Vapor lock in Jeep MPI setup plus poor gas mileage

[MomoJeep]

Hi Moses,

So I have the 4.0L head on the 258 block, with the Mopar/Hesco MPI kit installed. I love it, except for the vapor lock problem, and that the gas mileage still seems poor (14.5mpg)

MILEAGE AND HEAT

I experience vapor lock after long drives on hot days and/or going up mountains. There is a pressure and return line running up the driver side, where the exhaust is also located. External fuel pump mount near the exhaust pipe on drivers side of the rear cross member, filter also on cross member in center of vehicle. I tried wrapping the exhaust from manifold, over cat conv and muffler (and put a heat shield fabric tube over the fuel lines from firewall to injector rail). This did not fix the problem at all. However, the gas mileage jumped up to 16-17mpg! Why do you think this happened? I am guessing that since the air intake sits right about the exhaust, cooler air into the engine is more efficient since it can expand more in combustion than hot air? I also heard something about "Scavenging" with hotter (insulated) exhaust, but I don't understand that.. Why do you think the mileage got better? Any tradeoffs to keeping it this way (exhaust wrapped)?

VAPOR LOCK

Then with the vapor lock, there are a few ways to go, and I'm not sure which to try:

(1) Move fuel lines to passenger side, route under radiator and back up to injector rail. However, I've read some people have this set-up and still experience vapor lock. Some speculate because of the radiator, but I'm guessing it's the low pressure line before the pump.

(2) Switch to using a regulator near tank, instead of in the fuel rail. Hesco sells parts for this. The advantage is there will be no return line. I heard that gas vaporizes much easier at low pressures, but I'm really not sure if vaporized / aerated fuel returning to the tank is the problem? This could be combined with (1) above to make things easier and cleaner. http://www.hesco.us/products/7902/40l-conversion-parts/313496/hes409606-1-regulator-kit#.WbbvWmeWzIV

(3a) Novak sells an in-tank pump. I like this idea because currently fuel travels across the rear xmember from passenger to driver side at low pressure (the external pump is on the drivers side). I am suspicious that this low pressure run before the pump is where the vapor lock might be happening. The in-tank pump would eliminate this. (3b) I believe I'd also still have the option of locating the regulator in the rear and running a single line up to the engine (and could also be on the passenger side?) https://www.novak-adapt.com/catalog/fuel-system/cj-fuel-module

The drawback I'm concerned about is that I do a lot of outdoor adventure road trips and remote backcountry travel. I carry a spare external fuel pump. If the in-tank pump fails, wouldn't this be much harder to replace on the trial?

Do you think a spare external pump could still be installed inline if the in-tank pump failed to get me home? Or would the failed in-tnnk pump prevent fuel from flowing?

So, which option do you think makes the most sense? (1), (1) & (2) Combined? or (3a) or (3b)

Thanks!

 

[Moses Ludel]

On 9/11/2017 at 1:26 PM, MomoJeep said:

Some thoughts, MomoJeep.  See my red highlights...Moses

So I have the 4.0L head on the 258 block, with the Mopar/Hesco MPI kit installed. I love it, except for the vapor lock problem, and that the gas mileage still seems poor (14.5mpg)

MILEAGE AND HEAT

I experience vapor lock after long drives on hot days and/or going up mountains. There is a pressure and return line running up the driver side, where the exhaust is also located. External fuel pump mount near the exhaust pipe on drivers side of the rear cross member, filter also on cross member in center of vehicle. I tried wrapping the exhaust from manifold, over cat conv and muffler (and put a heat shield fabric tube over the fuel lines from firewall to injector rail). This did not fix the problem at all. However, the gas mileage jumped up to 16-17mpg! Why do you think this happened? I am guessing that since the air intake sits right about the exhaust, cooler air into the engine is more efficient since it can expand more in combustion than hot air? I also heard something about "Scavenging" with hotter (insulated) exhaust, but I don't understand that.. Why do you think the mileage got better? Any tradeoffs to keeping it this way (exhaust wrapped)?

Which exhaust manifold are you running on the engine?  If the OEM iron 4.2L type, this is inefficient, and there may be some benefit from scavenging if that's actually occurring.  Is it a 4.0L manifold or factory "header" type?  For sure, the exhaust is very hot with the wrap and shields, and this would more likely create a fuel mixture (O2 sensor driven) change that could be helpful to mileage.  Is the engine pinging?

VAPOR LOCK

Then with the vapor lock, there are a few ways to go, and I'm not sure which to try:

(1) Move fuel lines to passenger side, route under radiator and back up to injector rail. However, I've read some people have this set-up and still experience vapor lock. Some speculate because of the radiator, but I'm guessing it's the low pressure line before the pump.

