Fuel boiling at hot idle

I have 60 hours on my RV-8, Mattituck IO-360 (vertical induction) with Silver Hawk injection and two P-mags. After a few teething pains (to be expected), the thing runs great. The only outstanding issue is that when taxiing with hot engine, it starts to miss at low RPM settings after 3-4 minutes. This only occurs with a hot engine, on the ground, at low RPM, with ambient temperatures > 60F or so. I'm pretty sure this is due to fuel boiling somewhere in the lines, but I'm not sure what to do about it.

Here is some relevant information:

• Engine only misses when hot, on the ground, low RPM, outside air temperature > 60F or so.
• Increasing the RPM to 1100+ makes the problem go away.
• Temperature under the cowl, at the top near the oil filler door runs about 50-60F higher than outside air temperature. So it's 110-120F in there when this is happening, although it might be cooler above the cylinders on the forward side of the baffling.
• Turning on the electric boost pump has no effect, so the problem is not occuring upstream of the mechanical pump.
• Adjusting the mixture (manually and/or idle mixture screw) has no effect.
• Every fuel line firewall forward is firesleeved except the lines to the injector nozzles.
• No gascolator installed.
• Hot starts are never a problem -- only idling when hot.

I've talked to some experts and read just about every post I can find (on multiple forums) regarding this issue, but I still have some questions.

1) If this only happens on the ground with very little air moving over the engine, why would installing a blast tube to the mechanical pump make any difference? There wouldn't be much air miving through it...

2) Is there an easy way to tell where the fuel is boiling? Between the mechanical pump and servo, servo and divider, or divider and injectors?

3) At what temperature should 100LL be boiling? My under-cowl temperature probe is moveable, so I can put it just about anywhere to take measurements.

4) Has anyone tried putting firesleeve on the injector distribution lines? Do they even make it that small?

5) Should I just resign myself to the fact that a tightly-cowled engine is going to do this at idle sometimes? I never experienced this when operating similar engines in much hotter climates, but they weren't nearly as tightly-cowled as the RV-8.

6) Does anyone else experience this problem?

Any and all comments/ideas welcome.

Thanks.

[frankh]

I have an RV 7a with a Mattituck and an AFP system. I also run mogas exclusively and I don't experience this problem.

The only difference between my system and yours is that you run a mechanical pump (I don't have one), so while there may not be vapour locking going on at the inlet to the mechanical pump...It may be the engine block is transferring enough heat thru the pump to the fuel so that the fuel is warmer than it is in my system when it hits the flow divider.

Almost certainly the fuel is boiling in the SS feed lines to the injectors as there is a lot of surface area and very little volume...This is also the area of lowest pressure in a system that is at idle.

I would go get some small bore (1.8th or 3/16th) rubber hose and slit it down one side and slip over the lines..You can add a few wraps of fine safety wire (like the 020 wire) to keep it in place.

My bet is this will help a lot.

Frank

[Guy Prevost]

Geoff,

Here are my thoughts in no particular order.

The boiling temperature of 100LL is not fixed in your system, because it's not all at the same pressure. Areas of low pressure will have a lower boiling temp.

If you have any peaks in your tubing--a high spot followed by a low spot-- you invite boiling at that peak. It's a locallized low pressure area both because of it being higher and because of the flow characteristics. This is particularly true on the low pressure side of the system, but is also possible when the other parts are running at low pressure. i.e. the small tubing lines from the spider to the nozzles while at idle. These small lines are also more sensitive to little bits of vaporization, since the gas (vaporized fuel, not gasoline) bubble can easily be big enough to fill the whole diameter of the tube instead of just a small portion of it.

Frank's comments about the relationship between surface area, cross sectional area, and flow rate at this location make a lot of sense. Basically, you've got a little bit of fuel spending a lot of time (low flow rate), in contact with a lot of hot tubing surface area.

Any chance you could move your rich mixture stop a bit richer to prevent this? It might increase the fuel flow enough to prevent it.

Have you ruled out an ignition problem?

Cheers,
Guy

[AlexPeterson]

Geoff,

Here are my thoughts:

1. I second the thought that the boiling is probably occurring downstream of the fuel servo, causing "burps" of fuel to go in the cylinders in this low pressure area. I don't know the Silver Hawk system, but check with the manufacturer to see if you can put smaller restrictors in all the injectors. (I did this with my AFP system, and it greatly improved hot idle.) These smaller injectors will result in an increased pressure downstream of the servo. However, the idle mixture needed to be adjusted after this. Also, you will need to verify that the system will still deliver full throttle, full rich fuel flows without the restrictors limiting it.

2. I would caution against putting any insulation on the small injector lines. It could change the vibrational characteristics such that a line goes into resonance. It goes without saying how problematic that would be.

3. I don't know the typical idle pressure with the Silver Hawk system, but if it similar to the AFP system, the boiling temp of 100LL at that pressure (around 1 to 2 psi? - can't remember) is only around 145F. I have a chart of vapor pressure vs temp for 100 LL somewhere, I'll look for it.

Good luck - anywhere (except for the little injector lines) you can insulate things will help.

[Norman CYYJ]

You were wanting to know what the boiling temperature of 100LL is. Why not put some 100ll in a pot and place it on the stove with a thermometer and watch till it boils and note the temperature. You had best make sure your wife is not around she might object to you using her pots and stove.

[rjcthree]

I surely would not boil fuel inside. It should be a handbook thing anyway. Rick 90432

[cytoxin]

Originally Posted by Norman CYYJ

You were wanting to know what the boiling temperature of 100LL is. Why not put some 100ll in a pot and place it on the stove with a thermometer and watch till it boils and note the temperature. You had best make sure your wife is not around she might object to you using her pots and stove.

do what

[David-aviator]

Why would a partial vapor lock go away by increasing rpm unless the mechanical fuel pump pressure is low. You said the electric pump ON does not change the situation so it is not vapor lock due to low fuel pressure.

It may not be a fuel problem at all.

[AlexPeterson]

Originally Posted by David-aviator

Why would a partial vapor lock go away by increasing rpm unless the mechanical fuel pump pressure is low. You said the electric pump ON does not change the situation so it is not vapor lock due to low fuel pressure.

It may not be a fuel problem at all.

David, while the fuel pressure leading up to the fuel servo is around 20 to 25 psi, the regulated pressure to the injectors at idle is probably in the neighborhood of 1 psi. That's all a fuel servo is, a precision pressure regulator.

What Geoff is probably experiencing is quite common on FI RV's. The fuel in the small lines above the engine boils, momentarily burping out some fuel. It is not vapor lock, but an inconsistent fuel flow. The engine gets too much fuel, then too little, and so forth.

The problem goes away when the rpm is increased simply because the pressure increases accordingly. Additionally, the increased fuel flow reduces the residence time of the fuel in the "hot" area, reducing its temperature.

[Tandem46]

Geoff,

I have no solutions, but just know you are not alone. I have the same set up as you, IO-360, Silverhawk FI, and I've been flying my plane for over 4yrs. Every summer I get the exact same symptoms you describe, exactly. I don't like it, but I've learned to live with it. Just too tightly cowled and no way for heat to escape while on the ground.