Vapor lock with billet fuel rails?
11-30-2014, 03:55 PM
#1
Vapor lock with billet fuel rails? Trying to tune a lq4 swap 79 trans am and it acts like its having vapor lock issues. Its fine at first but then after a short drive dies and won't restart for a minute.
The car has black billet fuel rails and they get very hot to the touch. We can let the car cool a bit and the header tubes cool quicker than the rails after 10-15 minutes which doesn't seem right to me.
My car uses factory rails so I don't have experience with billet rails. Sound like they are the issue?
The car has black billet fuel rails and they get very hot to the touch. We can let the car cool a bit and the header tubes cool quicker than the rails after 10-15 minutes which doesn't seem right to me.
My car uses factory rails so I don't have experience with billet rails. Sound like they are the issue?
11-30-2014, 06:34 PM
#2
Sounds like fuel is heating up. Is it a returnless system w/ fuel pressure valve ahead of the rails? Fuel can get very hot when it is continuously dumped over a reducing valve. Since it's not a stock set up, am thinking that this may be a possibility.
11-30-2014, 08:51 PM
#3
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I would think that standard pump pressure would hold
down any fuel boiling problem. What's the pressure look
like when this is going on? If you purge at the Schrader
does it get back to right?
My manifold, rails and all that stay cool enough to touch
(I forget the readings now, from the heat-soak thermal
measurements post). Wonder if there is a cooling airflow
shortage in the engine bay. What kind of fans setup,
what kind of underhood air temps?
down any fuel boiling problem. What's the pressure look
like when this is going on? If you purge at the Schrader
does it get back to right?
My manifold, rails and all that stay cool enough to touch
(I forget the readings now, from the heat-soak thermal
measurements post). Wonder if there is a cooling airflow
shortage in the engine bay. What kind of fans setup,
what kind of underhood air temps?
12-01-2014, 08:14 AM
#4
I will have to get back to you. He didn't have a schrader valve on the rail to hook up my gauge. Has an aluminum radiator with a dual fan setup with a shroud. It's has the shaker hood but it's not open as they were just for looks in the later years.
I'm not certain this is the problem but it's the only thing I can think of that makes sense. The car idles perfectly until you take it for a drive. Later in the day we could drive it shorter and shorter before it would die. I gave it a stock lq4 throttle cracker table just to eliminate that.
I will report back once we can check pressure. I think he also said he was going to put thermal wrap around the rails and see if that helps. If not we may try pulling the hood and if that fails but pressure looks good he may just go to a return style system with a regulator instead of the corvette filter/regulator which currently dumps back into the sump on the tank.
I'm not certain this is the problem but it's the only thing I can think of that makes sense. The car idles perfectly until you take it for a drive. Later in the day we could drive it shorter and shorter before it would die. I gave it a stock lq4 throttle cracker table just to eliminate that.
I will report back once we can check pressure. I think he also said he was going to put thermal wrap around the rails and see if that helps. If not we may try pulling the hood and if that fails but pressure looks good he may just go to a return style system with a regulator instead of the corvette filter/regulator which currently dumps back into the sump on the tank.
12-01-2014, 10:41 PM
#5
I'm not certain this is the problem but it's the only thing I can think of that makes sense. The car idles perfectly until you take it for a drive. Later in the day we could drive it shorter and shorter before it would die. I gave it a stock lq4 throttle cracker table just to eliminate that.
I will report back once we can check pressure. I think he also said he was going to put thermal wrap around the rails and see if that helps. If not we may try pulling the hood and if that fails but pressure looks good he may just go to a return style system with a regulator instead of the corvette filter/regulator which currently dumps back into the sump on the tank.
I will report back once we can check pressure. I think he also said he was going to put thermal wrap around the rails and see if that helps. If not we may try pulling the hood and if that fails but pressure looks good he may just go to a return style system with a regulator instead of the corvette filter/regulator which currently dumps back into the sump on the tank.
Are the fuel lines running close to the exhaust or headers? There's no good reason for the fuel to be getting hot. Am sure that the engine tables are reacting to the hot fuel when shutting down after driving for a while. I don't think that heat wrap is gonna change anything. It's not residual under hood temps that are heating the rails. Either a small restriction like a reducing valve ahead of the rails (not the case since you stated stock regulator) or something like extremely hot like headers/exhaust running next to fuel line.
Maybe the regulator is piped backwards where dumped fuel is getting sent to rail & regulated fuel to the tank?
