Excessive fuel tank pressure

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my experience

I run a OEM subtank and had this very issue 3 weeks ago. I was moving slow on a two tracker for about 30 miles and began to smell fumes... Finally decided to vent the gas cap and it took 4-5 minutes for it to calm down. I was surprised and a little worried to discover this kind or pressure build up. I thought it was a result of the subtank but i guess its somewhat prevalent among non subtank cruisers. Do you guys think its only an ethanol issue or does the Charcoal cannister play a part. Are they theoretically suppose to need replacement or wear over time?
 
forgot to mention. I run 30K cst and was running the truck at 6000ft with ambient of low 80s. my experience is that i will have the same type of venting issue albeit not nearly as dramatic when the tank gets low.
 
Took the same trip as last year a couple of weeks back. Last year had 'boiling' sound and gas smell around the charcoal cannister. This year, same trip, similar temps and nothing. Only change to the truck...a new gas cap required when the 16 year old one failed emissions testing. Go figure? In fact, this year there was more stress on the engine because I was towing my trailer!
 
Hey Guys,

read through this whole thread and thought I'd throw in my 2 bobs worth, which may throw a spanner in the works.

I have had this happen to me twice both times whilst 4wding. the altitude and fuel mix is a minor issue, the main issue is the fuel is heating up due to heat in the engine bay and returning to the fuel tank as mentioned in a couple of the replies.

As well as the fuel heating up because the 4.5 petrol motors run hot due the the size of the beast. Theres also the factor of travelling at low speeds reduces air flow through the engine bay. So its understandable that the fuel will heat up in the return pipe. As well as stinking hot days not helping

I think the question that needs to be asked is why isn't the CC working properly and it all boils down to the VCV in the canister not activating, if your not getting a vaccuum in the intake hose that goes into the manifold the VCV in the CC wont work, if the VCV is stuck it wont work..............has to be in the CC. if the breather hose in the bottom the the CC is blocked it wont work

The fuel system is a one way system, the fuel tank works on vaccuum as the fuel is consumed by the engine a vaccuum is created, sucking in air through the filling cap (negative pressure) as the fuel heat up it creates positive pressure. As the filler cap only has a one way valve on it. The positive pressure has to go somewhere, it travels down the breather pipe into the CC where its supposed to vent into the intake manifold and when the pressure gets to great, its supposed to vent through the bottom of the CC to the vent hose. On the 105 it goes into the chasis rail.

Drilling a hole in the filler cap isnt the answer as its only a quick fix.

CeeJay
 
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I wonder if it is possible to run the return fuewl line through the A/C some way so it gets cooled down before returning to the fuel tank??
 
ceejay1501 said:
I wonder if it is possible to run the return fuewl line through the A/C some way so it gets cooled down before returning to the fuel tank??
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Why don't you just stick a couple of washers under the bonnet bolts to tilt the rear of the bonnet up to make a 2mm gap so the fan can blow out the engine heat like people in 75 series troopies used to do to keep their 2H's cool.
 
It seems to me that most of us on this thread have reached the conclusion that any venting more than a few seconds after the gas cap is removed is due to boiling fuel since there is no other plausible explanation. I'm not having any of these problems but it seems to me that a couple of things have been mentioned but never tried. I read through the thread and couldn't find where anyone had tried a fuel line cooler on the return line. Why not? The price of this is about the same as a couple of 5 gallon boil offs.
I think that someone needs to pay more attention to the fuel pressure regulator or possibly the fuel pump as being possible culprits here. I don't think that every 80 has a problem as described, so there must be something malfunctioning. The result of the malfunction is a fuel tank full of gas so hot that it boils. This is obviously exacerbated by both high engine bay heat and low atmospheric pressure found at high elevation. The heat is getting into the gas tank from the return fuel line coming from the hot engine compartment. The problem is probably more prevalent in the 80 because we already have engine compartments that are hotter than normal, especially when traveling at low speeds off road.
The heat transfer to the fuel tank is dependent on 2 variables, the temperature of the returning gas and the volume of the gas that is returning. It is going to be difficult to lower the temperature of the gas unless you use a fuel cooler and locate it where there is some relatively cool air to blow through it. I would put it somewhere just before it leaves the engine compartment on its return to the tank and route it to in front of the radiator then back to the tank. Insulating the fuel lines and rail should help, but it seems like a difficult thing to do.
The other possible solution may lie with the regulator because this is where the amount of fuel returning to the tank may be regulated. I'm not sure exactly how this works in the 80, but the system vacuum pressure often regulates the pressure in the fuel rail by way of this regulator. Someone should try troubleshooting this regulation system in the trucks that are having the problem. Another way to prevent too much heat getting back to the tank would be to either lower the fuel pump pressure or maybe constrict the fuel tank return line so less hot fuel gets back into the tank. I'm not really sure how to do these things on the 80 but it is something that some of you who are closer to the problem may want to think about.
I'd bet my life against a donut that a properly configured fuel cooler with enough cool air blowing through it will solve these problems of boiling fuel and venting tanks. Cooler fuel may also give you more power but the ECU may already compensate for that.
 
