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Have searched, read, etc, may be a very simple answer, but "old guy" needs some insight..........1989 S5TII
Rather than taking a guess, thought I'd ask:
How does air enter the tank as gas drops during usage and vapors are pulled from tank through the canister?
The cap is a pressure only cap, and realize there is a check/cut valve at rear as well as a vent on top of the tank that attaches to the c/c valve. The test on this port is .14/.71 psi, and the bottom is listed as a passage to atmosphere .78/1.00 psi.
Am I to assume that when vacuum exceeds the parameters on the passage to atmosphere it will open and draw in air?
The check/cut valve is standalone, fastened to the body, while the tank vent is positioned on the tank top.
I'm going to pull the c/c valve and test, but wanted some info before doing so. The passage to atmosphere actually goes into the body behind the right side bin.
Just to give the amount of vacuum, after driving and leaving the engine at idle, if I remove the cap, air rushes in with so much force that the engine will stall from a lean condition. I took the cap apart and placed a 1/32" hole and no problems except that the gas odor at back of car is something I don't like.
Your gas tank should respirate through the charcoal canister line. If the canister, or the line are clogged, you will need to fix this. When I was rebuilding my car that had sat for a long time (cough, the 90's, cough), the tank vacuum was crazy as well. That steel line does not see the same flow as the fuel feed and return lines to keep it clean. Stagnation in this line can cause it to corrode, in my case completely shut. I went and bought replacement brake line, bent and replaced the rusted vent line. To check, you can pull off the hose at the tank, and at the charcoal canister and see if you can send air through it. If you can get air through the hose easily, and through the fitting in the top of the tank, check the charcoal canister. Gas cap should be non-venting unless you like to stink. you can make a replacement line with the simple brake bending tool for very little expense.
The check and cut valve is a feature that is included for Jeep owners. It regulates the tank pressure when the car is upside down. You can clean it up (very gently, if I remember right, there are screws that can strip easily).
Thanks for the reply, but there has been 2 canisters installed, and when checking the line, tank to canister, there is no blockage when checked with a vacuum tester, and when the hose from canister to purge valve is disconnected, the fumes are there. In fact, as I stated earlier, when "excess" vacuum is present, remove cap and car will stall as it flooded with air, so the line from tank-c/c valve-canister-oil filler tube-purge valve-throttle body is clear/open.
Not disputing what you found, but difficult to understand how reverse flow of air can go from canister to the tank as the engine is not only "sucking" vapors through the purge valve, but also the check valve in the air intake is one way..
I shake when you mentioned Jeep as I've owned many. Not certain as to what Jeep you're speaking of, but the daughter's Hemi Grand Cherokee sitting waiting for me to remove the heads to do the lifters and that Jeep has a cap that, under certain conditions, has both vacuum and pressure relief.
The charcoal can should run with the vent line like we mentioned previously, as well as have a line that runs to a T. That splits off to both your oil filler neck, and the check valve in the turbo inlet ducting. If any vacuum were pulled into the ducting at that check valve, it would be at a minimal pressure/vacuum, because its right behind the air filter and MAF as the only restrictions. The charcoal canister has an additional hole on the bottom of it as well that is at atmospheric pressure and should allow a small amount of airflow. If plumbed correctly, the only point that any sort of high pressure vacuum would be intersected would be at the oil filler neck venting the crank case. That oil filler neck has a 2nd port on it that leads to the PCV valve. If the PCV is stuck open, either by failure, or vacuum routing issues, then you could have a constant hard vacuum on the evap system. A good way to check would be to disconnect the evap system at the oil filler neck and put a vac gauge there.
On the diagram, follow the brown line from the charcoal canister to the T and then off to the oil filler neck and turbo inlet ducting. From here, the light blue line from oil filler to PCV, with the red and yellow going from PCV to throttle body vacuum ports.
When we purchased our '88 TII new, the engine/car design was one that created a tremendous amount of pressure to build in the emission system (fuel tank), and in fact it would bleed out through that (canister) valve and be sucked into the cabin via the condensate drain, negative pressure. We had that problem and also the trailing plugs that stopped firing after only a few hundred miles, so Mazda actually bought the '88 back and I got the '89 TII at dealers cost.
The lines have been checked all the way to the purge valve and they are fine, in fact, just to be sure, I purchased a new purge valve, no change.
I do appreciate the replies and just to give some more info, over the years there has never been a problem with the '89, but it started after the car was at the body shop for 6 mos. What happened during that time, beats me.
Think tomorrow I'll get my lazy body out there and take the c/c valve out, take a couple tests and see if any problems.
I have a '91 TII, minus engine/trans, but everything else there, so can remove the c/c valve from that one.
Rob-No I didn't check the fuel return line, no harm checking.
Actually the car runs great with the 1/32" hole in the cap, 12lbs boost, pulls strong, but just wanted to get it back to stock, at least the emission system.
10-05-18, 09:33 AM
