imgon New Reader
Aug. 10, 2014 7:39 p.m.

I have an NA '87 RX7 track car, no serious mods to the engine and have owned it for past seven years. Last fall it developed a problem building up pressure in the fuel tank. When the ambient air temperature was hot unscrewing the gas cap after a session on track the gas cap would almost blow out of my hand when it was unscrewed and one time it was pushing fuel out the filler neck while on track. Over the winter I researched the problem and it seemed like the evaporator canister in the engine compartment was the issue. I pulled the canister from my parts car, installed it and things seemed fine. I have been to 4 events this year with no issues. Yesterday at an event I started smelling something like rotten junkyard oil. I thought it was coming from another car on the track, when I got back to the pits I could still smell it. I popped the hood and discovered fluid had dumped out of the "overflow" tube of the canister. This tube comes out the bottom of the canister. It was yellowish in color. I cleaned up the mess it made, it didn't smell like fresh gas but it did go through the canister which I think has charcoal in it, so the friend I was with suggested that the charcoal might change the smell of the fuel. He told me then when he had been behind me and I was accelerating a puff of black smoke would come out. I had noticed I was backfiring more than normal and the idle was a little rough. We decided that as a test we would block off the hose that goes from the canister to the engine block and also block the port coming off the canister that connects to that hose. I went out for the next session and after a few laps the smell was back and I went back to the pits. A check under the hood revealed a new puddle of the same yellowish fluid. All the fuel line components are OEM and 25 years old. I'm thinking I may have a failed hose somewhere in/around the fuel tank I can't see or maybe there is a problem with the check and cut valve. When checking for the pressure build up problem last fall there was a test for the valve that indicated it was OK. Any pointers on what to start looking for would be appreciated.

alfadriver UltimaDork
Aug. 10, 2014 8:23 p.m.

I may be the fuel/emissions guy, but I'm going to approach this from a different- you freaking need to fix this ASAP- position.

The key issue is that the tubing that vents the cannister back to the engine is blocked- so the pressure is being built up, and not vented via the engine. Outside of the emissions issues- there's no reason why to not have this hooked up, unless you want to just give fuel away via evaporation.

So the FIX THIS NOW reason- I actually had a similar problem with my gold GTV that you see in my avatar. So my GTV's hose got blocked, and for some reason, the line decided to vent the cannister by itself. Onto my brakes. When I was on the track.

I'll tell you what- your attention is grabbed pretty quickly when you see a corner station look under your car, and then the next corner station is shaking a fire extinguisher. I drove to the next corner station to put the fire, wait, make that FIRE out. No damage, thankfully. But to have the car love of your life on fire is pretty scary. (I even left a trail of fire on the grass as I drove straight to the corner station)

Fix the venting problem, and vent it to the engine.

Kenny_McCormic PowerDork
Aug. 10, 2014 8:28 p.m.

If its track car, just delete the whole system, run the vent line someplace high in the engine bay and stick a lawnmower fuel filter on the end, or a rollover valve from a jeep or something if you're feeling paranoid.

If you really care about a little VOC emission, do it like the early smog cars did, vent line into the canister, the other line just sitting in the air cleaner horn, the engine running put a slight vacuum on the line and cleared the charcoal can.

alfadriver UltimaDork
Aug. 10, 2014 8:42 p.m.
Kenny_McCormic wrote: If its track car, just delete the whole system, run the vent line someplace high in the engine bay and stick a lawnmower fuel filter on the end, or a rollover valve from a jeep or something if you're feeling paranoid. If you really care about a little VOC emission, do it like the early smog cars did, vent line into the canister, the other line just sitting in the air cleaner horn, the engine running put a slight vacuum on the line and cleared the charcoal can.

Vent a line of fuel vapor into the engine compartment? Right around a lot of hot items and spark sources?

Yea, that doesn't seem like a great idea.

There are better places.

MichaelYount Reader
Aug. 10, 2014 9:47 p.m.

Track car with no need to pass emissions inspection - vent the tank at the BACK. Eliminate the vent line, canister and engine connection; cap off any vacuum source that may have been connected to the canister.

Kenny_McCormic PowerDork
Aug. 10, 2014 11:15 p.m.

Sorry, was thinking in terms of quick and dirty desmog. If you're gonna rip it all out, it would be best to put the vent somewhere else, if not right on the cap via the drill a hole in it method.

alfadriver UltimaDork
Aug. 11, 2014 6:52 a.m.
MichaelYount wrote: Track car with no need to pass emissions inspection - vent the tank at the BACK. Eliminate the vent line, canister and engine connection; cap off any vacuum source that may have been connected to the canister.

Just as long as you have it vented at a safe spot. Even if it's streaming liquid fuel.

Assume that fuel will pour out of the vent. That should make you find a safe spot.

I really don't want to see someone elses beloved car catch on fire like mine did.

The other "duh" that you have noticed- vent it outside. If you can smell it, that's not good. If it gets so strong that you stop smelling it, that's worse.

imgon New Reader
Aug. 11, 2014 8:34 p.m.

