Gas Cap Question...
12-10-13, 10:01 AM
#1
Rookie
Thread Starter
Gas Cap Question...
What kind of gas cap is the SC400 supposed to have? Does the OEM cap vent vacuum as well as pressure? Is the tank vented through the EVAP system?
I've gone through about 15 CL threads and countless ask.com/justanswers/stupidBS self help websites trying to find the answer to a simple question. So, now that we got that out of the way... I recently drove my new-to-me, bone stock, 1992 SC400 more than 700 highway miles. After about 1/2 tank of gas (or ~150 miles of non-stop driving) the car started shuddering. Within four or five minutes, it would stall out at 80mph. Definitely suffering from fuel starvation.
There was enough of a vacuum on the gas cap the first time that I had trouble getting it off. Opening the gas cap and letting the pressure equalize fixed the problem, but only temporarily.
The car came with a non OEM Stant brand gas cap. After the second time it stalled, I went to the nearest parts store and they gave me the exact same Stant brand replacement. The new cap did not fix the problem. After 100 miles of driving there was so much vacuum on the gas cap this time that I considered leaving the car on the side of the road and getting a rental. I eventually did get it off and took periodic breaks for the rest of the trip to top off the tank.
I'm guessing that the cap is the problem. Any advice/insight/useful information is appreciated. As soon as I find a FSM, I'll leave you guys alone.
I've gone through about 15 CL threads and countless ask.com/justanswers/stupidBS self help websites trying to find the answer to a simple question. So, now that we got that out of the way... I recently drove my new-to-me, bone stock, 1992 SC400 more than 700 highway miles. After about 1/2 tank of gas (or ~150 miles of non-stop driving) the car started shuddering. Within four or five minutes, it would stall out at 80mph. Definitely suffering from fuel starvation.
There was enough of a vacuum on the gas cap the first time that I had trouble getting it off. Opening the gas cap and letting the pressure equalize fixed the problem, but only temporarily.
The car came with a non OEM Stant brand gas cap. After the second time it stalled, I went to the nearest parts store and they gave me the exact same Stant brand replacement. The new cap did not fix the problem. After 100 miles of driving there was so much vacuum on the gas cap this time that I considered leaving the car on the side of the road and getting a rental. I eventually did get it off and took periodic breaks for the rest of the trip to top off the tank.
I'm guessing that the cap is the problem. Any advice/insight/useful information is appreciated. As soon as I find a FSM, I'll leave you guys alone.
12-11-13, 05:10 AM
#4
Lexus Test Driver
iTrader: (6)
Join Date: May 2010
Location: Colorado Springs, CO
Posts: 1,442
Likes: 0
Received 4 Likes
on
4 Posts
yea no idea why the 12v jump was brought up.
what actual side effects are you having? from what i read, you said when you equalize your gas cap it fixes your problem temporarily? you sure that turning the car off and on isnt causing that temporary 'fix'?
what actual side effects are you having? from what i read, you said when you equalize your gas cap it fixes your problem temporarily? you sure that turning the car off and on isnt causing that temporary 'fix'?
12-11-13, 05:57 AM
#5
Rookie
Thread Starter
It's on my list of things to do, albeit very far down on the list. I'd like to fix the problems before I start the mods...
Actual problems... Starts sputtering, then bucking, then ultimately stalling out. Stomping the gas when it's starting to flutter seems to make the problem worse. Just turning the car off and on does not "fix" the problem. Taking the gas cap off does "fix" it, but only for a little while.
It's only happened after driving 100+ miles without stopping. The three mile romps to and from work have been totally uneventful.
I was digging through the receipts that came with the car and found one for a new fuel pump installed at a Lexus dealership 14 months ago.
It's only happened after driving 100+ miles without stopping. The three mile romps to and from work have been totally uneventful.
I was digging through the receipts that came with the car and found one for a new fuel pump installed at a Lexus dealership 14 months ago.
Last edited by McCloudsZJ; 12-11-13 at 06:01 AM.