what about plumbing 2 CC's in parallel, especially for you lucky suckers with aux tanks? that ASSumes that the restriction for pressure relief IS the CC and not the valve at the top of the tank.

i have had big pressure after slow sloshy trips, but never had fuel smell in engine bay. i just installed the vc120 from the thread above, and am heading to deathvalley next week, we'll see what happens... <fingers crossed>
 
CycloSteve said:
This issue has been solved in another thread. My rig with Subtank now has ZERO venting issues whatsoever. Cheap and easy fix. Happy reading.

https://forum.ih8mud.com/80-series-tech/412261-charcoal-canister-great-option.html

Cheers,

Steve
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I think that statement is an assumption that will have to bear out next summer.

If it is true that a replacement charcoal canister completely eliminates the problem then this will be a great thing for all of us. However, various age and mileage 80's have fairly recently started presenting this symptom, and that suggests the problem may lie beyond the CC.

If it was only the CC's useful life, then you'd expect something to be occurring like a the useful life of a lightbulb, i.e. failure at a certain mileage more than failure occurring across a range of usage at a point in time. This is why businesses will often change all the bulbs in a building all at once as the first ones start to fail - the labor cost is high enough and useful life predictable enough that it makes sense to do it all at once from an economic perspective. Correspondingly, we should know that the CC has a defined replacement interval, which it doesn't.

So...much like you can replace all of the lightbulbs in a building all at once and suggest that the problem is cured because they are all lit, you can also suggest that replacing CC's all at once has also cured this issue with our 80's. The problem with that assumption is similar to having a voltage spike problem drastically shortening the life of your bulbs - the failure is a symptom, not a cause.

Now, if you can address this symptom once every few years for $40 and 30 minutes of your time without any other adverse impact, and you can rely on the aftermarket CC availability over time, there is no reason to look any further. Both of those things, however, require a great measure of faith and we'll see how time weighs in on the matter.

It is reported on this thread that some people have replaced their CC's with OEM to no effect - I don't suppose we are purporting that a cheap GM CC will outperform the stock Toyota part, and that the problem is actually a Toyota design flaw?
 
I don't suppose we are purporting that a cheap GM CC will outperform the stock Toyota part, and that the problem is actually a Toyota design flaw?
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Why would such an assumption be so shocking? Not to start a flame war regarding reliability or quality, but give credit where credit is due. GM makes fantastic products which often surpass Toyota in reliability. In this case, yota's CC is total junk be it a design flaw or not.
 
Nay said:
It is reported on this thread that some people have replaced their CC's with OEM to no effect - I don't suppose we are purporting that a cheap GM CC will outperform the stock Toyota part, and that the problem is actually a Toyota design flaw?
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i figured i'd give it a shot for 40$...:meh: since no destruction or revert capability is lost, (the old canister is still safe in my old parts bin, ready to go back in if i have suspicion). i can say after holding a vc120, it by no means feels as rugged as the oem i just pulled, but container ruggedness has nothing to do with fuel vapor pressure management ability of course.

