Questions about deleting the fuel vapor canister in the trunk

[GGR]

Hi all,

I want to delete the fuel vapor canister which is on the left side in the trunk. But I didn't figure out a way to let air in the tank. Both the ports in the filling neck seem to continue under the level of the gas inside the tank, as I'm getting gas when I connect a pipe and apply vacuum. Is there something wrong there? How do I let air in the tank? Do I need to drill a small hole in the cap?

Thanks!

[mdsalemi]

Gael,

I thought I had answered this question somewhat, years ago. Sure enough, I did. Here's what I said some time ago...

Understand that we have two different things here, or at least two models.

1969 and earlier was a tank to accomodate expansion of the fuel in the main tank; it expanded into the overflow tank via vent lines, and the vapor (or fuel if the expansion was too great) vented to atmosphere via a small vent line from the expansion tank to the atmosphere; you can usually see this little pipe near the exhaust; sometimes it has a cone shaped end.  I've cut these open, there's nothing inside but the ends of three lines.  Two connect to the vent lines on either side of the filler neck where it enters the tank and the third vents to atmosphere.  Nothing else.

1970 saw (as the photo shows) the elimination of allowing fuel and vapor to escape to atmosphere (smog you know) and there was the secondary unit (were they all one??  I can't tell) that had valves and maybe internally, a carbon canister.  This kept the system closed.

Presuming you have a 69 or earlier, understand you don't have a vapor canister but an overflow tank. If your main fuel tank is full, and the ambient temperature rises, fuel will expand through those "vent lines" at the top of the tank, into the overflow tank. As it cools it will be drawn back in. One of the three connections on the overflow tank is a true overflow, and excess vapors or fuel that expands beyond the tank's capacity is vented to atmosphere. There's nothing inside that tank--no valves, no filters, no carbon canisters. Later USA models--1970 or so? may have altered the arrangement...

[wjsvb ✝︎]

Gael:  Another direction might be to replace the cap with another which is vented.  I will, later today, check my Stant catalogs to see if there is a vented cap listed for 113's.  If there is, it would be available at Autozone or such.  Amazing what you can find if you have old catalogs.  Unfortunately it won't be the original locking type but a shiny stainless cover.  Jon

[GGR]

Thanks. My car is a 71 model, and I have the bigger canister in the truck. I wanted to replace that system by the simple overflow system. However, it looks like the lines connecting into the filler neck are prolonged under the gas level in the tank. As a result, when pressure builds up in the tank, vapors are not evacuated via these little ports. Instead, pressure seems to push gas out of these ports, and I can hear gas in the canister when I shake it. When I open the tank, there is a big "whoosh" as pressure escapes.  According to Michael, vapors should be vented first, and then gas if the tank is full. That's not what's happening in my case. I would like to know if Michael's assumption is wrong or if there is something wrong with my tank. I checked on EPC and there is only one p/n for the 280SL tank, so it wasn't modified with the different venting set-ups.   

A vented cap would surely be the simplest solution. But I wonder if gas would be escaping from there when the tank is full or under hard acceleration.

[mdsalemi]

Do you have any operational issues?

Years ago, I had a tremendous whoosh as you describe. I also had a situation where the car was less than ⅓ full, I'd get misfiring. My local mechanic at that time said "we need to change the tank". The tank was changed, and all operational issues went away. Micro-rusting of the inside of the tank after 40 years. New tank: problem solved.

With the new tank, the whoosh sound decreased significantly. It's still there in the summer when hot. Operational problems gone.

What you describe sounds like it is performing as it should. I didn't say vapors should be vented first; I indicated that the overflow tank will do just that: accept an overflow of expanded gasoline (due to temperature and pressure) and then drain that out the back via the FST (funnel shaped thing) when needed or necessary. Since the fuel tank is "sealed" as it were with an unvented cap, you need somewhere for hot fuel to go upon expansion. The tank is the solution. There's no vapor recovery inside that tank...

