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Vapor lock?


linluv84

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As soon as the ambient temp here got up above 105deg, I started having fuel issues. After about 20mins of freeway driving, the ammeter creeps up and the fuel pump noise changes. The car will sputter and eventually stall, only restarting after a good 30mins of cooldown. The intake manifold and intercooler cold pipe are too hot to even touch after freeway driving. I am using the stock RB25det intake with a McKinney intercooler and it seems like the cold pipe is picking up heat where it runs behind the radiator. The intercooler itself is right at ambient. I've switched to a higher flow pump (pierburg 250lph) and checked the fuel lines for clearance from the exhaust. My next step is to check my fuel pump supply voltage and look for a lower temp t-stat. Anybody else running an RB in the desert and experiencing similar issues?

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As soon as the ambient temp here got up above 105deg, I started having fuel issues. After about 20mins of freeway driving, the ammeter creeps up and the fuel pump noise changes. The car will sputter and eventually stall, only restarting after a good 30mins of cooldown. The intake manifold and intercooler cold pipe are too hot to even touch after freeway driving. I am using the stock RB25det intake with a McKinney intercooler and it seems like the cold pipe is picking up heat where it runs behind the radiator. The intercooler itself is right at ambient. I've switched to a higher flow pump (pierburg 250lph) and checked the fuel lines for clearance from the exhaust. My next step is to check my fuel pump supply voltage and look for a lower temp t-stat. Anybody else running an RB in the desert and experiencing similar issues?

 

I just ran into a similar issue with some record high temps here in the midwest - it appears that the fuel in the tank is getting heated up to the point that it causes cavitation issues at the pump inlet. Next time this happens, try putting your hand on the tank and see how hot it is.

 

http://fuelab.com/customer-support/28-avoiding-cavitation/79-avoiding-cavitation

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The tank was quite hot, actually. Maybe that fram G3 prefilter is the culprit. A small amount of sediment has built up over the past couple years, might be time to change it...

 

Restrictions at the inlet definitely do not help, but in the link I provided they mention that fuel temperature alone can cause this. They are saying that the fuel only needs to exceed ~120degF, which isn't much of a stretch if the ambient temps are 105. In my case I suspect that the really high flow pump that I am using is not helping - the regulator ends up returning much more fuel from the engine to the tank than a lower flow pump would, so in addition to the pump trying to pull more fuel from the tank, it's also causing more hot fuel to come back from the engine bay.

 

I'm looking into some heat management strategies, including insulating the fuel rails and post filter in the engine bay (trying to come up with something that doesn't look like sh1t ;) ), and adding a heat exchanger on the fuel return line back near the tank. Also thinking about adding the competition hood vent/scoop thingy if I can find one. I'll post something up about it when I find out if it's actually helping.

Edited by TimZ
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After about 20mins of freeway driving, the ammeter creeps up and the fuel pump noise changes.

This is inconsistent with being due solely to warm fuel. The pump is likely to pull less current when it cavitates. Even if it pulls more current, you should not be seeing that as in increase in ammeter readout. The ammeter only displays the net current going into or out of the battery. It does not measure how much current is pulled by the other devices in the electrical system.

 

Not saying that warm fuel isn't part of the issue, just that you probably have something else going on as well with your electrical system.

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Strange that every load I put on the system shows up on the ammeter. Thought it was supposed to work that way...

 

Under normal conditions, what does the ammeter read?

 

If the fuel pump is powered from the battery instead of alternator, that would explain the higher reading under increased load. That means you should see around +10A under normal conditions. Is that what you see?

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A fuel cooler on your return line is not going to completely fix the problem. The heat due to friction of the fluid going through the lines would be the only thing you fix and that wouldn't even be perfect. An in tank pump will help. The cavitation is from the fuel pump's suction pressure being below the saturation pressure (pressure where the fuel will change state for the given temperature) of the fuel on the low pressure side (assuming the fuel is vaporizing on the inlet side to your pump or in your pump, if it is after your pump then you have fuel system design issues, cover that later). To fix this, the amount of suction that is required by the pump to get the fuel to it (suction lift) needs to be reduced as it affects the difference from pump suction pressure to fuel saturation pressure. Even if you used an in tank or external pump mounted to tank below normal fuel level to feed your current pump it should do the trick.

 

Now if your fuel is boiling after the pump (not in but after it leaves) then your fuel system is too small for the fuel at the ambient temperatures. The heat added by the pump and by the friction going through the pipe would be causing the temperature of the fuel to rise above that of saturation temperature causing the fuel to boil. To prevent that the fuel system would have to be increased in size. Example: if you had 3/8" fuel lines then switching to 1/2" fuel lines would change to amount of friction felt by the fuel for a set amount of fuel flow. This concept can be seen in refrigerant systems for things such as refrigerators.

 

Previously you probably didn't have the issues because you were right at the borderline but the increased ambient temperature can put you right over the top. As for the increased current draw on the fuel pump I have no idea, it should be erratic if the pump is cavitating, no resistance to pump then normal, no resistance than normal.

Edited by Tequilawormz31
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