Fuel System Vapor Lock

[Kempo]

Have any of you guys have had vapor lock in your fuel systems? I was having issues after doing some spirited driving or making a WOT pull to make a datalog with HP Tuners. After doing a pull and stopping the car for about five minutes to review the data and reflash the PCM it wouldn't start. I checked every possible problem and found out I had no fuel pressure. I could hear the pump working but it made a noise as if there was air in the system. I opened up a fitting in the system and only gas vapor was coming out. After letting the vapor go out the car would start right up with a steady fuel pressure of 58 PSI. After realizing what it was I did another test. Made another WOT pull and as expected it happened again. This time instead of opening up a line I poured water over My filter, pump and stock returnless fuel rail. The car started right up again.

I came to the conclusion that because the stock GM LS3 fuel rail is a returnless system the fuel gets heat soaked in the hot engine bay on a hot day while waiting on the rail to go down the injectors. My solution was to remove the returnless rail and install aftermarket ones that have the fuel flowing all the time. This way it would be pulling colder fuel from the tanks all the time and cooling the system as it travels thru it. So I did and it worked. Did a few pulls today and never had the issue again. I never took any temp readings with the stock rail but I did with the new setup. I did some driving close to my home then pulled in my garage. After allowing ten minutes for the engine bay to heat soak I took some readings with a laser temp gauge. The temperature at the filter was 112*F and at the fuel rail was 136*F I then started the car and within 1 minute the temperature went down. The filter went down to 97*F and the rail to 99* This was only with a normal drive not a WOT pull or spirited driving which make the engine bay temps a lot more hotter.

The rails I used are the ones made by FAST and plumbed it using my existing Aeromotive regulator,filter and pump.

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[The Stig]

Sorry to hear of the fuel vapor issues Hugo. We really need to come up with a solid answer to getting heat out of the engine bay. If we're going to drive these cars on a somewhat regular basis, we really don't have a choice...

I have an idea, but it involves buying another Gen-I hatch that I can use to do some fabrication with. I'm thinking that this may be another of those mini projects that you don't really want to do, but that you would be much better off with once you finish it. It looks like I'm going to get to learn those new aluminum shaping and fiberglass fabrication skills after all...

[VRaptor SpeedWorks, LLC]

I would guess that your problem has a lot more to do with the location of your pump than vapor-lock. If this is vapor-lock, it would be the first confirmed case of vapor-lock in a fuel injected car that I've ever heard of. I don't know what the boiling point of gas is at 58psi, but I'm guessing it's probably more than your plastic intake manifold could withstand. I would guess that with your fuel pump and pre-filter both being mounted above fuel level and not being able to prime. If your fuel was boiling inside the fuel rails and the pump was trying to pump against the vapor, you would still have 58psi of pressure.......it would just be that you'd be measuring the pressure of the vapor instead of liquid, but the gauge would still show 58psi. The fact you have no pressure tells me that the pump isn't pumping because it's sucking air because it can't prime because it's mounted too far above fuel level. That's my guess anyway. It would also make sense as to why going to a return type fuel rail would fix the problem. If the pump doesn't have liquid to it, it can't pump the air/vapor out of the line because it's dead-headed by the fuel rail.....no where to pump the air to in order to suck the fuel up to the pump. Once you put on the return-type system, then the pump is able to continuously flow fuel instead of being dead-headed by the fuel rail, so it is able to pump the air out of the feed hose to get fuel to the pump to build pressure.

[Kempo]

That makes sense Shane but, I have a check valve on the system that enables the pump and filter to stay full of fluid when the car is turned off. It only happens when I turn the car off immediately after a WOT pull and let it heat soak for about five minutes. At that point the fuel in the system isn't at 58 PSI as the pump is off. I guess that would take the fuel boiling point to a lower temp. After cooling it down by pouring water on the pump, filter and rails it would start right up. That is what lead me to think that the fuel was boiling in the system.

[Kalstar]

That makes sense Shane but, I have a check valve on the system that enables the pump and filter to stay full of fluid when the car is turned off. It only happens when I turn the car off immediately after a WOT pull and let it heat soak for about five minutes. At that point the fuel in the system isn't at 58 PSI as the pump is off. I guess that would take the fuel boiling point to a lower temp. After cooling it down by pouring water on the pump, filter and rails it would start right up. That is what lead me to think that the fuel was boiling in the system.

Not sure if this is a help or not. But I had the same issue. I too thought it was a failing pump or vapor lock. It was doing as you are discribing and "would only start when cooled down". I found out it had nothing to do with cool down, it was the fuel pump relay that had wiggled itself loose and would have just enough contact when cooled down that it would start. I replaced the relay (which was loose when I removed it) threw a zip tie around the new relay and bracket and have not had an issue since.