[GGR]

I had operational issues in summer similar to what you describe. A couple of times the car died on me, for no reason. Letting the pressure out of the tank did bring everything back to normal. Conversely, opening the tank to let pressure escape while the engine is running would also have the car die. It would then re-start with no problem.

I also suspect there is something wrong with the tank, though I would like to determine exactly what the issue is - clogged venting port? I'm asking as if I am to replace the tank, I would most seemingly have a shop build me a Franken tank with the top of the Pagoda one to keep the filling neck in the right place and the bottom part od a 6.3 105L one, to increase range during long trips. I have a spare 6.3 tank though it needs to be opened for cleaning and remove some dents at the bottom. I want to make sure I don't miss anything while both tanks are open.

[stickandrudderman]

GGR. If you lived locally I think we'd be mates!

[GGR]

GGR. If you lived locally I think we'd be mates!

Ha Ha! I think so too! I would be handing you wrenches and screw drivers while you're working on your plane!

[mdsalemi]

I had operational issues in summer similar to what you describe. A couple of times the car died on me, for no reason. Letting the pressure out of the tank did bring everything back to normal.

Please, do search and get to the bottom of this and report back. I suspect I know the problem: your tank. I suspect I know the solution: a new tank. But, it is (as I'm sure you'll agree) far more interesting to know the little details. Believe me, years ago I posted and asked and inquired and had all manner of people look at this...the only sure fire reliable answer was the mechanic who said new tank. Yes, it was $1,100 at the time but it solved a couple of years worth of living with not knowing whether or not I'd get home...

[Shvegel]

Interesting problem. Is the whoosh you are experiencing pressure escaping or is it relieving a vacuum?   I am assuming(since I don't have my cap available right now) that vacuum is relieved through the cap itself since there would be no harm from an emissions standpoint to let air into the tank. if this is the case there should be some signs of a vacuum vent on the inside of the cap like a small hole. This should have a a check valve to allow air in but fuel vapors out. Not sure about our cars but on more modern cars there should also be a relief valve in case there is too much pressure in the tank?

WJSVB might be on the right track here. I would walk into an auto parts store and see if you can find a generic cap for your car and see if it helps with the problem and If it does take your original cap apart and figure out what is wrong with it.

You also might want to remove the two small hoses in the trunk under the filler neck cover and make sure the fittings that attach to the tank are not clogged.  If they are it could be the root of the problem especially if one of the small fittings goes to the bottom of the tank as this would cause exactly the symptom of fuel being pushed up into the overflow under pressure. It is also very possible that such a small fitting attaching to the top of the tank could get clogged with rust.

Do not use compressed air to check or clean the fittings as this can create static electricity and cause a spark. If you do have a clogged tube try running some stainless wire down the fitting. Malin Co. makes various diameters of stainless aircraft lock wire.

[GGR]

Thanks Shvegel. The "woosh" is pressure coming out. I will see if I can use some wire in the ports next time I'm in there.

[mdsalemi]

I am laughing at Shvegel's and GGR here...these are all absolutely identical issues, recommended solutions, questions, etc. that I dealt with. YEARS ago.
Since we (me and several mechanics and intelligent people here too that we all know and respect) were trying every possible solution EXCEPT replace the costly tank, we did it all. We checked fuel pressure and volume, we changed the filler cap, we snaked out and blew out all the fuel lines, we changed fuel filters, we cleaned screens, we checked things in the tank and on and on it went.

The beauty of what we did is I did discover one issue compounding the runnability problems, and that was a compromised electrical feed to the fuel pump. That doesn't and didn't change the tank.

When I brought the car after all this to another mechanic (who was reported to have apprenticed in Germany, and worked at Sindelfingen in the late 1960s) who knew these cars well, he heard my story, and in one Herzschlag said "new tank". He pointed to a literal PILE of old discarded fuel tanks from Pagodas and other similar cars (250SE, 280SE, etc.) in his shop and said, "See? All similar problems. All similar solutions. Replace the tank and your problems will disappear."

