pressure buildup in tank.....

[erickmarciano]

Hi guys got a problem that I can't solve.
My car is a 71 euro with small compensating tank
Got a fuel stain on the rear bumper and this is what I did so far.
1-took the small tank off . tested all 3 inlets and they hold no vacuum . pressure tested and it holds vacuum < tank has no hole>.conclusion tank is good

2-changed all the hoses going to the top of the tank , lines were free but still cleaned them with a guitar wire .

3-pulled a vacuum on the 2 small lines going to the tank and they are ok did the same with the wire .
put all back together and the problem persits .if I pull on the cap I hear a shuuu of pressure.
4-drove for an hour and stopped the car. I know there was pressure in the tank since the stain was there.
went to the compensating tank removed all the lines and the pressure was still there
then remove the cap over the tank lines . removed one of the lines and pressurized fuel came out to release the pressure.
sorry for the long post

what shouild I do?

[Benz Dr.]

We had the same problem with a ' 69 that was here recently. It turned out to be a plugged vent hose or line. It was fine after we repaired it. The pressure would blow the cap right out of your hand.

Your system is not the same as a ' 69 but the problem is the same. 

[erickmarciano]

Checked all the lines again and they are free.
There was alot of gas in the comp tank . Looks like it does not drain Back

[Benz Dr.]

One of them is party blocked or a piece of hose is causing it to block up once a bit of pressure is introduced. If everything is free any pressure build up will be vented to the front of your engine. Did you check the line running under the car?

[erickmarciano]

My car is a euro it does not have a line going to the front

Bypassed the 2 metal line from the gas tank to the compensating tank with rubber hoses and i still get pressure.
The 2 little lines comming out of the gas tank , if i blow through them they are free.
To weird

[tel76]

You must have two fuel lines running under your car from the tank to the engine,one takes fuel to the engine the other returns the fuel to the tank

[erickmarciano]

My car is a euro the evap system is not the same as yours.
NO lines to the engineS

[mdsalemi]

Erick,

All of our cars have a supply and return line from the fuel supply pump at the rear, to the fuel injection pump at the front, and a return line to the "flower pot" (trademarked, Dr. Benz) at the back.  That's how the system works.

I had the same fuel pressure buildup in the tank, and the only thing that solved it was a new tank.  I suspect the flower pot was clogged up with 40 years of gunk.

[zoegrlh]

I did the same as Machael.  Ordered a new replacement.
Bob

[Benz Dr.]

How would a plugged flower pot cause that?

[erickmarciano]

was wondering the same.
the car runs amazing ,no problem what so ever.
Dr what can be wrong??

Thanks

[mdsalemi]

How would a plugged flower pot cause that?

I don't know that it was a plugged flower pot per se, but with a 40 year old tank, surely there's gunk in it.  You may not remember Dan, but you had the car; blew out all the fuel supply lines, snaked out all the vent lines, I changed the vent tank (not that it matters in this case) changed all the vent lines, and fought the problems for a couple of years.  Don't feel bad, you are only one of several mechanics who tried to solve the problem and never did. One local mechanic here, Karl-Heinz, told me that the tank is deteriorating, and things are getting plugged up. He changed the tank and the problem went away. I had the additional problem caused by the pressure build up of the car not running well unless it was F to 50% F; below that in fuel level and it missed.  The new tank did the trick.

Looking back, I believe the reality was a variety of small issues all caused by the deteriorating tank, which had been original, and did sit for 13 years at one point.  Karl said it is rusting, and lot of what is happening is extremely fine rust particles that start plugging things like filters and screens and getting into places you don't want them.  I didn't like the cost of a new tank, but he sounded reasonable, and believable, and the problem went away.  He was the only mechanic to suggest a new tank and the only mechanic to solve my problem.

Note there have been others here on our site who have struggled with one fuel issue after another.  Nobody likes to buy a new tank, but for several here this solved what ever their problems were too.

[Benz Dr.]

It's a problem that has many questions and not enough answers. Mike, if I replaced your $1,000.00 fuel tank and it didn't fix the problem, you wouldn't have been very happy, right? I like to be sure about things like that.

I can do a number of tests to see if the IP is working right before I send it out for rebuilding but fuel tanks are not so easy to call. I had one recently where the engine kept running out of fuel when it was nearer empty. I had the tank cleaned but it still didn't work right. In this case the flower pot was still plugged.
 
I believe Eric's car has the late exspansion tank with a line running to the engine. On a 280 we had here recently, all I had to do was change the vent hoses and the problem went away. This was on an early 280 which had so much pressure in the tank it would blow the fuel cap right out of my hand.

I still think that the line going to the front of your car could be plugged. If the tank is venting properly it will never build up pressure.

[mdsalemi]

Well, the interesting thing was, several mechanics looked at the issues, several suggested several possible solutions, and all of them were tried to no avail.  When I went to the last place, he said with 100% confidence "It's your tank; change it and the problem will go away".  We did, and it did.  I've never seen such confidence in his assessment and solution.  Lucky?  Maybe.  Whether there was a least costly solution, I don't know: many looked, nobody found it and I lived with the problem for at least 2 years and tired of it.  With new fuel lines, everything cleaned and snaked out it was worth a shot.