12-11-13, 09:49 AM
#6
Rookie
Thread Starter
For reference when I get off work this afternoon:
https://www.clublexus.com/forums/per...ng-issues.html
https://www.clublexus.com/forums/per...ne-stalls.html
https://www.clublexus.com/forums/per...-stalls-2.html
https://www.clublexus.com/forums/per...ing-issue.html
It really does look like the 12V fuel pump mod is the answer to everything... Guess I'll do it for kicks and see what happens.
https://www.clublexus.com/forums/per...tures-faq.html
https://www.clublexus.com/forums/per...ng-issues.html
https://www.clublexus.com/forums/per...ne-stalls.html
https://www.clublexus.com/forums/per...-stalls-2.html
https://www.clublexus.com/forums/per...ing-issue.html
It really does look like the 12V fuel pump mod is the answer to everything... Guess I'll do it for kicks and see what happens.
https://www.clublexus.com/forums/per...tures-faq.html
12-11-13, 01:12 PM
#7
Not sure if the 12v mod is going to fix your problem or make it worse due to the extra flow through the fuel rails.
sounds like the evap may not be working right, or someone hooked up the vac line wrong or bypassed the evap vsv improperly.
on the sc300 and 400 it is a sealed system, the cap will hold the pressure in and does not vent at all, if it does it is broken. the tank has a vent line that runs to the charcoal canister. from there the canister does its magic and there is a line that goes from that to a vsv for the evap controlled by the ecu, and the other side of the vsv goes to the intake manifold.
The ecu pulses the vsv to allow vacuum from manifold to pull excess vapor and pressure from the tank under cruise conditions and some other conditions where it richens up the mixture some without throwing the ecu off.
what is likely happened is someone bypassed the vsv and is applying vacuum to the tank at all times, which causes the car to run lean as the pump has to fight the vacuum to pull up gas. If you had a wideband it would swing several AFR points lean when this happens.
someone could have also improperly deleted the charcoal canister and blocked the tank vent line, which is bad and would pressurize the fuel tank, probably also making the pump act wierd and go lean.
basically check the routing on the canister and make sure a vent line on the firewall is running to it (bigger sized line), and look for the smaller vac line running to a vsv (vacuum switch) and then to the intake and check all that out.
you can even have a bad canister from someone topping off the fuel system too much, and the gas works its way into the canister and then all sorts of problems occur as the vsv sucks raw gas in instead of gas vapor.
you can bypass the evap as a test by connecting the large vent line from the tank to the large line under the canister that goes under the car. this bypasses the evap system. if doing that for a long period of time stick a fuel filter in between the 2 hoses and that will help with fuel smell.
If unsure post a picture of it. if it is not connected right or bypassed the normal way you can get those fuel issues, I have seen it several times but not sure if that is your problem 100%. take a look at it and report back.
sounds like the evap may not be working right, or someone hooked up the vac line wrong or bypassed the evap vsv improperly.
on the sc300 and 400 it is a sealed system, the cap will hold the pressure in and does not vent at all, if it does it is broken. the tank has a vent line that runs to the charcoal canister. from there the canister does its magic and there is a line that goes from that to a vsv for the evap controlled by the ecu, and the other side of the vsv goes to the intake manifold.
The ecu pulses the vsv to allow vacuum from manifold to pull excess vapor and pressure from the tank under cruise conditions and some other conditions where it richens up the mixture some without throwing the ecu off.
what is likely happened is someone bypassed the vsv and is applying vacuum to the tank at all times, which causes the car to run lean as the pump has to fight the vacuum to pull up gas. If you had a wideband it would swing several AFR points lean when this happens.
someone could have also improperly deleted the charcoal canister and blocked the tank vent line, which is bad and would pressurize the fuel tank, probably also making the pump act wierd and go lean.
basically check the routing on the canister and make sure a vent line on the firewall is running to it (bigger sized line), and look for the smaller vac line running to a vsv (vacuum switch) and then to the intake and check all that out.
you can even have a bad canister from someone topping off the fuel system too much, and the gas works its way into the canister and then all sorts of problems occur as the vsv sucks raw gas in instead of gas vapor.
you can bypass the evap as a test by connecting the large vent line from the tank to the large line under the canister that goes under the car. this bypasses the evap system. if doing that for a long period of time stick a fuel filter in between the 2 hoses and that will help with fuel smell.
If unsure post a picture of it. if it is not connected right or bypassed the normal way you can get those fuel issues, I have seen it several times but not sure if that is your problem 100%. take a look at it and report back.
Last edited by Ali SC3; 12-11-13 at 01:16 PM.
Trending Topics
12-11-13, 01:25 PM
#8
Rookie
Thread Starter
That sounds like a reasonable explanation. I have a wideband sensor, but my LC1/OT2 is for OBD2. I'm not sure if it'll read just the 02 sensor without hooking it up to an OBD2 port. It'd be worth a try if nothing on the EVAP canister looks suspect.
Since fixing the PCV valve and driving it almost 1,000 miles (long enough for the computer to relearn...) it definitely does not smell like it's running rich anymore.
