Vapor lock - check out these fuel trims

[J17GT]

Had the car out on Saturday running errands with lots of stop and go traffic. It was 90* out and I noticed after my last stop when the car sat for about 15mins that it was tough to start again. I had just under half a tank of fuel at the time.

My car has a gen5 whipple with an Arcane stage 3 fuel system on e85. I installed the fuel system and went to e85 last spring. I had my laptop in the car from an event a few weeks ago, so I hooked up my RTD to take a quick log. I downshifted and went WOT very briefly (you'll see below why this was the worst idea ever) and the car felt super lazy. I could not believe my eyes when I looked at the fuel trims. How was the car even running!? Fuel must have been vaporizing right in the rails. I feel extremely lucky that I did not hurt the engine.

I've driven the car plenty on hot days before, but I normally leave it pretty full on e85. I assume the passenger side saddle was pretty much empty with under half a tank, so no cooler fuel transferring from that side made matters worse. If I did not make a stop and notice the car was hard to start, I would have never known anything was potentially wrong. So yeah, about these return style fuel systems...

Here is the next morning with a full tank of e85 and 70* outside:

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[engineermike]

Did you include lift pressure in your log?

 

[J17GT]

Did you include lift pressure in your log?

Negative. Is that available on a gen2 car?

 

[engineermike]

Negative. Is that available on a gen2 car?

Oops, no it’s not.

 

[SheepDog]

I would say it is more likely that your pumps went dry - either because the cross over pickup/venturi isn't working properly, or that the pumps emptied the bucket and isn't refilling properly. (or perhaps an electrical issue) Vapor lock on a fuel injected return system isn't generally an issue, because your pressure regulator will relieve the pressure in the fuel rail, and the lift pump in the tank will force the vapor out of the rails. The stagnant fuel in the rail heats up and increases pressure, but the regulator bleeds that pressure off and back to the tank. Even at 1/2 tank, the pump bucket is completely submerged.

 

[J17GT]

Could the bucket go dry that fast? The trims were basically immediately bad. As soon as I hit WOT they jumped to +20% within 200rpms and then +40% within like 100rpms or so more.

It took 8 gallons to bring the tank back to full.

 

[engineermike]

What is your fuel pressure set to?

 

[J17GT]

What is your fuel pressure set to?

55psi with the boost reference disconnected.

 

[J17GT]

@SheepDog if there was an issue with the crossover / venturi from the passenger side, wouldn't this issue happen more often than not? To your point, the driver side saddle was most likely still full in my scenario that happened over the weekend.

I have so many logs of this car at the track, never have I seen this before. But I usually keep the tank at least 3/4 full.

 

[SheepDog]

Could the bucket go dry that fast

It depends-

If the umbrella valves at the bottom of the Radium bucket are blocked, or trapped by the Fuel pump filter socks, and the fuel level was just below the top of the bucket opening, the bucket could have been empty when you shut the engine off.

or, the cross over line is kinked or got disconnected

 

[SheepDog]

But I usually keep the tank at least 3/4 full.

In this scenario, fuel is filling the bucket from the top. There is a gap between the sides of the bucket and the top hat where fuel can basically just fill up the bucket with gravity. If the fuel level is below that gap, then it is imperative that the umbrella valves at the bottom of the bucket are flowing properly so that when the pumps are sucking in fuel, that suction draws in fuel from the bottom of the bucket, and also that your cross-over is working as intended.

The venturi that Radium uses isn't as good as the OEM system, but you shouldn't have starvation issues when everything is working properly. At least not on short WOT hits like this.

I'd pull the bucket out and make sure that

1-the cross over tube is connected, and not kinked
2- the umbrella valves move freely
3- there are no loose electrical connections

What pumps do you have?

 

[J17GT]

I'm pretty confident the cross over is working as intended. When I swap over to 93 for winter storage, I run a fuel drain tune that keeps the pumps on full-time with just the key on (no start). I tap into the return of the regulator and pump the e85 into cans. I can drain out the entire tank in no time, so I feel the cross-over is keeping up.

I run two TI 274 pumps. Juggernaut assembled the Radium bucket with the pumps for me. All I did was hook up the fuel level sender, connected the cross-over and dropped it in the car.

I did have a fuse issue early on due to a bad ground and one of the pumps was not running. That log showed just +10%ish compensation at this boost level I am running with only one pump. Obviously what I dealt with on Saturday is way worse.

I have another event coming up Friday/Saturday and unfortunately won't have time to pull the bucket out. I will inspect the connections as that is super easy. I am planning to swap out both pumps next spring just for maintenance reasons. They are cheap, so I figured why not. I'll inspect the bucket at that time. This car sees ~750 street miles and about 5 drag racing events per year.

Appreciate all the insight here!

 

[engineermike]

I was like @SheepDog doubting vapor lock but now I'm second guessing that. Check this out:

If the fuel pressure is 45 psi (vacuum reference connected), the boiling point at pressure is about 160 to 220 deg F, or as low as 120 deg F in the tank. Do we think it's plausible, with the full-on return style system and low fuel flow/residence time, that the fuel reached 160+ deg F in the rail or 120 in the tank? Does the return-style system circulate fuel through the rails or does the regulator bypass the rails? I'm starting to think maybe so.

If this is the case, I would suggest raising the fuel pressure. The gen3 runs 65 psi or higher when the rail temp is above 176 deg F. However, you will need to make a change in the tune to account for it, being a Gen2. This would only help in the rails, though, not if the problem is happening in the tank.

 

[J17GT]

Very interesting @engineermike my first clue of suspected vapor lock was the extended crank time to start the car after it sat for 10-15mins. I thought the starter was going to time out, but at the last second the car stumbled to life. The car never does that typically.

My rails run in parallel with the regulator AFTER the rails. So yes, all fuel cycles through the rails before returning to the tank. As you know, these whipple rails are tucked in nice and tight under the supercharger lid.

I probably had about an hour of run time on Saturday. The car was about as hot as you could get it. I popped the hood when I got home and the prop rod about burned my hand. I'm sure the fuel rails were HOT.

With no cooler fuel coming from the passenger side and constant recycling of hot fuel to the driver saddle with piping hot fuel rails....I'm talking myself into the majority of the fuel was vaporizing in the rails... But @SheepDog does point out some very good things to check as well.

 

[SheepDog]

Still though, unless your pressure regulator is making a perfect seal when the system is off, meaning that no fuel is leaking down and back into the tank ( I know mine doesn't, and leaks down almost instantly) the pressure in the rails is being relieved when the engine isn't running. It would seem concerning to me that, the fuel rails are getting so hot when the engine shuts off, that new fuel entering the rail upon startup, is then instantly boiling. One of the disadvantages of a PD blower I guess, that all that heat is being trapped and held on top of the motor.

Vapor lock can happen at the pump outlet as well, so I suppose that if the fuel in the tank was at or above 120F as @engineermike pointed out, and the pumps were hot as shit from running 100% as these systems do, it may be the problem.

My Fuel-it ethanol sensor reads temp as well, and there are times when the fuel returning to the tank is over 110 and even as high as 120 degrees on really hot days, and especially in slow moving situations where the engine bay temp is high. That fuel is then being dumped right on top of the pump in the bucket, but also being mixed with the fuel around it.

It probably wouldn't be too hard to wire in a temp sensor that lives in the bucket, and use some of the vacant terminals on the hat to send that to a gauge

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