[fastthings]

How old are you Kempo? Back in the day we used to put close pins on the fuel lines to keep them from getting hot, it worked. If it is vapor locking, letting the fuel circulate should do it. I wondr what kind of fuel your getting over there. The more RVP's you have in the fuel the more likely it could vapor lock. Winter fuel has more RVP's.

Without knowing, I like Shane's logic.

Let us know please if it's solved.

Gene

[Kempo]

Not sure if this is a help or not. But I had the same issue. I too thought it was a failing pump or vapor lock. It was doing as you are discribing and "would only start when cooled down". I found out it had nothing to do with cool down, it was the fuel pump relay that had wiggled itself loose and would have just enough contact when cooled down that it would start. I replaced the relay (which was loose when I removed it) threw a zip tie around the new relay and bracket and have not had an issue since.

At first that was my line of thinking but I could hear the pump working every time I cranked the engine.

[Kempo]

How old are you Kempo? Back in the day we used to put close pins on the fuel lines to keep them from getting hot, it worked. If it is vapor locking, letting the fuel circulate should do it. I wondr what kind of fuel your getting over there. The more RVP's you have in the fuel the more likely it could vapor lock. Winter fuel has more RVP's.

Without knowing, I like Shane's logic.

Let us know please if it's solved.

Gene

I'm still a young kid (at least I like to think so. Although the mirror tells me otherwise. Starting to get white on top) I'm 39. The fuel on the tank usually is Total 91 octane. Our cold winter is about 70-79* so don't think the fuel has a lot of RVP's. The problem has been solved with the changes made.

[VRaptor SpeedWorks, LLC]

That makes sense Shane but, I have a check valve on the system that enables the pump and filter to stay full of fluid when the car is turned off. It only happens when I turn the car off immediately after a WOT pull and let it heat soak for about five minutes. At that point the fuel in the system isn't at 58 PSI as the pump is off. I guess that would take the fuel boiling point to a lower temp. After cooling it down by pouring water on the pump, filter and rails it would start right up. That is what lead me to think that the fuel was boiling in the system.

If you had a check valve in the system to keep the pump and filter full of fuel, then you should have still had 58psi in the fuel rail, even with the engine turned off. This is why I've never heard of a case of vapor lock in an EFI car. Unless your check valve is bad, the fuel rail will always have pressure in it. In every fuel injected car I've ever worked on, you can shut off the car, and hours later, if you pull the cap off of the schrader valve (where you would hook up your pressure gauge) and push the valve in with a screwdriver, it will spray fuel out all over the place....at high pressure. With that much pressure retained in the rail, it can not boil. In the hours or days that it would take for that pressure to leak down, the engine will long ago be cooled off to ambient temp.

I would also guess that in almost every system, that check valve is AFTER the pump....not before it. If you have a spring loaded check valve that the pump must try to suck fuel past, that is going to compound your problem with getting fuel to your pump. Just as you say in your OP, that you can hear the fuel pump sucking air. My guess is that it is cavitating in it's attempt to suck fuel and trying to overcome both gravity and the check valve spring.

[Kempo]

Shane, you just hit the nail in the head. Now that you mention it my system used to hold the pressure whenever I turned the engine off or primed the system. Not anymore. It still holds fluid inside but not pressurized. I have installed the XRP flapper check valve part # 732608. There must be something wrong in there. I will take a look at the valve and see whats wrong in there. Thanks for sharing your knowledge.

[VRaptor SpeedWorks, LLC]

Let me know what you find out!

[LCD Gauges]

If you are having trouble with the fuel rail prime pressure, you can supply a constant voltage to the pump to allow for start-up. I'll provide details for my system if you like;
essentially the pump is connected to switched 12VDC, and I've installed a return line from the fuel regulator to the passenger side tank. This might not be desirable for those
using the complete PCM, and harness...but is an easy, effective solution for the guys like me running a 'custom' wired PCM.

[Kempo]

Let me know what you find out!

Here is what I found out. I removed the check valve and tested it using compressed air. It worked as it should. It had flow one way and was totally sealed the other way. After that I removed the fuel pressure regulator. Again I used compressed air to test it. With the regulator set at 58 PSI as it was I applied 40 PSI of air to the regulator's inlet and air came out of the return port. I increased the pressure on the regulator and applied the same 40 PSI of air and air still came out of the return port. I then searched for the install instructions of the regulator and Aeromotive has a note there that says "the enclosed regulator will not necessarily seal to hold fuel pressure after the fuel pump stops running" So I guess that's why my system holds fuel in the lines but without any pressure. At least my problem went away with the changes I made so I won't have to get a different brand regulator.

http://aeromotiveinc.com/wp-content/uploads/2013/01/13101-09-14.pdf