Vacuum in Fuel Tank. Problem?

[raers]

I've noticed a significant hiss of vacuum as I remove the fuel cap from my '91 22re 4R. I would guess that it creates more vacuum the more fuel I burn, as i've noticed that when I run the tank very low it's worse. I've checked the fuel cap visually for a mis shapen seal or a crack/break of some kind but can't see anything noticable.

Maybe a clogged charcoal canister? How long do those things last? 202K miles on the engine. Any ideas?

 

[turboSR5]

Bellingham, Wa. Small world! We do have whole heap of Toyotas in this town, I myself am holding on to 3 of them.

So what's the problem with your tank, you think the pressure that it is building up is too much? I noticed something similar with my 87 when it started getting cooler at night, the pressure in the tank seemed to have jumped much higher when I went to crack the cap to help relieve pressure on my fuel line when I changed a fuel filter last weekend.
I have never had a problem with a charcoal canister but you could be on to something there. If it is plugging up then vapors would be building beyond normal in the tank making more pressure than normal.. Maybe pull the hoses off and try to see if you can blow air through it. I am going to check into that some more when I get home from work.

 

[raers]

Yeah, Bellingham is a great town. Being so outdoorsy though, 4Runners catch a pretty high dollar in the used market. I looked for quite a while before finding a 4x4 5sp with the 4-cyl. Couldn't be happier with it though...I'm jealous of your turbo 22re though...

So, it has over 200k on the 22re and every once in a while I'll notice a little bit of surging while accelerating at 10-25 mph in second gear. It's pretty negligable but I'm just trying to think of all of the preventative maintenance I could do to keep from having any problems down the road. The idle seems pretty steady at ~850 once warm, not much hunting. That being said, I've noticed that when I take the gas cap off at fill staions there's a vacuum (sucking in) that's pretty noticable. The valve in the cap itself seems to be okay, but I'm not sure if it is the stock cap or not. Also, I took the charcoal canister off this morning and blew air into the inlet and was able to confirm the check valve was working, but I don't have an air compressor so I can't clean it like it shows in the repair manual with 45psi air.

I guess i'm just wondering if there is supposed to be a vacuum on the fuel tank always, or if that might be making the fuel pump struggle a little too hard? The other two cars I've owned ('85 Jeep Wagoneer, '04 Audi TT) have always let out a little pressure when you take the gas cap off. I assume that's from evap. due to temperature change...but I don't know exactly how the Toyotas are designed.

 

[turboSR5]

That is a good find, 22re, 5 spd..it really doesn't get much better than that.

Well, that sure is odd. Normally your tank gets pressurized and will let off the pressure when you open the filler cap. I have never encountered the problem you have where the tank wants to pull air inside of it. On my turbo build I had a problem where my fuel pump checked out ok but I could not get pressure to build up in my fuel rail so I crammed a rag in the filler and shot about 7 psi in there and capped it up. I finally got pressure to the rail and the rest of the story is too sad to continue with... Seems to me that if you had vacuum in the tank you would have some pretty hard starts to deal with if it sat for a day or so.

I am currently dealing with the surge at around 1,000 to 2,500 RPM (most noticeable in 2nd gear) on my 22RE 4runner myself. I checked the diagnostic codes and I have a code 11, TPS (most likely). Saturday I will be installing a new TPS and crossing my fingers that the surge goes away after I have the new one dialed in. I have a huge dead spot on the one I currently have. I at first thought the temp. sender was being misleading to the ECU but a couple of road tests later and the problem is there cold or warmed up.

 

[the_dingman]

Are you 100% positive that it's a vacuum (air entering in), not air escaping out? Both my 4Runners have let out a lot of air when I open the fuel cap and give a distinctive hiss.

 

[n8buddy]

my truck makes the hiss as well when i undo the gas cap, always has. i always assumed it was from vacuum pressure. why would the gas tank be under positive pressure? as the fuel is sucked out it would create a vacuum

 

[turboSR5]

You can not have vacuum in a fuel tank, you will never start the thing or if you did, you would soon stall out. Fuel vapors build pressure in your gas tank, and I'm sure other things factor in, I'm no expert but I'm sure of this much.

 

[n8buddy]

You can not have vacuum in a fuel tank, you will never start the thing or if you did, you would soon stall out. Fuel vapors build pressure in your gas tank, and I'm sure other things factor in, I'm no expert but I'm sure of this much.

from the factory service manual:

High pressure in gas tank - Charcaol canister check valve open. Check valve in fuel cap closed. HC (fuel Vapors) from tank is absorbed in charcoal canister

High vacuum in gas tank - charcoal canister check valve closed. check valve in fuel cap open. air is led into gas tank

so you could have a vacuum or pressure buildup in the gas tank, causing air to escape or rush in when cap is opened, depending on driving conditions.

 

[turboSR5]

Ahh, makes perfect sense. Open valve in canister, closed valve in cap . Or Closed valve in canister, open valve in cap. Lot's of tricky venting going on.

 

[raers]

I've looked into it a bit. I tested the fuel cap and it does allow air to come in if the vacuum is high enough, and I blew compressed air through the charcoal canister. I checked the gas cap using a small suction pump and the valve did open but I'm not sure at excactly how much vacuum. I checked the charcoal canister by following the instructions in the service manual and confirmed that the check valve in the canister works. Two things I'm still not sure about...one is whether or not I even have a problem, and two: When I blew air through the canister it seemed like a lot of resistance and not much flow, but again, this might be normal.

I'm gonna check into getting a new gas cap with a looser spring on the check valve, throw the canister back in and call it good. Then it's on to a new project...

Thanks for the tips guys...

 

[turboSR5]

Sounds like a bad canister, you should be able to freely blow through it without losing small pieces of charcoal.

 

[raers]

Are you sure there should be good airflow through the canister? I wish I had some pressure guages, but I was blowing 90psi air in and getting a very light flow out. The canister was very restrictive in my opinion. Do you think that's not normal?...I wish I had something to compare it to...

 

[turboSR5]

From 1988 FSM:

 

[raers]

Hmm...
Thanks turboSR5, that's almost the exact same thing as my repair manual says too, only without the "unrestriced" part. I'm thinking I should look for another canister because like I said, when I blew shop air into port A and plugged port B, there was very little flow out of the drain. I'll give it a shot and see if that frees things up a bit. Cheers!

 

[SKIDPLATE]

If it ain't broke, don't fix it!

I'm confused. Does it run fine? If so, leave it alone. Lifes too short to be looking for problems. They all hiss when you open the tank. Hell, it's a Toyota. My Tundra hisses. My Solara hisses. Both my 4runners hiss.

 

[fishrod]

Mine hisses too, I always thought it was an ingoing hiss but now im not too sure. It might be hissing out...... Ill check this next time I fill up.
Not sure if its related, but Im also having some surging/struggling issues when the car is cold and at low speeds in 2nd and 3rd gear. 3Vze engine though. The mechanic I took it to hooked it up to a computer and drove around while cold and said the errors coming up were fuel related, or lack of. So I was going to check the fuel filter next, maybe its to do with the pressure buildup in the tank?!?

 

[mwhite49]

Gas tank issues

I have an 82 pickup 4x4 with the 22r all original. It was given to my son. 270,000 miles. Ran when parked. When we opened the fuel cap it was under vacum. Added fuel. We got it running and drove it around the block. But it keeps dying. Also the heat riser valve looks to be carboned up. Never seen this on a valve before. It has a new carb put on by the old owner and he drove it. I'll check the charcoal canister today. It sounds like it's not getting enough fuel. Any other ideas?