What maintains the fuel line pressure when the care is off?

[Gary L]

I've got a 76 z with the EFI. When I break open the fuel line just above the fuel filter for example (with the car off of course) there isn't any pressure in the fuel line and when starting it takes a while because I think it has to build up pressure again.  I've recently put a new fuel pump on it and fuel pressure regulator. I did get rid of the fuel damper since it was the original and I don't think it was working and they aren't available any more. The engine does seem to run well, and smooth once started and warmed up.

 

So my question is, what component is responsible for maintaining the fuel line pressure when the car is off?  It's like somehow the fuel drains back to the fuel tank and the pressure is gone?

 

I should also mention I can't find any obvious fuel leaks and there isn't any gasoline smell

 

Thanks

Gary

Edited July 27, 2015 by Gary L

[SteveJ]

The fuel damper and fuel pressure regulator should work to maintain pressure in the fuel system when the car is off.  Refer to the EF section of the factory service manual.

[EuroDat]

Hi Gary,

Its not simply one thing that holds pressure.

Pressure "leak down" can be caused by several things and you can test the system to narrow it down by using a pressure gauge.

Things that can cause pressure loss are.

1. FPR is leaking back to tank. Common with aftermarket FPR's

2. Leaking fuel injector or CSV (cold start valve).

3. Check valve in pump discharge.

4. Leaking hoses.

The quickest way to test it is by fitting a Tee piece with a pressure gauge after the fuel filter. I would also replace the hoses from the filter and the return line. Use enough hose both sides of the tee piece so you can clamp the hose to block sections in.

1. Start and run the engine and then stop engine. Check for pressure drop.

2. Do this test again, but then clamp the hose between the gauge and the Injectors/FPR. If it still drops at the same rate your pump is leaking back to the tank or a hose is leaking fuel. If not them you problem is further down the system.

3. Repeat test, but now clamp the return line from the FPR. If it now holds pressure, then you have a leaking FPR.

4. If it still loosing pressure then its an injector or CSV. You can remove the two screws a d lift the cold start valve to check it.

5. If all those items check out, then its most likly one or more leaking injectors.

Hope that all makes sense...

Edit: Out of curiosity, why did you change the fuel pump and regulator?

I think you injectors will be ok from your description of how it starts. They generally start really rough on one or more cylinders until the fuel air mixture settles down on the cylinder with the leaking injector. Yours seems to start up ok.

Most likely causes are the FPR or check valve in pump.

Chas

Edited July 27, 2015 by EuroDat

[siteunseen]

I'll guess it's the check valve or lack of one in an aftermarket pump.  Like EuroDat says, aftermarket fuel pressure regulators are horrible about leak down, the universal ones.

[Gary L]

Thanks guys. I do see in the FSM the original fuel pump had some sort of a check valve and may be the aftermarket ones don't.

 

I should have also mentioned this has been a 3 year "refresh" project where I've not only replaced the fuel pump and regulator, but also the injectors and cold start valve.  I guess I'll try to find time to do the fuel pressure gauge and leak test at various section of the system. Gary

[mayitin]

back in the day the check valve was the problem, now car been this old I will say all of above that euroDat stated. the valve is a little thing on top of the fuel pump and the house is attatch.

[FastWoman]

In my experience (with my Z), almost EVERYTHING was leaky.  Replacing the check valve took care of 50% of the problem.  A dirty CSV was maybe another 25%.  Old, worn injectors were the final 25%.  I swapped one old FPR with another old one, and it made no difference.  Today my system holds pressure indefinitely.  Along the way I tried an adjustable FPR (Chinese knockoff of Aeromotive style), but it didn't even attempt to hold pressure.  I returned it.

 

The original check valves are NLA.  There's been a good bit of effort on this list to source alternative check valves.  When I publicized a plentiful source from Volvo on this list, some predator/parasite consumed the entire national supply in a single day.  I was very lucky to get just my one.  (I wanted a spare but couldn't buy one.)

[EuroDat]

The check valves are still available NEW. 

Now should I tell my secret.......mmmm.

 

Ill dig up the part number later if people are interested.

 

Here are some options for the FPR.

 

[FastWoman]

Chas, very cool about the check valves!  But believe me that if you tell your "secret" here on the board, the supply will be gobbled up instantly.  This sort of thing is better handled through private messages when we know someone legitimately needs a check valve (and isn't just trying to hoard parts supplies).  Food for thought.

[rcb280z]

Chas, sent a pm

[Gary L]

Hi Chas, I can't seem to find the PM.

 

Anyhow, since I've recently replaced the injectors, FPR, fuel pump, check lines for leaks, etc., etc, I thought I'd try placing an inline check valve just above the fuel pump where the fuel damper was. There is just enough space to fit the following check valve I picked-up at Advanced Auto.

 

http://shop.advanceautoparts.com/p/dorman-oe-solutions-fuel-check-valve-5-16-800-195/10611403-P?searchTerm=fuel+check+valve

 

 

The Dorman part number is 800-195

 

I'll let you know if it helps maintain fuel pressure.

 

Gary

Edited July 29, 2015 by Gary L

[SteveJ]

Be careful with the assembly of that check valve. Several of the reviews for it on Amazon indicated it tended to leak.