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Gary L

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

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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 by Gary L

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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.

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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 by EuroDat

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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.

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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

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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.

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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.)

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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.

 

post-25317-0-09038400-1438098335_thumb.j

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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.

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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 by Gary L

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

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Steve, thanks for the "heads-up". That being the case I'll definitely leave it up on the jack stands and run it for awhile to make sure its not going to leak. I hope I got the ends together correctly and snugged enough. Since its going in a rubber hose section, I used the nylon rings with the bevel towards the nut. The illustration in the instructions wasn't the greatest.

 

Gary

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Well, that didn't help, darn it  I got the check valve installed last night as described above. It fit well in that location and doesn't leak, but when I tried starting it this morning I had to crank it for awhile before it started. I didn't pull the line apart yet to see if there is any pressure, but I'll do than tomorrow and then probably proceed with a pressure gauge as outlined by Chas. The new FPR I put on it last year was an aftermarket one but I can't remember the brand. May be that's the culprit. If it was a leaky cold start valve for example, it seems I'd smell raw gas through the AFM/air filter for example. Once it starts, it does run well with great throttle response. Gary

Edited by Gary L

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Gary, Once you get yourself a gauge it will take all the guessing out og it. I would also replace the two hoses, the hose from the filter to the fuel rail and the one next to it for the return to tank. New line clamps shut a lot easier than the old hoses. The old hoses can also crack and get damaged when you pinch them shut.

PS: rcb280z sent me a pmabout the original check valve.

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I'm also having problems with my car losing fuel pressure when sitting.  I can't drive it due to a few shoulder surgeries so all I do is start it and backit out of the garage.  After a day or two, I have to crank it quite a bit to get the pressure up so it starts.  I've confirmed the loss of pressure with a gauge mounted before the fuel filter.

 

I'd love to know how more about a quality replacement check valve. Could somebody please PM me the info?

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FAIW, the check valve is important mostly for HOT restarts.  After the engine cools, the gasoline condenses, and any fuel vapor should be replaced with liquid fuel.  When you turn the key, the fuel rail should re-pressurize within a fraction of a second, and all should be good.

 

If it doesn't happen this way, it means you're getting air in the lines, probably when the fuel vapor cools and condenses, and a vacuum is formed.  The air would most likely come from leaky injectors.

 

There are obviously other reasons for hard starting, so you should explore those too.

 

Of course none of this is to say you don't need a new check valve, which would help remedy a hot restart issue.

Edited by FastWoman

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Chas describes a process for narrowing down what is leaking, which I haven't had a chance to do yet. When it comes to the injectors, if I do suspect an injector or two, is there a way to check them individually without pulling them? Its kind of a pain pulling them. And even if I do, how do I then test them?  If an injector is leaking gas into the cylinder shouldn't it be leaking down the cylinder walls and into the oil. That was one of the problems with Ford's 2.8L engines in the Bronco IIs --- the carbs were always flooding and running rich and the raw gas seeped past the rings into the oil diluting it.

 

I think I did mention the injectors are new, having put them in last year. I forget what brand they are. When I purchased the car a couple of years ago, everything was original and thats why I have been replacing many of the almost 40 year old parts ( a "refreshing" project).

 

Gary

Edited by Gary L

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I was suspecting problems with my injectors once. Lean on 3 and 4 spark plug. To check the injectors I lifted them out of the manifold and attached small beakers to them and cranked the engine 10 seconds. With the spark plugs removed it wasn't to much stain on the starter motor.

 

After seeing the results it was obvious the injectors were causing the lean condition. I removed then and reverse flushed them with a mixture of carb cleaner and fuel. They have been working great ever since.

 

You mentioned changing the injectors last year. I think I would leave them to last and try to test, repair or replace the other parts first. The leakdown test with the pressure gauge will tell you were to look. You probably have more than one culprit causing the pressure loss.

 

Like Sarah said, pressure loss is not that big a problem as long as its not getting air in the system it should built up pressure in seconds. When I first got mine it lost pressure in about 2 minutes. Mine was the check valve and a leaking fuel filter (leaking at the seam). After that it now holds pressure for 3 to 4 weeks.

 

Just a note on starting: When it has pressure mine starts 1 second and sometimes 2 seconds. After the pressure has dropped to almost 0 psi it takes 3 to 5 seconds to start.

 

Chas

post-25317-0-69842900-1438442560_thumb.j

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This morning I went to O'Reilly Auto Parts and they loaned me a fuel pressure gauge no charge. I hooked it up as described by Chas (in the rubber fuel hose just after the fuel filter). The first thing I noticed was that my fuel pressure was only 28, not the 36 per specs. When I shut the engine off the pressure immediate and quickly dropped to zero. After starting the engine again (and of course it has to crank for awhile), I shut it off and clamped the hose just after the gauge but before where the fuel rail starts. This stopped the pressure drop and so I think my check valves are OK. Then I repeated this process but clamped the return line hose after the fuel rail. This also stopped the dropping pressure. So if I'm looking at this correctly, not only does my FPR not maintain proper pressure of 36, but it must be leaking. This is good news since its easier to change the FPR than check and change the injectors.

 

Anyone have a suggestion for a good FPR that is reasonably priced? I know some are pretty expensive (eg > $100). Anyone try the Echlin from NAPA Auto that runs about $70 ? I know Echlin makes quality electronic components and I hoping their other products are also excellent.

 

Gary

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28 is normal for a running engine.  There's a hose from the intake manifold to the FPR that adjusts pressure based on manifold vacuum level.  Irrelevant to the leak-down but important to know for future work.

 

It does look like your FPR leaks down.  The valve in the FPR is just two pieces of steel that press together.  A little bit of rust could cause a leak.  If "recently" is three years ago, that might be what happened.

 

No recommendations.  I've had luck with the factory FPR's and have since installed a modified Bosch regulator.  I'd get one from a local store that will take returns.  Swap until you find a good one.

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^^^ Everything that Zed said!  I'll add that 28 is a very good fuel pressure with an idling engine.  It suggests your intake vacuum is quite good, which means your engine is running efficiently (good fuel/air mixture).  You must have lovely spark plug insulators!  ;)

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When I rev it up a little, the fuel pressure does jump from 28 to may be 29 for a second or two. But why isn't it closer to 36 as specified in the FSM?

 

When I put a vacuum gauge on my engine, it does read a steady 17 and when I flick the throttle, it drops to zero then up to about 25 and then settles back to a steady 17, so I do think I have a "healthy" engine.

 

Gary

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