Trying to find an EVAP leak on my 93 AWD 4.0 liter.
I removed the hoses from the carbon canister and tried blowing into them. The one going to the engine seems blocked or there is a valve shut somewhere. The one going to the gas tank seems clear but I have to blow into the hose quite hard.
I blew smoke into the canister in the hose from the gas tank and smoke came out of the top of the canister, I assume this is where the fumes vent? I don't know, is the smoke supposed to come out of the top of the canister or is it faulty.
I'm hoping someone can point me in the right direction to find this leak and maybe explain how this system works. It seems that one could just plug the system to pass the EVAP test, or is there a sensor to show that it's plugged?
This is all new to me, I'm from the East Coast originally and never dealt with smog tests until now in California, the pitts!
Thanks for any help...

 

The top of the canister has a vent hole that is supposed to open up when the engine side gets vacuum. There is a valve in the line going to the intake manifold that does this, when the EEC decides to. But supposedly the vent is only supposed to open when the valve opens up to allow drawing of the fumes into the intake. It's not supposed to allow fumes to leak into the atmosphere, which would defeat its purpose.

There may be more to it, but this is about all I remember.

 

Thanks, still looking for information...

 

(Faulty duplicate post was deleted)

 

How was your EVAP leak discovered? I'm guessing here that it was likely from CA emissions testing program. Unless they specifically told you the leak was from the EVAP canister system, a faulty gas cap could also trigger an emissions failure but the cap is usually tested separately.

The EVAP system is a very simple setup. A vacuum line from the gas tank vent to the carbon canister and a second vacuum line from the canister to a small purge valve and then a line from the purge valve to the intake manifold. The electronic engine control (EEC) determines when the purge valve opens and closes to pull canister fumes into the engine manifold.

The rubber 'elbow' connections on the canister vacuum line seem to break down after years of use and develop small cracks, almost like weather checking tire rubber. Eventually leaks can develop. My '91 4.0 had this problem. A short term solution is to just coat the elbows with black silicone or find replacements.

Another source of the problem may be the purge valve. It sits under/next to the bottom of the intake manifold and is exposed to a lot of engine heat. Again, over time, the plastic nipples the vacuum lines attach to becomes brittle and can easily break away from the valve body. This is especially true if you some has been rooting around in the engine compartment, tugging on lines to see where they go. Be very careful when trying to locate the purge valve and DO NOT tug on the vacuum lines. As one who committed this sin, you don't want to have to purchase a new valve just because one small nipple broke loose. There is no easy fix.

The vacuum line running from the canister body to the purge valve runs past heater hoses and a radiator hose. It is possible that a misrouted hose has rubbed a hole in the vacuum line. Since you have tried blowing into this line and find it "blocked" would tend to indicate there is no leak in this line. The blockage is the purge valve in a closed state.

There are diagnostic codes for the EVAP system. If you do not have these, most parts stores can pull the codes for you for free. Your model year likely uses OBD-1 codes, and will return both a 2 and 3 digit code. The two digit just specifies which major system is problematic while the 3 digit is more specific as to which component is faulty. A specific code number will greatly help you pin down the problem area.

Perhaps one of the posting gurus here can find a schematic for you to see the location and visual of what the various components look like.

 

Wealth of information, thank you. Yes on the failed Ca. leak test (they only told me "gross leak") but passed the emissions part, barely.
I am quite familiar with rotted vacuum lines but not emissions, and have tugged on none (so far).
I think my biggest problem is locating these evap parts in order to diagnose the issue. Then getting at them is another story.
Thanks much for the info, going to see what I can get accomplished now, thanks again.

 

I started to smell gas fumes from y Aerostar, both while parked and driving. At first I thought it might have been the gasket on the fuel sender plate, which I've re-seated before. But after replacing that, the odor persisted. On a hunch I looked at my evap canister (had to remove the air filter box to get to it), and found that elbow had somehow come off of the nipple on the canister. The elbow was also kind of deteriorated, as Aerocolorado described. It seemed the line leading up to it was a little tight, and maybe that's why it pulled off. But I noted that the elbow on my Mustang, 3 years older, was still in good shape. After verifying that the nipples were the same size, I got another elbow from my local salvage yard, from the same vintage Mustang, and replaced the rotted one on the Aerostar. The gas smell has stopped, so I think I fixed that problem.

Here are some simple things to check:

If you pull the tank vent line from the canister and pull off the gas cap, you should be able to blow through that line with no obstruction. If you replace the gas cap, you should not be able to blow through that line.