12-02-2014, 12:42 AM
#6
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Are the fuel lines running close to the exhaust or headers? There's no good reason for the fuel to be getting hot. Am sure that the engine tables are reacting to the hot fuel when shutting down after driving for a while. I don't think that heat wrap is gonna change anything. It's not residual under hood temps that are heating the rails. Either a small restriction like a reducing valve ahead of the rails (not the case since you stated stock regulator) or something like extremely hot like headers/exhaust running next to fuel line.
Maybe the regulator is piped backwards where dumped fuel is getting sent to rail & regulated fuel to the tank?
Maybe the regulator is piped backwards where dumped fuel is getting sent to rail & regulated fuel to the tank?
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12-02-2014, 08:41 AM
#8
Fuel filter is also new (along with tank and all lines), I asked because I suspected rust or dirt might have stopped up the filter.
I agree that we need to know pressure so that's next on the list.
12-02-2014, 10:34 AM
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The new tank I bought has a 5/16 vent line that comes off the top of the tank and drops over the edge and runs behind the fuel pump mounting plate that I fabbed up. The original sending unit is still in the top of the tank but the supply and return lines are looped togther by a piece of 3/8ths fuel line. I have a lower tank sump that was welded on by my buddy. It is a competition engineering 1/2 sump part number on jegs is 247-4041. Above where the sump is welded I drilled five 2 1/4" holes to feed the sump as per the instructions. The tank had just over 1/2 tank of new 94 octane in it when we were having the issues. As far as pressure I'm waiting on UPS to bring the aeromotive fitting that I ordered in order to run a guage and make sure the pressure is where it should be and also to run a leak down test.
Last edited by sicmyn; 12-02-2014 at 11:09 AM.
12-02-2014, 09:09 PM
#15
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Correct. I cut the hood scoop out today as well as added the nylon washers. Once the motor was up to 180 there was quite a bit of heat leaving the engine bay. I let the car idle for almost 1 hour and shut it off. The rails were warm but they were no where near as bad as before. Maybe due to the ls eating up much more space than the 403 did heat may have been building up under the hood with just small gaps to leave from. Now that there's a larger opening to vent from it may be good to go. I am still waiting on the fuel guage fitting so I'll check that before I trust it to go out and kill some mustangs lol.
12-03-2014, 01:46 PM
#16
Correct. I cut the hood scoop out today as well as added the nylon washers. Once the motor was up to 180 there was quite a bit of heat leaving the engine bay. I let the car idle for almost 1 hour and shut it off. The rails were warm but they were no where near as bad as before.
That would lead me toward the tune. Go drive it & see if it goes back to the original problem. Uses different parts of, as well as, different tables while idling than when moving. Every car has high under hood temps & you would see heat leaving the engine bay were you to cut a hole in the hood. Have never heard of under hood temps heating fuel to the point where the car wouldn't run. Maybe I'm wrong, but would go for a long drive before calling it solved.
12-03-2014, 01:59 PM
#17
the tune is built as a copy of the one from my car but with adjustments for injectors, cyl volume, and gearing...MAF scale was nearly dead on for the tuning we got to do, EFI Live's calc.vet was creating a VE table about 24% lower than mine so I tried reducing the table by 20% as a starting point and there was no change to the dying issue but the car did seem to be getting more responsive and LTFTs were getting in line as opposed to being very negative
our setups are very similar:
243 heads (mine are ported some on the intake side)
228/232 113+2 cam versus my 230/238 113+2 cam
62lb injectors versus my 42lb injectors
1 7/8" headers versus my 1 3/4"
ls6 intake versus my Fast 90mm
6.0l versus 5.7l
his lq4 with 243 heads should be slightly lower compression than my ls1 with the heads milled .010"
our setups are very similar:
243 heads (mine are ported some on the intake side)
228/232 113+2 cam versus my 230/238 113+2 cam
62lb injectors versus my 42lb injectors
1 7/8" headers versus my 1 3/4"
ls6 intake versus my Fast 90mm
6.0l versus 5.7l
his lq4 with 243 heads should be slightly lower compression than my ls1 with the heads milled .010"
12-05-2014, 04:50 PM
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Ok, so far so good on fuel system. I installed the fitting and hooked up a pressure tester. 60 psi as soon as I key on, (once it primes) 55-60 psi reving it up in park. 60 psi in second spinning the tires at 4000 rpm. It's been raining here so I haven't done any driving yet to test but it's dry tomorrow so I'll post back after a few runs and the related pressures under load. The pressure drops to 0 as soon as either the pump stops priming or once the motor is shut off. Can someone comment on the loss of pressure at key off/ pump off. Is that normal? Should I add a check valve between the pump and regulator/ filter?
