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Fuel tank pulls vacuum, carb overflow with gas cap out
This is my first post, so hello everyone! I've been lurking for a bit and done a considerable amount of searching on this subject but with no success. I'll try to keep this organized so it makes sense.
The car:
An FB Rx7 with apparent California emissions control (e.g. EGR valve and 4 solenoids in the rat's nest), but with a 1979 carburetor with "FED" configuration (e.g. no power valves, etc.)
Symptoms:
With the gas cap in, the fuel tank pulls a vacuum. This has the effect of the engine dying of fuel starvation at all revs after a few moments of acceleration without extremely careful throttle control and even that isn't a sure thing.
With the gas cap OUT, the float bowls in the carb overflow and fuel steadily trickles into the primaries on one side or both causing it to run extremely rich.
I suspect fuel may be getting into the oil pan but I'm not 100% on this.
Edit: I should mention, with the gas cap IN, the fuel level in the floats is perfect. I've rebuilt the carb and the floats are exactly per factory specs.
Troubleshooting steps so far:
All the fuel lines seem clear.
Edit: The carburetor is just about spotless. I let it soak in Simple Green for a few days and scrubbed what needed it with a brass brush. Accelerator pumps and other diaphragms are intact.
The ventilation and check valve (on the rat's nest) functions per the FSM.
Other solenoids seem to function per the FSM.
New fuel filter
Fuel pump reasonably new and apparently functional (fuel flows per the FSM specs with or without gas cap installed, at least while the car is stationary and testing is possible)
I haven't been able to perform some tests without a vacuum gauge, but I've done just about everything I can do as it is, even running Seafoam through the system a handful of times. I still feel like I'm missing something, though. What could it be? So far, a thorough search online, including here, hasn't really yielded me any results that have worked. I'm open to removing the EGR system, but I do need to retain the other emissions to pass local standards. I'm hoping someone here can help! Thanks!
Every carb rebuild thread I've read has the advice to *not* use the needles that come with the kit as they are generally poorly burnished and stick, leading to flooded bowls. Not suggesting this as a complete fix as that would seem to have nothing to do with the gas cap on/off and the flow stays the same according to your testing. But it could contribute.
People have used the original needles or absent that tried to polish the rebuild needles.
And edit: Welcome!
Last edited by Toruki; 05-07-20 at 11:38 AM.
Reason: politeness :)
I've swapped out the needles a couple of times and none seem to work any better or worse than the others. That said, did another test drive tonight with the gas cap out and noticed the front float seemed okay, but the rear was definitely overflowing still. At the very least this points more towards the carb in this regard, but I'm still not sure about the fuel tank vacuum issue. Gonna take the carb apart (again, ugh!) and quadruple check the valve and float to make sure nothing's sticking. I'd still like to be able to put the gas cap in and not have a vacuum issue, but at least this way I won't be running so rich! I worry about what that might be doing to the oil in the rear rotor which already runs pretty hot! We'll see what happens, either way. :>
Mary not be much, but have you checked your charcoal canister? I have an SE, and understand well the deferences between the fuel injected and carbed FBs. But one constant I have noticed is that these cars seem to demand a certain "balance" when it comes to fuel/air system. And this balance can easily be thrown off by either aging parts, deleted parts, etc. In my case, my SE had an "excessive" amount of pressure at the gas cap when opened shortly after running the car for a while. To the point that my fuel pump would whine, signaling fuel starvation at the pump. I changed the white check valve next to the tank, the one that goes between the tank and the charcoal canister, and a little change, but not much. I then tried to push air through the small tube of the charcoal canister, and NOTHING. Mind you, it's a very restricted tube, but there was NO air moving through it. I used my air compressor to push someasome through it and got it to flow, what I think is sort of "enough" air through it. Now I have what seems to be a fairly normal "whoosh" of air through the gas cap when opened at the gas station without getting almost bathed in gas. And no whine from the fuel pump after extended rides. Again, just thought I should share my experience. Who knows. Good luck, sir.
You indicated you have a FED carb. setup. Have you verified the condition of the fuel return line from carb. to tank? Make certain it is functioning should solve your issue.
I agree with Taino30. Check the gas tank vent line that goes to the charcoal canister, as well as the fuel return line. Look at them under the car and make sure they're not crushed. Normally, as gas is drawn from the tank air replaces it via the vent line from the charcoal can, so there's no vacuum built up in the tank. When the car is at rest the gas vapors travel the opposite way towards the charcoal can, so there's no pressure buildup.
There should never be any pressure or vacuum in the tank if the charcoal can lines are hooked up and working properly.
You indicated you have a FED carb. setup. Have you verified the condition of the fuel return line from carb. to tank? Make certain it is functioning should solve your issue.
When I swapped out the fuel filter a few weeks ago, I checked all the lines. They all look good, but given how easy a step it is to skip, it's a good suggestion!
