where they terminate? Still new at this so I have no idea what tickover is.

Just as a clarification this was an issue before I started working on the car. It originally had this code (stored) that the dealership ID'ed when doing some insurance work. They suggested new plugs, wires, and replace valve gasket cover. No mention of cleaning MAF or replacing O2 sensors. All of suggested work, I have completed. I do not have any oil trails indicating leaks in the valve cover or crank-case cover since the repair but the 0170 has tripped twice since the repairs. Had it reset and about 5 miles later it came on again.

Thanks for the help
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#6 Old 09-06-2013
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OK, i am actually reading the OP question, rather than following the pack.

The 0170 code indicates a fuel trim malfunction.
I read that as possible fuel control failure as well as possible air control failure.
Did a quick Google, and found this nice site
Essentially, the code is a mismatch between the values required for running the engine (lambda sensors) and teh fuelling scheduled (MAF and engine speed) so the trim logic tries to correct the mismatch,and reaches the end of its allowed adjustment before achieving the balance.
The fuel system drives the injectors based on the assumption the rials and injectors are to spec, and then the injector is opened for a defined period to allow the correct volume of fuel to pass. So you could be lean because of an under pressure rail (bad regulator or bad pump) or blocked injector nozzles (have you tried Techron?), or could be rich because of leaky nozzles (dirt, see Techron, or wear, see MB dealer).
The page mentions the crankcase breather tubes, so it may be that the engine is unhappy with the amount of air coming in through the crankcase breather, and so is adjusting the fuel to match. Double check your breathers, to be happy that all the tubes have stayed fitted as intended.

Is the duct around the MAF oily? If so, a nice "function" of the supercharged system is that this oily air will get pushed out of any ducts or tubes that are not making a good seal, so chase down the oil leaks (again).

More anon
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#7 Old 09-06-2013
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Thanks for the continued help. Put the techron in yesterday morning along with a full tank or gas and pulled the MAF and cleaned it using MAF cleaner. I was going to remove the sensor from casing and did not have the right torx for it. Really strange the T15 is too small and the T20 is too big. Are there T16, 17, 18 torx?

Set out this morning to run the techron through the system. Check engine light went off at 120 miles. Has not returned. Got off the hwy at 150 miles, slightly rough idle. Came home, unloaded the car, 5 minutes, got back in car to run a short errand. Got to the end of the street and the car nearly stalled. Almost turned around and came back home but did not. Stopped several more times in 10 minute round trip and no additional stalling. Still have 3/4 tank of gas, so I would assume that the techron has not run completely through yet.

The breather hose that attaches to the MAF does not fit a snugly as the prior on and you can easily slip it off the MAF. I have not tried it while the engine is running.

Yes, there was oil in the old breather going to the MAF but only a little in the MAF tube itself. I have a V6 not the supercharger. Been monitoring for oil since the tune up, but not today since the engine is still really hot from the drive. Will check later once the engine cools down.

You guys let me know if there is anything else I should be doing and I will keep you updated on the symptoms and hopefully the resolution

Thanks again for the help.
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#8 Old 09-06-2013
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Quote:
Originally Posted by DIYGirl View Post
...
I was going to remove the sensor from casing and did not have the right torx for it. Really strange the T15 is too small and the T20 is too big. Are there T16, 17, 18 torx?

...

The breather hose that attaches to the MAF does not fit a snugly as the prior on and you can easily slip it off the MAF. I have not tried it while the engine is running.

...I have a V6 not the supercharger.
...
The torx on my MAF is a security torx (ie the screw head still has a pin in the middle of the star) My set of torx keys does have the security hole, but being a really cheap set, the hole doesn't line up with teh pin! (D'oh!)
[EDIT: AFAIK, the Torx series advances in steps of 5, until you get down to T10, where the step size drops to 1. Notable exception; there is a T27, which is quite a common screw head size. I have a set that also has T24, which I'd not noticed before checking data for this thread. So T55 down to T10 in steps of 5 (including T24 and T27, excluding T35) and then down to T4 in steps of 1. Often, the next size down will work, so T30 screws can be driven with a T25 driver]

Again, on my car, the duct attached to the MAF has a large hose clip, and this holds the upstream end. The downstream side (upper side on the 230K) is held on with two clips, so should be good and tight.

I remembered you have the V6, so no supercharger, after the listing. Sorry to give a suggestion that is not useful for you.
And unfortunately, there are so many vents from the inlet ducting, that you cannot pressurise the system in the shop, and check the leaks.

We will all think of a good way to check for ways to generate the 0170 fault, including the leakages allowing air into the manifold between teh MAF and the cylinder inlets.

Last edited by Tolak; 09-07-2013 at 02:21 AM. Reason: Add info on Torx bits
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#9 Old 09-07-2013
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Originally Posted by DIYGirl View Post
Tolak- Would an improper seal on the breather cover(crank-case) itself cause the issue? So if not the breather hoses, do I just start tracking all of the hoses starting at the engine-test at the engine and then where they terminate? Still new at this so I have no idea what tickover is.
Sorry, missed the question.
The crankcase can have some airflow through it, but is not expecting much.
Originally, crankcases were open to the air; this tended to be down a tube, so that any oil that was entrained with the air would escape outside the car without making everything dirty.
Then the crankcase had oil scavengers, to reclaim the oil. And then they entered the "sealed" concept we have now, where all crankcase gasses are recycled into the inlet manifold (typically upstream of a turbo/supercharger)

The source of this airflow is in a very small part from water and other "stuff" outgassing from the oil, but in majority from blow-past for the rings, valve stems, and other seals that keep the high-pressure gasses away from the oil. Hence as teh engine gets older, so it beds in (and improves the seals, in the early stage of life) and then wears out (and slowly increases the blow-past). So an older car might have quite a flow of air, which would then entrain oil mist and help it get into the MAF flow. (Oil consumption goes the other way, too; the bottom ring is an oil scraper, and should decrease the amount of oil left on the cylinder walls, but if too much is left on the walls, it is burnt off and exits the system via the exhaust.)

So, yes, a small effect from a poor breather cover, but since this is typically upstream of the MAF, it will be included in the total air measured into the system.

Final comment; tick-over is the idle speed (500-700 rpm), and for carburetted vehicles was the bane of a mechanics life. Now we have an ECU to look after it, lumpy tick-over or wrong speed is much less useful as an indicator of the state of the tubes and carb configuration.
HTH.
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#10 Old 09-07-2013
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Quote:
Originally Posted by DIYGirl View Post
Set out this morning to run the techron through the system. Check engine light went off at 120 miles. Has not returned. Got off the hwy at 150 miles, slightly rough idle. Came home, unloaded the car, 5 minutes, got back in car to run a short errand. Got to the end of the street and the car nearly stalled. Almost turned around and came back home but did not. Stopped several more times in 10 minute round trip and no additional stalling. Still have 3/4 tank of gas, so I would assume that the techron has not run completely through yet.
The ECU learns how it needs to control the engine, so that each time it starts, it does not have to begin from default.
(I had that; disconnect the battery whilst working on the car, and it took almost two minutes to get the engine running properly. Mind you, once I cleaned the MAF, the same cycle was almost immediate)
In your case, it still remembers that it has to over-fuel the engine (beyond what it considers acceptable limits, hence the fault) and only finally works out that it can reduce the fuelling to within expected ranges, and hence removes the "active fault warning" = CEL.
The highway running will have changed the parameters for the highway running; you then had to do the same for the town running, hence the other part of the cycle.
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#11 Old 09-07-2013
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Wow Tolak- That is great information to have. Thank you
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