If you pull the engine line from the canister (engine off), you should not be able to blow through it, as the purge valve should shut off the line.

If you then re-install the engine side line, and install another line to the tank side nipple and try to blow into it, it should also be blocked. The vent cap on top of the canister should be one-way, only allowing air to go in when the engine is purging the canister (opens with negative pressure).

I would not recommend sucking on any of these lines, as they can all have fuel vapors in them.

 

Okay, here's what I got so far.
I think the line from the tank to the canister is good but the top of the canister leaks when I blow through the line from the tank.
I took off the hard plastic caps two of them, (they are supposed to be hard aren't they? I'm thinking they hardened up and now leak.) anyway, I blew back through the line-in and it was blocked good when I covered the holes with my thumbs. I figure that must be my "gross leak".
I have one free retest for smog and I'm gonna cap these things off and test it.
Thanks everyone.
Still open to any more thoughts!

 

If those caps are leaking, you might try to find one in the salvage yard that is not leaking.

If you plug the holes, then the next time the EEC tries to purge the canister, it will be drawing vacuum on the tank, and may suck in too much fumes. That's obviously going to make the engine run too rich, and if it happens during your test, it have a greater chance of failing.

 

So you're saying the canister has failed and needs to be replaced?
I didn't seal those holes off yet, just didn't seem right.
Those caps look as though they're designed to vent with ribs built into the mating surfaces. I imagine there is no fix for the canister itself huh?
I don't understand how this thing won't leak with caps like this. There must be some kind of internal valve in the canister, I'll take it out and see what I can do if anything.
Thanks.

 

I was sitting here wondering, aside from the gas cap test, how an EVAP test would detect a leaking EVAP system. I ran across this answer:

[I]Answer: The EVAP Functional Test (LPFET) is in addition to the smog check test. All 1976 to 1995 model year vehicles will be tested, which includes all pre OBDII vehicles subject to Smog Check. The most important impact on consumers is that the emission reductions will improve air quality and reduce their health risks. This test is designed to insure your vehicle's fuel evaporative system is not leaking gas fumes in to the atmosphere. It is estimated that over 7,000,000 vehicles will need to be tested each year and of those 11% will fail. The average cost to repair a failed EVAP system is estimated to be approximately $250.00.


Here's how the EVAP Test work... All vehicle's have an EVAP hose which runs from your vehicle's gas tank to the engine's charcoal canister. The charcoal canister is usually located in one of the far corners of the engine compartment. It is normally a black container filled with charcoal pellets that absorb fuel vapors. Not all charcoal canisters however are located in the engine compartment. Some vehicles manufactures have mounted their canisters closer to the vehicle's gas tanks in order to maximize efficiency.



The EVAP test insures that there are no leaks in the hose between the Gas Tank and Charcoal Canister... it that simple. The technician must clamp the end of the hose leading to the canister and pressurize the gas tank with Nitrogen via the EVAP test tool. The EVAP test tool will then calculate the drop in pressure due to any leaks. Technically there should be very little drop in pressure once the gas tank is pressurized.Should there be a pressure loss the machine will fail the EVAP portion of the smog test.[/I]

It would appear your problem may not be the carbon canister, purge valve or vacuum lines, but the vapor valve mounted on the fuel tank. This is the only way pressure could escape during the EVAP testing procedure if the hose leading to the canister is clamped off during the test procedure.

The valve can accessed from the top of the fuel tank, but that said, it is a difficult procedure to drop that big Aerostar tank. Especially one full of fuel.

To my knowledge yours is the first reported problem of this kind. The question then becomes what to do about it. That vapor valve also acts as pressure relief valve to allow excessive tank vapor pressure to vent to the atmosphere. (Near empty tanks develop the most vapor pressure in a hot sun.)
Perhaps you should try filling the tank completely full before the next emission test and wait for a cool morning to go. But this might be a moot point if the testing station pressurizes the valve beyond its current retention limit.

If you are a conspiracy theorist, you could make a case the State is deliberately over pressurizing tanks on these older vehicles to get them off the roads. Hmmm, yeah, I could go for that.

This all I can think of at this time but others will have some good suggestions as well.

 

If I blow into the hose leading to the tank from the canister, there is a great deal of resistance, as if I am trying to compress air. If I take the gas cap off, it blows freely. Leads me to believe that the "tank to canister" connections should be OK. If I blow into the canister, into the nipple from the gas tank, it leaks from the top of the canister.
There are two caps that look like they're supposed to vent but must be controlled by an internal check valve or something.
This canister should not be venting like it is.???
Hate to go buy a canister and find I don't need it.
Anyone else?