More reading for after work this evening:
https://www.clublexus.com/forums/per...onnect-to.html
Since fixing the PCV valve and driving it almost 1,000 miles (long enough for the computer to relearn...) it definitely does not smell like it's running rich anymore.
More reading for after work this evening:
https://www.clublexus.com/forums/per...onnect-to.html
Last edited by McCloudsZJ; 12-11-13 at 01:38 PM.
12-11-13, 01:57 PM
#9
that line with the large open ended check valve does not get hooked up to anything. that line is like a safety purge or too much pressure builds in the canister or something like that. alot of the definitions of these parts on the net are vague so I go off of part research/explanation and part experience with messing with it many many times.
12-11-13, 02:05 PM
#10
Rookie
Thread Starter
that line with the large open ended check valve does not get hooked up to anything. that line is like a safety purge or too much pressure builds in the canister or something like that. alot of the definitions of these parts on the net are vague so I go off of part research/explanation and part experience with messing with it many many times.
12-12-13, 05:19 AM
#11
Rookie
Thread Starter
I peeked under the hood before work. Charcoal canister is not hooked up to anything. Can't believe I missed that one. 
More on this after I look at it during lunch.
More on this after I look at it during lunch.
12-12-13, 06:28 AM
#12
Im having this same problem, although my symptoms are not as extreme, i do get an obscene amound of vacuum when i remove the gas cap when refuling. and my car does studder, but only when im at at least 1-4 tank and taking a hard turn. but its like you say, applying more gas doesnt help, i have to let off and then reapply to get it to go.
12-12-13, 08:41 AM
#13
Is there a readily (or not so readily) FSM available for these on the internet somewhere? I really feel like most of my questions could be answered if I had one. I appreciate forums and Google and all that, but I'd really like something a little more in depth than a Haynes manual... The one's on fleabay are all $200+, and that's just not going to happen.
and thats how you get fuel starvation lucky you don't have a turbo and were not trying to boost that lean mixture would have done more than stall you out!!Im having this same problem, although my symptoms are not as extreme, i do get an obscene amound of vacuum when i remove the gas cap when refuling. and my car does studder, but only when im at at least 1-4 tank and taking a hard turn. but its like you say, applying more gas doesnt help, i have to let off and then reapply to get it to go.
12-12-13, 10:40 AM
#14
Rookie
Thread Starter
It's 15 *F and windy outside, so I didn't spend a lot of time under the hood. But I did snap a couple pictures. It definitely looks like there are some vacuum lines that are MIA. I didn't find any of them capped off or tucked away anywhere, but I didn't spend a lot of time looking.
Just for my own curiosity, what would motivate someone to do this? I'm all about weight reduction, and have deleted a charcoal canister or two in my day in the name of shaving a pound or two and cleaning up the engine bay, but just unhooking it seems pointless.
Just for my own curiosity, what would motivate someone to do this? I'm all about weight reduction, and have deleted a charcoal canister or two in my day in the name of shaving a pound or two and cleaning up the engine bay, but just unhooking it seems pointless.
12-12-13, 01:41 PM
#15
that is the wierdest thing I have ever seen ever related to the evap system.
maybe they thought the canister was bad and tried to bypass it, but that is the wrong way to do it.
Or they thought they were breaking new ground in evap technology.. lol.
As I suspected someone just shoved the evap line from the intake into the gas tank vent line (the bigger hose in the picture right there)
The bigger line goes to the big nipple on the canister, and the vac line on the smaller nipple.
then the thing I cant see is if the smaller line goes to a vacuum switching valvs (VSV) that has 2 vac nipples on it and an electrical connector like it is supossed to be, and then another line from the vsv to the intake..... or does the line in the pic just go straight to the intake allowing constant vacuum on the gas tank.
connect the 2 hoses as above and let us know about the vsv there or not.
maybe they thought the canister was bad and tried to bypass it, but that is the wrong way to do it.
Or they thought they were breaking new ground in evap technology.. lol.
As I suspected someone just shoved the evap line from the intake into the gas tank vent line (the bigger hose in the picture right there)
The bigger line goes to the big nipple on the canister, and the vac line on the smaller nipple.
then the thing I cant see is if the smaller line goes to a vacuum switching valvs (VSV) that has 2 vac nipples on it and an electrical connector like it is supossed to be, and then another line from the vsv to the intake..... or does the line in the pic just go straight to the intake allowing constant vacuum on the gas tank.
connect the 2 hoses as above and let us know about the vsv there or not.