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MG MGB Technical - MGB fuel venting problem

I'm stumped. The fuel tank on my '71 MGB is pressurizing (greatly)...as evidenced by a near explosive rush of air when the filler cap is removed (car equipped with non-venting cap...standard). This happens after driving, or even after the car sits in the sun for 20 minutes or so. Also, raw fuel streams out the charcoal cannister (under the hood) after about 10 miles of driving. Not good. These 'incidents' are preceded by rough running of the engine...which eventually quits. The 'fuel streaming' can be stopped by plugging the line at the cannister, and waiting a while, or removing the fuel cap to release the pressure from the tank.

I've removed the charcoal cannister, cleaned the screen and filter element at the bottom of the cannister. Air flow through the cannister is good. I've also blown out all the fuel vent lines, and the vapor separator in the trunk with compressed air. No obstructions found...though it does take some effort to manually blow through the long piece of tubing from the charcoal cannister to vapor separator (in the trunk).

Could the in-tank vent box be faulty? I'm clue-less as to what could be causing this problem, but I have to fix it before I burn up my MG! Plus, I'm tired of smelling like gas. Any help/advice would be appreciated greatly.

dave
David

The easiest solution for me would be to drill a small hole in the cap and make it a venting cap.
Clive Skelhorn

David,

Clive's solution certainly would work, but it wouldn't get the thing working the way it's meant. I'd try to find the reason for the problem. A cut can still become infected underneath the band-aid, if you know what I mean.

I wonder if the hoses on the separator are reversed? The heat from your muffler and reflection of the New Mexico sun on asphalt cause the vapor in the tank to expand. The vapor goes into the top of the separator. In the relatively cooler separator, the fuel vapor condenses, and fuel runs into the bottom hose. Normally this would run back into the tank, but in your case, it's on its way to the charcoal canister. Enough fuel in a vapor line might also be keeping the pressure from venting, thus the pressure behind the gas cap.

Alternatively, what if your crankcase breather is blocked? That would leave the dipstick tube and the hose from the valve cover as the only vents. Could your blow-by be pressurizing the tank via the tank vent plumbing?

All of this is purely conjecture, but I hope it serves to help you "think outside the box" in the manner of diagnosing your problem.

Once you find it, please let us know what the problem was. This is an unusual and interesting subject.

Best wishes,

Matt K.
Matt Kulka

Clive, Matt,
Thanks for your thoughts. I agree with Matt...a vent hole in the gas cap would certainly solve the symptom, if not the problem. An expedient solution, but I want to get to the bottom of this. Matt, this problem started occuring rather suddenly. I've restored the car, and I 'assume' I got the hoses connected correctly on the vapor separator. It's worked fine for couple thousand miles. The only thing that changed: the car was stored for 3 years while I was overseas...then recently driven/towed from OKC to ABQ. Of course, we're at 5K feet in Albuquerque, for whatever that's worth. The weather here hasn't been too hot, so I discount ambient temperature as the cause...it was hotter in OKC before storing the vehicle. I drained the tank (ran the vehicle till the engine quit), sopped the fuel out of the carb bowls, and installed fuel stabilizer before doing any of the above, so any fuel residue in the tank might be 'stabilized'. I also don't think that crankcase pressure could be causing the problem because the cannister isn't blocked...any blowby through the valve cover ought to just pass through the cannister. I will double check the vapor separator hoses, on the chance it might have been possible to connect them backwards. Thanks for your comments/ideas! I'll post any solution I come across.

dave

Dave

All right. Another clue. Blowing through the short length of vent tube (the one normally connected to the vapor separator) to the tank, results in bubbling noise from the tank. It is clearly unobstructed, but I did not expect that blowing into it would result in bubbling noises. The implication is that the vent tube opening, inside the tank, appears to be submerged in fuel, if this is possible. BTW, the tank is less than three-quarters full. If the vent tube opening is indeed below the level of the fuel, I believe it would account for the symptoms I'm experiencing. Does this make sense? Has anyone had this happen to them?

dave
David

I'm not familiar with the SUs, but on the Zenith-
equipped Bs, a stuck metering needle can cause
a similar problem. The excess gas backs up into the
charcoal cannister from the carb float bowl, resulting
in gas coming out of the cannister. So maybe the
real problem is the float, metering needle?
Ronald

David, is it possible that the fuel pump has decided to pump at a higher than standard pressure? I had similar problems with my '71, before i fully de-smogged it. i replaced the fuel pump, which was non standard, and which i am covinced was overpressurizing fuel, and it seemed to cure it for a while, and then i got another dumping of fuel out onto the road via the canister. I ripped out everything and bought a new venting fuel cap. No problem since, no matter what the temp. Of course, i sealed off the tank fitting in the boot beside the filler tube. These cars are so old, and their systems so neglected, that it is sometimes difficult to pin a problem down to one source. My gas stench in the cockpit was coming from the above problem for sure- when it misbehaved, but when i took out the gas tank and found the metal around the filler tube to be totally gone for about 270 degrees, and countless other holes all over one half the tank top, I knew that the main culprit for stink was this area. After a rebuild of my gas tank, and the desmogging/new fuel pump, i have no smells and no gas dumping. The problem is solved- I just don't know ( or care) how many contributors there were. Go for the vented filler cap. And seal off that evaporative system.
Ken R
Ken Rich

David - are you sure it is not the expansion tank or vapour separator tank that is bubbling? If it the fuel tank then that would be most odd, and normal temperature changes would indeed pump fuel out of the vapour line. Time to get the tank transmitter out and peer inside, methinks.