Originally Posted by Taino30
Mary not be much, but have you checked your charcoal canister? I have an SE, and understand well the deferences between the fuel injected and carbed FBs. But one constant I have noticed is that these cars seem to demand a certain "balance" when it comes to fuel/air system. And this balance can easily be thrown off by either aging parts, deleted parts, etc. In my case, my SE had an "excessive" amount of pressure at the gas cap when opened shortly after running the car for a while. To the point that my fuel pump would whine, signaling fuel starvation at the pump. I changed the white check valve next to the tank, the one that goes between the tank and the charcoal canister, and a little change, but not much. I then tried to push air through the small tube of the charcoal canister, and NOTHING. Mind you, it's a very restricted tube, but there was NO air moving through it. I used my air compressor to push someasome through it and got it to flow, what I think is sort of "enough" air through it. Now I have what seems to be a fairly normal "whoosh" of air through the gas cap when opened at the gas station without getting almost bathed in gas. And no whine from the fuel pump after extended rides. Again, just thought I should share my experience. Who knows. Good luck, sir.
Originally Posted by Maxwedge
I agree with Taino30. Check the gas tank vent line that goes to the charcoal canister, as well as the fuel return line. Look at them under the car and make sure they're not crushed. Normally, as gas is drawn from the tank air replaces it via the vent line from the charcoal can, so there's no vacuum built up in the tank. When the car is at rest the gas vapors travel the opposite way towards the charcoal can, so there's no pressure buildup.
There should never be any pressure or vacuum in the tank if the charcoal can lines are hooked up and working properly.
In this case, we're talking about a '79/'80 where the "canister" is built into the air filter cap and isn't really removable. I'd have to find a complete replacement or do some serious work to bypass it and use a proper canister like later model years. That said, I'm starting to think this is the culprit more and more but can't be 100% sure without a vacuum gauge. I've gone through and cleaned out a few more lines and ports with q-tips and carb cleaner to make sure there's no blockages, but the one thing I can't clean out is that carbon trap! Would that I had an air compressor
An update of sorts:
I went for a long test drive yesterday. I put the fuel cap in and...it seems to run just fine now. I'll explain what I did in a moment, but I can't say with any certainty that what I did solved the issue or that the issue is really solved. But at any rate, I no longer have any issues with the car starving of fuel due to vacuum in the fuel tank. There was a brief period where it did stutter riiiight at the end of driving for about 45 minutes or so, but with the carburetor running rich, that may have caused the stall rather than a lean condition. I still have to rebuild it.
At any rate, for the sake of posterity (and future Google searches), I'd like to impart a quick tip. The FSM for this car is pretty detailed and if you run into this issue, you want to look under the section for Emissions more than Fuel/Exhaust (though it's useful, too.) But there was one thing in particular that was bothering me that I only just found. I initially thought there was only one line that goes to the oil filler tower. But in the FSM there's a line that kind of just...disappears near there and I was wondering what it was. Well...let me help you out. A photo of the oil filler tower base with a both breather lines in frame, one of which is highlighted with a red circle for reference.
On this 1979/1980 model, that bugger is directly connected from the charcoal canister from the top of the air filter cap and the air vent solenoid line and is buried behind a load of other stuff. The rubber line itself has hardened over the years and is practically like hard plastic now. I managed to flatten it out enough to see that more than half of it's diameter was either clogged or constricted. When the parts store opens in the morning, I'll get some replacement line, but for now I ran a q-tip soaked in carb cleaner through it several times to clear out the gunk. I did the same to the ports on either end, but I felt no resistance there. For now, I can only guess that that was the problem-- or at least a large contributor. I've been working on this issue for weeks and if I can save someone the headache of finding that line in the future, I'll at least have done something, I think. I can assure you it is FAR more obvious in this photo than when you're leaning over your engine bay, but that's what a creative use of cell phone cameras is for! Hopefully this has done the trick and when I swap out that line tomorrow, I won't have to worry about this ever again.
Im glad you figured it out. On later cars theres a charcoal canister on the right inner fender, but it does the same thing, in the same way, as what you have in the air cleaner lid. A clogged hose or crushed hard line prevents the gas tank from breathing, leading to fuel-starving tank vacuum. The flooding is from a separate carb issue I think.
Im glad you figured it out. On later cars theres a charcoal canister on the right inner fender, but it does the same thing, in the same way, as what you have in the air cleaner lid. A clogged hose or crushed hard line prevents the gas tank from breathing, leading to fuel-starving tank vacuum. The flooding is from a separate carb issue I think.
That does seem to be the case. Annoyingly, I assumed it was from this issue because I had just rebuilt the carb and triple checked everything since those 12mm bolts are a huge pain to get on or off! Alas...it looks like I missed something and I'll be checking the carb again later today.
05-06-20, 07:34 PM