Thanks for the posts/comments, I have no problem removing the EVAP system but I don't believe it is the cause of the problem, I think it is the symptom/result of the real problem. My limited knowledge would lead me to believe that just vapors are supposed to go to the charcoal canister. I seem to be pumping fluid into the canister. To me that means I am building up ALOT of pressure in a line or the tank and would guess if I was to just provide a vent line I will be leaving a trail of gas every where I go, not good! In my experiment at the track we took the "vacuum" from the engine out of the equation by blocking the line from the canister to the engine. Somehow fluid still found its way from the tank to the canister. This further confuses me as I would think if it is supposed to collect fumes the line in the tank must be located so that it is not in an area that it "sees" fluid. I'm thinking at the top of the tank there is a spot that never fills up, some sort of bulge that would be above the level of the filler neck otherwise it could suck up fuel accidentally. Every time I have had a pressure problem the tank has been relatively empty, 1/4 tank or less. At this point I am looking to see what components I should test for failure. Is my fuel pressure regulator failing and letting too much pressure by thus filling the fuel rail and pushing too much pressure back in the return line? Or is there some vent line somewhere else that might be blocked allowing pressure to build? Is the charcoal canister the problem, I replaced a 25 year old part with another 25 year old one? In looking at my shop manual the system seems painfully simple. There is a supply and return fuel line and a vent line that goes through a Cut and Check valve to the charcoal canister which goes to a port in the engine. One of these parts has a problem and I would prefer not to replace everything, just the broken component. Anyone have a good starting point?

Kenny_McCormic PowerDork
Aug. 11, 2014 9:28 p.m.

FPR wouldn't do that, its a closed system, to up the pressure you've gotta up the volume, only way that could happen is either the pump is sucking air(you'd know, it would barely run), or the fuel is heating up, quite a bit by the sounds of it.


Rotary engines have really hot exhaust right? How close do the pipes come to the fuel tank? Lower fuel level=less thermal mass, fuel now heats up faster(perhaps within the time of a track session) perhaps even boils in the tank, giving a large pressure rise and a lot of gasoline vapor trying to come out of the tank, maybe even enough to condense into an appreciable amount of gasoline like fluid on its way down the vent line. The lightest 25% of the compounds in gasoline boils around 100*f

imgon New Reader
Aug. 12, 2014 7:04 a.m.

Thanks Kenny, heat is certainly a concern with rotaries. The exhaust is mostly stock with a header and has been the same for as long as I have owned the car and this problem just started last year. But heat might explain why the issue crops up when the fuel level is lower.

iceracer PowerDork
Aug. 12, 2014 9:31 a.m.

Check the fuel vapor control valve that connects to the tank.

foxtrapper UltimaDork
Aug. 12, 2014 10:22 a.m.

The gasoline vapors can condense and drop out as liquid gasoline. Doesn't normally happen as the engine strips the vapors off/out of the charcoal. With time the charcoal becomes ineffective and everything becomes coated with gasoline goo, a yellow liquid. That's what you're likely seeing.

Problem is probably in the valve partially, and in the charcoal can itself. Vapors are not being routed to the engine (valve not opening). The pressure buildup pushes the fumes and liquid out through the fresh air intake of the charcoal can (saturated charcoal).

Generic easiest fix is a fully vented gas cap, it will vent both ways.

imgon New Reader
Aug. 12, 2014 5:12 p.m.

iceracer, would that be the "cut and check valve"? This component is located at the tank and the vent line goes from it to the charcoal canister. Or are you talking about the purge valve located in the engine compartment?

foxtrapper, in your scenario which valve are you referring to, the one in the canister? The experiment at the track was to take away possible vacuum that might be sucking fuel from the tank. We blocked off the vent port from the canister (goes from canister to engine block), in your description, we essentially closed the valve if it was working and forced it to dump out the fresh air intake. I am still confused about the liquid. Could the vapors alone condense to liquid in 10 minutes? I think there was at least many spoon-fulls of fluid on the frame rail and not sure if more spilled off the edge. It really didn't make much of a mess, contained to frame rail and some on the engine mount. In reading the testing procedure for the cut and check valve I am hoping to find that has failed, even better will be if I find it tests OK when cool but failing when heat applied to it. I really want to find out why I am pushing fuel around before I drive the car again. After that I can decide if I keep the canister or go with just an atmospheric vent.

Kenny_McCormic PowerDork
Aug. 13, 2014 3:38 a.m.

In reply to imgon:

Different fuel formulation that is more volatile is a possibility.

tr8todd HalfDork
Aug. 13, 2014 5:54 a.m.

What happens if you suck/blow on the vent line back to the tank? Disconnect the charcoal canister and suck on the line. If your face shrinks up, then you know there is an obstruction. Run a new vent hose to atmosphere somewhere. On fuel cells, there should be a rollover ball check to stop fuel from spilling out, and there should be a breather filter on the end so no creepy crawlies go in there and clog it up. Even once you fix the non venting issue, you still need to figure out why the tank is getting more air/fuel vapor volume when you are using fuel In theory, the tank should have less air/fuel volume, and a non venting issue would cause the tank to implode. The heat causing the fuel to turn to vapor is a good theory, but I don't think that is solely responsible for the kind of pressure you are seeing.

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