Static, hot fuel is prime for vapor lock.  Vapor lock risk should be reduced by the constant return flow of fuel to the tank if you have a two-rail EFI type system or earlier Mopar kit.  With a single rail ('97 up TJ type) Mopar EFI, I can see how vapor lock might occur between the fuel pressure regulator (mounted at or near the tank) and the single fuel rail.  The fuel could be somewhat static between the regulator and fuel rail on a single rail system.    You could have an issue on the pickup side of the pump if any of the hose connections are drawing air when hot or the hose is collapsing.  If pickup side of the pump hose connections are loose or swelling when hot, this could be a problem.  So could a clogged tank pickup sock.

(2) Switch to using a regulator near tank, instead of in the fuel rail. Hesco sells parts for this. The advantage is there will be no return line. I heard that gas vaporizes much easier at low pressures, but I'm really not sure if vaporized / aerated fuel returning to the tank is the problem? This could be combined with (1) above to make things easier and cleaner. http://www.hesco.us/products/7902/40l-conversion-parts/313496/hes409606-1-regulator-kit#.WbbvWmeWzIV

Since you do have a two-rail system now, and if the regulator works and the return line to the tank is not restricted, vapor lock should be a lower risk.  Check the return flow from the two-rail regulator to the tank, make sure fuel flows smoothly and with enough volume.  Look for kinks, restricted orifices and other causes that could restrict return flow.  The single rail system conversion places the regulator at or near the tank if you make that change, and the fuel rail itself is different.  What looks like a rail fuel regulator on the single rail system is actually a fuel damper.  There is no return port or return line back to the tank.

(3a) Novak sells an in-tank pump. I like this idea because currently fuel travels across the rear xmember from passenger to driver side at low pressure (the external pump is on the drivers side). I am suspicious that this low pressure run before the pump is where the vapor lock might be happening. The in-tank pump would eliminate this. (3b) I believe I'd also still have the option of locating the regulator in the rear and running a single line up to the engine (and could also be on the passenger side?) https://www.novak-adapt.com/catalog/fuel-system/cj-fuel-module

The low pressure or pickup side of the pump is always an issue if there is heat or an air leak on the draw side path.  Your pump is currently pulling fuel from the tank floor (via the sock) to the pump.  That's a considerable span for a pump designed primarily to push fuel.  (EFI kit Walbro type pumps are the same as OE pumps mounted inside the tank and designed to draw short spans then push long distances.)  And you're right, the pickup side is lower pressure and more vulnerable to vapor lock.  A clogged floor sock in the tank, pickup hoses drawing air, hose(s) overheating or hoses weak and collapsing can each be a problem.

The drawback I'm concerned about is that I do a lot of outdoor adventure road trips and remote backcountry travel. I carry a spare external fuel pump. If the in-tank pump fails, wouldn't this be much harder to replace on the trial?

Yes, the external pump is easy to reach or replace.  My son-in-law has been running a single rail Mopar EFI kit for some time, I installed it with the kit pressure regulator as near the tank as possible, using steel fuel pipes everywhere except at the flexing body/frame-to-engine and tank-to-frame sections, and he's had no problems whatsoever with vapor lock.

Do you think a spare external pump could still be installed inline if the in-tank pump failed to get me home? Or would the failed in-tnnk pump prevent fuel from flowing?

Good thinking...The failed in-tank pump has no bypass in the event of failure.  Attempting to draw fuel from an upstream pump through this stalled in-tank pump would be an issue.  I'd stay with your current system and concentrate on the tank pickup to the fuel pump section, routing steel piping along the frame and away from high heat sources.  Use high quality, stainless braid fuel hose if necessary at hoses from the tank to frame tubes and frame tubes to the engine rail(s).  The OEM chassis to fuel rail hoses generally serve well.  I also use amply sized steel pipe for the return line along the chassis with high quality hoses at the rail and tank ends.  

Note:  Steel brake/fuel pipe is much safer than fuel hose where steel pipe can be fitted.  I make "bubble flares", actually S.A.E. convex type or British Girling style, for the steel pipe-to-hose connections.  You'll find my how-to series on flaring tube and creating various flare types at:  http://www.4wdmechanix.com/video-series-how-to-flare-automotive-brake-tube-fuel-lines-and-cooler-tubing/.

So, which option do you think makes the most sense? (1), (1) & (2) Combined? or (3a) or (3b)

Concentrate on unrestricted fuel flow from the tank floor to the pump.  Use steel pipe routed away from heat along the frame rails.  Improve the tank-to-frame tubing hoses.  Make sure hoses will not collapse on the pickup side...Be certain there is sufficient fuel return flow, unrestricted between the rail and the tank's return pipe, listening at the fuel tank filler tube for actual fuel flow back into the tank...This is where I would start.

If that doesn't turn up a solution, check out your hose routings on the EVAP system.  EVAP hose routings or an improperly vented fuel tank can create symptoms of vapor lock.  A defective EVAP canister can also act like vapor lock.

Thanks!

 

 

[Lou]

Hi. I'm new to this forum and have an 82 CJ7 with the 258 and the MPI set-up. I believe I am having the same issue with vapor lock. I'm wondering what fixed the issue for you?

Thanks.

Lou