I did (well, he did) and the problems went away. His claim is that after 40+ years the tank is continually rusting away from the inside, even with microscopic rust. Even if you were to be able to remove the tank, open it up, clean it out, and put it back, your problems will quickly return. I had no choice but to believe him due to respecting his experience, and the fact that we tried darn near everything else INCLUDING a new fuel pump before getting to this guy.

What I did NOT do is salvage the old tank to cut it open and really get to the bottom of what gets plugged up inside, and why it causes the issues it does. But with a fine running car, that was the least of my concerns as I drove away (cue the music) into the sunset of a fine August day...  

Of course there might be disbelievers out there, and the possibility remains that the mechanic was a cad, a shyster, and what he really did was unnecessarily change my tank, AFTER tweaking the miracle, hidden screw that only he know about to fix all my problems. I honestly don't know. I do know for certain I did indeed get the new tank I paid for, and that all my problems I brought the car in for went away with the new tank. Whoosh. Just like that!

With the new fuels containing more ethanol than ever, I suspect my "new" tank will not last as long as the old tank did, which during its lifetime saw mostly fuel w/o any ethanol at all. As we all know ethanol has an affinity for water, and the water/moisture dissolved in the ethanol in the fuel accelerates the degradation of the fuel tank insides.

[GGR]

I would be interested in contacting this other mechanic who recommended replacing the tank. He may know exactly what is going on in there which would be helpful in the build of my extra capacity tank. You can PM me his contact if you prefer. Thanks in advance.

[andyburns]

Hey Michael,

Did you ever try acid.  I have had every single tank I have ever recommissioned acid stripped and pickled.  They come back clean as a whistle with every last trace of internal rust gone if the acid guy has done his job.

Modern epoxy tank sealer then applied, if applied correctly, should bring the tank internals up to a 'better than new' rust resistance.

I cant for the life of me imagine why this wouldn't be a permanent and easy solution if the tank was in reasonable condition to begin with.  If you can blow compressed air freely down the tank vent and also confirm the flower post fuel access holes are free then I cant see why you don't have a perfectly serviceable tank?  I own a cheap inspection camera that I snake into the tanks and check to see if everything is ok.

The issue I had with my injectors being blocked was 100% resolved with acid.  These would seem a much harder fix that the tank internals.

My next door neighbor is a very talented professional A grad mechanic.  I have had several discussions with him over the years regarding his approach after watching him working.  He gets things done very very efficiently and quickly.  He has to as for most situations he has the clock on him.  As such he has developed a replace rather than repair mentality which has become ingrained. 

I would hate to scare off anyone in here from trying to repair a tank if possible.  They cost over 1000US from the dealers.  I have now repaired about four of them for as little as 150USD with no problems showing thus far in the cars.  Internally they seem to be very very simple.  I still don't quite understand what all the fuss is about.  Perhaps I am missing something. 

[Mike K]

I have the same issues, pressure build up and WHOOSH when opening the tank cap. This is after installing a new tank.

When I purchased my 1971 280SL last year I had running problems, which with the help of members on this site we traced to a rusty old tank.

I replaced the tank with a new one, new fuel sender, new Bosch fuel pump and new fuel lines etc.
Running problems were instantly solved.

Since installing the new tank and during the warmer months, I've had the WHOOSH when opening the fuel cap, along with some intermittent back firing.
As the weather's cooled down in the past few weeks, this has stopped. I've also stopped filling the tank up completely when refuelling.

I've been thinking about disconnecting the 2 pipes from the fuel filler neck and blocking those holes. And rather than damage my locking chrome cap, I've purchased a €19.00 generic fuel cap which I plan to drill a small hole into? I'll have to try this in summer when the problem seems to occur.

More thoughts and suggestions please...

Best,

Mike


 

[GGR]

I have the same issues, pressure build up and WHOOSH when opening the tank cap. This is after installing a new tank.
 

Well, you may have to replace your tank by a new one again!

More seriously, if your tank is new, your issue may be with the expansion tank or the lines that go to and from it.

