[Subscribe to Daily Digest] |
 | [Main General Bulletin Board | BBFAQ |
Prev by Date | Post Followup ]
Member Login / Signup - Members see fewer ads. - Latest Member Gallery Photos
Re: ...also... Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Mon, 19 May 2008 05:27:52
In Reply to: ...also..., Robert Adams, Mon, 19 May 2008 00:22:45 Members do not see ads below this line. - Help Keep This Site Online - Signup |
If you didn't hear the Evap valve ticking, it could be the sign of a bad valve. The fact that you don't get a CEL is good to know. Did the problem start AFTER the tank was worked on, or before? That would be helpful.
Back to basics - the Evap valve opens up when the ECU tells it to, and fumes are sucked from the canister into the intake. This happens at a specific time in the engine cycle, very specifically when the engine is at idle, because it needs vaccum to pull the fumes into the engine to be burned. When the ECU opens the valve, the air pressure changes (as measured by the MAP). The ECU tests the valve by making sure it sees the pressure drop it expects. If it doesn't, it lights the CEL.
I'm thinking the valve has failed in an interesting way. It still works - it opens when the ECU commands it, and shuts when commanded off. But it's got a problem - when the turbo spools up, the pressure in the intake is forcing its way past the valve, pressurizing the tank. I've never dissected the Evap valve, but I'll bet there is a one-way valve component. I'll further bet that has failed on yours. So the valve opens up as normal when the ECU commands it, so there would be no CEL or drivability issues. The only time there is a problem is under boost, when some boost pressure is bled off into the tank. You wouldn't notice that.
I'd have to check the fuel system schematics - the one-way valve might be part of the Evap valve, or at the tank.
Otherwise, I can't see a good way to pressurize a fuel tank. Under normal driving, the pressure in the tank drops. Fuel is being sucked out and then spit back in, but on average, less fuel comes in than goes out, so the level drops. This would tend to de-pressurize the tank, not pressurize it. As the fuel heats up it will pressurize the tank, but the canister is there to catch those fumes - the tank doesn't vent to atmosphere.
Here's some experiments - Relieve the pressure by removing the cap the night before or early in the morning, when the car is cold. Don't drive it. After the car has been sitting for a while in the sun, remove the gas cap. If it spits up, then you're seeing heat causing the fuel to expand (and vapors to form), and they're not making it to the canister. I'd suspect the hose to the canister was crimped or kinked.
If it doesn't spit up, start the car and let it idle for 5-10 minutes. Shut the car down and remove the gas cap. It should actually hiss some as air is pulled in. That tells you the Evap valve is working.
Now, take the car for a short drive. Make sure you get the turbo boosting. The shorter the drive, the better, but hit decent boost. Shut the car down, and remove the gas cap. If it spits up, then the turbo is pressurizing the system.
posted by 192.249....
, Sun, 18 May 2008 19:34:45, Mon, 19 May 2008 05:27:52 <-- Viewing This Message
No Site Registration is Required to Post - Site Membership is optional (Member Features List), but helps to keep the site online
for all Saabers. If the site helps you, please consider helping the site by becoming a member.
 |
 |
 |
 |
 |