Joe Alexander likes to start with the inexpensive solutions and work up to the expensive ones, hoping you'll catch something along the way.  Well, we kind of did that, and the only thing left was a new tank.

On my car now, I don't think I have any lines plugged.  I think it is all OK.  Changing a fuel line for Erick is certainly a less costly alternative than a new tank, but my lines had been new just a few years earlier.  All of them.

If I were Erick, I'd change all the fuel lines, vent lines, and hoses entirely before a new tank is deemed necessary.  But I had done all that.

[49er]

If memory serves me right, my tank has always given a short "sigh" when removing the cap.

John

[erickmarciano]

My car is a late euro. It does not have a vent line going to the front just the small tank on the side of the trunk
 Like all euro models

[Naj ✝︎]

Then there is also a third leak-off pipe from the cannister which vents to the atmosphere,Exits near the exhaust box and has the infamous 'FSVT'

Naj

[mdsalemi]

I think what you are calling the compensating tank is a fuel overflow tank.

The way it is supposed to work (correct me if I'm wrong, anyone...) is that as temperature in the fuel tank heats up, the fuel expands, and needs a place to go.  Pressure pushes it into the small tank.  If the small tank gets too full, excess fuel is drained out the FSVT (funnel shaped vent thing).  Pressure and temperature alone, under design circumstances, will move fuel to and from the main tank to the overflow tank.

Are you having any issues beside a gas stain on your bumper?  Does the car run properly, when F, 1/2 and 1/4 tank?  At all speeds?  If you have no runnability problem, maybe you don't really have a problem...yet.

(as I mentioned earlier, I had the same issue but it was accompanied by runnability issues)

When warm (like it has been) I still get a minor whoosh when opening the fuel filler, but mostly this is when it is really warm out.

[erickmarciano]

Yes Michael you are correct
It is a fuel overflow tank
No running problems at all just the stain and whooss.
Guess i have to find another problem to keep me busy on the car
Thanks

[49er]

Spot on Michael, "whoosh" is much closer to the sound then "sigh"
John

[Dave]

I'm in the same boat as you guys.  Fuel starvation problem when awakened this year.  Rusty tank.  Drained a full tank of red gasoline, lots of rust.  Replaced tank, new gas filter and gaskets.   Disassembled and blew out the 3 lines going into the small tank inside the rear fender.  While I'm down there, replaced fuel pump.  Got it all back together.  So now I have the worst of both.  Did high dollar repair, and no better.  Next to do, disconnect return hose see if I'm pumping fuel back from the engine.  If I'm not getting gas back at the tank, then what?

[mdsalemi]

Dave, if you had a rusty tank, replacement was probably not a bad thing!  If you have a new tank, and a new pump, time to check the supply lines. (those little ones were overflow)

You need to check the lines going from the fuel pump to the fuel injection pump, and back.  Were these replaced, too?  They might be in bad shape; I've heard of fuel lines a lot newer than our 40 year old ones being compromised by rust.  They are steel, just like the tank.  If they are the same age as the tank you took out...

I believe there are both hard steel lines, and flexible connector lines on each end. 

[Dave]

Thanks Michael, you're right.  I'm not getting anywhere with polluted fuel.  Good tip on the lines. I'll be back at it this weekend.  I hope compressed air thru will be enough to clean or verify they're not blocked.  Next steps the fuel injection pump?

[ctaylor738]

With all these experts paying attention, this seems a good time to ask my questions.

As I understand it the system allows air to enter the tank when fuel is being consumed and allows vapor to vent when the car is sitting.  Correct?

Why are there two lines into the tank? 

Where do the lines run in the tank?

Since these lines are supposed to vent air, how is it that they seem to be pushing raw gas out?

[mdsalemi]

Chuck,

It would be worthwhile to see the inside of a fuel tank, to where these two small vent lines go. I did cut open an expansion tank once.

As I understand it, let's say you take the car outside on a warm day.  The fuel will expand and vaporize; the fuel vapor will then want to go somewhere.  We can't vent it directly to the atmosphere (smog and all that).  So, the vapor goes out one of those small lines into the expansion tank.  Once there, cooler, it condenses; and then drains back into the main fuel tank via the second small line.

I think that under certain conditions, such as extremely full tanks, high temperatures, etc. there will be a pressure build up and more fuel gets into the expansion tank than this crude little system can handle.  In that case, excess fuel will go to the  FSVT (funnel shaped vent thing) which should be visible "right near the hot muffler tips" at the rear of your car.

I think under normal operating circumstances, you have a small amount of fuel going to vapor; a small amount of fuel collecting in the expansion tank, and this same amount draining back to the tank.  Normally you should not have fuel dripping from the FSVT, and if all your lines have integrity, you should not smell fuel in your trunk, either.

Note that original expansion tanks have brass ferrules and used a combo of both rigid and flexible fuel lines to connect back to the tank.  Replacement tanks are all plastic one piece with integral lines; these go right onto the tank lines, and right down to the FSVT.