Hi all,

I asked this before but didn't get an answer. Now I'm trying to decide whether it's worth $35, plus a delay, to replace one part.

There's a check valve between the fuel/air separator and the carbon canister on my '78 Z. It's mounted near the fuel pump, and it ensures one-way airflow between the fuel tank system and the carbon canister. The problem is that mine passes air more easily in the reverse direction (towards the tank, in the direction it should actually block airflow) than in the forward direction (towards the canister). However, it does pass air in both directions.

So is this thing actually important in any respect, or is it simply federally mandated bovine byproduct? Is there any potential problem that would occur from it driving airflow towards the tank? Is it worth the $35+S&H to replace, or should I simply bypass it (or turn it in the "reverse" direction)?

Thanks!

Sarah

Edited August 23, 201014 yr by FastWoman

I never downloaded a FSM for a 78, but on the 79 FSM that part is called a "fuel check valve with vacuum relief valve." To me that description implies that it prevents FUEL from flowing from the tank to the carbon canister. (Which makes sense, that would be a bad thing.)

It seems logically that under some circumstances air would have to flow back into the tank from somewhere. The diagram isn't clear on how that happens. Since there appears to be no other path, perhaps the fuel check valve allows air to pass both ways, but prevents fuel from leaving the tank. (As it otherwise would if the tank was completely full.)

Hopefully someone here knows more than I do about this system.

Thanks, Walter! I'll have to check my FSM more carefully tomorrow to see if there is a test for the check valve. Maybe it's supposed to pass air in both directions, but under different pressures. I doubt it, though.

With regard to the liquid gasoline vs. gasoline vapor issue, the line from the gas tank system actually originates in the fuel/air separator, so I think that would already take care of the liquid phase. But yes, raw fuel in the canister would be a pretty bad thing!

I think I would spend the $35 if neccesary, to maintain the integrity of the system. I don't have an FSM for '78 either but I do know that the evaporative emissions system changed in '79. I gathered this from what little information I could find. I wouldn't be concerned with liquid fuel getting past the seperator as it is designed to only allow vapours to continue on to the carbon canister. Any liquid fuel is returned to the fuel tank. If you bypass the system, one of two possible problems could arise. Your fuel tank will not vent properly, possibly causing a fuel delivery shortage and depending on how you circumvent the system, it will vent to the atmosphere. There are different procedures for testing '76-'78 models and '79-'88 models. All the illustrations I found are generic in nature. I couldn't find a '78 specific illustration that explains the differences but did scan the procedures for each. The first scan comes from a Chilton's manual. The second and third scans are taken from a Haynes manual and explain the evaporative emissions system but again, it is not year specific and is a generic description.

Edited August 24, 201014 yr by geezer
added title for future searches

The oem gas cap is supposed to have a vacuum relief system in it, so there's no need to have any other way to allow air into the gas tank. the carbon cannister is different thing tho.

Edited August 24, 201014 yr by TomoHawk

Thanks, guys! Ron, the first image/text you posted hit the nail on the head. That's exactly how my check valve works. It's a bit sloppy, but it's still serviceable. I guess the idea is that the main airflow into the tank is actually through the carbon canister (which would make sense), but the gas cap would relieve the vacuum if the evap emission control system became blocked. (Better than having a buckled tank!) If the tank becomes pressurized, it will relieve its pressure through the carbon canister, but the check valve would keep these air movements to a minimum to keep fuel from evaporating from the tank. Makes perfect sense now.

Anyway, thanks for helping me out with this one!

The oem gas cap is supposed to have a vacuum relief system in it, so there's no need to have any other way to allow air into the gas tank. the carbon cannister is different thing tho.

Correct Tomo, I suppose that would prevent fuel starvation even if the system was inop.

wow i was thinking of something WAY different then this when you asked me about it in my thread . im sorry.

No prob, Shade. I appreciated your help anyway!

Here are the pages from the 78 FSM that cover the evaporative emission control system:

Thanks, Steve! It's actually page EC-25 that has the info I needed. Fortunately I have the FSM. I was just dumb enough to assume this works like an ordinary check valve (airflow ONLY in one direction) and not to confirm that by reading the section that explains how to test/check it! :stupid: If I had read the paragraph on EC-25, I would have seen that this is a very wonky sort of check valve and that the one on my car is working just fine. It's said there's no such thing as a dumb question, but perhaps this one would have qualified (since the answer was right in the book). Anyway, I appreciate your help!

EC-25 shows basically the same test as what Ron attached in post #4. I wanted to show the description of a 1978, however, since it is slighlty different from the ZX EECS.