:popcorn:
 
Well all I can say is based on my experience. My rig with a subtank went from venting quite a bit to zero venting at the cap after changing the CC. No other changes done, and it instantly went away. Have already driven the truck in the upper 70's and still working fine. Will see when it hits the 90's, but I for one am optimistic. $50 well spent and 100% reversable.

:cheers:

Steve
 
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Czar said:
Why would such an assumption be so shocking? Not to start a flame war regarding reliability or quality, but give credit where credit is due. GM makes fantastic products which often surpass Toyota in reliability. In this case, yota's CC is total junk be it a design flaw or not.
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I didn't say it was - I always favor design over brand. I was just asking a question: do people believe that the GM part is a cure vs. the OEM part that is not? If so, why?

The referenced thread was about the low cost of using a GM part vs. a $250 OEM ripoff, not that it has solved the problem of fuel vapor/pressure as a permanent fix.

If that low cost GM part has a design advantage, somebody should pipe up. Otherwise methinks this thread will be alive and well in the summer of 2011.
 
I am currently in Mexico on the coast. I had a large pressure build-up/venting issue on the drive down when I stopped to fuel in Lordsburg NM. When I got to Tucson the problem went away and I have had zero issues for the entire week. I am currently running Mexican fuel that I believe is straight gasoline. I bet the problem comes back when I get back to above 4,000 feet and the blended fuel.
 
cruiserdan said:
I am currently in Mexico on the coast. I had a large pressure build-up/venting issue on the drive down when I stopped to fuel in Lordsburg NM. When I got to Tucson the problem went away and I have had zero issues for the entire week. I am currently running Mexican fuel that I believe is straight gasoline. I bet the problem comes back when I get back to above 4,000 feet and the blended fuel.
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I would not bet against you on that one. Enjoy the trip!
 
I do not have excessive pressure in my 91 but do have a fuel smell venting from my CC canister. I changed the CC canister the other day with the GM part and fresh charcoal helps but did not fix it. I started looking at the VCV as the culprit and if the post that I found has the correct test procedure from the FSM then I would say this might be the problem. I read that:

VCV - Cold engine - No vacuum at idle, vacuum at 3000 rpm.
VCV - Hot engine - No vacuum at idle, vacuum at 3000 rpm.

If this is correct then it does not make any sense. No vacuum at idle but vacuum at 3000 rpm? What is the point in the valve then? My truck rarely sees 3000 rpm and on a trail I can’t imagine it. Have I misread something? If the intake is not pulling vacuum at idle or low rpm then I would expect pressure to build in the fuel system and if the CC canister vent is clogged there is no place for it to go. If the CC canister vent is not clogged then it is going to vent and you are going to smell fuel. Am I missing something? Is the VCV test I found wrong? IF not is there any reason not to hook the CC canister vent directly to the intake manifold or just put a basic check valve in line to it? Anyone tried this and I missed it?

 
remac226 said:
VCV - Cold engine - No vacuum at idle, vacuum at 3000 rpm.
VCV - Hot engine - No vacuum at idle, vacuum at 3000 rpm.
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That is not the correct procedure for testing the TVV(cold test) and VCV(hot test) - at least from the 1997 FSM. It should read:

TVV - Cold engine - No vacuum at idle, NO vacuum at 3000 rpm.
VCV - Hot engine - No vacuum at idle, vacuum at 3000 rpm.
-both tests are to check for suction at the same location at the VCV where the canister hose connects to it

However the 1997 FSM says to "gradually increase engine speed to 3000 and check for suction". So I don't know for sure when the valve starts to open vs when it's fully open.

I agree that a hot day rock crawling is a tough situation for this design with the heat and sloshing fuel, and with the canister located in front of the driver, I expect that we'll continue to smell that sweet fragrance. You're right in that once the canister is overwhelmed with vapors and if the engine is not purging the canister(sucking the vapors out), then the excess is vented thru the charcoal and out the bottom of the canister. The factory canister had a hose that seemingly brought these vented fumes down to the frame rail. But I, like you, had the sweet smell with both the factory canister and this aftermarket unit.

I vote for low-range and first gear - keep the RPMS up and the oil pressure up while sucking up fuel vapors!
 
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