PaulH.
Paul Hunt

Ron, Ken, Pete;
Don't think it's the carburator (or fuel pump). Gas is entering the cannister from the tank vent. Ken, I think a vented cap would solve the problem...I'm just too stuborn to resort to that just yet. Pete, I know the bubbling isn't coming from the vapor separator because I'm blowing directly into the tank through the short length of tubing (disconnected from the vapor separator). Your idea of removing the tank transmitter to look inside has merit. Frankly, I didn't know I could gain enough access to look in there through the transmitter opening. But it looks like that's what I'll be doing. Saturday's project, I expect. Maybe I won't have to drop the tank. I'll let ya'll know.

dave
David

From the other posting it looks like the vent pipe inside the tank has corroded through below the 'water'-line. Short of replacing the tank (what else is on the verge of doing the same?) you could cut and seal off both ends of the pipe between tank and vapour separator and fit a vented cap.
Paul Hunt

David,

As usual, Paul Hunt seems to have said all that needs to be said. I only chime in here to assure you that I hadn't left the conversation after making only a single wild-eyed comment.

If you've ruled out the lines and separator, there isn't much else that could be at fault. To grasp at straws, your fuel pump may have lost a valve, pressurizing the wrong side. But even so, you'd think the designers would have built the system to simply pump vapors out the vent, not fuel.

And so, as any sane person would do, I agree with Paul. Pull the sender unit and peek inside. At worst, replace the tank. Fuel going to unexpected places is dangerous.

Again, I hope you'll let us know the outcome. Your experience is valuable to us all.

Matt K.
Matt Kulka

Roger that. Paul's description of the vent system (at the other posting, under 'questions'), is complete, and offers the most viable explanation yet of my symptoms. I'll certainly let ya'll know what I find this weekend.

dave
David

Epilogue (I hope): I removed the tank to have a look. The tank itself, though dented, appeared to be in relatively good shape (though I know it's been 'sealed' once, by the previous owner). No sludge in the bottom and though there's rust, it's not flaking. There's no accumulation of debris in the bottom near the pickup. But, back to the problem. I found the opening for the vent pipe, inside the tank, to be about 2 and one-fourth inch below the top of the tank. The opening of the vent pipe is (was) positioned at the 'waterline' level of fuel when the tank is about five-eighths full. This is why I was getting the bubbling sound, when blowing through the vent, when the tank was three-quarters full. The vent pipe in the tank is a rather simple arrangement. It enters the tank at the tank fitting, then bends 90 degrees inside the tank, as described by Paul H. The pipe runs about 12 inches, and is simply open on the end away from tank fitting. I was able to reach into the tank with a long screwdriver, and just bend the pipe towards the top of the tank. It's now positioned about one-quarter inch from the top of the tank, and appears solid...no looseness. I don't have a clue as to how the pipe became positioned as low as it was in the tank... nor do I know why this problem (fuel venting) hasn't occured before.

Possible contributing factor: The vapor separator in my trunk was positioned in a non-standard way (by the previous owner). The fitting where the vapor enters the separator from the tank was slightly lower than the fitting leading to the cannister...but maybe not *enough* lower. Now it's positioned correctly, with the outlet fitting on top, and the inlet fitting on bottom.

Sadly, my 'poor running' problem may or may not be resolved. If you remember, the fuel venting incidents coincided with very rough running, to the point where the engine would simply quit. Well, this morning, before doing the tank removal, I drove about 25 miles (in two legs), and experienced the very rough running problem at about the 10 mile point...running fine till then. I had a rag in the fuel filler opening (instead of my non-venting cap), so the tank wouldn't pressurize. And it didn't. I popped the hood/bonnet, and there was no fuel venting, which is good. I lowered the hood, and blasted down the road, and the problem seemed to clear up. No problem on the second leg of the trip. So go figure. A couple of things I found out about my fuel delivery in the process of emptying the tank: The pump (after market centrifugal) is putting out about 21 gallons an hour, at 6 PSI. I know that's a bit much...don't know if that had anything to do with the rough running problem this AM, but I plan to put a pressure regulator on.

I'll replace the tank tomorrow (new paint is wet tonight) and see what happens. Keep your fingers crossed for me! Thanks to all of you for your advice and encouragement! This is an excellent forum for MG owner/mechanics.

dave
David

Just a thought......if you are indeed using a after market pump running @ 6psi and when the normal gas cap
(unvented) is on, when the car is driven the vapor pressure in the tank will increase, effectivly icreasing the head pressure to the pump possibly by 1-2+ psi and the pump is adding approx 6psi to this!
giving possible 8psi.Any way i think this might be enough pressure to over come the float/needle-seat
causing a flooding condition,and the extra fuel goes straight to the carbon canister..........I would check this by disconnecting the line/s that goes from carb/s
to carbon canister @ the can and put it in a jar,taped under the hood next to the carbon can. Go drive a bit then check the jar. Any fuel in the jar and i would try a different pump. Good luck!
Dave Ross

Thanks for the comment, Dave. I think I've solved my problem. The tank is properly vented now, eliminating the tank overpressurization problem. Also, I installed a fuel pressure regulator in the line going to the carburator, eliminating any overpressurization problem at the float seats. I've been commuting in the car, and so far, the car is running well.

dave
David Tye

This thread was discussed between 04/11/2001 and 16/11/2001

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