Hi all,

I had a eureka moment this morning in my half-groggy state. (Apparently I had been dreaming about fuel injectors, seized screws and bolts, and other frustrations all night. Go figure.) My '78 has this problem that it sometimes gets vapor locked and also sometimes gets air (as opposed to fuel vapor) in the injector rail. The result is hard starting.

Part of the problem is worn injectors (which I'm trying to replace). Part of the problem is heat. I figure it this way: I drive to where I'm going and shut down. Then the area over the intake gets quite hot, and the fuel in the injection rail boils. Fuel is ejected to the return line via the pressure regulator and/or dribbles out through worn injectors into the intake manifold. Then the engine cools, and the fuel vapor condenses, creating a vacuum in the injection rail. Air is then drawn from the intake manifold, through the worn injectors, into the injector rail. When I go to restart the car, all the air has to be purged before the engine will run right. It takes several attempted starts, and the engine will run very roughly (on only a few cylinders) until it smooths out. I think I'm learning this is a fairly common problem.

Apparently this problem can also occur without fuel vaporization if the fuel pump check valve is bad. Gravity will pull the fuel from the rail, and apparently there is enough air leakage from even newer injectors as to allow air to enter the fuel rail from the intake manifold.

But here's what I figured out this morning: When you get air in your fuel rail for any reason, Bosch's "trumpet" design does not help you to purge it. If one end of the fuel rail is any higher than the other (which it probably is), then the air will settle at that end. Fuel enters one side of the trumpet (i.e. not the end, but the side), and it ordinarily flows around both ends of the trumpet to the 6 injectors. The excess flows from the trumpet on the other side (not end) and is returned via the fuel pressure regulator to the tank. However, when there's a big air pocket in the higher end of the trumpet, the fuel will only flow around the lower end of the trumpet (the one without the air pocket. Thus the air pocket will not be flushed by the normal circulation of the fuel. Instead, the air will have to be purged through the injectors.

A much smarter design would be a system without a bifurcated route. Fuel would be introduced into one end of a straight line (e.g. the aluminum billet rails), flow past the 6 injectors, exit the other end, travel directly to the pressure regulator, and then return to the tank. That way any air pockets would be purged by the normal flow of fuel through the rail, not having to be bled through the injectors.

The aluminum billet fuel rails I've seen seem to be dead-end streets, perhaps with the pressure regulation occuring prior to the rail. (Am I correct about this?) This would require purging of the air through the injectors and could be even worse than Bosch's "trumpet." The secret to purging air and fuel vapor is to put the fuel pressure regulator on the far side of the injectors.

Anyway, even though I have a new check valve and will soon have new injectors under the hood, I think I'm going to redesign the rail system. Anyone know where I can find a pre-made aluminum billet rail, or do I have to make my own?

Hello FastWoman... I have found it very difficult to vapor lock an EFI equipped engine. My understanding is that since EFI systems operate at much higher pressures on the supply side (38 to 45) psi that it is next to impossible to vapor lock one. Here is a link to a company that makes all kinds of "build your own" EFI parts, including universal fuel rails. http://www.rossmachineracing.com/intakepartspage.html My suggestion for your car without anymore information would be to take a close look at the injector connectors, Coolant Temp sensor and related connectors, and the Airflow Meter. My 75 would start up and run fine, but after driving it for a while it would fuel foul all the plugs up and be almost impossible to start. I turned out to be the AFM. Also take a look at ignition parts (coil, cap, rotor.. ect. ), My current Z uses the stock fuel system components, but I use the MegaSquirt standalone computer. I had a hard starting issue on that one that I traced back to a faulty "performance" ignition coil. Hope some of this helps, good luck.

I recall many a heated discussion in the past on this site about vapor lock on EFI cars. The only way you could actually show vapor lock type symptoms would be if you had low fuel pressure for whatever reason. The handful of times I've helped out someone with a 280Z or ZX complaining of vapor lock it always turned out to be something else. I live in Southern California and it gets pretty hot in the summer. Like I said, I've only seen the symptoms a handful of times so, based on my experience, its not what I'd call a "fairly common problem". The EFI system on these cars is fairly simple. Compare it to modern day systems. Overthinking it is often the fairly common problem.

Well, maybe not vapor lock, then. I know gasoline boils very easily, but I also realize it can be compressed back to its liquid state.

However, I do get air in my fuel rail, whichever way that might happen, and if the car is sitting at even the slightest angle, it's harder to purge it. (I installed a priming switch on my fuel pump system, so I do know this.) I had been confused as to why the air isn't generally purged rather quickly by priming. (It sometimes takes maybe 10 sec of priming). However, the bifurcated flow path explains it.

Billy, I've been going through all that stuff! Thanks for the help, though!

Edited September 22, 201014 yr by FastWoman

I have a similiar issue and i've narrowed it down to the check valve.

Ztrain, I recently replaced my check valve, and it made a MAJOR difference. The Nissan check valves are NLA, but you can MAYBE still find a Volvo or Saab check valve that fit and function the same: See here:

http://www.classiczcars.com/forums/showthread.php?t=36255&highlight=volvo+check+valve

Unfortunately when I had sourced out the Volvo check valves (which were plentiful) in public view, they almost immediately went NLA. I was lucky to buy just one.

I have an aluminum fuel rail that I never used, but it is for o-ring type injectors and AN fittings or pipe fittings on either end. If you are keeping the barb-type injectors, I suggest Pallnet (Peter Allnet). He sells fuel rails for L28 engines that will bolt up to the stock intake manifold and have proper alignment for your injector bosses, but I took a look at his website and it looks like he is having machinery trouble. He supplies an email on his website: http://pallnet.webs.com/

I have an adjustable fuel pressure regulator after my fuel rail bolted to my firewall. It seems to dump pressure when I shut down the engine withing a couple of seconds (I guess there's no check valve), but I have never had a starting problem even in 98 degree summer days after shutting down. It's on the passenger side and blurry in this picture (red Aeromotive FPR)