Andy, I also had good success with the process you describe. I wanted to fit a used 6.3 tank on my Coupe which was full of gunk and the lines to the pump and for fuel return were blocked in spite of going in there with all kinds of rods and wires. I had the tank dipped into acid. This got rid of all the gunk and the lines got freed. I immediately treated the internals with the POR-15 tank sealer and then also painted the outside with POR-15. The tank has been on my car for about four years and 70.000 miles now with no problems, and for a fraction of the price of a new tank.

For the Pagoda I will have to proceed differently as I want to mate together the top and the bottom of two different tanks. There is a shop close to me which opens the tanks in the process of renewing them. I may go to them, have both tanks opened and then check out the internals of the parts I want to use before they mate them together. The bottom needs dents removed anyway. In the meantime I may see if I can have the tank vent through the cap.

[Shvegel]

In response to Michael:  You can fix a swollen toe by cutting off the leg but wouldn't you rather just fix the toe?  This is not a complex issue. It is a few hoses, a gas cap and a tank.  The question is how is the tank building pressure when theoretically the fuel level is dropping. There will be some heating of the fuel since both the electric pump and the injection pump circulate the fuel through warm areas.  The early cars that used the overflow bottle rather than the complete evap system should have some provision for venting the tank so it is only a question of finding out what it is and fixing the problem.  I am not adverse to throwing money at my car but given the choice of simply buying a new tank and  trying to fix the old one I will at least try to fix it and if successful I will send my wife off to a spa for a couple days and be money and relationship ahead.

I have also seen tanks cleaned using electrolysis.  All that said if your tank is rusty you should replace it.  Tanks are expensive but injection pumps are even more so.  Fuel filters can only do so much but they will never get all the rust flakes and tiny rust flakes running through an injection pump can wreak havoc. 

[Novamonte]

I had exactly the same issue with my -66 - pressure in the tank that would not vent through the overflow tank and a big whoosh eveytime I removed the gas cap, especially on hot days. Turned out to be clogged vent lines from the tank. I cleaned them with steel wire - problem solved.

[Mike K]

I did the same exercise yesterday, pushed a steel wire through both vent lines, then blew some air through with a compressor.
After a long drive, opened the cap & WHOOSH is gone...

Thanks,
Mike

[mdsalemi]

In response to Michael:  You can fix a swollen toe by cutting off the leg but wouldn't you rather just fix the toe?  This is not a complex issue. It is a few hoses, a gas cap and a tank.  The question is how is the tank building pressure when theoretically the fuel level is dropping. There will be some heating of the fuel since both the electric pump and the injection pump circulate the fuel through warm areas.  The early cars that used the overflow bottle rather than the complete evap system should have some provision for venting the tank so it is only a question of finding out what it is and fixing the problem.  I am not adverse to throwing money at my car but given the choice of simply buying a new tank and  trying to fix the old one I will at least try to fix it and if successful I will send my wife off to a spa for a couple days and be money and relationship ahead.

I have also seen tanks cleaned using electrolysis.  All that said if your tank is rusty you should replace it.  Tanks are expensive but injection pumps are even more so.  Fuel filters can only do so much but they will never get all the rust flakes and tiny rust flakes running through an injection pump can wreak havoc. 

Bringing medical issues into this is funny, but that's about it. Makes no sense otherwise. Often times here, people will spend an inordinate amount of time and money trying to repair or refurbish something where the "clinical diagnosis" should be replace. In my particular issue, that overflow tank was cleaned out; all the lines to this tank were replaced (there are hard and soft lines); the entire fuel system was "gone over" by experts some of whom you know here. Hard fuel lines had been replaced just a few years earlier; soft ones too. Still, they were all disassembled and blown out. Filters where changed. This was over a 2 year period.

The last mechanic said that there is rusting on the inside of the tank; small micro particles that will return soon after you clean everything up. So, the only solution is to replace the tank. It was the last thing I did since nothing else was left to do, and everything else had been looked at.