Hi all. I recently took my 5-speed 93 4Runner (with 310,000 miles, 4wd, 3.0 engine) on a 2800 mile round trip from New Mexico to Wisconsin, getting a high of 23.4 mpg at the start and down to as little as 12.4 coming back, pulling two snowmobiles on an aluminum snomo trailer on the return trip. It was very damp in Wisc and the engine didn't want to run right until after a 15 mile warm-up (arcing wires? cracks in cap?) I thought it needed a tune-up (74K on previous plugs, wires, cap and rotor) so I did that in the Wisconsin northwoods and it stopped the jerking I was experiencing on start up, but still had performance issues, which got worse after hitching the trailer to it and starting the drive back. The trip back was bad- declining mpg with every tank of gas, and loss of power all the way through Iowa, Neb and Colorado. Stopped for gas in Walsenburg, CO before tackling La Veta pass and as I unscrewed the gas cap noticed the gas tank venting OUT big time for up to a minute, which had never happened before. A clue! So looked it up and read that a clogged evap system is typically the cause of that and can affect performance and mpg. After limping up to the summit, I turned the engine off, unscrewed the cap and timed the venting- gave up after a minute, walked around a bit and it was still venting almost a minute later. Decided on a backwoods fix to run with the gas cap almost completely unscrewed and drove the last 400 miles home with all the power I needed, and finished with 19 mpg on the last fill. So clearly the Evap system is intermittently not working. I read up on the issue a bit, will blow through the check valves/ ports on the canister to check them. In looking around, I found the hose from the canister labeled 'purge' was just laying there, not hooked to anything. I looked as best I could to see where it should have been hooked to, and plugged it in (with great effort in the tight space) to what may be the BVSV/purge valve, but I was not able to find any pic on the net for what the BVSV valve looks like. It's in the middle of the back of the engine, about 6 inches down. Almost have to hook it up by feel--can't see it without lying down on top of the engine.

Any advice on finding/clearing any blockage in the system? And what the heck does the BVSV purge valve look like (i.e., did I get the hose hooked up to the right place?)

 

Here's a picture of a Thermal Vacuum Valve (TVV) / Bimetallic Vacuum Switching Valve (BVSV): https://www.yotatech.com/forums/f116.../#post52420298

I don't think the Evap system is your problem. Out-venting from a near empty and warm fuel tank is normal. The Evap system absorbs vapors at a certain rate, and warming up an empty tank produces vapors faster than they can be adsorbed. (Once the tank cools, those vapors re-condense.) At worst, this vapor pressure is slightly helping your fuel pump.

But with your purge line disconnected, there is no mechanism to empty the canister. At some point, so much fuel is pushed into the canister that it re-liquifies and drips out the bottom vent. Usually, before the drips happen, you'll get a strong smell of gasoline on the passenger side of the engine bay.

Avoid driving with the fuel cap loose. Not only does it contribute a lot of HC pollution, the cap is part of the fuel safety system. If you get rear-ended, a big cloud of gasoline will be sprayed into the air.

 

Without the purge line hooked up all those fuel vapors have nowhere to go, and just saturate the activated charcoal in the canister. Depending on how long it's been like this, it can lead to the charcoal losing it's ability to absorb fuel vapors or lead to a clogged canister. That last one would account for the excess pressure in the tank. You can try pulling it and cleaning it out with compressed air: https://itstillruns.com/clean-charco...r-8104632.html

I've also seen some done by shooting WD-40 into it through the top inlet port, letting it soak a bit then blowing it out. I did this to my '78 pickup and it worked to unclog it, but I can't vouch for the charcoal inside still being any good. So far I'm not getting any fuel smell from it.

X2 on what scope103 said about driving with the fuel cap loose. We don't want you to become a crispy critter on the road if you get into an accident.

 

Okay, but... my recent understanding is that the sometime venting heard from unscrewing the gas cap is actually air being sucked in, with the tank under normal conditions being in a vacuum state (negative pressure) or neutral state. This episode was serious out-venting, like totally unscrewing a tire valve from a truck tire under pressure and add in heat and fumes pouring out. As I unscrewed all the way and pulled the cap away, I could see gas actually flitting up from the hole underneath. NEVER had that before. It was about 80 degrees, and the tank was 2/3 full of 85 octane with 10% ethanol which is why I stopped to gas up with 91 octane since the vehicle seemed to be worse with the lower octane. (I know, I should have spent the extra 40 cents per gal and put in 87- this in Colorado! I'm used to $1.80 for 87 octane in New Mexico not $2.49 for 85 octane). At first I thought maybe the high ethanol was causing more pressure. I read that there are three check valves in the canister, activated by engine vacuum. I have two lines in the top and the third going down somewhere under the engine- this is the overflow? Would a saturated canister at some point return to vapor and purge? I haven't blown low pressure air in yet to see if all three are clear (but doesn't a check valve go only one way? seems to contradict what the book says...) Now that I took pics, I can see that there are two fittings for hoses and a screw-in below. so this must be the purge. The top looks melted.
Really hard to see down there with both eyes as there is little room.

middle of engine, looking straight down between firewall and engine

 

The fuel tank is never under negative pressure. The fuel cap lets air in, but does not let air/vapors out. If the cap didn't vent in, the fuel pump could actually crush the tank (the fuel pump needs to hit about 50psi; only one psi of vacuum over a 2'x3' tank is more than 800 lbs!)

I'm going to guess someone tried to fix your TVV with something like JB Weld. Maybe they even succeeded; you could disconnect the purge line at the canister and see if you get engine vacuum with the coolant warm.

Here's the manual page for testing the canister: http://web.archive.org/web/201003261...76evaporat.pdf

 

Okay! Makes sense. I'll test tomorrow and see if things are working as they should. I think I can replace it if it's not working properly. Thanks for the replies! Trying to keep this old Rolls Can'ardly rolling for a few more miles. Lots of original parts on it, and still on the original clutch! Things are sure to break down eventually.

 

Followup: I pulled the canister, checked for cracks and found none. I, applied 45# air pressure to the tank nozzle and observed there was air exiting the bottom vent but no air out the purge tube. The test procedure stated the purge and bottom vent lines should both vent the air. I then applied air to the purge, and no air exited the other two lines. For chits and giggles I blew air into the bottom vent and heard the tank valve flapping with a sort of throaty gurgling sound.. Assuming a stuck check valve in the purge line, I gave it a quick shot of carb cleaner and did the same on the tank valve. After again applying air blasts to the tank and purge nozzles, the purge line cleared and starting expelling air. I then cleaned the canister by applying air through the tank valve for about 45 seconds. No charcoal particles came out. Reinstalled it and make sure there were no kinks in the lines.
I will try it for awhile and see if the problem repeats itself.. Thanks again for the advice!